SYSTEMIC EFFECTS OF INHALATIONAL AGENTS
Effect on respiratory system:
- Bronchodilation:
- All inhalational agents.
- Halothane – Maximum bronchodilatation.
- DOC in asthmatics (intravenous anesthetics) = Ketamine.
- Respiratory depression (minimal):
- All inhalational agents.
- Enflurane – Maximum depression.
- Halothane – Maximum inhibition of ventilatory response to increased CO2 & hypoxia.
- Vasodilation:
- Vasodilate pulmonary vascular bed – All inhalational agents.
- By blunting hypoxic pulmonary vasoconstriction (HPV) response.
- Halothane – Maximum effect; Also by isoflurane
- Phosgene production:
- By Trilene -> Causes ARDS.
Effects on CVS:
- Cardiac output:
- CO best maintained with isoflurane.
- Isoflurane – Most cardiostable inhalational agent & agent of choice for cardiovascular surgery.
- Due to reflex tachycardia acting as protective baroreceptor reflex is minimally inhibited by isoflurane.
- CO decreasing drugs:
- All inhalational agents decrease CO, except ether & cyclopropane.
- Cardiovascular stability in decreasing order:Isoflurane (most stable) > desflurane > Sevoflurane > Halothane > Enflurane.
- Baroreceptor reflex:
- Halothane – Only agent suppressing baroreceptor reflex –> causes tachycardia.
- Myocardial contractility:
- All inhalational agents cause directly depress contractility.
- Isoflurane, desflurane & sevoflurane cause minimum depression to similar level.
- Systemic vascular resistance (SVR):
- All inhalational agents decrease –> result in hypotension.
- Except cyclopropane, ether, halothane & N20 – Not decrease SVR.
- Isoflurane, Desflurane & Halothane = Cause maximum hypotension.
- Isoflurone – Inhalational DOC for controlled hypotension.
- Note: Halothane causes hypotension by myocardial depression, not decrease SVR.
- All inhalational agents decrease –> result in hypotension.
- Cardiac adrenaline sensitization:
- Halothane – Sensitizes heart to adrenaline –> Results in arrhythmias.
- Contraindicated in pheochromocytoma & along with adrenaline.
- Sympathetic stimulation:
- Cyclopropane & Ether -> maintain CO & not decrease SVR.
- (Note: Other inhalational agents cause those effects).
- Coronary steal phenomenon.
- By Isoflurane
- Mainly in patient with coronary artery stenosis.
- Causes coronary artery vasodilatation in non-ischemic area & diverts blood away from ischemic zone.
- N20 has no effect on BP, CO & HR.
Effect on liver:
- Hepatotoxicity:
- Mild toxicity – All inhalational agents.
- By decreasing hepatic blood flow.
- Direct hepatotoxicity (hepatitis, hepatic necrosis)
- Halothane, chloroform, trilene & methoxyflurane.
- DOC in liver disease:
- Isoflurane – Due to least effect on hepatic blood flow.
Effect on kidney:
- Renal function depression:
- All inhalational agents.
- By decreasing renal blood flow.
- Nephrotoxic agent:
- Most nephrotoxic – Methoxyflurane causes high output (polyuric) renal failure.
- Sevoflurane & enflurane – No renal toxicity in normal person; contraindicated in renal patients.
- By producing nephrotoxic compound A.
- Desflurane has no nephrotoxicity.
- Note:
- Direct toxicity caused by fluorinated compounds due to inorganic fluoride (F-) production.
- Renal threshold beyond which fluoride levels are toxic is 50 mm.
Effect on Brain:
- Increasing ICT:
- All inhalational agents.
- By increasing cerebral blood flow –> ICT.
- Provide cerebral protection by decreasing cerebral metabolic rate.
- Maximum cerebral protection – Isoflurane.
- Inhalational DOC for neurosurgery.
- Enflurane – Provokes seizure.
- Contraindicated in epilepsy.
- Neurotoxicity:
- Dichloroacetylene involving most commonly 5th CN.
Effect on hematopoietic system:
- Bone marrow depression (aplastic anemia) & Vit B12 deficiency – By nitrous oxide.
- Vit B12 deficiency causes megaloblastic anemia & subacute degeneration of spinal cord.
