Anaesthesia Teaching Package PDF

Ministry of Higher Education and Scientific Research Middle Technical University, College of Health and Medical Technique, Anaesthesia Technique Department, 4th stage. Teaching package for anaesthesia technique By Dr. Hussam Kareem...

Ministry of Higher Education and Scientific Research Middle Technical University, College of Health and Medical Technique, Anaesthesia Technique Department, 4th stage. Teaching package for anaesthesia technique By Dr. Hussam Kareem 2023-2024  Pregnancy-Related Physiological and Anatomical Changes Numerous changes occur in a woman's body during pregnancy. These changes affect almost every organ system. Changes early in pregnancy are largely due to increases in hormones (i.e., progesterone and estrogen) and increased metabolic demands of the fetus, placenta, and uterus. Later changes are due to the expanding uterus and growing fetus. All of these changes impact anesthetic care. I. Cardiovascular System changes 1. Blood Volume Blood volume will increase progressively starting at 6-8 weeks. At the time of delivery, the average blood volume is increased by 1-1.5 liters. There is a greater increase in plasma volume compared to red blood cell mass, resulting in relative anemia. The clotting components of the patient’s blood also increase to enhance clotting, reducing excessive bleeding during delivery. These components include fibrinogen, and factors VII, X, and XI. Increased blood volume meets the mother and fetus’s metabolic demands, allowing the mother to tolerate blood loss during delivery. The average blood loss associated with vaginal delivery is 400-500 ml. With a cesarean section, it is 800- 1000 ml. 2. Cardiac Output Cardiac output will increase up to 40% at term. Most of the increase occurs in the 1st and 2nd trimesters. The exception is during labor when cardiac output peaks secondary to increased heart rate and stroke volume (the volume of blood ejected from the heart with each 1  contraction). To handle the increased blood volume, the myocardium and chambers enlarge. Cardiac output will return to normal 2 weeks after delivery. Cardiac output can decrease after 28 weeks of pregnancy due to mechanical changes. 3. Blood Pressure Despite increases in the patient's cardiac output and blood volume, the patient’s blood pressure does not normally increase from pre- pregnancy levels unless there is an abnormality such as pregnancy- induced hypertension. A decrease in blood pressure occurs by about 8 weeks of gestation. By mid-pregnancy, the diastolic blood pressure and mean arterial pressure reach their lowest point (16-20mmHg below pre-pregnancy values), returning to pre-pregnancy levels by term. The overall decrease in diastolic blood pressure and mean arterial pressure is 5-10 mmHg. 4. Venous System The venous system has an increased capacity for distension and dilation (up to 150%). This can reduce blood flow, delaying the absorption of subcutaneous or intramuscular medications. Distention of the vessels within the epidural space may increase the risk of vascular damage and bleeding during neuraxial blockade. This, along with hormonal changes, reduces the required amount of local anesthetics by 30%. Using the same dose in the pregnant patient as normally would in the non-pregnant patient results in a high neural block. Anesthetic considerations due to Cardiovascular Changes in pregnancy: a) Never lay the patient supine. Always place a wedge or roll under the right hip so the patient is “tipped” to the left. This maneuver will 2  prevent a decrease in cardiac output, maternal hypotension, fetal distress, or asphyxia that can result from supine hypotension syndrome/aortocaval compression. b) The pregnant patient is dependent upon sympathetic outflow to maintain systolic blood pressure. Always pre-load the patient with 1-2 liters of crystalloid fluids before the neuraxial blockade. c) Vessel distention in the epidural space may increase the risk of vessel damage during the neuraxial blockade. d) Vessel distention also decreases the intrathecal and epidural spaces. Decrease the dose of local anesthetics by 30% to avoid a high neuraxial block. e) Delayed absorption of subcutaneous and/or intramuscular medication. II. Respiratory System changes 1. Lung Volumes As the gravid uterus grows in size, it places pressure on the abdomen. At term, the pregnant patient favors thoracic breathing over abdominal breathing. The most important is the change in functional residual capacity (FRC- the volume of air that is in the lungs at the end of a normal breath). FRC decreases 20% by term, returning to normal 48 hours after delivery. A decrease in FRC reduces the patient’s reserve. In the event of apnea, the patient can become hypoxic quickly. In addition, the pregnant patient will increase her tidal volume (normal volume with each breath) by 40%. 2. Respiratory Gas Exchange 3  Minute ventilation (amount of air breathed in one minute) increases by 50% by the second trimester. The respiratory rate will increase by 15% (2-3 breaths per minute). These changes speed the uptake of inhaled anesthetics. Alveolar ventilation (air that participates in gas exchange) will increase by 70% at term. Oxygen consumption increases by 20-50%. The combination of a decreased FRC and increased oxygen consumption can result in hypoxia. 3. Respiratory Tract As mentioned earlier, the pregnant patient has venous vascular engorgement, which results in a swollen respiratory tract and a diminished view during laryngoscopy. The obstetric population is more difficult to intubate compared to the non-pregnant population. A smaller-than-usual endotracheal tube may be required. Manipulation during laryngoscopy can result in bleeding, obscuring the view of the glottic opening. Anesthetic considerations due to Respiratory changes in pregnancy: Patients undergoing regional anesthesia should have supplemental oxygen. a) Patients undergoing general anesthesia should be pre-oxygenated with 100% O2 before induction. b) Patients may desaturate despite pre-oxygenation due to increased oxygen consumption and decreased FRC. c) Be prepared for difficult intubation. Swollen mucous membranes may decrease visualization. Ensure the patient is positioned optimally for laryngoscopy. 