Primary FRCA in a Box, 2nd Edition PDF

https://t.me/Anesthesia_Books Primary FRCA in a Box, 2nd Ed Sarah Armstrong MA MB BS FRCA Consultant anaesthetist Frimley Health NHS Foundation Trust Barry Clifton MB ChB FRCA Consultant anaesthetist...

https://t.me/Anesthesia_Books Primary FRCA in a Box, 2nd Ed Sarah Armstrong MA MB BS FRCA Consultant anaesthetist Frimley Health NHS Foundation Trust Barry Clifton MB ChB FRCA Consultant anaesthetist Barts Health NHS Trust Lionel Davis MB BS FRCA Consultant anaesthetist Barts Health NHS Trust and Homerton University NHS Trust CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2019 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business https://t.me/Anesthesia_Books No claim to original U.S. Government works Boxed Cards ISBN: 978-1-4441-8063-3 This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reli- able data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright. com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for- profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com For Mark, Jamie and Alex Sarah Armstrong For Subha and Matilda Barry Clifton For my family Lionel Davis Introduction The Primary FRCA exam is a daunting task facing any trainee anaesthetist. Many currently available books cover the enormous syllabus in detail, and we have not attempted to duplicate these. The sec- ond edition of these revision flashcards is precisely mapped to the FRCA curriculum. We have added a new anatomy section and clinical ‘nuggets’ to relate the basic sciences to clinical practice. The cards focus on the premise that structuring your answers into an initial definition followed by a catego- rization of the topic will increase your chances of saying something sensible and reduce that feeling of impending doom in a viva situation. The cards are designed to fit into a scrubs pocket for quick reference and thus act as a more convenient revision aid than conventional texts. They will also be of use to those studying for the Final FRCA who wish to have a concise summary of Primary FRCA topics. We wish you the best of luck in your exam! SA, BC and LD Contents Physics Basic Principles of Measurement 10 Types of data and their Thermodynamics 1 Measuring system characteristics representation 19 Heat and laws of Measurement systems – static 11 Choice of simple statistical tests thermodynamics characteristics for different data types Heat definitions 2 Measurement systems – Frequency distributions 20 Temperature definitions dynamic characteristics 21 Measurement of temperature Basic measurement concepts SI Units Graphs relating to temperature 3 Resonance and damping, 12 Base SI units 22 Clinical aspects of heat and frequency response Derived SI units temperature 4 Calibration 13 Electrical and magnetic Patient warming systems Calibration and damping derived units 23 Humidity Non-SI units of relevance to 24 Humidifiers Mathematical Concepts and anaesthesia Statistics Electricity and Magnetism 5 Mathematical concepts Simple Mechanics and Pressure 25 Basic concepts 6 Logarithms and exponentials 14 Simple mechanics 26 Electrical interference 7 Exponential processes Pressure definitions Biological signals Hierarchy of evidence 15 The gas laws – for ideal gases 27 Measurement of neuromuscular 8 Randomised controlled trials Pressures and temperatures blockade 9 Errors 16 Pressure measurement in 28 Electrical hazards – causes Power gases 29 Electrical hazards – prevention Sensitivity Bourdon pressure gauge 30 Principles of lasers Specificity Piezo-electric effect 31 Laser safety PPV 17 Pressures of anaesthetic Electrical symbols Accuracy equipment 32 Wheatstone bridge RRR Manufacture and storage of Circuit breakers, fuses, ARR gases transformers, transistors, NNT 18 Gases and vapours diodes Isotherms for nitrous oxide 33 MRI scanners and anaesthesia 34 Principles of cardiac 48 Measurement of cardiac output Physiology pacemakers 49 Capnography General Principles 35 Defibrillators 50 Pulse oximetry 62 Effects of old age on 36 Diathermy 51 Measurement of gas and anaesthesia Problems with diathermy vapour concentrations Paediatric anatomy and 52 Measurement of gas and physiology Fluids and Flow vapour concentrations 63 The cell 37 Laminar and turbulent flow Measurement of pH 64 Cell membrane characteristics 38 Bernoulli principle 53 Measurement of pCO2 Genes and their expression Venturi masks Measurement of PO2 39 Surface tension 54 Derived measurements – Biochemistry Surfactant bicarbonate and 65 Acid–base balance 40 Measurement of volume and base excess 66 Buffers flow in gases and liquids Oxygen consumption, carbon 67 Sodium 41 The rotameter dioxide production and 68 Potassium 42 Ultrasound and the Doppler effect respiratory quotient 69 Causes of acid-base 43 Simple tests of disturbances pulmonary function Equipment 70 Bicarbonate and phosphate 44 Tests of gas exchange 55 Classification of vaporizers 71 Calcium Coanda effect Factors that affect delivered 72 Magnesium concentration of anaesthetic 73 Chloride Clincal Monitoring and Measurement agent Enzymes 45 Minimum (essential) monitoring 56 Types of vaporizers standards 57 Methods of killing Body Fluids ECG – principles contaminating 74 Body fluid compartments Placement of leads organisms Measurement of fluid 46 Principles of pressure Classification of ventilators compartment volumes transducers 58 Breathing systems 75 Osmolality and osmolarity NIBP measurement 59 Scavenging Starling’s forces 47 Principles of pulmonary 60 Carbon dioxide absorbers 76 Cerebrospinal fluid artery and wedge pressure 61 Suction Blood–brain barrier measurement CO2 removal systems Specific gravity Contents (Continued) 77 Other body fluids: Pleural, 92 Spinal cord 109 Valsalva manoeuvre Pericardial 93 Spinal cord anatomy 110 Exercise 78 Other body fluids: Lymph, Spinal cord injury 111 Pressures within the normal heart Intraocular 94 Pain pathways and pulmonary circulation Haematology and Immunology Gate control theory of pain Shock 79 Blood groups 95 Intraocular pressure 112 Coronary artery blood supply 80 Immune responses Pupillary responses 113 Special circulations Hypersensitivity 96 Nausea and vomiting 114 Pulmonary vascular resistance 81 Inflammation Muscle Factors that affect pulmonary 82 Haemostasis 97 Neuromuscular junction vascular resistance Peripheral circulation 98 Muscle types and contraction 115 Central venous pressure 83 Clotting cascade 99 Muscle components Venous waveform Coagulation – cell-based 100 Sarcomere Renal model Disorders of neuromuscular 116 Structure and function of the 84 Haemoglobin function kidney Red blood cells 101 Myopathies Renin–angiotensin–aldosterone Nervous System Sphincters system 85 Resting membrane potential Heart and Circulation 117 The glomerulus Neurones and nerve fibres 102 Cardiac action potentials 118 Renal tubular function 86 Action potentials 103 Cardiac definitions Proximal convoluted tubule 87 Organisation of the nervous 104 Control of blood pressure 119 Loop of Henle, DCT and system 105 Cardiac cycle collecting duct Motor pathways 106 Pressure–volume loop for the left 120 Renal glucose handling 88 Special senses ventricle Sodium and potassium handling Somatic sensation Autoregulation by the kidney 89 Intracranial pressure and 107 Cardiovascular response to 121 Renal blood flow cerebral blood flow haemorrhage Clearance 90 The vasomotor centre Response to the rapid infusion 122 Assessment of renal function Autonomic nervous system of 1000 mL saline Micturition 91 Sympathetic nervous system 108 Frank–Starling law of the heart 123 Pathophysiology of acute Parasympathetic nervous system Heart failure kidney injury Respiration Liver, GI and Metabolism 150 Hypothalamus 124 Oxygen dissociation curve 138 The liver Pituitary gland 125 Work of breathing Functions of the liver 151 Adrenal gland Shunt 139 The pancreas 152 Catecholamines 126 FRC and closing capacity 140 Gastric secretion Adrenergic receptors Functions of FRC 141 Gut motility – functional 153 Thyroid gland 127 Hyperbaric pressure anatomy Thyroid hormones Hypobaric pressure 142 Nutrition overview 128 Carbon dioxide stores Carbohydrates, proteins Pregnancy and transport and fats 154 Changes in pregnancy 129 Oxygen stores and transport 143 Essential