anesthesiology exam 2 - inhalent anesthetics

the pressure exerted by vapor moleucles once in equilibrium with liquid

gases/vapors can be administered in a range (0-100%), the greater the vapor pressure the greater concentration possible, vapor pressure is compared to atmospheric pressure at sea level (760 mmHg)

28/760 = ~3%max

760 x .32 = 240

temperature; increased temperature = increased vapor pressure and increased potential inhalent concentration

dilutes vapor with O2 and/or N2O, permits the administration of gases in clinically relevant concentrations, are agent specfic and color coded

flow from vaporizer chamber/total gas flow

the extent gas/vapor dissolves in solvent (ie how much ISO dissolves into blood, fat, tissue)

chemical nature of gas, partial pressure of gas, nature of solvent, temperature

gas is in all layers (ie gas, oil, water), but gas numbers will vary

the more soluble the gas, the more uptake, the longer it takes to become saturated = slower induction

the more soluble the gas, the longer it takes to be removed from the body, slower recovery

minimum alveolar concentration of anesthetic

minimum alveolar concentration of anesthetic that renders 50% healthy non-premedicated subjects non reactive to noxious stimuli (remember, 50% do move, the MAC of ISO in dogs is ~1.3%)

comparisons between different inhalents, estimation of where to set vaporizer

MAC values are additive

hyperthermia, CNS stimulation (excitement, drugs, disease)

age, hypothermia (nost anesthetized patients get cold), other CNS drugs/depressants/analgesics (opioids, alpha-2 agonists, acepromazine), pregnancy, concurrent disease

ether, chloroform, methoxyflurane

halothane?, isoflurane, sevoflurane, nitrous oxide (N2O)

desflurane, xenon?

relatively conserved across species

1.3 to 1.5 times MAC, 95% of patients will remain asleep and allows estimation of vaporizer setting

MAC of ISO in dog is ~1.3%, 1.3% x 1.5 (surgical) = 1.95%, 1.95 x 0.50% (decrease from acepromazine) = ~1%

diethyl ether

cheap, good skeletal muscle relaxation, minimal cardiovascular depression

expolosive, very slow and unpleasant induction, prolonged recovery, respiratory irritation

nonflammable, non-explosive, mild cardiovascular depression, non-irritating to respiratory tract

vapor concentration is limited to a maximum of 3.0%, very soluble so induction takes 20-30 min and recovery is prolonged, renal toxicity (flouride ion metabolite produced in the liver)

nonflammable, non-explosive, less soluble than methoxyflurane resulting in 7-10min induction and 5-15min recovery, potent myocardial depressant, sensitizes heart to catecholamines, does not irritate mucous membranes of respiratory tract but does cause respiratory depression, ~25% is metabolized by the liver and hepatitis has been diagnosed 2-5 days following anesthesia

less soluble than halothane so rapid induction and recovery ~5min, dose dependent vasodilation but less myocardial depression than halothane and does not sensitive myocardium to catecholamines, respiratory depression and irritating ot mucous membranes of respiratory tract, almost no liver metabolism (~0.2% metabolized)

less soluble than isoflurane for rapid onset and recovery, dose dependent vasodilation (similar to ISO) and does not sensitize the myocardium to catecholamines, dose dependent respiratory depression but non-irritation, ~3.0% metabolized by liver

minimal solubility so very rapid onset and recovery but the high vapor pressure necessitates special vaporizer (expensively specialized and requires power supply, does cause airway irritation, but has minimal liver metabolism (0.02%)

unique analgesic inhalent, can cause increased sympathetic stimulation, incomplete anesthetic in animals, administered in very high concentartions (~67%; the MAC is ~200% in animals therefore oxygen is delivered in smaller percentages), N2O diffuses 37x faster than nitrogen and is tetragenic when used in first trimester, contraindicated with any gas filled disease (GDV, pneumothorax) and hypoxemia/pulmonary disease

safety in terms of cardiovascular, respiratory, metabolism, CNS effects, pleasant induction/recovery, cost

isoflurane, halothane, sevoflurane

halothane

isoflurane and sevoflurane

halothane

myocardial depression, arrhythmias, catecholamine release (pheochromocytoma)

neither

decrease minute ventilation resulting in hypercapnia, alter responses to hypoxia and hypercapnia but there are species differences between inhalent and respiratory depression (apneic index)

halothane

% inhalent when apneic/MAC of inhalent. The higher the number the better the ventilation

pulmonary disease/hypoxemia, obesity/respiratory muscle weakness, high intracranial pressure (increased CO2 will further increase ICP), may need ot assist or control ventilation

it's not, rather it is exhaled from body

halothane (~25% metabolized), might avoid SEVO as well (3% metabolized)

decrease cerebral autoregulation in a dose depenent manner (ISO less than HALO)

Isoflurane (in smallest amount possible, but may want to avoid inhalents completely)

Sevoflurane is least soluble, so quickest

Isoflurane

sevoflurane

Sevoflurane (less irritating/fighting)

Sevoflurane

halothane ($0.10), isoflurane ($0.28), sevoflurane ($0.88)?

profit, client affordability, patient care

inhalents < 2ppm, N2O < 25 ppm

~50ppm

leaky machine, poor cuff, from patient during and after extubation

risks are unknown, but veterinary careers are long, be careful

insoluble inhalent

CNS

anesthetics