General Anesthesia Flashcards
The state of “general anesthesia” includes:
- Analgesia
- Amnesia
- Loss of consciousness
- Suppression of reflexes
- Skeletal muscle relaxant
Balanced anesthesia
Used of several drugs to produce an anesthetic state
Types of General Anesthesia
- Inhaled anesthesia
- Gases
- Volatile halogenated hydrocarbons - IV anesthesia
Common features of Inhaled anesthetics
- Inc brain perfusion
- Bronchodilation
- Dec minute ventilation
- Potency correlates w/ liposolubility
- Rate of onset inversely correlates to Blood solubility
- Recovery is due to redistribution from the brain
Minimum Alveolar Concentration (MAC)
Conc. that results in immobility in 50% of pts. when exposed to a noxious stimulation
Greater the MAC — Lower the Potency
Lower the MAC — Greater the Potency
NB: MAC values are additive
MOA of Inhaled Anesthetics
- +ve modulators of GABAa & glycine receptors
- Inhibit nicotinic receptors
Direct interaction w/ ligand gated ion channel
Meyer-Overton Correlation
Oil: Gas Partition co-efficient —-ratio of conc. of a compound in one solvent to the conc. in another solvent
-Measure of liposolubility
High liposolubility = High potency
High oil:gas = Low MAC = High potency
Factors that determines the rate of anesthesia induction
- Solubility of anesthesia (Low Blood: Gas = Faster onset of action)
- Conc. in inspired air (Higher = Faster rate of induction)
- Pulm. ventilation rate (Higher = Inc rate of induction)
- Pulm. blood flow (Higher = Dec rate of induction)
- AV conc. gradient (Higher = Dec rate of induction)
Blood: Gas Partition Coefficient
Relative solubility of an anesthetic in blood compared to air
- Index of Solubility
- Inverse relationship between blood solubility & rate of rise of its tension in arterial blood
Low Blood: Gas = Fast onset of anesthetic
Increases in the rate of induction
- Inc in anesthetic conc
- Inc in ventilation rate
- Dec Pulmonary blood flow –> Dec Cardiac output
- Inc AV conc (Inc in arterial blood & tissue)
Elimination of inhalation anesthetics
Low Blood & tissue solubility - Recovery mirrors induction (regardless of duration of administration)
High blood & tissue solubility - Recovery depends on duration of anesthetic administration (bc anesthetic accumulates in fat)
CVS effects of Inhaled anesthetics
- Depress normal contractility (Dose-dependent)
- Vasodilation –> Dec MAP
- Halothane & Enflurane - Dec MAP by myocardial depression; little effect on PVR
- Isoflurane, Desflurane & Sevoflurane - Vasodilation; little effect on CO (Better choice for pts. w/ impaired myocardial function + less risk of ventricular arrhythmias) - Sensitization of Myocardium to circulating catecholamines –> Ventricular arrhythmias
- Halothane
Resp. effects of Inhaled Anesthetics
- Bronchodilation
- Resp. depressant
Mostly w/ Isoflurane & Enflurane
Least w/ N2O
CNS effects of Inhaled anesthetics
- Inc ICP (least w/ N2O)
- Tonic-clonic seizures @ high conc. (Enflurane)
- EEG changes
Other effects of Inhaled Anesthetics
Inc V/P of cavity
- Avoid in pts. w/ pneumothorax, obstructed middle ear, air embolus, obstructed bowel, intra-ocular air bubble, pulmonary bulla & Intracranial air
Toxicities of Inhaled Anesthetics
- Hepatotoxicity
- Halothane - Nephrotoxicity
- Methoxyflurane - Malignant Hyperthermia
- Halothane & Succinylcholine (Depolarizing skeletal muscle relaxant) - Hematotoxicity (Megaloblastic anemia)
- N2O chronic use
Halothane-induced Hepatotoxicity
Can develop severe & life-threatening hepatitis
- No specific treatment
- Liver transplant may be needed
Methoxyflurane-induced Nephrotoxicity
Due to fluoride released during metabolism
Malignant Hyperthermia
AD disorder of skeletal muscles
- Main cause of death due to anesthesia
CFs of Malignant Hyperthermia
- tachycardia
- HTN
- Severe muscle rigidity
- Hyperthermia
- Hyperkalemia
- Acidosis
- Acute kidney failure (Myoglobinuria + Hyperkalemia)
MOA of Malignant Hyperthermia
Defect in Ryanodine Receptor gene (RYR1) –> Unregulated Ca release from the SR –> Inc muscle contraction –> Heat
Depletion of O2 & ATP & Inc CO2 (aerobic) –> Switch to anaerobic –>Inc acidosis & lactate –> Depletion of energy stores –> Muscle fiber death –> Hyperkalemia & Myoglobinuria
Treatment of Malignant hyperthermia
Dantrolene
- Blocks Ca2+ release form SR
- Measures to dec body temp & restore electrolytes and acid-base balance
How does the use of Succinylcholine & Halogenated anesthetic affect pts w/ Muscular dystrophies?
