INHALED ANESTHETICS - STUDENT.pptx

INHALED ANESTHETICS NRAN 80424 SPRING 2024 RON ANDERSON, M.D. 1 OUTLINE CURRENT INHALED ANESTHETICS ORGAN SYSTEM EFFECTS NEUROPHYSIOLOGY CIRCULATORY PULMONARY HEPATIC NEUROMUSCULAR CHRONIC EXPOSURE MATERNAL/FETAL EFFECTS DEGRADATION METABOLISM CLINICAL USES INDUCTION MAINTENANCE 2 KEY POINTS The low...

INHALED ANESTHETICS NRAN 80424 SPRING 2024 RON ANDERSON, M.D. 1 OUTLINE CURRENT INHALED ANESTHETICS ORGAN SYSTEM EFFECTS NEUROPHYSIOLOGY CIRCULATORY PULMONARY HEPATIC NEUROMUSCULAR CHRONIC EXPOSURE MATERNAL/FETAL EFFECTS DEGRADATION METABOLISM CLINICAL USES INDUCTION MAINTENANCE 2 KEY POINTS The low boiling point of desflurane requires a special vaporizer to assure delivery of the desired concentration. Sevoflurane, with its low pungency and low blood:gas solubility, is preferred for inhalation induction. All of the volatile agents will trigger MH in a susceptible patient, while nitrous oxide and xenon will not. MAC of the inhalation agents is increased or decreased by multiple factors. Cerebral blood flow is increased, and CMRO 2 decreased by the volatile anesthetics. 3 KEY POINTS The volatile anesthetics produce predictable effects on evoked potential monitoring which vary by type of monitor. The volatile anesthetics all produce similar decreases in blood pressure secondary to relaxation of vascular smooth muscle. The volatile anesthetics trigger a cascade of intracellular events similar to that produced by a brief ischemic period. This results in some degree of protection from ischemia for the myocardium and potentially other organ systems. The volatile anesthetics produce a dose-dependent decrease in compensatory autonomic nervous system responses. 4 KEY POINTS The inhalation agents produce a dose-dependent decrease in tidal volume with an incomplete tachypneic compensation resulting in decreased minute ventilation. The volatile anesthetics produce a dose-dependent decrease in uterine smooth muscle tone. Degradation of volatile anesthetics in CO2 absorbents mat result in production of Compound A and carbon monoxide. 5 CURRENT INHALED ANESTHETICS Volatile agents Halothane Isoflurane Desflurane Sevoflurane Gases Nitrous oxide Xenon 6 BARAS H 7 HALOTHANE Nonflammable liquid at room temperature Sweet, non-pungent odor which is well accepted for inhalation induction Susceptible to decomposition, requiring a preservative, thymol, and packaging in an amber bottle Significantly metabolized (~20%) Properties High potency MAC = 0.75% Higher blood solubility than the other volatile agents Blood:gas partition coefficient 2.50 8 ISOFLURANE Nonflammable liquid at room temperature Pungent odor Extremely stable – no deterioration with: 5 years storage Sunlight CO2 absorbents Properties Most potent of the currently available volatile agents MAC = 1.17 Intermediate blood solubility Blood:gas partition coefficient 1.46 9 DESFLURANE Very similar structure to isoflurane, but markedly different properties Most pungent of the volatile agents May result in: Coughing Breath holding Salivation Laryngospasm Essentially zero metabolism to triflouroacetate Extremely rare incidence of immune mediated hepatitis Degrades to CO in a dry CO2 absorber 10 DESFLURANE Properties Lowest potency of the volatile agents MAC = 6.6 Lowest blood solubility of the volatile agents Blood:gas partition coefficient 0.42 Low tissue solubilities Significantly higher vapor pressure and lower boiling point than the other volatiles 11 BARAS H 12 SEVOFLURANE Nonflammable liquid at room temperature Non-pungent with minimal odor Least amount of airway irritation of the volatile agents Potent bronchodilator Properties Intermediate potency MAC = 1.8 Low blood solubility Blood:gas partition coefficient 0.65 13 SEVOFLURANE Metabolism 2-5% is metabolized which is much greater than isoflurane or desflurane Results in increased inorganic fluoride But has not been associated with injury to renal concentrating mechanisms in humans Not metabolized to triflouroacetate Interaction with CO2 absorber In a dry CO2 absorber may: Form carbon monoxide Produce an exothermic reaction and absorbant fire