Abstract
Haemodynamic responses and antidiuretic hormone (ADH) were measured during body position changes designed to induce blood volume shifts in 10 cardiac transplant recipients to assess the contribution of cardiac and vascular volume receptors in the control of ADH secretion. Each subject underwent 15 min of a control period in the seated posture, then assumed a lying posture for 30 min at 6 degrees head-down tilt (HDT) followed by 30 min of seated recovery. Venous blood samples and cardiac dimensions (echocardiography) were taken at 0 and 15 min before HDT, 5, 15 and 30 min of HDT, and 5, 15 and 30 min of seated recovery. Blood samples were analysed for haematocrit, plasma osmolality, plasma renin activity (PRA) and ADH. Resting plasma volume (PV) was measured by Evans blue dye and per cent changes in PV during posture changes were calculated from changes in haematocrit. Heart rate (HR) and blood pressure (BP) were recorded every 2 min. In the cardiac transplant subjects, mean HR decreased (BP less than 0.05) from 102 b.p.m. pre-HDT to 94 b.p.m. during HDT and returned to 101 b.p.m. in seated recovery while BP was slightly elevated (P less than 0.05). PV was increased by 6.3% (P less than 0.05) by the end of 30 min of HDT but returned to pre-HDT levels following seated recovery. Plasma osmolality was not altered by posture changes. Mean left ventricular end-diastolic volume increased (P less than 0.05) from 90 +/- 5 ml pre-HDT to 105 +/- 4 ml during HDT and returned to 88 +/- 5 ml in seated recovery. Plasma ADH was reduced by 28% (P less than 0.05) by the end of HDT and returned to pre-HDT levels with seated recovery. PRA was also reduced by 28% (P less than 0.05) with HDT. These responses were similar to those of six normal cardiac-innervated control subjects and one heart-lung recipient. Therefore, cardiac volume receptors are not the only mechanism for the control of ADH release during acute blood volume shifts in man.
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