Can develop life-threatening Hyperkalemia & Rhabdomyolysis due to upregulation of extra-junctional Ach receptors depolarizing the whole cell
What tests is used to establish susceptibility to Malignant hyperthermia?
Caffeine-halothane muscle Contracture test
- Muscle sample removed from thigh –> Halothane & caffeine response assessed
Genetic testing
N2O-induced Hematotoxicity
Prolonged exposure to N2O –> Dec Methionine synthase activity –> Megaloblastic anemia
RF: Working in poorly ventilated dental operating suites
IV Anesthesia (names)
- Propofol
- Ketamine
- Etomidate
- Barbiturates (Ultra-short acting)
- Thiopental
- Methohexital
Use of ultra-short acting Barbiturates
Induction of anesthesia for short surgical procedures
Metabolism of Ultra-short acting Barbiturates
Anesthetic effects are terminated by redistribution from brain to other tissues
Elimination from body requires hepatic metabolism
Actions of Ultra-short Barbiturates
- Dec ICP
- Do NOT produce analgesic
AEs of Thiopental & Mathohexidate
- Hyperalgesia
- Apnea
- Coughing
- Chest wall spasm
- Laryngospasm
- Bronchospasm
Concern for asthmatics
Actions of Propofol
- Induction & maintenance of anesthesia
- Antiemetic (Post-operative vomiting uncommon)
- NO analgesia
Liver metabolism
AEs of Propofol
- Resp. depression
- CV depression (Dose dependent)
- Hypotension (Dec PVR)
- Dec ICP
Prodrug of Propofol
Fospropofol
Actions of Etomidate
- Anesthetic induction of pts. w/ RISK FOR HYPOTENSION
- NO analgesic effects
AEs of Etomidate
- Minimal CV & Resp depression
- Dec ICP
- Nausea & vomiting
- Inhibits 11-beta-hydroxylase
- Adrenal suppression (prolonged infusions)
Actions of Ketamine
Dissociative anesthesia - Catatonia, amnesia, analgesia, w/ or w/o loss of consciousness
- Blockade of NMDA receptors
- Analgesic effects
- CV stimulation
AEs of Ketamine
- Inc ICP
2. “Emergence phenomena” - sensory & perceptual illusions & vivid dreams
Drugs used to reduce “Emergence phenomena”
- Diazepam
- Midazolam
- Propofol
Adjuvant drugs to General anesthesia
- Benzo - Anxiolytic & Anterograde amnesia
- Opioids - Analgesic
- Neuromuscular blockers - Muscle relaxation
- Antiemetic (Ondansetron) - Post-op N&V
- Antimuscarinics
- Scopolamine - Amnesic effect
- Glycopyrrolate - Prevent salivation & bronchial secretions
- Atropine - Prevent Bradycardia caused by inhalation agents & neuromuscular blockers