Reacts with absorbant to produce Compound A Produces dose-dependent nephrotoxicity in rats but appears safe in humans even at low fresh gas flows 14 NITROUS OXIDE Nonflammable gas at room temperature But will support combustion Odorless to sweet smelling Not an MH trigger Has inherent analgesic properties which are brief in duration Produces no skeletal muscle relaxation Properties Very low potency MAC = 104 Low blood solubility Blood:gas partition coefficient 0.46 15 NITROUS OXIDE Concerns Increased postoperative nausea and vomiting Inactivation of vitamin B12 Anti-neuroprotective effect Expansion of an air-filled structure or bubble Effects on embryonic development 16 XENON Nonflammable inert gas present in air Nonpungent and odorless No significant myocardial depression or change in coronary blood flow Provides some analgesia –less than N2O Not an MH trigger Properties Low potency MAC = 71 Extremely low blood solubility Blood:gas partition coefficient 0.115 17 ORGAN SYSTEM EFFECTS NEUROLOGIC MAC PHYSIOLOGY CMRO2 and EEG CBF, COUPLING and AUTOREGULATION ICP CSF CBF RESPONSE to HYPER and HYPOCAPNIA CEREBRAL PROTECTION POCD MONITORING NITROUS OXIDE 18 MINIMUM ALVEOLAR CONCENTRATION The alveolar concentration of an anesthetic at one atmosphere that prevents movement in response to a surgical stimulus in 50% of patients. Classically assessed by abdominal incision. 1.3 MAC of an inhaled anesthetic consistently, but not universally, will prevent movement in patients 0.5 MAC typically results in loss of self awareness and recall MAC values are roughly additive 19 THINGS THAT INCREASE MAC Increased CNS neurotransmitter levels MAOIs, ephedrine, levodopa Acute cocaine or amphetamine intoxication Upregulation of CNS response due to chronically reduced neurotransmitter levels Chronic ETOH abuse Hyperthermia Hypernatremia Red hair ~ 19% increase in MAC 20 21 THINGS THAT DECREASE MAC BARAS H 22 MAC CHANGES WITH AGE MAC decreases ~ 6% per decade from 1 year of age onward. Prior to 1 year of age MAC is decreased BARASH THINGS THAT DO NOT CHANGE MAC Duration of administration Gender Type of surgical stimulation Thyroid function Hypo- or hypercarbia Metabolic alkalosis Hyperkalemia Magnesium levels 23 MAC-awake and MAC-BAR MAC-awake Alveolar concentration at which the patient opens eyes to command 0.15-0.5 MAC Hysteresis exists between losing and regaining consciousness MAC-BAR Alveolar concentration that blunts adrenergic response to noxious stimuli ~ 1.5 MAC 24 NEUROPHYSIOLOGY OVERVIEW The modern volatile agents, Isoflurane, Desflurane and Sevoflurane: Have similar effects on: CMRO2 CBF Coupling EEG Vary in effect on: ICP CSF production and resorption Vascular response to CO2 Autoregulation Cerebral Protection 25 CMRO2 and EEG CMRO2 decrease consistent with the decrease in spontaneous cortical neuronal activity Once an isoelectric EEG is attained, no further decrease in CMRO 2 occurs Isoflurane Desflurane Sevoflurane EEG Changes with Isoflurane 1 MAC, isoflurane, desflurane and sevoflurane begin to produce similar mild increases in ICP Desflurane might produce slightly greater increase Sevoflurane is the least pungent and produces the least airway irritation, so may produce less coughing or bucking on the tube which can markedly increase ICP This increased ICP may be blunted by hyperventilation or the coadministration of propofol or barbiturates Nitrous oxide May be as potent a vasodilatory agent as the volatile agents, but can only be delivered at sub-MAC concentrations 29 CSF PRODUCTION and RESORPTION AGENT CSF PRODUCTION CSF RESORPTION ISOFLURANE NO CHANGE INCREASED** DESFLURANE NO CHANGE TO SLIGHT INCREASE ---------- SEVOFLURANE DECREASED UP TO 40% ---------- NITROUS OXIDE NO CHANGE ---------** Depending on what you read, isoflurane may increase, decrease, or produce no change in CSF resorption. Changes in CSF production and resorption play an insignificant role relative to ICP changes associated with changes in cerebral blood flow. 30 31 CBF RESPONSE to HYPER and HYPOCAPNIA Cerebral blood flow changes ~3% from baseline for each 1mmHg change in pCO2 Hypocapnia will blunt