Altitude, Inspired Oxygen, and the Tour de France

Tomorrow, during a grueling stage 18 of the Tour de France, riders will be making a 2409 m (7902 ft) climb from Revello to the Col Agnel.  This is the first of three major climbs on the day (classified as Haute Category, or Beyond Category), culminating in the highest finish in tour history at Galibier Serre-Chevalier (2645 m/8676 ft).

Image: letour.fr

Tour de France riders are amongst the fittest humans on the planet.  They are the elite of the elite.  Five time winner Miguel Indurain had a resting heart rate of 28 beats per minute and a cardiac output of 50 liters/minute.  In the case of the latter, a fit amateur cyclist has a cardiac output of about 25 liters.

Nevertheless, even at at this elite level, athletic performance is affected by altitude.  Oxygen comprises 21% of the atmosphere.  At sea level, where the mean pressure is about 1000 mb, the partial pressure of oxygen is about 210 mb.  At Revello, it is about 203 mb, and at the Col Agnel it is 152 mb.  That's about a 25.1% reduction during the climb.

But, the partial pressure of inspired air in the lungs is somewhat lower than it is in the atmosphere.  This is because the lungs are an extremely moist place and saturated with water vapor at body temperature (37C).  At that temperature, the saturation vapor pressure for water vapor is 63 mb, and this affects the partial pressure of oxygen in inspired air.

What this means is that 63 mb of the the inspired air will always be associated with water vapor in the lungs.  So, to estimate the partial pressure of oxygen in the lungs, you must first deduct 63 mb from the ambient air pressure and then multiply by 21%.  So, at Revello, the partial pressure of oxygen in inspired air is:

(966.5 mb - 63 mb)*.21 = 190 mb

whereas at Col Agnel it is 

(725.6 mb - 63 mb)*.21 = 139 mb

Because of the effects of water vapor in the lungs, the relative reduction of oxygen in inspired air during the climb is about 26.8%, slightly larger than the relative drop in ambient oxygen.

The graph below illustrates pressure of partial pressure of inspired air versus atmospheric pressure.  

Source: Dill and Evans (1970).  Scales are mmHg, with mb scale
for atmospheric pressure on left.

There are also a host of physiological effects that influence rider performance with increasing altitude, although I'll skip that as it is outside of my area.  The bottom line, however, is that riders who excel at climbing at high altitude will be at an advantage tomorrow.  With eight riders within 4 minutes of the overall lead, it is going to be a very interesting stage.