Monitoring The Anaesthetised Patient PDF

CP 3.14.22 2022 • Year ILO CP3017 • Select and explain appropriate diagnostic techniques to investigate common veterinary emergencies, including problems arising under anaesthesia, and interpret the findings of these technique • Lecture ILOs • Understand the rationale for basic anaesthetic monit...

CP 3.14.22 2022 • Year ILO CP3017 • Select and explain appropriate diagnostic techniques to investigate common veterinary emergencies, including problems arising under anaesthesia, and interpret the findings of these technique • Lecture ILOs • Understand the rationale for basic anaesthetic monitoring and recognise values out with the normal range in companion animals • Explain the basic principles of anaesthesia monitors (ECG, pulse oximeter, capnograph and blood pressure monitors), including how they work, the information they provide and their limitations • Explain how abnormalities in monitored parameters may arise and the implications of these abnormalities for the anaesthetised animal Depth • Stages of ether anaesthesia proposed in 1937 • Less relevant now with modern anaesthetic drugs Stage Name 1 Analgesia 2 Delirium 3 Surgical 4 Respiratory Arrest & Death • Movement • Physiological measures • Heart Rate/Respiratory rate • Tend to increase at very light planes but very variable and large range. Not a reliable indicator of being at a surgical depth of anaesthesia • Eye • Reflexes • Position • Jaw tone • Eye rotates ventro• • • • medially Palpebral reflex abolished Corneal reflex abolished Eye rotates centrally Pupil dilates TOO DEEP • Can be measured by • Watching the chest • Watching the reservoir bag on the breathing system • Normal rate is • 8-20 for dogs • 15-30 for cats • Respiratory rate is a relatively poor indicator of respiratory adequacy • CO2 levels by capnography are better • Pulse palpation is one of the most important skills you can learn • Palpating pulses is infinitely preferable to auscultation of the chest during anaesthesia • Heart rate • Rhythm • Pulse quality/strength give some indication of peripheral perfusion Auricular artery Lingual artery Brachial artery Metacarpal artery Femoral artery Coccygeal artery Dorsal pedal artery • Feel for pulsation in the artery with your fingers not your thumb! • Normal rate is roughly (size dependent) • 60-140 for dogs • 100-180 for cats • Arrhythmias may be detected as ‘dropped beats’ • Pulse rate and heart rate may not be the same! • Palpation of peripheral pulses gives some information about perfusion to the periphery • Do good pulses equal good blood pressure? • Pulse pressure is the difference between systolic and diastolic blood pressure and does not reflect the absolute values! • A full pulse generally implies an adequate blood pressure – but not always! • Legal/RCVS requirement! • Identifies trends • Records drugs/interventions too • Complications may arise at any time • Procedural • Equipment • Animal • Good monitoring should allow earlier detection and better outcome • Additional monitoring is not necessarily better • May increase anaesthetic time in short procedures! • Good interpretation essential • I.e. A monitor is only as good as the person interpreting the information, and is not a substitute for good basic monitoring! • Respiratory • Pulse Oximetry • Capnography • Cardiovascular • ECG • Blood Pressure • Urine output • Central venous pressure • Arterial blood gas analysis measures partial pressures of O2 and CO2 • Invasive, technically difficult and not continuous • Therefore less invasive measures have been developed • Pulse oximeters are commonly used to assist with monitoring of anaesthesia • A sensor is placed on a suitable part of the body • • • • • Tongue Nail Bed Ear Tip Vulva/Prepuce Lip fold • Two different wavelengths of light (660 and 940nm) are emitted, passed through the tissues and their absorbtion measured. • They are absorbed differently by oxy- and deoxy- haemoglobin • Pulse oximetry measures SpO2 • • Percentage saturation of haemoglobin with oxygen They also measure pulse rate • Blood gas analysis (Gold standard) measures PaO2 • Partial pressure of oxygen dissolved in plasma • SpO2 should be • Above 90% in animals breathing room air • Above 95% in anaesthetised animals breathing 100% oxygen • Values of less than 90% will cause morbidity/mortality • If the reading is reliable! • • • • • • • • • Pigmented skin Movement (heavy breathing) Compression of the vascular bed Ambient light Poor Contact Peripheral vasoconstriction (medetomidine) Low blood pressure Pulsatile veins (tricuspid regurgitation) Abnormal haemoglobins • Carboxyhaemoglobin (Reads 100% even if not!) • Methaemoglobin (Reads towards 85%) • All pulse oximeters have some way of assessing the signal quality • Plethysmograph • Flashing light (green/orange/red) • Only believe the reading if you are satisfied with the signal • Replacing