Altitude Physiology

Altitude causes a number of physiological effects, related to:

  • Reduce atmospheric pressure
  • Reduced temperature
  • Reduced relative humidity
  • Increased solar radiation

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.

Respiratory

  • 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 hypocapnoea
    • 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 comepensatory increase in 2,3-DPG to right-shift the curve and improve oxygen offloading at the tissues

Cardiovascular

  • 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

Haematological

  • Increased risk of thrombotic events to due increased haematocrit
  • Increased red cell mass due to EPO secretion

References

  1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
Last updated 2017-10-04

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