4  d) Have smaller endotracheal tubes available for intubation. A smaller endotracheal tube may be required for intubation due to swollen tissue. e) Be very gentle during laryngoscopy as bleeding may obstruct the view. III. Renal system changes The pregnant patient’s renal plasma flow and glomerular filtration rate will increase by 50-60% at term. This correlates with increased cardiac output and blood volume. Increases in renal plasma flow and glomerular filtration rate result in increased clearance of blood urea nitrogen and serum creatinine, which may be reduced by 40%. Obstructive changes to the renal system can occur due to the enlarging uterus. This may result in increased urinary tract infections and decreased blood flow to the kidneys. IV. Gastrointestinal system changes Mechanical and hormonal alterations result in several changes within the gastrointestinal system. As the uterus enlarges, pressure is placed on the stomach resulting in an incompetent lower esophageal sphincter. In addition, progesterone will reduce the competence of the lower esophageal sphincter. Placental gastrin causes an increased secretion of gastric acid. These changes lead to the reflux of gastric acid into the esophagus and delayed gastric emptying and place the pregnant patient at risk for aspiration during anesthesia. Anesthetic considerations due to gastrointestinal changes in pregnancy: Pregnant patients should be considered to have “full stomachs”, regardless of fasting. 5  a) If available, medications should be administered before anesthesia to reduce gastric acidity and volume. A non-particulate antacid (i.e., sodium citrate), Metoclopramide (Plasil) 10 mg IV should be administered 30-60 minutes before anesthesia to stimulate gastric emptying and increase lower esophageal sphincter tone. The use of histamine H2 blockers 30-60 minutes before surgical intervention may help to reduce the acidity of stomach contents. b) Position the patient with a roll under the right hip. A slight reverse Trendelenburg position may help prevent passive reflux. c) Cricoid pressure should be applied and held until the patient is intubated. Cricoid pressure should not be released until it is confirmed that the endotracheal tube has been placed in the trachea. d) Do not routinely administer positive pressure ventilation, with a mask, before intubation. Positive pressure ventilation should occur if the patient’s pulse oximetry reading starts to decline or a difficult airway is encountered. Unnecessary positive pressure ventilation before intubation may result in gastric distention, placing the patient at risk for aspiration. Hepatic system changes The overall function and blood flow to the liver are unchanged during pregnancy. There is a 25-30% decrease in pseudocholinesterase function at term. In the immediate delivery period, this should not produce a clinically significant prolongation of succinylcholine, mivacurium, or ester local anesthetics. 6 VI. Central nervous system changes Changes in hormones result in a decrease of up to 40% in minimal alveolar concentration (MAC). By the 3rd-day post-delivery MAC levels return to normal. Hormonal changes and venous dilatation contribute to a 30% decrease in local anesthetic requirements for spinal and epidural anesthesia. Anatomical changes may create an epidural space that has positive pressure instead of negative pressure. Anesthetic considerations due to Central nervous system Changes in pregnancy: a) Reduces the dose of inhaled anesthetics by up to 40%. b) Reduces the dose of local anesthetics for spinal and epidural anesthesia by up to 30%. c) Positive pressure in the epidural space may make it slightly more difficult to identify the epidural space. MCQ test 1- The average blood loss associated with vaginal delivery is: a) 200-250ml. b) 250-400ml. c) 400-500ml. d) 500-1000ml. e) 800-1000ml. 2- The dose of local anesthetics for spinal anesthesia in pregnancy reduced by up to: a) 30%. 7  b) 20%. c) 40%. d) 60%. e) 5%. 3- Regarding anesthesia for pregnant lady(all true except one) a) Pregnantlady should be considered to have “full stomachs”, regardless of fasting. b) Reduces the dose of inhaled anesthetics by up to 40%. c) Cricoid pressure should not be released until it is confirmed that the endotracheal tube has been placed in the trachea. d) Always administer positive pressure ventilation, with a mask, before intubation. e) A non-particulate antacid and Metoclopramide given before anesthesia to increase lower esophageal sphincter tone. 4- All the following parameters are increase in pregnancy except one a) FRC. b) Blood volume. c) GFR. d) Minute ventilation. e) Cardiac output. 5- Which one is true regarding physiological changes in pregnancy a) the pregnant patient has venous vascular engorgement, which results in a swollen respiratory tract. b) The overall function and blood flow to the liver are huge changes during pregnancy. c) Patients may desaturate despite pre-oxygenation due to decreased oxygen consumption and increased FRC. d) Cardiac output will return to normal 20 weeks after delivery. e) increase in plasma volume compared to red blood cell mass, resulting in polycythemia. 8  Pediatric anatomical and physiological differences Several anatomical and physiological differences impact the effects and techniques of anesthesia administration. Pediatric patients can be divided into four groups based on age. I. Newborn: from birth to the first 24 hours. II. Neonate: from 1 to 30 days of life. III. Infant: from 1 month to 12 months of age. IV. Child: from 1 year to the onset of puberty. Cardiovascular Physiology Cardiac output for a neonate is 30-60% greater than an adult. This helps meet the increased oxygen consumption requirements. Cardiac output in the pediatric patient is dependent on heart rate. Monitoring of the pediatric patient’s heart rate can be accomplished with a precordial stethoscope, ECG, and pulse oximetry. Prompt recognition and treatment of bradycardia are critical. Bradycardia is less than 80 in children 1-8 years, less than 100 beats per minute in infants aged 1-12 months, and less than 120 beats per minute in neonates. It is the most common rhythm before cardiac arrest in the pediatric patient. The most common cause of bradycardia in pediatrics is: 1. Hypoxia. And then: 2. Vagal stimulation (suctioning, surgical traction, etc.). 3. An overdose of anesthetic medications. 4. Hypothermia. 