the probe is the first step if you are concerned about the accuracy of the reading (Some examples – not exhaustive) 2 • Low inspired oxygen • • • • Disconnection Incorrect gas mixture (100% N2O!) Kinked tube Airway obstruction • • • • • Pneumonia (Aspiration) Pneumothorax Pulmonary oedema Pulmonary embolus Bronchoconstriction (Asthma) • Lung Disease (V/Q mismatch & Diffusion Impairment) • R to L Shunting • Congential Heart Disease (Patent ductus, septal defects, tetralogy of fallot) • Hypoventilation (although not on oxygen supplementation) • Cyanosis is the blue colouring of arterial blood when deoxyhaemoglobin is present • SpO2 is more sensitive for the detection of desaturation than cyanosis • Cyanosis may never occur in some patients (anaemic) • Cyanosis is very subjective • Problems with getting reliable readings in some patients • Doesn’t tell you about perfusion/blood pressure • Doesn’t tell you anything about CO2 levels • I.e. an SpO2 of 100% (when on >40% oxygen) does not mean the animal is breathing adequately! • Less commonly seen in practice • Possibly the most reassuring and least error prone monitor available • Monitors • Respiratory function • Detects breathing system/ETT malfunction and other problems • Gives some indication of changes in cardiac output • Measures CO2 at the end of the endotracheal tube • May be • Mainstream • Analyser actually placed at end of tube • Sidestream • Gas is sampled down a sampling line to a distant monitor • Gives the following information • Respiratory rate/EtCO2/Inspired CO2/’The trace’ 2 • Normal value for dogs is 4.6 – 6 kPa (35-45 mmHg) • In animals with normal lungs it is an approximation of arterial CO2 partial pressure • This value is directly linked to the minute volume • Hypoventilation increases EtCO2 • Hyperventilation decreases EtCO2 • Hypoventilation is common under anaesthesia • Action is required above EtCO2 of 8.0 kPa (60mmHg) • Most commonly due to excessive anaesthetic depth • A capnometer will just give values for RR/EtCO2 • The shapes of the traces on a capnograph may also be indicative of problems • Kinked ETT/Blocked ETT/Bronchconstriction • Rebreathing of CO2 (problem with anaesthetic breathing system) • ‘Bucking’ the ventilator • Disconnection • Cardiovascular collapse (Cardiac arrest/severe hypotension) Capnography • Adequacy of ventilation (CO2) • Confirms perfusion to lungs • Doesn’t measure oxygenation Pulse oximetry • Measures oxygenation • Doesn’t measure • Ventilatory adequacy • Perfusion (to lungs or tissues) • Measures electrical activity of the heart • Continuous measure of heart rate and rhythm • Easy and non-invasive to apply • Detects arrhythmias • May result from • Electrolyte imbalance • Cardiac disease • Systemic disease e.g. shock • Trauma (thoracic) • Useful to identify cardiac rhythm in CPR • Therefore often indicated in sick patients • Gives NO indication of adequate cardiac output and hence perfusion to tissues • Can be normal with severe hypovolaemia or hypoperfusion • Can even be normal when the heart is not beating! • Pulseless electrical activity BP = Cardiac Output x Total Peripheral Resistance • Driving pressure for perfusion of tissues • Commonly low in critically ill or anaesthetised animals • Animals should have a • Systolic BP >90 mmHg • Mean BP > 60 mmHg • Diastolic >40 mmHg • Gold standard • Cannulation of an artery allows direct measurement of blood pressure • Real-time changes • Allows arterial blood sampling • Requires skill & equipment • Oscillometric • Doppler • A cuff is placed over a peripheral artery • Metatarsal • Ulnar • Tail • The cuff occludes blood flow in the limb and senses oscillations • Oscillations • Start at systolic BP • Are maximal at mean BP • Disappear at diastolic BP • The cuff size is critical • Width should be 4060% of the limb diameter • Doesn’t measure well with • Arrhythmias • Severe hypotension • Vasoconstriction • Movement • An ultrasonic probe is placed over a peripheral artery • This gives a continuous audible signal • A proximally placed cuff is inflated until the sound disappears • The sound returns at systolic BP (mean BP in anaesthetised cats) Urine Output • Renal perfusion is required for the production of urine • Normal urine output is 12ml/kg/hr • A closed collection system is necessary (i.e. an indwelling urinary catheter attached to a collection bag) • Lower values require treatment to improve perfusion • Pressure measured in vena cava • Measured via a transducer/manometer connected to a central venous cannula • Central venous cannula usually placed via the jugular vein and is longer than a regular intravenous cannula • Acts as an indirect measure of cardiac preload • ‘Normal’ = 0-10cmH2O • Traditionally used to assess adequacy of fluid therapy although has fallen out of favor recently in medicine • Can be useful to detect volume overload in patients with cardiac disease