‘Non‐hypotensive’ hypovolaemia reduces ascending aortic dimensions in humans.

Abstract

1. The notion that small, 'non-hypotensive' reductions of effective blood volume alter neither arterial pressure nor arterial baroreceptor activity is pervasive in the experimental literature. We tested two hypotheses: (a) that minute arterial pressure and cardiac autonomic outflow changes during hypovolaemia induced by lower body suction in humans are masked by alterations in breathing, and (b) that evidence for arterial baroreflex engagement might be obtained from measurements of thoracic aorta dimensions. 2. In two studies, responses to graded lower body suction at 0 (control), 5, 10, 15, 20 and 40 mmHg were examined in twelve and ten healthy young men, respectively. In the first, arterial pressure (photoplethysmograph), R-R interval, and respiratory sinus arrhythmia amplitude (complex demodulation) were measured during uncontrolled and controlled breathing (constant breathing frequency and tidal volume). In the second, cross-sectional areas of the ascending thoracic 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) and a decreased arterial pulse pressure at moderate levels (40 mmHg; ANOVA, P < 0 05). Both R-R intervals and respiratory sinus arrhythmia were negatively related to lower body suction level, whether group averages (general linear regression, r > 0 92) or individual subjects (orthogonal 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 detection by conventional means.

Short-term variability of blood pressure: effects of lower-body negative pressure and long-duration bed rest

“…However, straightforward observations of the time course of mean arterial pressure (MAP) following rapid onset of mild LBNP (17, 18) evidenced a transient decrease followed by a recovery of pre-LBNP onset values, suggesting that arterial baroreceptors are affected by mild LBNP as well. More detailed analyses of such transient decreases of MAP in LBNP (13) showed that also systolic blood pressure (SBP) and diastolic blood pressure (DBP) fall and are restored within ϳ15 heart beats, confirming a direct involvement of arterial baroreflex, consistently with previous reports (23,35,45). However, to our knowledge, the quantification of the relative dynamic contribution of arterial and cardiopulmonary baroreflex has not been addressed yet.…”

Lower capacitance response and capillary fluid absorption in women to defend central blood volume in response to acute hypovolemic circulatory stress

“…During LBNP of 11 mmHg, a level generally ascribed to unload cardiopulmonary receptors (33), although challenged by Taylor et al (46), FVR increased in women but not in men 30 s after the initiation of LBNP (data not shown). Concomitantly, the initial arm capacitance response was greater at LBNP of 11 mmHg in women (Fig.…”

Parallel resetting of arterial baroreflex control of renal and cardiac sympathetic nerve activities during upright tilt in rabbits

“…These observations have been interpreted as reflexes triggered by cardiopulmonary baroreceptors (5,12). However, Taylor et al (17) showed that small reductions of effective blood volume reduce aortic baroreceptive areas and trigger hemodynamic adjustments that are so efficient that alterations in AP escape detection by conventional means. In addition, Fu et al (2) reported that arterial baroreceptors are consistently unloaded during low levels (i.e., Ϫ10 and Ϫ15 mmHg) of lower body negative pressure in humans.…”