Preferred Anaesthetics

The preferred inhalation agents are those that are minimally irritant and nonflammable, and comprise nitrous oxide and the fluorinated hydrocarbons, e.g., isoflurane.

PHARMACOKINETICS (VOLATILE LIQUIDS, GASES)

The level of anaesthesia is correlated with the tension (partial pressure) of anaesthetic drug in the brain tissue and this is dependent on the development of a series of tension gradients from the high partial pressure delivered to the alveoli and decreasing through the blood to the brain and other tissues. These gradients are dependent on the blood/gas and tissue/gas solubility coefficients, as well as on alveolar ventilation and organ blood flow.

An anaesthetic that has high solubility in blood, i.e., a high blood/gas partition coefficient, will provide a slow induction and adjustment of the depth of anaesthesia. This is because the blood acts as a reservoir (store) for the drug so that it does not enter the brain easily until the blood reservoir has been filled. A rapid induction can be obtained by increasing the concentration of drug inhaled initially and by hyperventilating the patient.

Agents that have low solubility in blood, i.e., a low blood/gas partition coefficient (nitrous oxide, sevoflurane), provide a rapid induction of anaesthesia because the blood reservoir is small and agent is available to pass into the brain sooner.

During induction of anaesthesia the blood is taking up anaesthetic agent selectively and rapidly and the resulting loss of volume in the alveoli leads to a flow of agent into the lungs that is independent of respiratory activity. When the anaesthetic is discontinued the reverse occurs and it moves from the blood into the alveoli. In the case of nitrous oxide, this can account for as much as 10% of the expired volume and so can significantly lower the alveolar oxygen concentration. Thus mild hypoxia occurs and lasts for as long as 10 minutes. Though harmless to most, it may be a factor in cardiac arrest in patients with reduced pulmonary and cardiac reserve, especially when administration of the gas has been at high concentration and prolonged, when the outflow is especially copious. Oxygen should therefore be given to such patients during the last few minutes of anaesthesia and the early postanaesthetic period. This phenomenon, diffusion hypoxia, occurs with all gaseous anaesthetics, but is most prominent with gases that are relatively insoluble in blood, for they will diffuse out most rapidly when the drug is no longer inhaled, i.e. just as induction is faster, so is elimination. Nitrous oxide is especially powerful in this respect because it is used at concentrations of up to 70%. Highly blood-soluble agents will diffuse out more slowly, so that recovery will be slower just as induction is slower, and with them diffusion hypoxia is insignificant.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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