Altitude Physiology

Altitude causes a number of physiological effects, related to:

Pressure Effects

Reduced air pressure results in a proportional decrease in PO₂:

At 3,000m, alveolar PO₂ is 60mmHg
At 5,400m, consciousness is lost in unacclimatised individuals
At 10,400m, air pressure is 187mmHg
With 47mmHg of water vapour and an alveolar PCO₂ of 40, breathing 100% O₂ gives an alveolar PO₂ of 100mmHg.
At 14,000m, consciousness is lost despite 100% O₂
At 19,200m, the ambient pressure is so low that the boiling point of water is 37°C
This is the Armstrong limit.

Fall in PaO₂ is compensated by increasing minute ventilation, which decreases PACO₂ and therefore increases PAO₂
- Limits of compensation are reached on 100% oxygen at 13,700m
Effective compensation is limited by the respiratory alkalosis, this is known as the braking effect:
- Peripheral chemoreceptors detect hypocapnea
- Central chemoreceptors detect alkalosis
The subsequent respiratory alkalosis generates a compensatory metabolic acidosis
This acidosis relaxes the braking effect and allows further hyperventilation, and is therefore am important part of acclimatisation.

There is an initial left-shift of the oxygen-haemoglobin dissociation curve due to alkalosis
This stimulates a compensatory increase in 2,3-DPG to right-shift the curve and improve oxygen offloading at the tissues

PVR increases due to HPV
Heart rate increases due to increased SNS outflow
Stroke volume falls (cardiac output remains the same) due to decreased preload:
- Plasma volume falls due to:
  - Pressure diuresis
  - Insensible losses from hyperventilation and reduce relative humidity
Myocardial work increases
- Increased HR
- Increased viscosity of blood due to high haematocrit
- Increased RV afterload from high PVR
  Increased pulmonary capillary hydrostatic pressures lead to fluid transudation and pulmonary oedema

Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.

Last updated 2021-08-23