5. Increased intracranial pressure. The blood volume of the pediatric patient is highest as a neonate and declines with age. Knowledge of the approximate blood volume is important when calculating total blood volume and estimated blood loss. e.g., a 4 kg neonate’s total blood volume would be calculated as follows: 1 4(kg) X 85 (ml/kg) = 340 ml total blood volume. Pulmonary Physiology and Airway Anatomy The functional reserve capacity is much smaller in infants and neonates, during anesthesia, airway obstruction can result in hypoxia very quickly. For this reason, pulse oximetry is essential. Induction and emergence are especially critical periods to monitor for these complications. Oxygen consumption for a neonate is two times greater than that of an adult. The anatomy of the pediatric airway is different compared to an adult: The pediatric patient is prone to airway obstruction related to a proportionally larger head, short neck, and large tongue. Positioning of the patient's airway is an important consideration. Overextension can result in airway obstruction in the neonate. Infants and neonates exchange air primarily through their nasal airways. The larynx is higher in the infant and child (cervical vertebrae 3-4) than in the adult (cervical vertebrae 5-6). The epiglottis is large, stiff, and U-shaped. The trachea is short, and the right main bronchus is less angled. This increases the risk of a right mainstem intubation. Choosing the correct-size endotracheal tube and approximate length of insertion is important. This can also be accomplished by a simple calculation. The equation that can be used (age/4+ 4) will approximate the correct size of the endotracheal tube. Regardless of the calculations, an endotracheal tube should slide easily into the trachea, and never be pushed or forced. The calculation for the correct endotracheal tube depth insertion is to multiply the diameter of the endotracheal tube by 3. For example, an endotracheal tube that is a size of 3.0 would be multiplied by 3 to equal a depth insertion of 9 cm. Auscultation of equal, bilateral lung sounds (including the axilla) after endotracheal tube placement should always be performed. Renal System and Extracellular Fluid Volume 2  At birth, the kidneys have a decreased glomerular filtration rate, decreased sodium excretion, and decreased concentrating ability. The glomerular filtration rate will increase and reach adult levels by 12-24 months of age. Neonates and infants up to 24 months are not able to compensate for alterations in fluid balance as well as adults. This makes fluid replacement very important. The extracellular fluid volume in an infant is twice that of an adult. Approximately 40% of the body weight in infants is extracellular fluid compared to 20% of adults’ body weight. Neonates, infants, and children fasting for anesthesia can become dehydrated more quickly than an adult. Careful fluid calculation includes NPO deficit, maintenance fluids, 3rd space fluid loss, and estimated blood loss replacement. For a pediatric patient that is dehydrated, it is important to correct pre- existing deficits before anesthesia to avoid hypotension. Temperature Regulation Neonates and infants can rapidly lose heat, even in warm environments. They are at greater risk for hypothermia than adults due to: a) Relatively high surface-to-volume ratio. b) High metabolic rate. c) Insufficient body fat for insulation. Infants less than 3 months do not shiver to generate heat. It is important to take steps to minimize heat loss including a warm operating room, warm blankets, or a heating blanket. Monitoring the patients’ temperature before, during, and after the anesthesia is important to detect abnormal drops or increases in temperature. Pharmacology in Pediatrics Pediatric patients respond differently to anesthetic medications when compared to adults. This is due to physiological differences that include: a. Increase extracellular fluid. b. Decrease skeletal mass. c. Increase metabolic rate. d. Decrease renal function. 3 e. Receptor maturity. Inhaled Anesthetics Uptake, distribution, and potency of volatile anesthetics are different in neonates and infants than in adults. Induction of general anesthesia occurs faster in neonates and infants. Emergence also occurs faster. The differences between adult and pediatric patients during induction and emergence are: 1. Smaller functional residual capacity (smaller lung volume). 2. Greater blood flow to the vessel-rich tissues such as the brain, heart, liver, and kidneys. In infants and neonates, the vessel-rich tissues comprise about 22% of total body weight. In adults, the vessel-rich tissues compose about 10% of the total body weight. MAC varies according to the age of the patient. In general, MAC is lower in neonates than in infants. MAC increases until about 2-3 months of age, peaks during infancy, and then steadily declines. During puberty, there is a brief increase in MAC. After puberty, MAC will continue to decline. Lower MAC requirements for volatile anesthetics in neonates are due to an immature central nervous system. Intravenous Anesthetic Agents Neonates are sensitive to intravenous anesthetic agents. They have an immature blood-brain barrier and a decreased ability to metabolize medications such as opioids and barbiturates. In general, lower doses of intravenous anesthetic medications are required to produce the desired effects. There are some generalized exceptions. For example, to induce general anesthesia higher doses of propofol (on an mg/kg basis) are required when compared to an adult. Pediatric patients less than 6 months old may be more sensitive to respiratory depression resulting from opioid administration. Caution should be used when administering opioids to this age group. The pediatric patient should be carefully monitored during the postoperative period. Nondepolarizing Muscle Relaxants 4 Neonates and infants may be more sensitive to the effects of nondepolarizing muscle relaxants. The neuromuscular junction of the infant is immature. The duration of action of nondepolarizing muscle relaxants may be prolonged due to immature renal and hepatic systems. Depolarizing Muscle Relaxants Neonates and infants require higher doses, on an mg/kg basis, of succinylcholine than the adult patient. This is due to an increased extracellular volume and volume of distribution. The dose of succinylcholine in pediatrics is 1.5-2 mg/kg IVP compared to 1 mg/kg IVP in adults. Routine use of succinylcholine in pediatric anesthesia is not recommended. The use of succinylcholine in pediatrics should be reserved for emergency intubation, rapid sequence induction, and laryngospasm. MCQ test 1- Pediatric anesthesia (all true except one) a) Pediatric patient is small adult b) Oxygen consumption for neonate is two time greater than adult. c) Alveolar ventilation in neonate is twice the adult rate. d) Overextension of neonate can cause airway obstruction. e) Neonate means 1-30 days of extra uterine life 2- Pharmacology in pediatrics (which one is true) a) Neonates are resistant to intravenous anesthetic agents. b) The routine use of succinylcholine is recommended. c) MAC of inhalational agents is lower in infants than neonates. d) Duration of action of non-depolarizing agents may be shorter. e) Neonates and infants may be more sensitive to nondepolarizing relaxants. 