NONHYPOTENSIVE HYPOVOLEMIA REDUCES ASCENDING AORTIC DIMENSIONS IN HUMANS

1. The notion that small, 'non-hypotensive' reductions of effective bl ood volume alter neither arterial pressure nor arterial baroreceptor a ctivity is pervasive in the experimental literature. Mie tested two hy potheses: (a) that minute arterial pressure and cardiac autonomic outf low changes during hypovolaemia induced by lower body suction in human s are masked by alterations in breathing, and (b) that evidence for ar terial baroreflex engagement might be obtained from measurements of th oracic aorta dimensions. 2. In two studies, responses to graded lower body suction at 0 (control), 5, 10, 15, 20 and 40 mmHg were examined i n twelve and ten healthy young men, respectively. In the first, arteri al pressure (photoplethysmograph), R-R interval, and respiratory sinus arrhythmia amplitude (complex demodulation) were measured during unco ntrolled and controlled breathing (constant breathing frequency and ti dal volume). In the second, cross-sectional areas of the ascending tho racic aorta were calculated from nuclear magnetic resonance images. 3. Lower body suction with controlled breathing resulted in an increased arterial pulse pressure at mild levels (5-20 mmHg; ANOVA, P < 0.05) a nd a decreased arterial pulse pressure at moderate levels (40 mmHg; AN OVA, P < 0.05). Both R-R intervals and respiratory sinus arrhythmia we re negatively related to lower body suction level, whether group avera ges (general linear regression, r > 0.92) or individual subjects (orth ogonal polynomials, 12 of 12 subjects) were assessed. 4. Aortic pulse area decreased progressively and significantly during mild lower body suction, with 47 % of the total decline occurring by 5 mmHg. 5. These results suggest that small reductions of effective blood volume reduce aortic baroreceptive areas and trigger haemodynamic adjustments which are so efficient that alterations in arterial pressure escape detecti on by conventional means.