amino acids and 155 Placenta 130 Control of breathing fatty acids 156 Fetal circulation 131 Ventilation–perfusion Vitamins and minerals Changes in fetal circulation relationships in the lungs 144 Carbohydrate metabolism – at birth West’s zones overview 157 Lactation 132 Dead space 145 Metabolism Massive obstetric 133 Lung volumes and capacities Starvation haemorrhage 134 Respiratory failure 146 Obesity Treatment of respiratory failure Anaesthesia for 135 Oxygen cascade and alveolar obese patients gas equation 147 Temperature regulation Pharmacology 2,3-Diphosphoglycerate Hypothermia Principles and myoglobin 148 Control of blood glucose 158 Drug interactions 136 Airways resistance Stress response 159 Lipid solubility and and compliance protein binding Compliance curve Drug mechanism of action Intrapleural pressure Endocrinology 160 Isomerism 137 Flow-volume loops of the lungs 149 Hormones 161 Malignant Hyperpyrexia Non-respiratory functions of Control of secretion 162 Materno-fetal drug distribution the lungs of hormones 163 Addiction and dependence Contents (Continued) Pharmacokinetics 180 Propofol 192 Rocuronium and atracurium 164 Absorption of drugs Thiopentone Sugammadex Drug entry into cells 181 Ketamine 193 Suxamethonium 165 Drug distribution Etomidate Suxamethonium apnoea Dose–response curves 182 Benzodiazepines – general 194 Local anaesthetics Compartmental models principles 195 Local anaesthetic toxicity 166 Drug metabolism 183 Midazolam 167 Elimination and excretion Diazepam Cardiovascular System Drugs 196 Inotropes Pharmacodynamics Inhaled Anaesthetics 197 Adrenaline and noradrenaline 168 Agonists and antagonists 184 Inhalational anaesthetics – 198 Phosphodiesterase III inhibitors 169 Ionization and pKa general principles 199 Alpha-antagonists 170 Receptors Theories of anaesthetic action 200 Alpha agonists 171 G protein–coupled receptor 185 Minimum alveolar 201 Antihypertensive drugs NMDA receptors concentration 202 Glyceryl trinitrate 172 GABA 186 Speed of onset of volatile Sodium nitroprusside Adverse drug reactions anaesthetics 203 Hydralazine 187 Important structures – inhaled Nifedipine Analgesia anaesthetics 204 Beta blockers 173 Aspirin Important structures – 205 Drugs used in ischaemic Paracetamol IV anaesthetics heart disease 174 Opioids – definitions and receptors 188 Inhaled anaesthetics – 206 Antiarrhythmics 175 Opioids – common effects common properties 207 Digoxin Opioids – dosage and Inhaled anaesthetics – other properties noticeable differences Central Nervous System Drugs 176 Comparative features 189 Oxygen 208 Antidepressants of opioids 190 Nitrous oxide Amitriptyline 177 NSAIDs Nitric oxide 209 Phenytoin IV Anaesthetics Carbamazepine and 178 General principles Other Anaesthetic Drugs sodium valproate 179 TIVA and context-sensitive 191 Non-depolarizing muscle 210 Antiemetics half-time relaxants Anticholinergics Miscellaneous Drugs Anatomy 230 Cutaneous nerves of 211 Cyclizine Respiratory System upper limb Drugs acting on the uterus 221 Anatomy of the larynx Cutaneous nerves of lower limb 212 Anticholinesterases 222 Anatomy of the nose 231 Brachial plexus Neostigmine 223 Mediastinum Stellate ganglion 213 Warfarin Diaphragm 232 Anatomy of the vagus nerve Heparin 224 Tracheobronchial tree 233 Trigeminal nerve 214 Oral drugs used in diabetes 225 Thoracic inlet and first rib 234 Anatomy of the orbit Insulin Intercostal space 235 Cranial nerves 215 Antimicrobial drugs 236 Internal jugular vein 216 Diuretics Cardiovascular System 217 Corticosteroids 226 Structure of the heart and great Vertebral Column Drugs used in thyroid disease vessels 237 Sacral anatomy and caudals 218 Colloids Cross section of the neck at C6 Crystalloids Nervous System 238 Vertebrae 219 Drugs that act on 227 Bones of the skull gastrointestinal tract Base of the skull Surface Anatomy 220 Antiparkinsonian drugs 228 Cerebral arterial supply 239 Antecubital fossa Respiratory stimulants Venous drainage of head and Axilla neck 240 Abdominal wall 229 Epidural and paravertebral Femoral triangle and lumbar space plexus 241 Sacral plexus and sciatic nerve 242 Lower limb blocks  Physics Basic Principles of Measurement 1 Measuring system characteristics Measurement Measurement converts a value of a physical quantity into a form that can be observed and recorded and that is repeatable and calibrated In medical monitoring, data can be collected directly if it is electrical Data may also be an evoked signal, such as in monitoring of neuromuscular blockade, or it may be a transduced signal A transducer will convert the input into usable data or a signal, usually an electrical signal, e.g. a thermistor (temperature to electrical) or a piezoelectric device (pressure changes to electrical) Signal-to-noise ratio (SNR) Signal-to-noise ratio is a measure of the amplitude of the electrical signal compared with the amplitude of the background interference (noise) It is defined as a ratio of the signal power to the noise power The noise is mainly the result of power line frequency signals and radiofrequency signals as well as muscle activity added to the ECG A poor SNR can be improved by eliminating the noise, differential amplifiers, filters and averaging a repetitive signal When the amplitudes are measured in Volts or Amperes: signal amplitude SNR = 20 log10 noise amplitude Measurement systems – static characteristics Static characteristics define the performance when the input is not changing Accuracy is the degree of conformity of a measured or calculated quantity to its actual (true) value. This is often quoted as a percentage Precision (also called reproducibility) is the degree to which further measurements or calculations will show the same or similar results. If a result is accurate and precise, it is called validity Sensitivity is the relationship between the change in output reading and the measured quantity. Less sensitive systems allow for a greater range Linearity is a measure of the degree to which the displayed value is proportional to the true value. On a graph of input against output, the ideal shape would be a straight line with the gradient being the sensitivity Non-linearity can be expressed as the maximum difference between the displayed and the actual value or as this difference as a percentage of the maximum output. The rotameter is an example of an instrument which is intrinsically non-linear Hysteresis is a measure of the difference between the displayed value and the true value depend- ing upon whether the true value is increasing or decreasing. It is seen with stretching and relaxing of solid materials (e.g. in pressure transducers) and is due to loss of energy as friction and heat. A graph of true value versus displayed value will have two lines: one for the true value increasing and one for it decreasing Drift is a measure of the degree to which the displayed value changes over time and is usually caused by temperature changes or unstable components in the system. Drift is corrected by zeroing  Physics Basic Principles of Measurement 2 Measurement systems – dynamic characteristics Dynamic characteristics reflect the ability of the measuring system to respond to rapidly changing inputs: The dynamic response can be one of three types: Zero-order response – the displayed value tracks the measured value exactly First-order response – the displayed value moves towards the true value exponentially (e.g. in a temperature probe) Second order – the displayed value may approach the true value like a first-order response or may oscillate around the true value (for example, in invasive blood pressure monitoring) Step response: The response to a rapid increase of the system being measured It is reflected by the: Response time – the time from the ‘step’ increase in the system being measured to 90% of the output being displayed Rise time – the time taken for the output measurement to increase from 10%–90% of the final value Phase shift response: Any signal can be broken down into component frequencies (Fourier analysis) Each of these frequencies will undergo a different time delay as it passes through the measur- ing system and this can distort the measurement Damping Frequency response Basic measurement concepts Input