Cerebral blood velocity regulation during progressive blood loss compared with lower body negative pressure in humans

Abstract

Lower body negative pressure (LBNP) is often used to simulate blood loss in humans. It is unknown if cerebral blood flow responses to actual blood loss are analogous to simulated blood loss during LBNP. Nine healthy men were studied at baseline, during three levels of LBNP (5 min at -15, -30, and -45 mmHg), and during three levels of blood loss (333, 667, and 1,000 ml). LBNP and blood loss conditions were randomized. Intra-arterial mean arterial pressure (MAP) during LBNP was similar to that during blood loss (P ≥ 0.42). Central venous pressure (2.8 ± 0.7 vs. 4.0 ± 0.8, 1.2 ± 0.6 vs. 3.5 ± 0.8, and 0.2 ± 0.9 vs. 2.1 ± 0.9 mmHg for levels 1, 2, and 3, respectively, P ≤ 0.003) and stroke volume (71 ± 4 vs. 80 ± 3, 60 ± 3 vs. 74 ± 3, and 51 ± 2 vs. 68 ± 4 ml for levels 1, 2, and 3, respectively, P ≤ 0.002) were lower during LBNP than blood loss. Despite differences in central venous pressure, middle cerebral artery velocity (MCAv) and cerebrovascular conductance were similar between LBNP and blood loss at each level (MCAv at level 3: 62 ± 6 vs. 66 ± 5 cm/s, P = 0.37; cerebrovascular conductance at level 3: 0.72 ± 0.05 vs. 0.73 ± 0.05 cm·s(-1)·mmHg(-1), P = 0.53). While the slope of the MAP-MCAv relationship was slightly different between LBNP and blood loss (0.41 ± 0.03 and 0.66 ± 0.04 cm·s(-1)·mmHg(-1), respectively, P = 0.05), time domain gain between MAP and MCAv at maximal LBNP/blood loss (P = 0.23) and low-frequency MAP-mean MCAv transfer function coherence, gain, and phase were similar (P ≥ 0.10). Our results suggest that cerebral hemodynamic responses to LBNP to -45 mmHg and blood loss up to 1,000 ml follow a similar trajectory, and the arterial pressure-cerebral blood velocity relationship is not altered from baseline under these conditions.

Keywords: cerebrovascular; hypovolemia; simulated hemorrhage.

Figures

Fig. 1.

Study protocol. Lower body negative pressure (LBNP) and blood loss conditions were counterbalanced. Duration of the rest period between LBNP and blood loss depended on which protocol was performed first, with more time required after the blood loss protocol.

Fig. 2.

Linear regression for amalgamated values for central venous pressure (CVP; A) and stroke volume (SV; B) responses between LBNP and blood loss conditions.

Fig. 3.

Individual plots of mean arterial pressure (MAP) vs. mean middle cerebral artery velocity (MCAv) for all 9 subjects for LBNP (white circles) and blood loss (gray circles). Group responses are presented at bottom right (n = 9).

Fig. 4.

Low-frequency (LF) and very-low-frequency (VLF) power spectral density for MAP (A and C) and mean MCAv (B and D) during LBNP and blood loss. Values are means ± SE.

Fig. 5.

LF coherence, phase, and gain between MAP and mean MCAv during LBNP and blood loss. Values are means ± SE.