Altitude anaesthesia

While working abroad, at an altitude of 5000m, it becomes necessary to administer a

general anaesthetic with an FiO2 of 0.9, to a healthy patient who lives locally. The

operating theatre is heated and equipped with an anaesthetic machine that uses variable

orifice flowmeters and a Tec5 isoflurane vaporiser, out of circuit. Regarding your anaesthetic

management, the following statements are true EXCEPT for which one?

a) The delivered concentration of isoflurane will be more than that shown on the dial

of the vaporiser

b) The oxygen rotameter will accurately read the delivered flow of oxygen

c) The alveolar concentration of isoflurane will need to be higher than at sea level to

achieve the same degree of anaesthesia

d) The partial pressure of isoflurane in the vaporiser is the same as it would be if you

returned with the same vaporiser to sea level

e) The patient’s oxygen saturation is more likely to be 90% than 96%

Answer: b

Explanation

In individuals living at altitude a number of compensatory changes occur including

hyperventilation, increased erythropoietin secretion, increased 2,3-diphosphoglycerate,

adaptation of the respiratory centre to a lower PaCO2 and an increase in peripheral

capillaries. At altitude there are a number of issues pertinent to giving an anaesthetic.

The first is that although the FiO2 remains constant at any altitude the PO2 decreases the

higher you go. The sigmoid-shaped oxyhaemoglobin dissociation curve compensates for

this, maintaining haemoglobin saturation in the high nineties up to about 3000m when

the PO2 hits the steep part of the curve and haemoglobin saturation starts to fall. The

position of the oxyhaemoglobin dissociation curve does not shift much as the respiratory

alkalosis shifts it to the left and the hypoxia shifts it to the right. In terms of delivering an

inhalational anaesthetic agent at altitude it is important to remember a few basic

principles. The first is that the saturated vapour pressure of a volatile agent, while

affected by temperature, is unaffected by atmospheric pressure hence at altitude the

partial pressure of the isoflurane in the vaporiser is the same as at sea level. This means

that the delivered concentration (or minimum alveolar concentration (MAC)) of isoflurane

the patient receives will be more than is dialled up due to the drop in atmospheric

pressure (as explained by Dalton’s law). However, the action of an anaesthetic agent

depends on the effect site concentration, which is determined by the alveolar partial

pressure not the concentration. Although the MAC is up, the effect will be the same so

dialling up the same amount as at sea level will have the same effect. At altitude due to

the reduced atmospheric pressure there are fewer gas or vapour molecules per unit

volume. This means that at a given flow rate dialled up on a rotameter fewer molecules

pass the bobbin per unit time, therefore the bobbin will under-read at altitude due to

fewer molecules hitting the underside of the bobbin per unit time.