the increase in CBF seen with the volatile agents Should not decrease below 3035mmHg Short-lived phenomenon Hypocapnia is contraindicated in TBI BARASH CEREBRAL PROTECTION Studies are mixed General concepts When blood pressure is maintained there is likely minimal difference between isoflurane and other drugs such as thiopental in reduction of a focal cerebral injury following incomplete ischemia When anesthetic agents are used to lower blood pressure, isoflurane and other volatile agents appear to improve cerebral tissue oxygenation versus the sedative-hypnotics This difference is likely due to the reduction in CMRO 2 combined with maintenance of, or a modest increase in, cerebral blood flow 32 POSTOPERATIVE COGNITIVE DYSFUNCTION Impairment to the mental processes of perception, memory, and mental processing. Studies are mixed, and mechanisms are unclear. General concepts All modern anesthetic agents appear to be associated with development of POCD to some extent Desflurane and sevoflurane appear to show improvement over isoflurane in terms of cognitive recovery N2O is also associated with POCD and delerium 33 NEUROMONITORING All volatile agents produce dose-dependent effects on EEG, sensory-evoked potentials, and motor-evoked potentials EEG Similar effect with all three volatile agents < 1MAC or N2O at 30-70% shifts to increasing frequencies 1-2 MAC shift to decreasing frequency and increasing amplitude >2 MAC creates burst suppression or electrical silence 34 NEUROMONITORING Evoked Potentials Sensory-evoked potentials All volatiles cause a dose-dependent increase in latency and decrease in amplitude of all cortical SEPs Subcortical monitoring (BAEPs) are little affected by the volatile agents Visual evoked potentials are affected to a greater extent than somatosensory Gradual changes are better tolerated than sudden (>0.5MAC) changes in anesthetic depth Motor-evoked potentials Best to avoid volatile agents when using this technique as these are exquisitely sensitive 35 NITROUS OXIDE Data on effects of nitrous oxide on CBF, CMRO2, and ICP is conflicting Injury is greater in the face of temporary ischemia when nitrous oxide is added to isoflurane Bottom Line Probably not the best choice if cerebral ischemia is anticipated or tight control of ICP is critical 36 ORGAN SYSTEM EFFECTS CARDIOVASCULAR HEMODYNAMICS CONTRACTILITY OTHER CIRCULATORY EFFECTS CORONARY STEAL, ISCHEMIA and CARDIAC OUTCOMES CARDIOPROTECTION AUTONOMIC NERVOUS SYSTEM 37 38 HEMODYNAMICS Blood pressure Dose related decrease due to: Potent relaxation of vascular smooth muscle No significant differences between volatiles Nitrous oxide may increase BP slightly due to sympathetic stimulation Heart rate Isoflurane and Desflurane increase HR 5-10% Mays see greater increase with rapid increase in delivered concentration Sevoflurane No change at < 1 MAC Modest increase at greater concentrations BARASH HEMODYNAMICS BARASH 39 40 CONTRACTILITY Healthy patient No reduction in contractility by any of the volatiles Cardiac patient (EF< 40%) Slight reduction in contractility No impairment of functional reserve Ventricular response to acute increase in preload Volatiles – no change Propofol - reduced BARASH 41 OTHER CIRCULATORY EFFECTS Redistribution of blood flow Dose-dependent Decreased Liver, kidneys, gut Increased or unchanged Brain, muscle, skin Sensitization of the myocardium to catecholamines Produced by halothane No change with isoflurane, desflurane, sevoflurane BARASH 42 OTHER CIRCULATORY EFFECTS Automaticity and Conduction Slow SA node discharge rate Slowed conduction in His-Purkinje and ventricular conduction pathways Prolongation of the QT interval All volatiles may slightly prolong QT intervals Only Desflurane produced significant prolongation No increase in dysrhythmias noted BARASH CORONARY STEAL, ISCHEMIA, and CARDIAC OUTCOMES Coronary steal All the volatile agents increase coronary blood flow far in excess of myocardial oxygen requirements, producing the potential for steal from diseased vessels, however there is no evidence this occurs or that cardiac outcomes are worsened. Cardiac Outcomes Bottom Line “Most studies would suggest that determinants of myocardial oxygen supply and demand rather than the anesthetic, are of far greater importance to patient outcomes.” 