3- Temperature regulation in infant and neonate (all true except one) a) Neonates and infants can rapidly lose heat. b) They are at greater risk for hypothermia. c) Infants less than 3 months do not shiver to generate heat. d) Monitoring the patients’ temperature before anesthesia only. e) Hypothermia is treated by warm operating room or warm blanket. 4- The most common cause of bradycardia in pediatrics is: a) Vagal stimulation. b) Hypothermia. c) Hypoxia. d) Traction. e) Atropine. 5 5- The total blood volume for 4 kg. neonate is approximately a) 100ml. b) 120ml. c) 220ml. d) 340ml. e) 440ml. 6  Geriatric Anesthesia The geriatric population (The elderly) experiences significant alterations of many organ systems as a result of the aging process. They also have several co-morbidities including hypertension, cardiac disease, diabetes, cerebrovascular disease, and renal dysfunction. Geriatric patients are considered vulnerable and especially sensitive to the stress of trauma, surgery, and anesthesia. GERIATRIC ANESTHESIA STRATEGIES A. Preoperative Evaluation Tests should be directed toward the type of surgery, known co- existing disease, and history and physical examination findings. Electrocardiogram (ECG), hematocrit (Hct), and hemoglobin (Hgb) are often the most useful tests. 1. Cardiac Major indicators of cardiovascular risk are unstable coronary syndrome, decompensated heart failure, significant or unstable dysrhythmias, and severe or critical valvular disease, especially aortic stenosis. Cardiac testing should be reserved for patients undergoing intermediate- or high-risk surgery. 2. Pulmonary Referral to a pulmonologist may be indicated if the patient has signs and symptoms of undiagnosed or decompensated lung dysfunction. Risk factors for postoperative pneumonia include the inability to carry out the activities of daily living, weight loss of 10% or more in the previous 6 months, history of stroke, long-term steroid use, smoking, and underlying lung disease. 1 3. Renal It is wise to obtain serum electrolyte levels and creatinine concentration before procedures that carry a significant risk of renal failure (e.g., cardiopulmonary bypass, aortic aneurysm resection, or surgeries in which large fluid shifts or significant blood loss are anticipated). 4. Hepatic Baseline liver function tests may be reasonable before surgeries that involve significant liver manipulation. 5. Diabetes Mellitus Poor glucose control (blood sugar higher than 200 mg/dL) is associated with a risk of aspiration, poor wound healing, infection, cardiac and cerebral events, and autonomic dysfunction. Whenever possible, control of serum glucose to levels of 120 to 180 mg/dL is desirable before surgery. 6. Malnutrition Serum albumin below 3 g/dL with hypocholesterolemia and low body mass index is indicative of malnutrition. B. Pharmacokinetics and Pharmacodynamics There is no evidence that any specific inhaled or injected anesthetic agent is preferable in elderly patients. Changes in body composition can affect the distribution, metabolism, and clearance of drugs. 1. Total Body Water: Total body water is decreased, leading to a higher plasma concentration of hydrophilic drugs for a given dose. 2. Adipose to Lean Muscle Ratio: The ratio is increased, and the volume of distribution of lipophilic drugs is greater, facilitating the accumulation of drugs and prolongation of effects. This is even more pronounced in the face of impaired hepatic or renal function. 3. Circulation Levels of Drug-Binding Proteins: Levels decrease, leading to increased free drug and drug effects. 2 4. Decreased Cardiac Output. This may prolong circulation time and may result in more rapid uptake of volatile agents. 5. Muscle Relaxant Effects Decreased muscle blood flow delays the onset of action. Reduced clearance by the liver and kidneys may prolong the action of some relaxants. 6. Multiple Drug Prescriptions Commonly, elderly patients are taking multiple medications, which may lead to undesirable drug effects or drug interactions. 7. Minimum Alveolar Concentration (MAC) of Volatile Agents MAC decreases with age, about 4% per decade after 40 years of age. C. Anesthetic Plan: Anesthetic management for elderly Patients require consideration of many details 1. Anesthetic Technique Retrospective and prospective studies have failed to show the benefit of regional versus general anesthesia. The technique should be based on patient choice, the anesthesiologist’s experience with the technique, the American Society of Anesthesiologists (ASA) status of the patient, and the planned operation. 2. Monitoring Monitoring should be based on potential risks and benefits, the potential for large blood loss or fluid shifts, the ASA status and comorbidities, and the planned surgery. 3. Optimal Analgesia Treatment of analgesia may be challenging due to pharmacokinetic and pharmacodynamics changes and the side effects of analgesic drugs. Dementia, delirium, and problems with hearing and vision can complicate pain assessment. The physiologic consequence of inadequate analgesia (tachycardia, hypertension, agitation) may be poorly tolerated. 3 Postoperative Delirium Delirium is defined as an acute alteration in cognitive function n that progresses over a brief period lasting for a few days to a few weeks. Risk Factors 1. Advanced age (>70) 2. Underlying dementia. 3. Various comorbidities 4. Drugs (narcotics and benzodiazepines). 5. Alcohol abuse. 6. Previous episodes of delirium. 7. Visual impairment. 8. Certain types of injuries (e.g., hip fractures). 9. Elevated blood urea nitrogen (BUN). Treatment Treating underlying disorders, 0.25 to 2 mg of oral haloperidol for acute control of delirium is the preferred treatment, but diazepam, droperidol, and chlorpromazine are also often used with good results. MCQ test 1- All the following are decreased in elderly except one a) Cardiac output. b) Total body water. c) Circulation level of drugs binding proteins. d) Adipose to lean muscle ratio. e) Muscle blood flow. 2- The physiologic consequence of inadequate analgesia in elderly is a) Hypotension. b) Tachycardia. c) Sedation. d) Fever. 4  e) Bradycardia 3- Risk factors for postoperative pneumonia in elderly include (all true except one) a) history of stroke. b) Smoking. c) underlying lung disease. d) short-term steroid use e) inability to carry out the activities of daily living 4- anesthetic plan for elderly patient (all true except one) a) anesthetic technique is based on patient choice and anesthesiologist experiencing b) analgesia may be challenging due to pharmacokinetic and pharmacodynamics changes. c) MAC for inhalational agents is increases with age, about 4% per decade after 40 years of age. d) Risk of post operative delirium. e) prolong the action of some relaxants. 