INTRODUCTION TO HYPOXIA

A. Hypoxia: deficiency of oxygen at the cellular level; low tissue O₂ partial pressure, low P_t-O2 (formerly termed "anoxia")

B.    Physiological Causes: must be due to defect in one or more of the stages by which O₂ is transported from air to tissue -- "Oxygen Cascade" (see figure)

If a value is low while the preceding value is normal, the cause must be

1. P_I-O2 low: low inhaled oxygen partial pressure (e.g. high altitude or breathing an oxygen-deficient mixture)

2. P_A-O2 low: alveolar hypoventilation, due to

a. restrictive pulmonary disease
b. obstructive pulmonary disease
c. neurogenic disease (low CNS drive or muscle strength)
d. high dead space (artificial)

3. P_a-O2 low: due to high AaΔO₂; possible causes:

a. reduced pulmonary oxygen diffusing capacity
b. shunt (right-to-left)
c. ventilation/perfusion non-uniformity (V'_A/Q mismatch)

Note: AaΔO₂ = P_A-O2 - P_a-O2

4. C_a-O2 low: due to

a. low amount of hemoglobin (anemia)
b. abnormal hemoglobin (hemoglobinopathy)

5. CV Transport low, due to

a. low cardiac output (e.g., heart failure)
b. local ischemia

Note:  index of ischemia is a-v extraction, C_a-O2 – C_v-O2, at rest

6. Tissue O₂ Uptake low: due to

a. abnormal cellular metabolism

Note: In exercise, the normal inspired, alveolar, and arterial values would be expected to be the same as at rest; the cardiac output and tissue O2 uptake would increase; and the venous values would decrease