Inhalational Agents Flashcards

Question

What effects do volatiles have on the kidneys?

Answer 1

* inorganic fluoride production is a mode of direct nephrotoxicity * levels needed are \>50 micromols/L for methoxyflurane * isoflurane less nephrotoxic than enflurane due to higher defluorination resistance * Des and halothane are not defluorinated * sevo is significantly defluorinated * Compount A (triflurormethyl vinyl ether) is renal toxic in rats but no real clinical impact on humans

Answer 2

Malignant hyperpyrexia, is an uncommon pharmacogenetic disorder of skeletal muscle induced by exposure to suxamethonium and any volatile anaesthetic agents. It's autosomal dominant with variable penetrance and incidence of 1:10000.

Answer 3

* hypermetabolism * muscle rigidity * muscle injury * initial signs: * unexplained increased CO2 production * tachycardia * late signs: * unstable BP, decreased sats * increased body temp * raised CK, myoglobinuria * cardiac arrhythmias * hyperkalaemia * DIC

Answer 4

* Unregulated calcium flux from the muscle SR to the muscle fibres themselves * due to a mutation in ryanodine receptor, RYR1, chromosome 19 on the SR surface * RYR1 receptors are linked to the L-type calcium channel, which is voltage dependent * RYR2 receptors in the heart open by calcium ions inducing their activity

Answer 5

In vitro contracture testing (IVCT) at specialist centres. The tension in muscle from MH-susceptible patients increases at lower halothane and caffeine concentrations.

Answer 6

Dantrolene 2.5 mg/kg bolus with further 1mg/kg boluses to a maximum of 10mg/kg.

Answer 7

* consider regional anaesthesia * avoid sux and all volatiles * TIVA * use a clean machine - no volatiles used by the machine beforehand (or O2 at 10L/min for 30mins through anaesthetic machine and ventilator) * have dantrolene ready * monitor temp and end-tidal CO2 closely

Answer 8

Central core disease, a rate congenital myopathy

Answer 9

MAC is expressed as a percentage, but the clinical effect of volatiles is determined by their partial pressure in tissues. Eg. the output of a vapour is calibrated to atmospheric pressure (101.325 kPa). So 1% on the dial is a vapour pressure of 1% of atmospheric. Sevo's SVP = 22.7 kPa (22.7% of atmospheric pressure) If you half atmospheric pressure, the SVP of Sevo is still 22.7kPa, but atmospheric pressure is now 50kPa = Sevo = 45.4% of atmospheric pressure. So now 1% on the vapouriser is no longer 1% of atmospheric pressure (101kPa) it's 2% because the pressure has halved because it hasn't been recalibrated. The resultant partial pressure is 2% of 50kPa ie 1kPa - same as sea level.

Answer 10

Where: * %1 is the agent % delivered * p1 is the pressure at a given altitude * %cal is the agent % dialled on the vaporizer * p cal is the pressure at an altitude where the vaporizer is calibrated

Answer 11

The partial pressure and therefore clinical effect does not change because the concentration delivered is increased.

Answer 12

Temperature decreases with altitude - this is "lapse rate" and varies according to the moisture content of the air. International standard is 6.49 degrees C per 1000m from sea level to 11000m. This reduces the saturated vapour pressure with altitude and therefore the clinical effect of the agent. However, modern vapourizers are temperature compensated, so can essentially be used at altitude/sea level/hyperbaric conditions without temperature correction.

Answer 13

The desflurane vapourizer is pressurized to 2 atmospheres, there is no compensation for ambient pressure. So, concentration delivered is stable, regardless of ambient pressure. Eg. if ambient pressure halved to 50kPa but dial constant at 2% - the partial pressure is 2% of 50 = 1kPa so the dial must be increased to maintain partial pressure of 2kPa at altitude