5- All the following are risk factors for delirium postoperatively in elderly except one a) Visual impairment. b) Underlying dementia, c) Alcohol d) Rib fracture. e) Advance age. 5  Anesthesia effects on the respiratory system The primary functions of the respiratory system are: 1. Ventilation; the movement of air into and out of the lungs. 2. Gas exchange; is the transfer of oxygen into the blood and carbon dioxide removal. General anesthesia has several effects on both of these key functions. The passage of gas into the lungs may be impaired by obstruction of the airway; the distribution of gas within the lungs may change and the transfer of oxygen (and anesthetic gases) into the blood may be impaired. Most of these adverse effects can be seen during anesthesia and in many patients, these extend into the postoperative period. HOW ANAESTHESIA AFFECTS VENTILATION 1. Airway obstruction General anesthesia, with or without the use of neuromuscular blocking drugs, results in the loss of airway patency due to the relaxation of the pharyngeal muscles and posterior displacement of the tongue. The ability to manage secretions is lost, and saliva and mucous can obstruct the oropharynx. The loss of the cough reflex allows secretions (or refluxed gastric contents) onto the vocal cords, causing laryngospasm, or to enter the trachea and lungs causing bronchospasm. These effects result in airway obstruction and prevent the passage of gases into and out of the lungs resulting in hypoxia and hypercapnia. 2. Reduced ventilation All anesthetic drugs (except ketamine, ether, and nitrous oxide) cause a dose-dependent reduction in minute ventilation. This can be due to either a reduction in the respiratory rate (e.g., opioids), a 1 reduction in the tidal volume (e.g., volatile anesthetics), or both (e.g., propofol). The ventilatory response to carbon dioxide is reduced by all anesthetic drugs, as a result, anesthetized patients become hypercapnic. HOW ANAESTHESIA AFFECTS GAS EXCHANGE Oxygen is the gas of primary importance, but the exchange of anesthetic gases will also be influenced. Oxygenation is dependent upon 1. The inspired oxygen content. 2. The presence of a patent airway. 3. Adequate alveolar ventilation. 4. Appropriate matching between ventilation and perfusion in the lung. 5. The transfer of oxygen across the alveolar and endothelial membranes. Anesthesia affects gas exchange by 1. Changes in functional residual capacity (FRC) 2. Changes in ventilation and perfusion. 3. Hypoxic pulmonary vasoconstriction (HPV) HOW MECHANICAL VENTILATION CAN DAMAGE LUNG TISSUE 1. Acute Respiratory Distress Syndrome ARDS Mechanical ventilation can directly damage lung parenchyma. Large tidal volumes (>12ml/kg) cause alveoli shearing stress, releasing inflammatory substances. These inflammatory mediators result in edema in the interstitial wall of the alveoli, which reduces lung compliance and gas transfer causing hypoxia. This is termed acute respiratory distress syndrome (ARDS) and results in prolonged stays in the intensive care unit (ICU) with a mortality of up to 40%. 2  2. Pneumothorax (Barotrauma) Pneumothorax occurs when air is trapped between the two pleural layers of the lungs with a loss of negative pressure that causes the lung to collapse. High inspiratory pressures or large tidal volumes can cause pneumothorax, which is more likely in the stiff, noncompliant lungs caused by ARDS or in the non-elastic lungs of chronic obstructive airway disease. The pressure required to keep one group of alveoli open may rupture another group causing a pneumothorax. THE PHARMACOLOGICAL EFFECTS OF ANAESTHESIA Table 1. Summary of the effects of the main anesthetic drugs on the respiratory system. Inhalational Positive effects Negative effects agents Isoflurane Bronchodilation Reduced MV Reduced response to hypoxia & hypercarbia Pungent – causes coughing Increased secretions Sevoflurane MV stable Hypercarbia Bronchodilation Depressed response to CO2 Non-irritant 3  Halothane Bronchodilation Reduced MV, Blunted response Non- irritant to hypoxia and hypercarbia Reduced bronchial secretions Iv Induction Positive effects Negative effects agents Thiopentone Dose dependent respiratory depression Increased bronchial smooth muscle tone with increased bronchospasm & laryngospasm Propofol Laryngeal relaxation – ease Respiratory depression of Reduced response to hypoxia & LMA insertion hypercarbia Bronchodilation Ketamine Preserved laryngeal Increased saliva and mucous reflexes production Maintain patent airway Less respiratory depression Reduction in bronchial smooth muscle tone Opiates Anti-tussive Respiratory depression Chest wall rigidity Bronchospasm MV = Minute Ventilation TV = Tidal Volume RR = Respiratory Rate HPV = Hypoxic Pulmonary Vasoconstriction LMA = Laryngeal Mask Airway MANAGING THE EFFECTS OF ANAESTHESIA ON THE RESPIRATORY SYSTEM I. Pre-operatively a. Positioning patients at a 45° angle before induction helps to reduce the fall in the FRC. b. Pre-oxygenation to maximize the oxygen content of the FRC can significantly increase the time from apnea to hypoxia. c. Antimuscarinic drugs (atropine, glycopyrrolate) given before induction reduce the quantity of saliva in the airway. II. Intra-operatively 4  a. Mechanical ventilation, in particular for obese patients, reduces airway collapse and atelectasis. Positive end- expiratory pressure (PEEP) helps to maintain alveolar patency and prevent hypoxia. If the patient is breathing spontaneously, continuous positive airway pressure (CPAP) will have the same effect. b. PEEP and recruitment maneuvers can be used to open collapsed portions of the lung. Recruitment is achieved by prolonged periods of high PEEP, Lung protective strategies that are used to treat ARDS can also be used safely for obese patients, bariatric and laparoscopic surgery, and the elderly during anesthesia to reduce atelectasis and increase oxygenation. This effect is not continued after extubating. III. Post-operatively Oxygen can be continued into the postoperative period in patients at risk of hypoxia. Head-up tilt increases the FRC and helps prevent atelectasis. In obese patients, extubation onto a CPAP mask may help prevent airway collapse and atelectasis and maintain arterial oxygenation. Similarly, extubating ICU patients onto bi-level noninvasive ventilation has been shown to reduce the rate of re-intubation. Make sure the patient has good postoperative analgesia. Patients should be able to take deep breaths and cough. MCQ test 1- All the following are positive effects of ketamine on respiratory system except one a) Reduction in bronchial smooth muscle tone b) Maintain patent airway. c) Less respiratory depression d) Anti-tussive. e) Preserved laryngeal reflexes. 