Transducer Device to convert one form of energy into another Transmission path Amplification Signal conditioning – Processes signal for Filtering unit display/storage Conversion analogue digital Display/storage component Output  Physics Basic Principles of Measurement 3 Resonance and damping, frequency response Any system that oscillates (such as a pendulum) does so at a natural frequency; it is known as the resonant frequency and is determined by inertial and compliance elements Energy imparted to a system at its resonant frequency will amplify the signal. In invasive blood pressure measurement, resonance must be avoided because it causes distortion of the wave- form and leads to errors in measurement Fourier analysis is the mathematical separation of waveforms into sine wave components at the fundamental (slowest) frequency and harmonics (multiples of the fundamental frequency). The more harmonics reproduced, the more accurate the signal breakdown. Analysis up to the tenth harmonic is required for accurate invasive BP measurement In order to avoid distortion, the resonant frequency of the invasive blood pressure measurement system must be manipulated so that it is out of range of the operating frequency. This manipulation must include the fundamental frequency and all harmonics to the tenth harmonic of the blood pressure waveform. This is achieved by using short, stiff-walled, wide catheters with no blood clots or air bubbles and minimal connections The bandwidth is the range of frequencies over which the measurement system will respond and should be 0–20 Hz for invasive arterial blood pressure Resonance and damping, frequency response (cont.) Frequency response is the response of the system (gain) plotted against the signal frequency. As the frequency response is variable, there may be inaccuracy in the measured output; this is maximal at the resonant frequency Damping is a progressive decrease in the amplitude of oscillations because of dissipation of energy. Mechanical damping is necessary with direct arterial blood pressure monitoring to pre- vent amplification of the waveform by subsequent pulsations With an underdamped system, the oscillations continue for a long time; falsely high systolic and falsely low diastolic pressures will be displayed with invasive blood pressure monitoring Critical damping occurs when there is a rapid fall in pressure just avoiding overshoot; the damp- ing factor, D, is 1.0 Although overdamping avoids overshoot, the system will be slow to respond, therefore causing falsely low systolic and falsely high diastolic pressures but with accurate mean arterial pres- sure (MAP). Overdamping is common and is caused by air bubbles or blood clots Optimal damping occurs when the damping factor, D, is 0.64; this produces the most rapid response while avoiding excessive oscillations  Physics Basic Principles of Measurement 4 Calibration Calibration is the setting up or correction of a measuring device or base level, usually by adjust- ing it to conform to a dependably known and unvarying measure A calibration curve is the graphical representation of the functioning relationship between the expected value of the observed signal to the measured amount Calibration aims to remove the effects of drift on the measurement. Drift may be gradient drift (measurement value increases disproportionately to increasing input value) or offset drift (where every measured value has a constant drift). These require one-point calibration. A combination of offset and gradient drift requires two-point calibration Calibration and damping Underdamped Offset + gradient drift Gradient drift Critical damping Displayed value Overdamped Pressure Offset drift Valid measurement True value Time Physics Mathematical Concepts and Statistics 5 Mathematical concepts Physics Sinusoids Repetitive processes of the body, such as traces from ECGs or invasive blood pressure, can be represented by waveforms. These waveforms can be formed by combining sine waves of different amplitude and frequency Amplitude The maximum displacement of the wave from the horizontal axis Wavelength Literally, the length of one wave from one corresponding peak to the next or one trough to the next. One cycle is one complete wavelength. This corresponds to 360° (that is, one complete revolution). One sine wave that is 360° after another identical sine wave will superimpose on the first and is therefore in phase. Two otherwise identical sine waves that are 180° out of phase will cancel each other out Frequency The number of complete cycles in one second; the SI unit of frequency is hertz. The period is the time taken for one cycle to occur and is the reciprocal of the frequency (Period = 1/Frequency) Velocity Frequency × Wavelength Parabolas Conic section (as are the circle, ellipse and hyperbola) that has an equation of the form y = Ax2 + Bx + C. A reflector with a parabolic cross-section may be used to focus light into a parallel beam. This is used in theatre lights Mathematical concepts (cont.) Differentiation and integration Equations can help us understand and demon- strate how relationships behave in the natural Differentiation will find the slope of the curve world. Equations can be plotted on a graph to (the rate of change at a given point in time) give a visual image d n d d x x = nx n−1 sin x = cosx e = ex Straight line y = mx + c Parabola y = ax 2 + c dx dx dx y Integration will find the area under the curve Circle c Ellipse 1 ∫ n x dx = n +1 x n+1 + c Parabola Example: o x Hyperbola 1 ∫ x 5 dx = x 6 + c 6 100 Concentration (µg/mL) 80 Sinusoidal y = sin x Rectangular hyperbola AUC y 60 AUC segment 1 y= x 40 20 x 0 0 2 4 6 8 10 Time (hour)  Physics Mathematical Concepts and Statistics 6 Logarithms and exponentials Logarithm The general form of a negative exponential If y = ax, the logarithm to the base a is defined equation is y = Ae−kt With a true negative exponential process, the by x = logay For example, the log10 of 10 is 1, the log10 quantity will never reach zero because the of 100 is 2, the log10 of 1000 is 3, etc. rate of change becomes smaller and smaller, The natural logarithm (loge) is also denoted therefore the process goes on indefinitely Taking the natural logarithm of both sides of ln, where e = 2.718 and is known as Euler’s the negative exponential equation y = Ae −kt number gives: Exponential ln(y) = ln(Ae−kt) = lnA + ln(e−kt) = lnA – kt As this is an equation for a straight line, a In an exponential process, the rate of graph of the natural logarithm of an exponen- change of a quantity at any given time is tial function against time will be a straight line proportional to the quantity at that time Exponential build-up Exponential decay An example of a wash-in or build-up expo- An example of a negative exponential pro- nential processes is the intake of volatile cess is the rate of metabolism of certain anaesthetic agents, in which the concen- drugs being directly proportional to their tration of agent in the alveoli exponentially concentration in the plasma; this produces approaches that in the inspired gas a wash-out curve Another example is lung volume during infla- In normal lungs, the rate of ‘wash out’ of tion with a pressure generator ventilator nitrogen in pulmonary function tests is pro- This follows the equation y = (1 – Ae−kt) portional to its concentration Logarithms and exponentials (cont.) Exponential (cont.) Time constant for the lung = compliance mul- Positive exponential tiplied by resistance The reciprocal of the time constant is equal Bacterial growth is a tear-away (positive) to the rate constant, k exponential function, because the rate of The rate constant, k, is also known as the con- growth is proportional to the concentration at stant of proportionality, because the prod- any given time uct of the quantity and k is equal to the rate of The general form of an exponential equation change of the quantity for a tear-away function is: y = Aekt Rate of exponential processes Half-life is the time taken for the quantity to Clinical nugget decrease to half the current value Lungs with high compliance (e.g. emphy- Time constant (τ, tau) is the time that it would sema) and high resistance (e.g. broncho- have taken to