How is the function of vaporisers affected by altitude?

Vaporisers are designed to deliver an accurate and precise concentration of a volatile agent. In plenum vaporisers, gas leaving the vaporising chamber is fully saturated with anaesthetic vapour. A concentration dial controls a valve within the bypass chamber, which enables the user to change the concentration of volatile delivered to the patient by altering the splitting ratio.

With increasing altitude, atmospheric pressure falls in a non-linear manner. However, vapour pressure does not change with altitude. Therefore, a vaporiser used at altitude will deliver a higher percentage concentration of volatile than is selected, but the partial pressure of the volatile agent remains the same. It is the partial pressure of the volatile anaesthetic agent that is important in determining its anaesthetic effect and not the concentration. Because the partial pressure remains unchanged at altitude, the desired isoflurane concentration can be set to the same value as it would at sea level.

Atmospheric pressure is 760 mmHg at sea level, which falls to approximately half at an altitude of 5500 m. If a plenum isoflurane vaporiser were set to deliver 3% isoflurane concentration, at 5500 m above sea level it would actually deliver a 6% concentration. In both instances, the partial pressure would be unchanged at 22.8 mmHg (3% of 760 mmHg = 6% of 380 mmHg = 22.8 mmHg).