43 CARDIOPROTECTION – ISCHEMIC CONDITIONING Preconditioning Following a brief period of coronary occlusion and ischemia Ischemia signals a cascade of intracellular events which reduces myocardial injury from ischemia and reperfusion Provides 2 – 3 hours of protection Volatile anesthetics trigger a similar cascade of events given either before or immediately after an ischemic event Preconditioning Postconditioning Other drugs which also create this ischemic preconditioning Adenosine Opioid agonists KATP channel openers 44 CARDIOPROTECTION – ISCHEMIC CONDITIONING A preconditioned heart may tolerate ischemia for 10 minutes longer than a non-conditioned heart prior to injury May also offer protection for kidney, liver and brain (neural tissue) Meta-analysis comparing CABG with sevoflurane or desflurane to TIVA revealed: 50% reduction in MI Reduced troponin I levels Reduced need for inotropic support Decreased all-cause mortality Shorter period of mechanical ventilation and ICU stay Sulfonylurea drugs close KATP channels, abolishing ischemic preconditioning and should be discontinued 24-48 hours preop in high risk patients 45 CARDIOPROTECTION – ISCHEMIC PRECONDITIONING MILLER 46 CARDIOPROTECTION – ISCHEMIC CONDITIONING FLOOD 47 AUTONOMIC NERVOUS SYSTEM The volatile anesthetics produce a dose-dependent impairment of reflex control mechanisms Patient’s ability to compensate for hypovolemia with vasoconstriction and tachycardia is reduced Baroreflex function returns to normal more rapidly following discontinuation of the less blood soluble agents Sevoflurane and desflurane faster than issoflurane 48 49 AUTONOMIC NERVOUS SYSTEM Desflurane Unique in producing an increase in sympathetic nervous system outflow Despite this increase, BP is decreased similarly to the other volatile agents Produces a transient significant surge in catecholamine levels with rapid increases in concentration BARASH AUTONOMIC NERVOUS SYSTEM BARASH 50 ORGAN SYSTEM EFFECTS PULMONARY VENTILATORY EFFECTS VENTILATORY MECHANICS RESPONSE TO CO2 and HYPOXIA BRONCHIOLAR SMOOTH MUSCLE TONE MUCOCILIARY FUNCTION PULMONARY VASCULAR RESISTANCE 51 VENTILATORY EFFECTS All volatile agents and N2O produce a dose-dependent reduction in minute ventilation and subsequent increase in PaCO2 via: Decreased tidal volume An incomplete compensatory increase in respiratory rate Note: isoflurane does not increase respiratory rate beyond 1 MAC Nitrous oxide causes less decrease in minute ventilation and increase in PaCO2 than the volatile agents due to: Less decrease in tidal volume Greater increase in respiratory rate 52 VENTILATORY EFFECTS BARASH 53 VENTILATORY MECHANICS General anesthesia produces a reduction in FRC Inspiration ~40% Intercostal muscles Compromised with anesthesia ~60% Diaphragm Remains essentially intact Expiration Passive due to elastic recoil of the lungs Not all patients will tolerate these changes and persist in adequate spontaneous respirations 54 55 RESPONSE TO CO2 In the absence of anesthesia, minute ventilation increases ~ 3 L/min for a 1 mm HG increase in PaCO2 All inhaled anesthetics produce a dose-dependent reduction in ventilator response to hypercarbia Apneic threshold PaCO2 below which respiratory drive ceases ~ 4-5 mm Hg below resting PaCO2 in a spontaneously breathing patient BARASH 56 RESPONSE TO HYPOXIA All inhaled agents produce a dose-dependent reduction in the ventilator response to hypoxia Apparent at subanesthetic concentrations of anesthetic As little as 0.1 MAC will reduce ventilator drive to hypoxia by 2575% Isoflurane produces greater and longer lasting reduction in ventilatory drive to hypoxia than sevoflurane and desflurane BARASH 57 BRONCHIOLAR SMOOTH MUSCLE TONE Airway smooth muscle descends to the terminal bronchioles Volatile agents relax airway smooth muscle by two mechanisms: Direct reduction in smooth muscle tone Compromised with endothelial damage from asthma or URI Indirectly via inhibition of reflex