Effects of hyperventilation and hypocapnic/normocapnic hypoxemia on renal function and lithium clearance in humans

Background: Using the renal clearance of lithium as an index of proximal tu bular outflow, this study tested the hypothesis that acute hypocapnic hypox emia decreases proximal tubular reabsorption to the same extent as hypocapn ic normoxemia (hyperventilation) and that this response is blunted during n ormocapnic hypoxemia. Methods: Eight persons were studied on five occasions: (1) during inhalatio n of 10% oxygen (hypocapnic hypoxemia), (2) during hpperventilation of room air leading to carbon dioxide values similar to those with hypocapnic hypo xemia, (3) during inhalation of 10% oxygen with the addition of carbon diox ide to produce normocapnia, (4) during normal breathing of room air through the same tight-fitting face mask as used on the other study days, and (5) during breathing of room air without the face mask. Results: Hypocapnic and normocapnic hypoxemia and hyperventilation increase d cardiac output, respiratory minute volume, and effective renal plasma flo w. Glomerular filtration rate remained unchanged on all study days. Calcula ted proximal tubular reabsorption decreased during hypocapnic hypoxemia and hyperventilation but remained unchanged with normocapnic hypoxemia. Sodium clearance increased slightly during hypocapnic and normocapnic hypoxemia, hyperventilation, and normocapnic normoxemia with but not without the face mask. Conclusions: The results indicate that (1) respiratory alkalosis with or wi thout hypoxemia decreases proximal tubular reabsorption and that this effec t, but not renal vasodilation or natriuresis, can be abolished by adding ca rbon dioxide to the hypoxic gas; (2) the increases in the effective renal p lasma flow were caused by increased ventilation rather than by changes in a rterial oxygen and carbon dioxide levels; and (3) the natriuresis may be se condary to increased renal perfusion, but application of a face mask also m ay increase sodium excretion.