complete the process had the constriction) will have an abnormally high initial rate of change continued time constant and will need additional After one time constant, the quantity has time for gas emptying. decreased to 37% of its initial value; time con- stant is therefore longer than half-life  Physics Mathematical Concepts and Statistics 7 Exponential processes Wash out/exponential decay y For example y = Ae–kt most drug elimination wash-out of nitrogen when a patient switches to breathing oxygen from a non-rebreathing circuit y y = (1 – Ae–kt) Wash in/build up t     Preoxygenation (wash-in of oxygen) y t y = Aekt Positive exponential For example development of action potential growth of bacterial colony t Hierarchy of evidence 1. Systematic reviews/meta-analyses 2. Randomised controlled trials 3. Cohort studies 4. Case-control studies 5. Cross-sectional surveys 6. Case reports THE COCHRANE Systemic review COLLABORATION® Explicit objectives using stated methods and materials Funnel plot Explicit and reproducible methodology A funnel plot is a scatterplot of treatment effect Clear question to be answered against a measure of study precision. It is used Widespread search primarily as a visual aid for detecting bias or sys- Assess quality of studies found, apply eligibil- tematic heterogeneity. A symmetric inverted fun- ity criteria and exclude without prejudice nel shape arises from a ‘well-behaved’ data set, Gain as much raw data as possible (contact in which publication bias is unlikely. authors) Apply meta-analysis, plot findings (see 0 ‘Forest plot’) 0.2 Publication bias? (funnel plot) Standard error 0.4 Find explanation for findings 0.6 Forest plot 0.8 The logo for the Cochrane Collaboration is a for- 1 est plot which represents the meta-analysis of a 1.2 systemic review of multiple studies testing ante- –4 –3 –2 –1 0 1 2 3 4 natal steroids and respiratory distress in neonates Log risk ratio  Physics Mathematical Concepts and Statistics 8 Randomised controlled trials The gold standard design of clinical trials because it is considered the best known way of eliminating bias Requirements Clearly defined aims, methods and statistical analysis Relevant Original Robust methods Ethics committee Must be obtained before patients recruited to a trial approval Committee has professional and lay members who determine whether the trial is justified Patients must be given relevant explanation of the purpose of the study and a consent form Power analysis Calculation of the sample size necessary to detect a certain statistical difference between treatment groups if a true difference exists Part of the design of the study Required result must be clinically significant as well as statistically significant Recruitment Strict inclusion and exclusion criteria Randomisation Carried out to make sure that all participants are equally likely to end up in any treatment group, i.e. to minimise bias Stratified randomisation can be used to minimise the differences in age, weight, ASA status, etc. Treatment compared with placebo, existing treatment or no treatment Randomised controlled trials (cont.) Blinding Used to reduce the chance of ascertainment bias In single-blind studies, the patient does not know what treatment they are receiving In double-blind studies, neither the patient nor the investigator knows which treatment is being given Possible causes Selection bias: methodical difference in acceptance or rejection for inclusion in of errors a trial or treatment group Ascertainment bias: knowledge of treatment given Drop-out bias: drop out more common in one treatment group Poor randomisation Unreliable data collection Poor choice of statistical tool Variability May result from instrument precision or observer variability May be reduced by using identical techniques, healthcare professionals, surroundings, etc. for all patients Data collection Guidelines should be drawn up and available, and data collectors appropriately trained The means of data collection and analysis should be established before the study is started Machinery and monitors must be tested and calibrated Endpoint Must be determined with the total number studied or by periodic analysis and termination of the trial when results become significant Publication Should include a comprehensive account of the methods used so that readers can assess their validity  Physics Mathematical Concepts and Statistics 9 Errors Sensitivity α is a type I error or false positive The number of true positives divided by the The probability of a positive finding from a total number with the condition study being wrong Describes the ability of a test to identify true Represented as the p value and 0.05 is positives (or exclude false negatives) usually the maximum accepted As the p value is reduced, the risk of reject- Condition Condition ing a statistically significant result (false present absent negative) increases β is a type II error or false negative Test positive a b The chance of not picking up a difference Test negative c d when a difference actually exists Highest acceptable value is usually 0.2 Sensitivity = a ÷ (a + c) Power Likelihood of the null hypothesis being cor- Specificity rectly rejected The number of true negatives divided by the Equals 1 − b total without the condition With a power of 0.8, there is an 80% chance of Describes the ability of a test to identify true showing a statistical difference if such a differ- negatives (or exclude false positives) ence actually exists Specificity = d ÷ (b + d) Positive predictive value (PPV) The number of true positives divided by the If events are common (i.e. A is large), relative total number with an abnormal test risk reduction underestimates the treatment Describes the ability of a test to predict true effect abnormality Positive predictive value = a ÷ (a + b) Absolute risk reduction (ARR) Negative predictive value = d ÷ (d + c) The absolute decrease in percentage risk of an adverse event by giving a certain treatment Accuracy If the risk of the event is reduced from A% to The sum of the true positives and true nega- B%, the absolute risk reduction is (A – B)% tives divided by the total Number needed to treat (NNT) Relative risk reduction (RRR) The number of patients needed to be given a Ratio of the probability of an adverse event certain treatment for one patient to have the occurring in a treatment group versus the desired effect ­control group Number needed to treat = 1 ÷ absolute risk If the risk of the event is reduced from A% to reduction B%, the relative risk reduction is [(A − B) ÷ A] If events are rare (i.e. A is small), relative risk reduction overestimates the treatment effect  Physics Mathematical Concepts and Statistics 10 Types of data and their representation Statistics describe data from samples that are Distribution parts of a population of similar items, events Central tendency or observations Median Mode Null hypothesis Median There is no difference between two Scatter ­samples – they are both taken from the same Percentiles population Standard deviation The object of trials is to reject the null hypothesis Types Descriptive statistics simply describe the The p value sample data The probability of a given result occurring by Inferential statistics are used to infer some- chance thing about the population itself p < 0.05 means that there is a less than 1 Qualitative data are names or labels in 20 probability of that result occurring by Quantitative data are numerical chance; it is the usual value required for sta- tistical significance and to reject the null hypothesis Types of data and their representation (cont.) Qualitative data Parametric or normally Nominal or categorical data have no partic- distributed data ular order, e.g. type of operation; represented Characteristic symmetrical bell-shaped curve by the mode (most common category) Mean is sum of values divided by number of Ordinal data are sequential but not numeri- values cal in that, for example, twice the value is Standard deviation is equal to the square not double the magnitude, e.g. pain scores; root of the variance represented by the median (middle value ­ Variance = Σ(x − x )2 ÷ (n − 1) when placed in ascending order) and Approximately 