neural pathways Less reduction in airway resistance with desflurane due to direct effect on bronchial smooth muscle due to it’s pungency BARAS H MUCOCILIARY FUNCTION Volatile anesthetics and N2O Impair ciliary movement Thicken mucous Similar changes occur with ventilation of dried gases Problem is compounded with cigarette smoking where mucociliary function is already impaired 58 PULMONARY VASCULAR RESISTANCE and HPV Volatile agents Minimal to no effect on pulmonary vascular resistance Nitrous oxide Typically increases PVR only slightly, but may be markedly increased with Pre-existing pulmonary hypertension Neonates Hypoxic pulmonary vasoconstriction Serves to minimize V/Q mismatch by reducing blood flow to under-ventilated lung Impaired to some extent by all the inhaled agents 59 HEPATIC EFFECTS Hepatic Blood Flow All volatiles reduce hepatic blood flow to some extent Greatest reduction seen with halothane Desflurane at 1 MAC reduces flow by ~ 30% Isoflurane 1 MAC – minimal reduction in flow >1 MAC – dose dependent reduction in flow Sevoflurane – minimal reduction in flow regardless of dose Hepatic arterial buffer response preserved with isoflurane and sevoflurane Volatile anesthetics appear to provide organ protection following ischemic injury Sevoflurane versus propofol for CABG – reduced incidence of hepatic dysfunction with sevoflurane 60 HEPATIC EFFECTS Current volatile anesthetics undergo minimal hepatic metabolism AGENT % METABOLIZED TO TRIFLOUROACETYLATED INTERMEDIATES Halothane 20 Enflurane 2.5 Isoflurane 0.2 Desflurane 0.02 Sevoflurane 0.0 Frequency of hepatic injury correlates with the degree of metabolism to these TFA intermediates Sevoflurane metabolism results in production of fluoride and hexaflouroisopropanol which are conjugated in the liver for renal excretion, but these produce no apparent injury to the liver Following a known injury to the liver from a volatile anesthetic, there is debate about whether a different volatile anesthetic is safe to use or would predispose to further injury 61 NEUROMUSCULAR EFFECTS Direct relaxation of skeletal muscle Effect most pronounced at > 1 MAC of volatile Not seen with nitrous oxide Significantly increased effect in myasthenia gravis Potentiation of neuromuscular blockers 30-40% reduction in rocuronium requirement in presence of volatile anesthesia Mechanism uncertain Believed to occur postsynaptically at the nicotinc Ach receptor of NMJ Produced by synergism of the volatile agent with the NMB Isoflurane, desflurane, and sevoflurane all produce a similar degree of synergism 62 NEUROMUSCULAR EFFECTS Malignant hyperthermia Mutation in the ryanodine receptor in skeletal muscle may result in MH susceptibility Exposure to a triggering agent (all volatile anesthetics and succinylcholine)results in: Uncontrolled release of calcium Increased skeletal muscle metabolism Increased ETCO2 Tachycardia Tachypnea Metabolic acidosis Muscle rigidity Rhabdomyolysis potentially Increased temperature is a late finding, but can be pronounced N2O and xenon are considered non-triggers 63 CHRONIC EXPOSURE TO INHALED ANESTHETICS NIOSH has set exposure limits at: Volatile anesthetics 2 ppm Nitrous oxide 25 ppm Ames test reveals no mutagenicity or carcinogenicity from isoflurane, desflurane, sevoflurane or nitrous oxide The volatile anesthetics have not been shown to produce teratogenicity in humans Nitrous oxide decreases the activity of methionine synthetase which is involved in the formation of myelin and DNA Importance of this is unclear, but some believe this may have an impact on embryonic or fetal development 64 MATERNAL EFFECTS Uterine smooth muscle tone Dose-dependent decrease Reduces contractility of uterus and frequency of contractions At greater than 1 MAC uterine atony may be a problem When this is bad Following delivery when uterine contraction is important in reducing ongoing blood loss Impairs the response to oxytocin When this is good Need for brief uterine relaxation E.g. Retained placenta following delivery 65 FETAL EFFECTS Spontaneous abortion Previously thought to be increased in OR personnel Now believed not to be the case Contributed to the institution of scavenging systems Neonatal effects of anesthesia for cesarean section Apgar score No difference between general anesthesia and regional anesthesia More sensitive tests reveal transient depression following general anesthesia which resolves by 24 hours 66 FETAL EFFECTS Long-term effects of neonatal or early childhood exposure to anesthetics Lots of work being done here, but no definitive answers yet Rodents Following exposure to volatile anesthetics there is clear evidence of: Accelerated neuronal apoptosis Behavioral changes Humans ????? 