Question 10

College Answer

Any description of the changes in inspired and alveolar PO2 with altitude required the
description of, and an understanding of, the equations for calculation of PiO2
(PiO2=0.21(BMP-47)mmHg or 0.21(BMP-6.3)kPa) and PAO2 (PAO2 =PiO2 – PaCO2/R). It
was important to mention alteration in atmospheric pressure (BMP) with altitude and that
saturated vapour pressure (SVP) is constant for the same body temperature.
A good answer to the second part of the question (respiratory physiological responses to
altitude) was one that had some structure, eg responses divided into acute and chronic,
respiratory, renal, haematological, cardiovascular and CNS categories, and relevant detail for
each category. Candidates were expected to at least mention, but not restrict themselves to the
following: acute responses, eg carotid/aortic body chemoreceptor stimulation
hyperventilation, proportional to reduced BMP and hypoxia, limitations to hypocarbia (due to
alterations in CSF pH) etc: physiological responses to early and chronic acclimatisation (eg
renal HCO3-excretion, respiratory centre CO2 response curve resets to left, altered peripheral
chemoreceptor sensitivity, 2,3, DPG levels and related right. Shift in HbO2 curve, alterations
in pulmonary diffusing capacity due to increased alveolar surface area and pulmonary blood
volume, etc).
It is important that current and future candidates use the examination reports to guide their
learning. Thus it is essential that candidates not just read the feedback provided to each exam
question but also refer to the reference(s) supplied below and other relevant sources they find
to be useful.
Syllabus – B1k, 2i
Reference – Nunn’s Respiratory Physiology, Ch 17 and 5.

Power and Kam 432-4.

Discussion

What follows probably represents some sort of passing minimum. For those interested in more detail, something insane is also available.

• With altitude:
  • Barometric pressure decreases and is ~ 200 mmHg at 10,000m altitude
  • Saturated vapour pressure remains stable at 37 ºC (47 mmHg)
  • Alveolar PO₂ decreases, and is ~ 30 mmHg at 8,000m altitude
  • PaCO₂ decreases due to hypoxic drive, and is  ~ 10mmHg at 8,000m
• The physiological responses to altitude are:
  • Acute:
    • Respiratory:
      • Minute volume increases due to hypoxic reaspiratory drive, mediated by peripheral chemoreceptors
    • Cardiovacular:
      • Tachycardia and increased cardiac output due to increased sympathetic drive
      • There is also a mild blood pressure increase
    • Neurological:
      • Decreased cognitive function
      • With profound hypoxia, delirium can develop
    • Renal and electrolyte:
      • Diuresis
      • Decreased serum bicarbonate (due to hypcapnia)
  • Chronic:
    • Respiratory:
      • Minute volume remains the same
      • Tidal volume may gradually increase due to thoracic remodelling
    • Cardiovascular:
      • Heart rate and stroke volume return to normal values as the haematocrit adapts
    • Haematological:
      • Haematocrit increases over days/weeks, largely due to haemopoiesis and haemoconcentration

Wyatt, Frank B. "Physiological Responses to Altitude: A Brief Review." Journal of Exercise Physiology Online 17.1 (2014).

Brown, James PR, and Michael PW Grocott. "Humans at altitude: physiology and pathophysiology." Continuing Education in Anaesthesia, Critical Care & Pain 13.1 (2013): 17-22.

Martin, D., and J. Windsor. "From mountain to bedside: understanding the clinical relevance of human acclimatisation to high-altitude hypoxia." Postgraduate medical journal 84.998 (2008): 622-627.