68% of the data lies within one ­p ercentiles (percentage at or below a standard deviation on either side of the mean ­p articular value) and about 95% within two standard deviations Mean = mode = median Quantitative data The standard error of the mean represents Continuous data can be any number includ- how certain we can be that the mean of a ing fractions, e.g. age sample corresponds to the population mean Discrete data can only be a whole number, It equals the standard deviation divided e.g. heart rate by the square root of the number of Ratio data – the zero value is truly equal to observations naught, e.g. degrees Kelvin There is a 95% chance of the true mean Interval data does not include a true zero, lying within two standard errors of the e.g. degrees Celsius population mean; this is known as the 95% confidence interval  Physics Mathematical Concepts and Statistics 11 Choice of simple statistical tests for different data types Qualitative data may be compared using the chi-squared test, which compares the frequency of observed results to the expected frequency if there were no difference between groups. Fisher’s exact test is used if any expected frequency is less than five The type of test used to compare quantitative data depends upon whether or not it is normally distributed Normally distributed data from two groups may be analysed with the Student’s t-test, which compares the mean and standard deviation for each group Non-normally distributed data from two groups may be compared with the Mann–Whitney U-test With more than two groups, multiple comparisons are made and there is a 5% chance of obtain- ing statistical significance by chance alone if the above tests are used For normally distributed data, therefore, analysis of variance (ANOVA) is used instead For non-normally distributed data of more than two groups, the Kruskal–Wallis test is used If paired data is used, paired Student’s t-test or paired ANOVA tests may be used for normally distributed data For non-normally distributed paired data, the Wilcoxon signed-rank test can be used or the Friedman’s test can be used for more than two groups Frequency distributions Normal (Gaussian) distribution Parametric (skewed) distribution Mode Median 34.1% 34.1% Mean 2.1% 2.1% 0.1% 13.6% 13.6% 0.1% f –3σ –2σ –1σ µ 1σ 2σ 3σ Outliers The mean, median and mode are identical in a normal distribution. Physics SI Units 12 Base SI units Measure SI unit Description Length Metre (m) The distance light travels in a vacuum in a specified time Mass Kilogram (kg) The mass of a specific piece of platinum–iridium alloy kept at Sèvres near Paris Time Second (s) The frequency of radiation emitted by caesium-133 Current Ampere (A) The current that produces a force of 2 × 10−7 N/m between two straight parallel wires of infinite length that are one metre apart in a vacuum Temperature Kelvin (K) 1/273.16 of the temperature of the triple point of water – the point at which water vapour, ice and liquid water exist in equilibrium. Temperature is the property of a substance that determines whether heat is transferred to or from it Luminous Candela (cd) A description of a physical process that will produce one candela intensity of luminous intensity Amount of Mole (mol) The quantity of substance that contains as many particles as there Substance are atoms in 12 g of carbon-12 Derived SI units Measure Derived SI unit Description Temperature Degree Celsius The magnitude of a degree Celsius is equal to that of a degree Kelvin. The relationship is °C = K − 273.15 Force Newton The force required to accelerate a mass of one kilogram at one metre per second per second. 1 N = 1 kg ⋅ m ⋅ s−2 Pressure Pascal One pascal is equal to one newton per square metre. 1 Pa = 1 N ⋅ m−2 Energy Joule The amount of energy required to move the point of application (or work) of a force of one Newton a distance of one metre. 1 J = 1 N ⋅ m Electronvolt (eV) The work done moving one electron through a potential difference of one volt in a vacuum Power Watt (W) The rate of expenditure of energy in joules per second. 1 W = 1 J ⋅ s−1 Frequency Hertz (Hz) The rate in cycles per second 3 Volume Cubic metre (m ) Litre (L) One thousandth of a cubic metre Density Kilograms per cubic metre (kg ⋅ m−3) Velocity Metre per second (m ⋅ s−1) Acceleration Metre per second per second (m ⋅ s−2) Physics SI Units 13 Electrical and magnetic derived units Measure Derived SI unit Description Charge Coulomb (C) One coulomb is the amount of charge transported in one second by a current of one ampere. C = A ⋅ s Electrical potential/ Volt (V) The potential difference between two points that imparts an electromotive force energy of one joule per coulomb. V = J/C. But C = A ⋅ s and W = J ⋅ s−1 so V = W ⋅ A−1 (Power = Current × Potential difference) Resistance Ohm (Ω) A potential difference drop of one volt producing a current of (also reactance one ampere gives the conductor a resistance of one ohm. and impedance) Ω = V ⋅ A−1 Capacitance Farad (F) A capacitance of one farad produces one volt of potential difference for an electric charge of one coulomb. F = C ⋅ V−1 Induction Henry (H) When an electric current that is changing at one ampere per second causes an electromotive force across the inductor of one volt, the circuit has an inductance of one henry. H = V ⋅ s ⋅ A−1 Magnetic flux/ Weber (Wb)/ A magnetic flux of one weber, passing through a conducting magnetic flux tesla (T) loop and reduced to zero at a uniform rate in 1 second, density induces an electric potential of one volt in the loop. One weber is equal to one volt second. Wb = V ⋅ s. T = Wb ⋅ m−2 Non-SI units of relevance to anaesthesia Name of unit Symbol Quantity Definition Calorie cal Energy 1000 cal = 1 Kcal = 4.184 kJ Electron volt eV Energy 1 eV = 1.602 × 10−19 J Dyne dyn Force 1 dyn = g ⋅ cm ⋅ s−2 Barye (cgs unit) Pressure 1 dyn ⋅ cm−2 = 0.1 Pa Torr torr Pressure 1 torr = 1/760 standard atmosphere Bar bar Pressure 1 bar = 105 Pa Atmosphere atm Pressure 1 atm = 101325 Pa Pounds per square inch psi Pressure 1 psi = 6.894 × 103 Pa Millimetres of mercury mmHg Pressure 1 mmHg = 133.332 Pa Centimetres of water cmH2O Pressure 1 cmH2O = 98.06 Pa Gauss Gs Magnetic flux density 1 Gs = 10−4 tesla Maxwell Mx Magnetic flux 1 Mx = 10−8 weber  Physics Simple Mechanics and Pressure 14 Simple mechanics Mass is the quantity of matter in a substance and is measured in kilograms Force is a physical influence that changes the state of rest or motion of a body. It causes the body to accelerate in the direction of the force and is a vector (i.e. has both direction and magni- tude). The SI unit of force is the newton Work is a form of energy; whenever energy is expended work is done. The amount of work done is equal to the force multiplied by the distance moved in the direction of the force. The SI unit of work is the joule Power is the rate of working. It can therefore be measured in joules per second, also known as watts Newton’s first law states that a body remains at rest at a state of constant velocity unless acted on by a force Newton’s second law states that Force = Mass × Acceleration Hence N = kg ⋅ m ⋅ s−2; J = kg ⋅ m2 ⋅ s−2 and W = kg ⋅ m2 ⋅ s−3 Newton’s third law states that for every action there is an equal and opposite reaction Pressure definitions Pressure = Force ÷ Area The derived SI unit of pressure is the pascal, Clinical nugget – Myocardial which is equal to one newton per square metre pressure/volume loops 1 Pa = 1 N ⋅ m −2 P = F/A. Then multiplying top and bottom by D Pressure equivalents Atmospheric pressure is the pressure in the P = (F × D)/(A × D) Earth’s atmosphere caused by the weight of air 1 atmosphere ≈ 101.3 kPa ≈ 760 mmHg ≈ But F × D = Work and A × D = Volume 760 torr 1033 cm of H2O 1.013 bar, so that: P = Work/Volume. Thus Pressure × Volume = 1 kPa ≈ 7.6 mmHg ≈ 10 cmH2O Work (energy) Partial pressure That is demands on the myocardium is Partial pressure is the pressure exerted by determined by pressure and stroke volume each gas in a mixture of gases It is equal to the pressure the single gas would exert if alone (Dalton’s law of partial pressures) Tension is the partial pressure of a gas in ­solution, for example oxygen in blood Physics Simple Mechanics and