Pharmacokinetics of inhaled anesthetics

The pharmacokinetics of inhaled anesthetics are explained here with detail

• Absorption of agent from alveoli to blood

• Distribution in the body

• Metabolism (liver)

• Elimination (lungs mainly)

Uptake and Distribution

The pharmacokinetics of inhaled anesthetics depends upon:

(1) Respiratory uptake, (2) Alveolar ventilation, (3) The partial pressure of the agent in the inspired gas, alveolar gas, arterial blood and in the brain tissues.

1. For the pharmacokinetics of inhaled anesthetics , the alveolar uptake depends upon various physical- principles:

a) Saturated vapour pressure is the pressure exerted by the molecules of a liquid in an enclosed space, when there is equilibrium of the number of molecules leaving the liquid and the number returning is equal during pharmacokinetics of inhaled anesthetics .

b) Dalton’s law of partial pressures states that the total pressure of a mixture of gases and vapours administered during anaesthesia will be the sum of the partial pressures which the individual components would exert if present alone and occupying the same volume as the mixture.

c) Ostwald solubility coefficient defines the solubility of a gas in a liquid. It is the, volume of gas dissolved in a unit volume of liquid at a given temperature. If an anaesthetic vapour is in contact with two different phases, i.e., blood or gas, there is a different affinity for each phase and at equilibrium the ratio of proportion is known as partition coefficient or Ostwald solubility coefficient.

2. In pharmacokinetics of inhaled anesthetics and during uptake of the anaesthetic in the alveoli, three physical factors are involved, viz., tension, concentration and solubility of the inhalation agent. Uptake depends upon the inspired concentration of the agent (F1), the alveolar concentration (FA) and the alveolar ventilation.

Increase in alveolar concentration (FA) toward that of inspired concentration (F1) with time compared with different agents.

3. Wash-in and wash-out of the agent occurs in an exponential manner when pharmacokinetics of inhaled anesthetics is studied. A wash-in of oxygen during preoxygenation at 0.5 minute, there is a 63% change, 86% change in 1 minute and 95—98% in 2 minutes (Eger 1992)

4. For pharmacokinetics of inhaled anesthetics , the anaesthetic uptake depends on three factors, viz:

a) Solubility (L)

b) Cardiac output (Q)

c) Alveolar to venouspttia1- pressure difference (Pa-Pv)

Since uptake is a sum of three factors; if any factor becomes zero, then the uptake will be zero or minimal.

Solubility or blood gas partition coefficient is the relative affinity of an anaesthetic between blood and alveolar gas. For example in pharmacokinetics of inhaled anesthetics , halothane has a blood gas coefficient of 2.5, which means that its concentration in blood is 2.5 times that of the alveolar phase at equilibrium. This ranges from 0.45 for desflurane to 15 for methoxyflurane.

 

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