The principle described in relation to the uptake of volatile anaesthetic agents into the blood is based on Henry's Law.
Understanding Henry's Law and Anaesthesia
Anaesthesia involves the administration of agents that affect the central nervous system. Volatile anaesthetic agents are gases or liquids that are vaporized and inhaled by the patient. For these agents to work, they must be absorbed from the lungs into the bloodstream and transported to the brain.
The movement of the anaesthetic agent from the air spaces in the lungs (the alveoli) into the blood is governed by physical principles, notably Henry's Law.
How Henry's Law Applies
According to Henry's law, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. In the context of anaesthesia:
- The gaseous phase: The air in the alveoli contains the volatile anaesthetic agent at a certain partial pressure.
- The liquid phase: The blood flowing through the capillaries surrounding the alveoli acts as the liquid into which the gas dissolves.
Based on this law, the relationship can be summarized as:
- The partial pressure of the anaesthetic agent in the blood is proportional to the partial pressure of the volatile agent in the alveoli.
This means that the higher the concentration and thus partial pressure of the anaesthetic gas in the lungs, the more of it will dissolve into the blood.
Impact of Inspired Concentration
A direct practical consequence of this principle is how we administer volatile anaesthesia:
- Increasing the inspired concentration: If the concentration of the volatile anaesthetic agent in the gas mixture the patient breathes is increased, this directly raises the partial pressure of the agent in the alveoli.
- Increased blood concentration: Following Henry's Law, this higher alveolar partial pressure leads to a corresponding increase in the partial pressure and therefore the concentration of the anaesthetic agent in the blood.
This increased blood concentration is essential for delivering enough anaesthetic agent to the brain and other tissues to achieve the desired state of anaesthesia. The speed at which this blood concentration increases depends on various factors, but the fundamental relationship between alveolar and blood partial pressure is described by Henry's Law.
