Ventilation/perfusion concepts.

Respiratory therapists confront hypoxemia on a daily basis. Some forms of hypoxemia are amenable to oxygen therapy, but other forms are refractory to oxygen administration. Some types of hypoxemia can be corrected by applying common low-flow oxygen delivery devices, but refractory hypoxemia

This article will explore the physiologic basis for hypoxemia by first describing how the diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane occurs, then by discussing the relationship between alveolar ventilation and pulmonary capillary perfusion, and last by presenting clinical implications of [dot.V]/[dot.Q] abnormalities.

GAS DIFFUSION ACROSS THE ALVEOLAR-CAPILLARY MEMBRANE

The primary function of the lungs is gas exchange. Gas exchange is accomplished through the process of passive diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane. Passive diffusion of these gases requires the presence of a pressure gradient. Gases diffuse down a pressure gradient from areas of high concentration to areas of low concentration.

The pressure gradient for oxygen diffusion begins to establish itself when atmospheric air with a partial pressure of oxygen of 159 torr (i.e., [760 torr - 0 torr P[H.sub.2]O] 0.2093) enters the upper airway. By the time the inspired gas reaches the trachea, the gas becomes 100% humidified at body temperature, acquires a P[H.sub.2]O of 47 torr, and assumes a P[O.sub.2] of 149 torr (i.e., [760 torr - 47 torr P[H.sub.2]O] 0.2093). Because the pulmonary capillary blood continuously takes up alveolar oxygen, the P[O.sub.2] decreases further to 100 torr after the inspired air enters the alveoli.

[ILLUSTRATION OMITTED]

The P[O.sub.2] of the mixed venous blood entering the pulmonary capillaries is approximately 40 torr. Therefore, oxygen diffuses down the pressure gradient from the alveoli where the P[O.sub.2] is 100 torr into the mixed venous blood in the pulmonary capillaries where the P[O.sub.2] is 40 torr.

The pressure gradient for C[O.sub.2] across the alveolar-capillary membrane develops when atmospheric air, which has a partial pressure of C[O.sub.2] of essentially 0 torr, enters the alveoli where C[O.sub.2] is continuously being deposited. Metabolically produced C[O.sub.2] leaving the mixed venous blood in the pulmonary capillaries produces an alveolar carbon dioxide tension ([P.sub.A][O.sub.2]) of 40 torr.