High Altitude Physiology

I. Altitude Affects Oxygen Delivery to Tissues

A. Atmospheric Pressure Determines the PIO2

B. PIO2 is a Primary Determinant of PAO2 and PaO2

1. PAO2 = PIO2 – PACO2/RER

2. PAO2 is the upper limit for PaO2

3. PaO2 is usually 3-10 mmHg less than PAO2

C. Arterial O2-Hb saturation depends on the PaO2

D. Oxygen Delivery to Tissues – F(x) of CaO2 and Cardiac Output

1. Arterial Oxygen CONTENT (CaO2)=

Hb(g/100ml blood) * saturation * 1.39 mlO2/g Hb

E.G. 15g/dL*0.85*1.39mlO2/gHb = 17.7 mlO2/100ml blood

2. Oxygen delivery in ml O2/min =

CaO2 (mlO2/Liter)*CO (L/min)

E.G. 177 mlO2/Liter * 5 L/min = 886 ml O2/min

**Examples are Flagstaff – Pb ~600 mmHg – try sea level or Everest

II. Acute Physiological Responses to High Altitude

Upon arrival at altitude, oxygen delivery – ml O2/min to tissues - is maintained by increasing:

• Cardiac Output (due to increase in HR)

• Ventilation Rate – lowers PaCO2, raises PAO2

• At altitudes above about 4,300 m (roughly 15,000 ft), max HR may be depressed due to myocardial hypoxia

III. Altitude and Athletic Performance

A. Changes in Anaerobic Performance at Altitude

B. Changes in Aerobic Performance at Altitude

C. Performance at Extremes of High Altitude

1. The first summit of Mt. Everest occurred in 1953 with the aid of supplemental oxygen

2. Previous attempts w/o supplemental O₂, were unsuccessful but close (w/in 300m)

3. Later attempts to summit w/o extra O₂ have been successful (1978), but rare

4. How do they do it?

• Those who were successful at higher altitudes have a greater capacity to hyperventilate. This drives down the PaCO₂ (which raises PAO2) and lowers the [H⁺] in the blood and allows more O₂ to bind with hemoglobin at the same PaO₂ (left shift)

• High altitude climbers also battle against dehydration (high respiratory water loss, loss with bicarb excretion, loss with vomiting), anorexia, and loss of muscle mass which may compromise exercise performance independent of O₂ changes

D. Immediate Adjustments to High-Altitude

1. Within the first 2 days of exposure to altitude the following adjustments occur:

• Loss of body weight, mostly water

• Increase in Hb concentration

• Originally due to a decrease in PV (hemoconcentation)

• Later increases result from greater Hb production (EPO)

• Resting HR and CO decline toward sea level values

• 2,3-DPG levels increase resulting in a right-shift in the Hb-O₂ curve (compensates for initial left shift)

* Kidney excretes bicarbonate (HCO3-) to compensate for respiratory alkalosis, water lost with HCO3-

2. Pathological Response to High Altitude - HAPE

• Hypoxia causes vasodilation in most blood vessels

• However, hypoxia causes vasoconstriction in pulmonary vessels

• Vasoconstriction leads to increased pulmonary vascular resistance

• Increased PVR leads to increased pulmonary BP

• In some people (not understood why some people and not others) the elevated pulmonary BP, and probably other factors, cause high altitude pulmonary edema (HAPE)

• Impairs exercise capacity by slowing diffusion at the BGB

• Only treatment is decent

E. Acclimatization to High-Altitude

Chronic altitude exposure results in:

• Increase in red blood cell production

• Kidneys produce erythropoetin which stimulates RBC production

• Increased Hct increases the O₂ content of the arterial blood

(CaO2 = Hb * saturation * 1.39)

• However, since PV remains the same, blood viscosity is increased which increases the TPR, afterload, and venous return, reducing SV

_• Persistant elevation in V_E

• Complete acclimatization in non-altitude natives in unlikely

F. “Ergogenic” effect of High-altitude Training

1. Due to the observed physiologic adaptations to high altitude exposure, HA is used as a means to improve exercise capacity and performance

2. The latest research suggests individuals live at high altitude to elicit changes in RBC mass and,

3. Train at a low enough elevation to maintain high training intensity