Pressure 15 The gas laws – for ideal gases Boyle’s law At a constant temperature, the volume of a fixed mass of gas is V ∝ 1/P inversely proportional to the absolute pressure Charles’ law At a constant pressure, the volume of a fixed mass of gas is directly V∝T proportional to the absolute temperature Gay-Lussac’s law At a constant volume, the absolute pressure of a fixed mass of gas P∝T is directly proportional to the absolute temperature Avogadro’s Equal volumes of ideal gases at constant temperature and pressure hypothesis contain equal numbers of molecules At standard temperature and pressure, one mole of a substance contains 6.023 × 1023 particles and one mole of gas occupies 22.4 litres The ideal gas A combination of the above PV = nRT equation Dalton’s law The pressure exerted by a fixed mass of gas in a mixture of gases is the same as the pressure it would exert alone Henry’s law At a constant temperature, the amount of gas dissolved in a solvent is proportional to its partial pressure above the solvent Abbreviations: P = pressure, V = volume, n = number of moles of gas, R = universal gas constant (8.3), T = temperature Standard temperature and pressure = 273.15 K and 100 kPa. Pressures and temperatures Gauge pressure is total pressure minus atmospheric pressure. For example, the gauge pressure of an ‘empty’ oxygen cylinder is 0, but the cylinder contains gas at atmospheric pressure Absolute pressure = Gauge pressure + Atmospheric pressure (that is, true total pressure) Critical temperature is the temperature above which a substance cannot be liquefied no mat- ter how much pressure is applied. Below this temperature, the substance is a vapour; above this temperature, it is a gas Critical pressure is the pressure required to liquefy a vapour at its critical temperature Pseudocritical temperature, at a given pressure, is the temperature above which a mixture of gases will not separate out into its constituents At a pipeline pressure of 4.1 bar, the pseudocritical temperature of an equal mixture of nitrous oxide and oxygen is −30°C whereas for Entonox cylinders (137 bar) it is −5.5°C Clinical nugget If the ambient temperature is above the critical temperature (which it could be with N2O of 36.5°C), then the cylinder could be in danger of pressure damage or even exploding. If the ambient temperature, is below the pseudocritical temperature which is feasible for Entonox in cold conditions, (< −5.5°C) then there is a danger of separating to oxygen gas and liquid nitrous oxide.  Physics Simple Mechanics and Pressure 16 Pressure measurement in gases The manometer One type is the Bourdon gauge: Spiral tube of oval cross-section The simplest method of pressure measurement The tube uncoils as it becomes circular in It does not need calibration, so it can be used cross-section with increasing pressure to calibrate other devices Does not require a power supply The pressure is balanced against a column of Mechanically tough liquid of known density – usually water for low Cannot be used for very low pressures pressures and mercury for higher pressures Difficult to recalibrate The pressure is equal to the depth multiplied by the liquid density multiplied by the accel- eration due to gravity, hence the commonly Piezoresistive strain gauge used units are cmH2O and mmHg Piezoelectric effect is the generation of Mercury is 13 times more dense than water charge in some solid materials, e.g. crystals, The vertical height gives the pressure value; when they undergo mechanical stress the tube itself does not need to be vertical Resistance varies with mechanical strain, It is bulky and does not provide a direct and the gauge forms one arm of a bridge reading circuit that produces a small current that is amplified and transduced Aneroid gauges Versatile and suitable for measuring high and Mechanical devices that contain a mecha- low pressures nism that moves a pointer according to the Requires a power supply and is susceptible pressure, e.g. by expansion of a sealed cap- to interference sule of gas Bourdon pressure gauge Piezo-electric effect The generation of electricity by some materi- als (e.g. crystals) when undergoing mechani- cal stress The reverse effect is seen when an electric Direction of movement charge is passed across this material and with increase there is a change in its size and shape in pressure Used in the ultrasonic transducer for ultrasonography Piezo-electric material Pressure Cross section of tube changes from Pressure applied Flattened to circular on exposure to high pressure  Physics Simple Mechanics and Pressure 17 Pressures of anaesthetic equipment Cylinder pressures Pressure Gas Phase in cylinder (kPa) (bar) Oxygen Gas 13,700 137 Nitrous oxide Mixed liquid and vapour 4,400 44 Entonox (50:50 O2:N2O) Gas 13,700 137 Anaesthetic machine pressures Pipeline pressure Gases supplied at 400 kPa (4 bar) except medical air for driving surgical instruments, which is supplied at 700 kPa (7 bar) Pressure regulating valves* 400 kPa (4 bar) Flow restrictors* Placed upstream of flow meters Protect machine from damaging surges in pipeline pressure (100–200 kPa) Flow control valves Govern transition from high to low pressure Non-return pressure relief Downstream of vaporizers on the back bar and prevent damage to flow meters safety valves* Open at 35 kPa Emergency oxygen flush Supplied by high pressure system directly Provides 37–75 L/min at 400 kPa Oxygen failure alarm Sounds at pressures lower than 200 kPa Adjustable pressure limiting Needs 0.7 is required for clinical recovery from neuromuscular blockade Measurement of neuromuscular blockade (cont.) Tetanic stimulation is 5 seconds of 50 Hz stimulation. Normally, no fade in tetanic stimulation occurs. With non-depolarizing blockade, there is fade and post-tetanic facilitation. This is used clinically when there is no response to the train of four. After applying tetanic stimulation for 5 seconds, the post-tetanic count is the number of muscle responses to 1 Hz single twitches. Depolarizing blockade does not cause fade or post-tetanic facilitation Double-burst stimulation applies two short bursts of 50 Hz tetanic stimulation 0.75 s apart. The tetanic stimuli used elicit a bigger response and reportedly make fade easier to detect than with train of four Objective measurement of neuromuscular blockade Acceleromyography uses a piezoelectric crystal attached to the thumb to measure accelera- tion of the crystal because of thumb movement in response to ulnar nerve stimulation Mechanomyography measures the force of thumb contraction after ulnar nerve stimulation Evoked electromyography measures the evoked compound muscle action potential in response to nerve stimulation  Physics Electricity and Magnetism 28 Electrical hazards – causes Mains electrical current According to Ohm’s law: the higher the volt- AC at 50 Hz with the live wire at 240 V age or lower the resistance, the greater the and the neutral connected to earth at the current and therefore the damage The path of the current will determine its pos- sub-station If a patient or member of staff forms a con- sible effects. Current through the chest may nection between the live wire and earth, a cause respiratory arrest or ventricular fibril- current will flow through. lation (VF). Current passing up and down the Electricity may cause body may cause unconsciousness or spinal electrocution cord damage With a 50 Hz current conducted across the burns – see diathermy a fire or explosion chest from one hand to the other, the effects would depend on the magnitude Electrocution 1 mA causes tingling 5 mA is the maximum safe current Causes damage according to 15 mA causes tetany (‘let go’ current), the current pain and asphyxia the path it takes 75 mA may cause VF its density the type of current (AC or DC) its duration Electrical hazards – causes (cont.) Electrocution (cont.) a connection from the electrical source to earth. This may result in burns to or electrocu- With AC current, mains frequency is the most tion of the patient or a member of staff dangerous, because it is the frequency most likely to cause arrhythmias and it also causes Capacitive coupling muscle spasm, which prevents release from If the body acts as the