5 2- All the following methods used to decrease the effect of anesthesia on respiratory system except one a) Oxygen can be continued into the postoperative period in patients at risk of hypoxia. b) atropine, glycopyrrolate) given before induction reduce the quantity of saliva in the airway. c) Head down position before induction helps to reduce the fall in the FRC. d) Positive end-expiratory pressure (PEEP) helps to maintain alveolar patency and prevent hypoxia. e) In obese patients, extubation onto a CPAP mask may help prevent airway collapse and atelectasis. 3- Negative effects of Isoflurane on respiratory system (all true except one a) Increased secretions b) Reduced minute ventilation. c) Sweet odor. d) Reduced response to hypoxia. e) Reduced response to hypercarbia. 4- All the following anesthetic drugs cause a dose-dependent reduction in minute ventilation except one a) Ketamine. b) Propofol c) Thiopental d) etomidate. e) Isoflurane. 5- Anesthesia affects gas exchange by (all true except one) a) Change in FRC, b) Change in ventilation. c) Change in perfusion. d) Hypoxic pulmonary vasoconstriction. e) Change in tidal volume. 6 Definition: An inhalational anesthetic is a chemical compound possessing general anesthetic properties that can be delivered via inhalation. They are administered through a face mask, laryngeal mask airway or tracheal tube connected to an anesthetic vaporizer and an anesthetic delivery system. The main use of inhalational was to maintain anesthesia, some of them can be used as induction The dose of inhalational anesthetic was mentioned as MAC MAC: The minimum alveolar concentration (MAC) of an inhaled anesthetic is the alveolar concentration that prevents movement in 50% of patients in response to a standardized stimulus (eg, surgical incision). e.g : Halothane MAC 0.75% Factors Increasing MAC (need more volatile to maintain anesthesia) Hyperthermia Hypernatraemia Sympatho-adrenal stimulation Chronic alcohol abuse Chronic opioid abuse Increases in ambient pressure Hypercapnia Decreasing age Thyrotoxicosis Factors decreasing MAC (need less volatile to maintain anesthesia) Nitrous oxide Hypothyroid/myxedema 1  Hypocapnia Hypothermia-decrease is roughly linear Hyponatraemia Increasing age Hypoxaemia Hypotension Anemia Pregnancy CNS depressant drugs Other drugs: lithium, lidocaine, magnesium, Acute alcohol abuse HINT: Sex, Weight and Duration of anesthesia does not affect MAC Nitrous oxide MAC105% , Halothane (Fluothane) 0.75% ,Isoflurane 1.2% ,Desflurane 6.0% ,Sevoflurane 2.0 % Characteristic of IDEAL inhalational: Non-flammable, non-explosive at room temperature Stable in light. Liquid and vaporizable at room temperature Stable at room temperature, with a long shelf life Stable with soda lime, as well as plastics and metals Environmentally friendly - no ozone depletion Cheap and easy to manufacture Non toxic Rapid induction and rapid recovery Safe with no toxic side effect There was no ideal inhalation till now Pharmacokinetic of inhalational anesthetics The forward movement of inhalational agent is determined by a series of partial pressure gradients, beginning at the vaporizer of the anesthetic 2 machine, continuing in the breathing circuit, the alveolar tree, blood, and then tissue. The principal objective of that movement is to achieve equal partial pressures on both sides of each single barrier. The alveolar partial pressure governs the partial pressure of the anesthetic in all body tissues; they all will ultimately equal the alveolar partial pressure of the gas. After a short period of equilibration the alveolar partial pressure of the gas equals the brain partial pressure. So there was an uptake , ventilation and concentration that effect the induction rate and awaking time Partial pressure is the ratio of the amount of substance in one phase to the amount in another phase 3 Recovery from anesthesia depends on lowering the concentration of anesthetic in brain tissue. Anesthetics can be eliminated by biotransformation, transcutaneous loss, or exhalation. Biotransformation usually accounts for a minimal increase in the rate of decline of alveolar partial pressure. Diffusion of anesthetic through the skin is insignificant. So The most important route for elimination of inhalation anesthetics is the alveolus. Mechanism of Action The exact mechanism of action for inhaled anesthetics remains mostly unknown. they work within the central nervous system by augmenting signals to (GABA receptors) while depressing neurotransmission pathways of acetylcholine to muscarinic and nicotinic receptors Nitrous oxide (N2O): Inhalational anesthetic agent, being a useful analgesic for dental extraction Properties: 4  Colorless Slightly sweet-smelling gas MAC 105%. Non-flammable but supports combustion Breaking down to O2 and nitrogen at high temperatures. Supplied as a liquid/gas in French blue cylinders Ice often forms on the cylinder during use CNS effect: Fast onset and recovery; strongly analgesic but weakly anaesthetic. Increases cerebral metabolism, cerebral blood flow and ICP slightly. Respiratory System: Non-irritant. Depresses respiration slightly. May cause diffusion hypoxia at the end of surgery. Cardiovascular system: Little effect on heart rate and BP usually Other: Post operative nausea and vomiting Does not affect hepatic or renal function, nor uterine or skeletal muscle tone. Prolonged use may cause bone marrow depression, megaloblastic anaemia and peripheral neuropathy. Generally considered as being safe during pregnancy Halothane: Halothane is a halogenated alkane ,it was nonflammable and no explosive. Sensitive to light so thymol preservative and amber-colored bottles retard spontaneous oxidative decomposition. Its use rapidly spread because of its greater potency, ease of use, non-irritability and non 5 inflammability , Risks of arrhythmias and liver damage on repeated administration (halothane hepatitis) Properties: Colorless liquid Pleasant smell MAC 0.75%. Non-flammable. Supplied in liquid form with thymol 0.01% Decomposes slightly in light. Central nervous system Effects: Smooth rapid induction, with rapid recovery. Anticonvulsant action. Increases cerebral blood flow but reduces intraocular pressure. 