Inhalation Anaesthetic Agents Overview

Introduction

Inhalation anaesthetics are commonly used after induction with injectable agents but can also themselves be used for the induction of anaesthesia. However, inhalation agents are poor analgesics and have varying muscle relaxant ability. As expected, if used alone, much higher concentrations of inhalation agents are required to produce general anaesthesia than if used in combination with analgesics and/or muscle relaxants - a balanced anaesthetic technique'. They are unusual in respect to most other drugs, as they are adminstered and removed via the same route i.e. as the patient breathes. This also allows for easy adjustment to the depth of anaesthetic of a patient as it does not rely on the metabolism of the agent.

General Properties

Most inhalation agents are halogenated organic compounds, but nitrous oxide is an inorganic compound. This improves the stability of the agent and increases the potency of the agent. They exist as either vapours or gases depending on their state at room temperature and sea level pressures - a vapor is liquid under these conditions and requires a vaporiser to produce the gaseous form required for inhalation. Nitrous oxide is one of the very few gases in clincal use.

General Pharmacokinetics

To produce a state of general anaesthesia, it is necesary to reach a partial pressure in the brain sufficient to depress the central nervous system. This means that the depth of anaesthesia is based on the partial pressure of agent in the brain. This is reached by diffusion of agent from inhaled air to the brain via alveolar air and blood. Diffusion occurs until an equilibium has been reached, which occurs rapidly bettween the alveoli and blood, and blood and brain. This means that the alveolar partial pressure is almost identical to that in the blood, and therefore brain.

Changes in anaesthetic depth

Factors that affect depth of anaesthesia include -

Inspired concentration of the agent.
Ventilation
Agent solubility both in blood and tissues.
The patient's cardiac output.

Metabolism and Elimination

Elimination of inhalation agents is mostly via the lungs as the patient breathes out. However, there are varying degress of hepatic metabolism.

Minimum Alveolar Concentration

Minimum Alveolar Concentration (MAC) is a measure of the potency of an inhalation agent. It is the minimum alveolar concentration of agent required to produce immobility of 50% of patients when exposed to a noxious stimuli. Therefore, the lower the MAC, the more potent the agent.