plate of a capacitor, the source. The longer the duration, the more capacitive conductance occurs damage is done With DC, current flows only very briefly A microshock of only 50 mA could induce until the plate is charged to the same VF through a central venous catheter or potential as the electrical source pacemaker, because bypassing the skin With AC current, the capacitor is impedance causes high current density in charged but then changes polarity at the heart. Impedance is the AC equivalent to the frequency of the electrical source; a resistance current will therefore continue to flow Sparks The changing magnetic fields in an MRI scanner may induce currents in the wires May cause fires or explosions by igniting of standard monitoring, which can, in turn, inflammable vapours cause a patient burn through capacitive Resistive coupling coupling A direct physical connection results in resis- tive coupling. Faulty equipment or leakage currents can cause electricity to flow through  Physics Electricity and Magnetism 29 Electrical hazards – prevention General measures Regular testing and maintenance of equipment High impedance shoes Ensuring that the patient is not in contact with earthed objects All medical equipment must comply with British standards for safety. These standards include two classifications of electrical equipment protection provided against electric shock caused by connection to the mains supply maximum permissible leakage current Protection from electric shock from the mains supply Class I equipment must have any conductible part accessible to the user connected to earth through the earth wire. If the conductible part becomes connected to the live wire through a fault, the current will be conducted through the lower resistance earth wire, which can result in breakage of the fuse and removal of the source current Class II equipment has double or reinforced insulation of any conductible accessible part. It does not have an earth wire Class III equipment uses batteries at voltages unlikely to cause electrocution but may result in microshock Electrical hazards – prevention (cont.) Maximum permissible leakage current Type B equipment has a maximum leakage current of 100 mA for IIB and 500 mA for IB and should not be directly connected to the heart. It may be class I, II or III Type BF is type B but also uses a floating circuit. Use of an isolating transformer that consists of two coils electrically insulated from each other means there is no direct electrical connection between the mains circuit and the patient circuit. The mains circuit is earthed, but the patient circuit is not. Therefore, connection between the electrical source in the patient circuit and earth will not result in completion of a circuit, and so no current will flow Type CF has a floating circuit and maximal leakage current of 10 mA for IICF and 50 mA for ICF and therefore can be safely connected directly to the heart Circuit breakers Current-operated earth leakage circuit breakers have coils of the live wire around a transformer An equal number of coils of the neutral wire are also wound around the transformer A third coil connects to a relay that operates the circuit breaker With equal currents in the live and neutral wire, the magnetic fluxes are equal and opposite and therefore there is no magnetic field With a small leakage current, the magnetic fluxes are different, and a magnetic field that induces a current in the third winding results in the relay breaking the current  Physics Electricity and Magnetism 30 Principles of lasers Laser stands for light amplification by the stimulated emission of radiation Properties of laser light Monochromatic (single colour and frequency) Almost non-divergent High-intensity beam In phase May be of a very small cross-sectional area Principles of laser light Electrons have energy but can exist only at certain energy levels according to quantum theory. They can move between these energy levels by absorbing or emitting a specific amount (quanta) of energy as radiation, which may be in the visible spectrum. With non-laser light, random pro- cesses result in the light waves being out of phase or incoherent Energy is absorbed from a flashlight or high-voltage discharge and is released as a photon, with energy equal to the difference in the two energy levels The photon is reflected back and stimulates an excitable electron (in a higher energy level) to fall to a lower energy level, resulting in emission of a wave of energy equal to the stimulating photon, which can go on to stimulate another Principles of lasers (cont.) Amplification occurs through a chain reaction, and as the photons have equal energy (and thus frequency), they are coherent If the photon stimulates an atom with a lower energy state into a higher energy state, the energy is absorbed. If many more atoms have electrons in the higher energy state than in the lower state, then, on average, more stimulated emission than absorption will occur Emission restricted to a single direction forms laser light Types of lasers An argon gas laser produces blue–green light that passes through the humour of the eye and is used for retinal surgery and also removal of birthmarks. It may be used endoscopically with optical fibres A carbon dioxide laser produces infrared light that vaporizes water in tissues, cutting with hae- mostasis and low penetrance. It is the laser used most frequently in surgery. It is not suitable for endoscopic use A neodymium-doped yttrium aluminium garnet (Nd:YAG) laser produces near infrared. As it is not absorbed by water, it is used for coagulation and cutting with deep penetration into tissues. It may be used endoscopically  Physics Electricity and Magnetism 31 Laser safety Burns disconnect the circuit and remove the ET High-intensity laser light may burn the tube if possible bag and mask the patient with air retina or optic nerve, which can result in inspect the airway with a rigid a permanent blind spot or partial or total bronchoscope blindness; infrared light cannot be seen, observe patient in ITU, and keep intu- so it may cause worse damage, especially bated for several hours to the cornea, lens, and aqueous and vit- give dexamethasone and humidified oxygen reous humour repeat bronchoscopy in a few days skin Safe use of lasers Fire The laser operator must ensure that it is used The danger of fire can be reduced by safely using air and oxygen that is less flammable An appropriate fire extinguisher and 50 mL than nitrous oxide and oxygen syringe filled with saline should be available a fraction of inspired oxygen (FiO2) ≤0.25 Personnel should wear suitable eye using non-flammable endotracheal (ET) protection tubes with cuffs inflated with saline All doors should be locked and windows using non-reflective, matte-black surgical covered instruments protecting nearby tissue with wet swabs If an airway fire occurs switch off the laser and flood the site with saline Electrical symbols Cell A.C. supply Battery Capacitor Ground Resistor Inductor Fuse + Variable resistor Diode – Thermocouple Switch Transistor V Thermistor Volt meter A Ammeter Transformer  Physics Electricity and Magnetism 32 Wheatstone bridge A Wheatstone bridge contains four resistors (two Rmeasure fixed, one adjustable and one strain gauge) arranged with a battery and galvanometer A strain gauge is a type of resistor in which the resis- Radjustable tance alters when it is stretched or strained In a pressure transducer, movements of the dia- Rmeasure R2 = phragm result in changes in tension in strain gauges Radjustable R1 Kinetic energy thus is converted to electrical A energy The variable resistor is used to balance the bridge so R2 no current flows. It is therefore known as a null deflec- R1 tion system. In practice, the four resistors are all strain gauges and the Wheatstone bridge is arranged so that as the resistances of two of the strain gauges on one side of the bridge increase, the resistances of the two on the other side of the bridge decrease. It thus works to amplify the electrical signal Circuit breakers, fuses, transformers, transistors, diodes Resistors, fuses and circuit breakers Power lines have high voltage (400 kV–700 kV) from step-up transformers