Cardiovascular and renal responses to head-out water immersion in canine model.

Water immersion (WI) of anesthetized dogs to the midcervical level in the vertical sitting position under thermoneutral conditions (38 degrees C) leads to generalized increases in regional blood flows (radiolabeled microspheres) to most tissues. The increased flows can be accounted for by increased local VO2 in some tissues (heart, respiratory muscle), or by local heating (skin, fat). However, an alteration in the relation between cardiac output (Q) and VO2 appears to play a role in the large flow increments observed in skeletal muscle and abdominal viscera in WT. Studies of conscious, chronically instrumented dogs during 100 min of thermoneutral WI at 37 degrees C in the quadruped position indicate that both left atrial and aortic transmural distending pressures and left ventricular contractile performance (LV dP/dtmax) increase, while total peripheral resistance is unchanged. Cardiac output increases largely in association with an increase of heart rate. In addition, urine flow, sodium excretion, and osmolal clearance all increased. The cardiovascular adjustments occurred more rapidly than the renal responses. These results indicate that the conscious dog may be useful for investigation of physiological mechanisms involved in the WI response.