6 Respiratory system: Non-irritant. Pharyngeal, laryngeal and cough reflexes are abolished early Respiratory depressant, with increased respiratory rate and reduced tidal volume. Bronchodilatation and inhibition of secretions. Cardiovascular system: Myocardial depression and bradycardia. Hypotension is common. Myocardial O2 demand decreases. Arrhythmias are common, e.g. Bradycardia ,ectopic Sensitizes the myocardium to catecholamines, e.g. Endogenous or injected adrenaline. Other: Dose-dependent uterine relaxation. Nausea/vomiting is uncommon. May precipitate Malignant Hyperthermia. Up to 20% is metabolized in the liver. Repeat administration after recent use may result in hepatitis. 7 Isoflurane: Properties: Colorless liquid Pungent odor MAC 1.20 Non-flammable, non-corrosive. With no additive. Central nervous system Effects: Smooth, rapid induction, but speed of uptake is limited by respiratory irritation. Recovery is slower than with sevoflurane and desflurane. Anticonvulsant properties Reduces Cerebral Metabolic Rate of O2. Increases cerebral blood flow and ICP. Decreases intraocular pressure. Has poor analgesic properties. 8 Respiratory System: Irritant; more likely to cause coughing and laryngospasm Respiratory depressant, with increased rate and decreased tidal volume. Causes bronchodilatation. Cardiovascular system: Myocardial depression is less than with halothane Vasodilatation and hypotension commonly occur Compensatory tachycardia is common Myocardial O2 demand decreases, but tachycardia may reduce myocardial O2 supply. Coronary steel may occur Other: Dose-dependent uterine relaxation. Nausea/vomiting is uncommon. Skeletal muscle relaxation May precipitate MH. Widely used in neurosurgery Sevoflurane 9 Properties: Colorless liquid Pleasant smelling MAC 2. Non-flammable, non-corrosive. Supplied in liquid form with no additive. Interacts with soda lime to produce compounds A Central nervous system Effects: Smooth, extremely rapid induction and recovery. Early postoperative analgesia may be required as emergence Is so rapid. Increases the risk of emergence agitation Anticonvulsant properties. Reduces cerebral metabolic rate of O2 Decreases intraocular pressure. Has poor analgesic properties. Respiratory System: Well-tolerated Minimal airway irritation. Respiratory depressant, with increased rate and decreased tidal volume. Causes bronchodilatation. Cardiovascular system: Vasodilatation and hypotension may occur Myocardial O2 demand decreases. Arrhythmias uncommon 10 Other: Dose-dependent uterine relaxation. Nausea/vomiting occurs. Skeletal muscle relaxation May precipitate MH. Tracheal intubation may be performed easily with spontaneous Respiration. Considered the agent of choice for inhalational induction in pediatrics because of its rapid and smooth induction characteristics. Has also been used for the difficult airway, including airway obstruction. Desflurane Introduced in the UK in 1994, a colorless liquid with slightly pungent vapor, boiling point: 23°C, MAC: 5%–7% in adults; 7.2%– 10.7% in children, non-flammable, non-corrosive, supplied in liquid form with no additive, may react with dry soda lime to produce carbon monoxide, requires the use of an electrically powered vaporizer due to its low boiling point. Effects: On central nervous system: 1) Rapid induction (although limited by its irritant properties) and recovery. 2) May increase cerebral blood flow, although the response of cerebral vessels to CO2 is preserved. 3) ICP may increase due to imbalance between the production and absorption of CSF. 4) Reduces CMRO2 as for isoflurane. 11 5) Has poor analgesic properties. On respiratory system: 1) Causes airway irritation; not recommended for induction of anesthesia because respiratory complications (e.g. laryngospasm, breath-holding, cough, apnea) are common and may be severe. 2) Respiratory depressant, with increased rate and decreased tidal volume. On cardiovascular system: 1) Vasodilatation and hypotension may occur, similar to isoflurane, may cause tachycardia and hypertension via sympathetic stimulation, especially if high concentrations are introduced rapidly. 2) Myocardial ischemia may occur if sympathetic stimulation is excessive, but has cardioprotective effects in patients undergoing cardiac surgery. 3) Arrhythmia as uncommon, as for isoflurane, little myocardial sensitization to catecholamines. 4) Renal and hepatic blood flow generally preserved. Others: 1) Dose-dependent uterine relaxation (although less than isoflurane and sevoflurane). 2) Skeletal muscle relaxation; non-depolarising neuromuscular blockade may be potentiated. MCQ TEST 1- Factors increase MAC(all true except one) a) Hyporthermia b) Hypernatraemia c) Chronic alcohol abuse 12  d) Chronic opioid abuse e) Hypercapnia 2- Not affect MAC a) Hyponatraemia b) Increasing age c) Hypoxaemia d) Hypotension e) Sex 3- Pungent odor a) Isoflurane b) oxygen c) Halothane d) Sevoflurane e) Nitrous oxide 4- Which one is true regarding MAC a) Halothane=1.2 b) Isoflurane=0.75 c) Sevoflurane=2 d) Desflurane=5 e) Nitrous oxide=105 5- requires the use of an electrically powered vaporizer due to its low boiling point. a) Desflurane b) Isoflurane c) Xenon d) Halothane e) Sevoflurane 13 6- Characteristic of IDEAL inhalational agents a) flammable at room temperature b) un stable in light. c) Un Stable at room temperature d) short shelf life e) Rapid induction and rapid recovery 14  Anesthesia for pediatrics and geriatrics Pediatric anesthesia Definitions: Neonate = less than 28 days old. Infant = 28 days to 1 year old. Child = 1 year to 16 years old (dependent on local laws of consent). Anatomical differences of pediatrics than the adults: 1) Larger head, shorter neck and larger tongue. 2) Glottic inlet is higher, epiglottis is longer and curved. 3) The narrowest part of the larynx is the cricoid ring. 4) Ribs are more horizontal and breathing is diaphragmatic rather than intercostals. Physiological differences in pediatrics than the adults: 1) Functional residual volume (FRC) lies close to the closing volume (CV) in the infants and the reduction in FRC with anesthesia or disease can lead to atelectasia and segmental collapse unless positive end-expiratory pressure (PEEP) is applied. 2) In infants, alveolar ventilation and oxygen demand are much higher than in the adults, but the FRC/VA (alveolar ventilation) is much lower (half), so the reserves of oxygen in the lung of the infant are lower. 3) Unlike in adults, mild hypoxia in the neonate causes hypoventilation leading to apnea. 4) Basal metabolic rate, caloric requirements and O2 uptake is are higher. 