Clearance of filtered fluid from the lung during exercise - Role of hyperpnea

During strenuous exercise in sheep, lung lymph flow increases within second s and rises to levels 7- to 10-fold over baseline. Concomitant with the flo w increase, the lymph protein content rapidly decreases to levels consisten t with severe capillary hypertension. This pattern of clearance of filtered fluid is quite different than is seen with the passive capillary hypertens ion that results from mechanical obstruction of the mitral valve. In passiv e capillary hypertension, the increase in lymph flow and reduction in lymph protein content develop over several hours. The purpose of this study was to discover if these observed differences in edema clearance are related to the hyperpnea that accompanies exercise. Sheep were instrumented for conti nuous measurement of pulmonary arterial, left atrial, and systemic pressure s, cardiac output by ultrasound, lung lymph flow, and ventilation. First, h emodynamics, ventilatory, and lymph clearance variables were measured durin g moderate exercise at 2.8 mph on a treadmill. Second, on a separate occasi on, sheep were induced to hyperventilate to the same minute ventilation as during exercise, using modest CO2 stimulation. Lymph flow and hemodynamics were unaffected by this hyperpnea. The third arm of the experiment was to r aise pulmonary microvascular pressure at rest to the level seen with exerci se by means of a balloon catheter placed in the mitral valve. Lymph flow ro se and protein content decreased slowly and to a lower degree than seen wit h exercise despite a comparable microvascular pressure. Finally, left atria l hypertension and induced hyperpnea were combined in sheep at rest, and th e resulting lymph flow and protein content were the same as seen with exerc ise at similar pressures and ventilation. We conclude that hyperpnea is a m ajor mechanism of interstitial liquid clearance during exercise, and may be largely responsible for preventing pulmonary edema that might occur at the high microvascular pressures of strenuous exercise.