Abstract
Abstract
Head‐up tilt (HUT) and lower‐body negative pressure (LBNP) both reduce central blood volume but differ in heart orientation and regional fluid shifts. Whether they evoke comparable cardiovascular regulation when matched for preload remains uncertain. We characterized differences in fluid redistribution and haemodynamic responses during graded HUT and LBNP at intensities eliciting equivalent reductions in thoracic blood volume (TBV). In a randomized crossover design, 30 young adults (16 healthy control subjects and 14 with type 1 diabetes) underwent both HUT (22°, 42°, 58° and 80°) and LBNP (−10, −20, −35 and −50 mmHg). Segmental bioimpedance, haemodynamic and ECG responses were recorded continuously. Equivalent reductions in TBV were obtained for 22° vs. −10 mmHg, 42° vs. −20 mmHg, and 80° vs. −35 mmHg (all
P
> 0.9). However, regional fluid shifts differed, in that HUT induced splanchnic pooling, whereas LBNP promoted pelvic pooling. At matched reductions in TBV, stroke volume decreased more during HUT than during LBNP (
P
= 0.016). Cardiac output was maintained during HUT owing to more pronounced tachycardia, but it declined during LBNP at −35 mmHg. Type 1 diabetics exhibited consistently higher heart rate across all conditions, but preserved stroke volume, cardiac output and mean arterial pressure relative to control subjects. Although HUT and LBNP generate comparable reductions in TBV, they elicit distinct cardiovascular adjustments. HUT engages gravitational influences that help to maintain cardiac output, whereas LBNP isolates baroreflex‐mediated responses, underscoring the unique contribution of gravity to orthostatic regulation. Collectively, these findings refine interpretation of orthostatic models and highlight their utility for detecting early reductions in autonomic reserve in type 1 diabetes.