67 DEGRADATION Compound A Produces renal injury in rats A vinyl ether produced from breakdown of sevoflurane in CO 2 absorbents Increased production in: Low-flow or closed circuit anesthesia Warm or desiccated CO2 absorbents Bara lime > soda lime > Amsorb Humans Prospective, multicenter, randomized studies have shown no adverse renal effects following low flow sevoflurane for long duration in either normal patients or those with preexisting renal disease Zero case reports of renal injury attributable to sevoflurane since its introduction FDA recommendation: > 1 L/min fresh gas flow up to 2 MAC hours > 2 L/min fresh gas flow after 2 MAC hours 68 DEGRADATION TO COMPOUND A BARASH 69 SEVOFLURANE, FGF, AND COMPOUND A FDA 70 DEGRADATION Carbon monoxide and Heat CO2 absorbent normally contains 13-15% water When water content is reduced to soda lime > Amsorb Desflurane >> Sevoflurane Increased a higher absorbent temperatures Greatly increased with desiccated absorbent Heat Produce by interaction of volatile with absorbent, particularly desiccated absorbent Greatest with sevoflurane Greatest with bara lime 71 72 DEGRADATION – CARBON MONOXIDE MILLER 73 METABOLISM Flouride Metabolism of some of the volatile anesthetics produces significant fluoride concentrations and, in some cases, injury to the renal collecting tubules Methoxyflurane and enflurane can produce a high output renal insufficiency thought related to: Peak serum fluoride concentration Duration of elevated fluoride concentration Sevoflurane Undergoes ~ 5% metabolism and increases fluoride concentration Does not produce a renal concentrating injury Potentially due to rapid washout of sevoflurane relative to enflurane, resulting in a briefer exposure MILLER CLINICAL USES - INDUCTION Inhalation induction Halothane or sevoflurane due to the low pungency and high acceptance of these drugs Historically halothane in a N2O/O2 mix Required a slow escalation of inspired concentration Currently sevoflurane + nitrous oxide in oxygen Can be escalated or begin with high inspired concentration for a 1-2 breath technique Advantages of inhalation induction Maintenance of spontaneous respirations Self-limiting Generally well-accepted 74 CLINICAL USES - INDUCTION Notes on Sevoflurane inhalation induction Used primarily in pediatrics with very occasional use in the adult population High inspired concentrations of sevoflurane may avoid Stage II and the associated excitement, etc A 1-2 breath technique with high inspired concentration of sevoflurane provides rapid loss of consciuosness, but does not imply an adequate depth of anesthesia for instrumentation of the airway Acceptance, success, and speed of onset improved with prior administration of benzodiazipines 75 CLINICAL USES - MAINTENANCE Advantages of volatile agents for anesthetic maintenance Easily administered Easy and rapid titration of depth of anesthesia Highly effective in preventing recall Easily monitored via end-tidal concentrations Additional Benefits Relaxation of skeletal muscle Typically well preserved cardiac output and CBF Predictable recovery Provide some protection from ischemic injury 76 77 PHARMACOECONOMICS Direct costs Cost of the drug Potency of the drug Fresh gas flows Indirect costs OR time PACU time Costs of dealing with side effects BARASH SOURCES Barash Clinical Anesthesia 8th edition. 2017 Flood Stoelting’s Pharmacology and Physiology in Anesthetic Practice 6th edition. 2022 Evers Anesthetic Pharmacology 2nd edition. 2011 Miller Miller’s Anesthesia 9th edition. 2020 78