Regional circulatory responses to head-out water immersion in anesthetized dog

Thermoneutral head-out water immersion (WI) of humans leads to a sustained elevation of cardiac output (Q̇(CO)), while systemic V̇(O₂) is unchanged. However, the regional blood flow responses (Q̇r) to WI are largely unknown. In eight dogs anesthetized with pentobarbital sodium, relative to the control vertical position in air with the chest and abdomen upright (Erect₁), after 30 min of WI at 38°C the Q̇(CO) (thermodilution), stroke volume, and aortic pressure were elevated (P<0.01) by 88.5 ± 6.4% (SE), 55 ± 9%, and 29 ± 4% from levels of 2.24 ± 0.01 l.min^-1, 18.4 ml ± 1.7, and 109 ± 7.9 mmHg, respectively. Q̇r (ml·min^-1·100 g^-1), measured by 15 μm-radiolabeled microspheres at WI₃₀(min) relative to the Erect₁ levels, increased (P<0.01) in the stomach (104 ± 27%), jejunum (78 ± 19%), ileum (102 ± 23%), liver (hepatic artery) (485 ± 46%), pancreas (200 ± 44%), spleen (88.7 ± 33%), renal cortex (88 ± 30%), left ventricle (157 ± 15%), skin (726 ± 30%), fat (680 ± 43%), gastrocnemius (200 ± 36%), triceps (414 ± 61%), intercostals (317 ± 44%), and diaphragm (477 ± 24%). Q̇r responses at WI₁₀(min) were largely similar. Rectal temperature and V̇(O₂) were unchanged during WI. In the lateral recumbent position in air, Q̇(CO) rose by 44.3% (P<0.01), but only hepatic arterial Q̇ rose significantly (89 ± 29%). Therefore, there may be a partial uncoupling of the normal Q̇/V̇(O₂) relationship in WI.