It is well known that the shortfall of PIO2 due to an ascent to high altitude is compensated by hyperventilation that brings about hypocapnia and respiratory alkalosis.

If a subject hyperventilates on ascent to high altitude why is the PaO2 low?

The most probable answer would be because the PIO2 is low. But it is necessary to consider another factor.

Ventilation is defined as:

1. The renewal of air in the lungs.
2. oxygenation of blood in the capillaries of the lungs.

To hyperventilate then would mean that there is a greater renewal of air in the lungs.

Atmospheric air is fundamentally composed of oxygen 20.98% and nitrogen 78.1%.

At sea level if one hyperventilates there is a greater renewal of air; naturally ,more molecules of air are moved within the lungs and hence PaO2 rises. Therefor, at sea level hyperventilation increases the number of molecules exposed to the lung capillaries.

On ascent to high altitude, and by using the Ideal Gas equation

[where, P = pressure, V = volume, n = # of moles, R = gas constant and T = temperature]

one can see that provided n, R and T remain constant, the volume increases as a result of lowering the pressure.

Hence, if the mean ventilation (BTPS) in L/min/m2 at sea level is 4.82, then in order to ventilate the lungs with the same # of molecules at 3600 m above sea level, provided that the temperature remain constant, we would need to hyperventilate (increase the ventilation) to:

[where P1 is the barometric pressure at sea level, V1 is the the VE(BTPS) at sea level, P2 is the barometric pressure at 3600 m and V2 is the expected VE (BTPS) at 3600 m.

In reality, the mean ventialtion for chronically adapted individuals at 3600 m is around 5.07 L/min/m2 or actually 31 % less than at sea level. This is why the PaO2 is only 60 mmHg ( 1/3 less than at sea level..

For if the ventilaiton would be 7.40 L/min/m2 then the PaO2 would be 95 mmHg similar to the value at sea level. Energy expenditure would be too great and the organism has "less expensive" means of adaptation.

This is why the term relative hyperventilation at high altitude is used.

Gustavo Zubieta Jr.