in order to reduce energy loss A rise in temperature in a resistor is the basis of the (the current will be very small) which are stepped resistance thermometer down by transformers to domestic voltages (240 V) As a resistor is stretched its resistance increases; this is the basis of the strain gauge which transduces mechanical energy to electrical energy Diodes and transistors A fuse is a wire of a certain resistance that is chosen A diode is a component of an electrical circuit which to heat and melt at a certain current thus breaking only allows current to flow in one direction the circuit and preventing inadvertent electrocution It can be used to convert AC to DC and to protect Circuit breakers act like fuses to protect circuits and circuits from back currents, for example when motors equipment from current surges but then can be reset are turned off rather than replaced. They use the heating or magnetic Most are made from semiconductors (e.g. silicon) effects of the current to cause an interruption of flow with p-n junction (positive-negative) A transistor is a semiconductor device that can be used Transformers as a switch or to amplify small currents.The simplest type are two diodes placed back to back (NPN or PNP) Transformers are used to step up or step down the voltage in alternating currents. They work on induc- 1 watt audio amplifier tion and do not work on direct current A primary AC with a primary coil will induce a sec- ondary current in a circuit with a secondary coil. This 100k Ω 3 Speaker Collector induction can be made more efficient if the coils are linked by a ferromagnetic core MPSW45A The step up (or step-down) in voltage depends on + input 2 Base 50 Ω the ratio of the number of turns in each coil 10k Ω As the primary voltage increases the current will 100k Ω + 9V decrease by the same factor (so the overall energy Emitter 1 stays the same) Chargers for small devices (e.g. mobile phones) – input have transformers included in their leads  Physics Electricity and Magnetism 33 MRI scanners and anaesthesia History Open scanners (useful for claustrophobic patients or for interventions) have lower strength Nuclear magnetic resonance (NMR) first described High magnetic flux density is achieved by cryogenic in 1946 (Bloch and Purcell) superconducting magnets close to absolute zero Wide-bore superconducting magnets introduced in (0 K) by immersing in liquid helium the 1970s The receiver and transmitter coils for RF pulses and First clinical MRI imaging introduced in 1980 detection can be separate or combined Principles All atomic nuclei (protons in hydrogen atoms) have a pos- T1 and T2 weighted MR imaging itive charge and spin, thus behaving like a bar magnet After the RF pulse ceases the nuclei returns to thermal Application of a strong external uniform magnetic force equilibrium–relaxation (B0) aligns some of the protons, either in a low-energy T1 is the time constant (time taken for 63% return to (parallel) or high energy state (perpendicular to B0) equilibrium) for longitudinal relaxation, T2 for trans- A second radiofrequency (RF) magnetic field (B1) verse relaxation applied perpendicular to B0 will excite the nuclei that Water and CSF have long T1s (3–5 s), whereas fat has possess spin a much shorter T1 (260 ms); T1 is good for white/grey The RF is applied in short bursts (µs) causing absorp- matter contract, whereas T2 is good for tissue oedema tion of the energy by the nucleus There are other groups of sequences which can be This energy is emitted on relaxation and can be used depending on what type of pathology and tis- detected and amplified; the voltage is displayed as sue is being investigated; e.g. free induction delay (FIDs) gradient echo for cardiac MRI In practice multiple RF pulses are applied obtaining inversion recovery (which can attenuate fat or multiple FIDs which are averaged thus improving SNR water signals) diffusion and perfusion weighted looking for The MRI scanner cerebral infarction Typically 1–3 tesla in the centre of the coil (c.f. Earth’s magnetic field is 0.00005 T which is 0.5 gauss), the fringe field around the scanner will be lower MRI scanners and anaesthesia (cont.) Clinical applications of MRI scanners Remote anaesthesia Neuroimaging MRI scanners are usually some distance from theatre MR angiography and cardiac MRI suites; help, drugs and equipment are not immediately Prostate and uterine cancer staging available MR spectroscopy to detect various metabolites in Recovery might not be easily available body tissues Lack of familiarity with the environment unless there is Operative MRI (hybrid theatres with attached MRI a regular team and list. Equally significant, radiogra- scanners) phers are not familiar with the needs and priorities of the anaesthetic team MRI scanners and anaesthetic Limited access to patient in the scanner; it is vital considerations that any airway/adjunct is secure before putting the patient into the magnet; it will not be possible to bring Sedation or anaesthesia can be chosen depending a laryngoscope (unless it is plastic) close to the mag- on the availability of staff. net in an emergency There needs to be clear guidelines dictating which patients are appropriate for sedation and to ensure Dangers to the patient and staff sedation practitioners are all appropriately trained. Projectiles causing trauma (e.g. cylinders, wheel- Drugs used for sedation – chloral hydrate, propofol, chairs, syringe pumps) midazolam, dexmedetomidine Interference with implantable devices (e.g. pace- If using general anaesthesia (GA) with laryngeal makers, defibrillators, programmable pumps) mask airway (LMA) it is important to remember to Interference with infusion devices (it might be neces- tape the pilot balloon to stop it being drawn into the sary to extend infusion pump lines so they are kept magnet next to the head and causing artefacts outside the Faraday cage) Patient population Monitoring interference–full monitoring is necessary and the Association of Anaesthetists of Great Britain The patients who will require anaesthetic care for and Ireland (AAGBI) suggests having the monitor in their MRI carry their own particular challenges the control room Infants and children Monitoring applied to the patient must be done in such ITU patients (especially neurological ITU) a way to avoid induction currents causing contact burns Patients who are unable to lie flat (e.g. due to intrac- Foreign bodies can shift (e.g. metal shards in the eye, table pain) aneurysm clips if not titanium) and must by approved Psychiatric patients and extreme claustrophobia by the supervising MRI radiographer (MRIsafety.com) Acoustic damage Risk of anaphylaxis and renal damage with IV con- trast media (though much less with gadolinium)  Physics Electricity and Magnetism 34 Principles of cardiac pacemakers A cardiac pacemaker provides cyclical pacing. It uses large-area skin electrodes electrical stimulation of cardiac activity for ­ and pulse duration of up to 50 ms to decrease the treatment of bradyarrhythmias or tachyar- nerve and muscle stimulation rhythmias. It is indicated for disease of the ­conducting system of the heart Permanent pacing Temporary pacing Permanent pacing may be used for sick Transvenous pacing is achieved via a cen- sinus syndrome, heart block or after MI if tral vein under x-ray control and may be the arrhythmia causes dizziness, syncope indicated after MI. The pulse duration is or heart failure. The pacing wire is usually less than 1 ms, and the potential difference placed endocardially in the atrium or ven- required is usually less than 4 V. There is a risk tricle, or both, and is connected to a subcu- of microshock. Transvenous pacing should be taneous battery-powered pulse generator considered preoperatively for The pacemaker code consists of five letters bradyarrhythmias (in order) indicating third-degree heart block chamber paced (0 = none, A = atrium, second-degree heart block if it is Mobitz V = ventricle, D = dual) II or if there are associated symptoms or chamber sensed (0 = none, A = atrium, extensive surgery V = ventricle, D = dual) bundle branch block if it is bifascicular response (0 = none, T = triggered, I = or has a prolonged PR interval inhibit