Effect on muscle relaxation & analgesia:
- Muscle relaxation:
- All inhalational agents except N2O.
- By neuro-muscular blockade.
- Maximum relaxation – By ether (unused), desflurane.
- Analgesic action:
- All inhalational agents except halothane.
- Maximum – Trilene, N2O & ether.
- No analgesic action – Halothane.
Effect on uterine relaxation:
- All fluorinated anesthetics.
- Halothane (maximum), isoflurane, desflurane, sevoflurane, enflurane.
- Halothane:
- DOC for assisting external or internal version during late pregnancy.
- Contraindicated during labor 4 – Due to post-partum hemorrhage.
Effect on Inflammability & reaction with sodaline:
- Inflammable agents:
- Ether, Cyclopropane, ethylene oxide, ethyl chloride.
- Cautery should not be used in vicinity (up to 25 cms).
- Inducing inflammability:
- Agents can be made non-inflammable by adding fluorine atoms.
- Trichloroethylene (Trilene) & sevoflurane reacts with soda lime -> Not be used in closed (circle) system.
Effect on metabolism:
- Most inhaled anesthetics are eliminated from lung.
- Hepatic metabolism also occurs in order:
- Methoxyflurane (> 50%) > Halothane (20%)> Ether (10 -15%) > enflurane (3-5%) > sevoflurane (3%) > isoflurane (0.2%) > desflurane (< 0.1%) > N20 (0%).
- N20 does not have any metabolism in body.
- Some agent cause hyperglycemia; chloroform (most profound), ether cyclopropane.
Effects on anesthetic circuit:
- Corroding metals & vaporizing in breathing circuit – Halothane.
- Halothane & methoxyflurane – Absorbed by rubber of anesthetic circuit & polyvinylchloride of ET tube.
- Can be used only on replacing plastic tubing.
Exam Important
- Halothane causes maximum bronchodilatation.
- DOC in asthmatics (intravenous anesthetics) is ketamine.
- All inhalational agents vasodilate pulmonary vascular bed, by blunting hypoxic pulmonary vasoconstriction (HPV) response.
- Halothane has maximum vasodilatory effect.
- CO best maintained with isoflurane.
- Isoflurane is most cardiostable inhalational agent & agent of choice for cardiovascular surgery.
- Halothane, only agent suppressing baroreceptor reflex –> causes tachycardia.
- Isoflurane, desflurane & sevoflurane depress myocardial contractility to similar level.
- Isoflurane is inhalational DOC for controlled hypotension.
- Halothane sensitizes heart to adrenaline resulting arrhythmias, contraindicated in pheochromocytoma & along with adrenaline.
- Isoflurane causes coronary steal phenomenon.
- DOC in liver disease is Isoflurane, due to least effect on hepatic blood flow.
- Direct hepatotoxicity (hepatitis, hepatic necrosis) are caused by Halothane, chloroform, trilene & methoxyflurane.
- Most nephrotoxic is Methoxyflurane causes high output (polyuric) renal failure.
- Desflurane has no nephrotoxicity.
- Sevoflurane & enflurane causes no renal toxicity in normal person;contraindicated in renal patients.
- Isoflurane provides maximum cerebral protection by decreasing cerebral metabolic rate, hence inhalational DOC for neurosurgery.
- Enflurane provokes seizure & contraindicated in epilepsy.
- Nitrous oxide causes bone marrow depression (aplastic anemia) & Vit B12 deficiency.
- Vit B12 deficiency causes megaloblastic anemia & subacute degeneration of spinal cord.
- Maximum analgesic action is seen with Trilene & nil analgesic action with halothane.
- Halothane (maximum), isoflurane, desflurane, sevoflurane, enflurane can cause uterine relaxation.
- Halothane DOC for assisting external or internal version during late pregnancy.
- Halothane is contraindicated during labor 4 – Due to post-partum hemorrhage.
- Inflammable agents are ether, Cyclopropane, ethylene oxide, ethyl chloride & cautery should not be used in vicinity (up to 25 cms).
- Trichloroethylene (Trilene) & sevoflurane reacts with soda lime, hence not be used in closed (circle) system.
- Most inhaled anesthetics are eliminated from lung.
- Neurotoxicity is caused by dichloroacetylene involving most commonly 5th CN.
- Sevoflurane produces nephrotoxic compound A.
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