Second Gas Effect

The second gas effect embodies the same principles as the concentration effect. However, in this case the administration of a rapidly absorbed gas given in high concentration (typically nitrous oxide) together with a volatile agent of lower solubility, produces an increasing alveolar concentration of the second agent thus promoting its absorption.

Figure V.1

Wash-in curves for volatile anaesthetic agents

Circulatory Phase

The following factors influence the transport of the volatile agent dissolved to the brain:

• Cardiac output

• Cerebral blood flow

• Distribution to other tissues

The distribution of blood to various tissues and compartments is described on page xx. While these principles apply to drugs that are intravenously administered, volatile agents behave in a similar fashion with the driving force of partial pressure (tension), tending toward equilibrium. The uptake of agent by the tissues is proportional to tissue perfusion, solubility and arteriovenous tension difference. The formula is:

Uptake = tissue blood flow x tissue/blood solubility x arteriovenous tension difference.

The formula produces time constants for the exponential function of tissue anaesthetic tension against time. High cardiac output and low tissue solubility produce a low time constant.

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