5) Glycogen stores are relatively low, but brain and myocardium are more glucose dependent. 1 6) Cardiac output average (relative to body weight) and heart rate are greater. 7) The neonate has limited responses to cold (vasoconstriction rather than shivering) and there is an increased propensity to bradycardia. 8) In general, infants have larger volumes of distribution for most drugs, and even susceptible infants may require larger initial doses of drugs to achieve adequate plasma concentrations. Normal respiratory rate and heart rate according to age Age group Heart rate Respiratory rate Beat/ min Breath/ min Less than 1 year 110 - 160 30 - 60 1 to 3 years 100 - 150 24 - 40 3 to 6 years 95 - 140 22 - 34 6 to 12 years 80 - 120 18 - 30 12 to 18 years 60 - 100 12 - 16 Practical conducts for general anesthesia Fasting guidelines: Neonates and pre-weaned infants will become irritable, dehydrated and hypoglycemic if starved for extended periods. For elective surgery a widely used scheme is: a) Solids- morning case, no solid food overnight; afternoon case, light food at breakfast; no solid food for 6 hours before surgery. b) Milk- up to 4 hours before surgery for bottled milk, up to 3 hours before surgery for breast milk. c) Clear liquids- up to 2 hours before surgery. Children with major organ dysfunctions, or those actually ill with infection or trauma, should be treated as though they had a full stomach regardless of fasting interval because these conditions are associated with delayed gastric emptying. Small infants 2 should be scheduled first on an operating list to improve planning, but it may still be necessary to commence intravenous fluids. Premedication: The advent of local anesthetic creams prior to venipuncture has reduced the necessity for sedative premedication. Occasionally, a sedative drug required, this is particularly useful for child who, in spite of good preoperative preparation, remains apprehensive. Currently, oral midazolam is a widespread popular, an alternative to midazolam is oral ketamine, in that case, an antisialagogue (e.g. atropine) should be added to prevent excess salivation. If profound degrees of sedation are required, it is possible to combine midazolam and ketamine. The incidences of nausea and vomiting and of excess sedation in the postoperative period are increased. Intramuscular premedication is generally not tolerated well by children. Rectal administration of induction agents has been used (such as thiopental), this form of premedication may be used only under the direct supervision of the anesthetist, as respiratory depression is a distinct possibility. Induction of anesthesia: Unlike adult practice, it is not possible to have all the necessary monitoring devices placed on the child before induction. In most cases, if should be possible to place an appropriately sized pulse oximeter probe on a digit. Most children also allow the placement of precordial stethoscope. ((Precordial stethoscopes)) 3  The appropriate monitoring should be placed as soon as possible after the start of anesthesia. When inhalational induction is planned, clear scented plastic masks are much more acceptable to little children. Clear masks allow respiration and the presence of vomitus to be observed. Gas induction has become increasingly preferred since the introduction of sevoflurane. It is usually elected from the outset together with nitrous oxide and oxygen. Opting for intravenous induction depends on child’s preference, suitability of veins, technical expertise and state of the child. Doses should be titrated to effect: neonates and sick infants may require reduced dose, whereas 3–5-year-old need relatively larger doses than adults (e.g., required propofol dose is 2.5-5mg/ kg while it is in adults 1-2.5 mg/ kg). The pain on induction with propofol can be reduced by adding 20 mg lidocaine to 200 mg propofol. Thiopental provides a smooth induction but can delay postoperative recovery. Maintenance: Most simple short procedures require only spontaneous ventilation under a volatile or intravenous anesthetic agent and analgesia that will extend into the postoperative period. Neonates are usually intubated and ventilated for surgical procedures to ensure adequate gas exchange, and are given local anesthetic blockade where possible to limit CNS depressant drug usage. In complex procedures where postoperative ventilation is planned, high-dose opioid techniques are often used to minimize stress responses. Regional analgesia: Regional blocks reduce intraoperative anesthesia requirements and provide a postoperative analgesia and, unlike systemic analgesia, may provide complete analgesia without systemic side-effects. 4 I.V fluids: Crystalloids: Intraoperative hypoglycemia can occur in neonates, but is unusual owing to the effects of the stress response on glycolysis and gluconeogenesis. In contrast, excessive perioperative administration of glucose solutions can lead to hyponatremia, water intoxication and cerebral edema. Hartmann’s solution (Ringer’s lactate solution) can be given as a sole agent during surgery, but it is wise to measure blood glucose hourly during prolonged cases, alternatively, a fixed maintenance infusion of a glucose-containing solution should be continued throughout, with additional fluid replacement of Hartmann’s given independently. A recognized formula for maintenance fluid hourly rates is: 1-10 kg = 4 ml/ kg 11-20 kg = 40 + 2 ml/ kg over 20 kg = 60 + 1 ml/ kg It has been shown that a mixture of glucose 2.5% in Ringer’s lactate can maintain normal glucose while avoiding hyponatremia. Increased replacement fluids may be required if the gut remain exposed. Colloids and blood: The threshold for transfusion will vary the child’s overall condition and associated pathologies. For otherwise healthy children it is acceptable to let Hb drop to 8-9 g/dl, but neonates and children with cardiac or pulmonary conditions may benefit from a Hb raised to 10-13 g/dl. A volume formula for transfusion is: (Hb required – Hb actual) × (body weight in kg) × 5 = volume of red cells required (using resuspended SAGM blood). Fresh frozen plasma and platelets may need replacing earlier than in adults to prevent coagulopathy. These colloids contain citrate and will require additional calcium administration to prevent significant hypocalcemia if infused quickly. Endotracheal tubes: For children over 1 year: Appropriate tube internal diameter (ID) can be approximately estimated by the formula: age / 4 + 4. Appropriate tube length in cm. can be approximately estimated by the formula: age / 2 + 12 oral (+15 for nasal). 5  In infants: Appropriate tube ID sizes for preterm: