The effects of superimposed tilt and lower body negative pressure on anterior and posterior cerebral circulations

Abstract

Steady-state tilt has no effect on cerebrovascular reactivity to increases in the partial pressure of end-tidal carbon dioxide (PETCO2). However, the anterior and posterior cerebral circulations may respond differently to a variety of stimuli that alter central blood volume, including lower body negative pressure (LBNP). Little is known about the superimposed effects of head-up tilt (HUT; decreased central blood volume and intracranial pressure) and head-down tilt (HDT; increased central blood volume and intracranial pressure), and LBNP on cerebral blood flow (CBF) responses. We hypothesized that (a) cerebral blood velocity (CBV; an index of CBF) responses during LBNP would not change with HUT and HDT, and (b) CBV in the anterior cerebral circulation would decrease to a greater extent compared to posterior CBV during LBNP when controlling PETCO2 In 13 male participants, we measured CBV in the anterior (middle cerebral artery, MCAv) and posterior (posterior cerebral artery, PCAv) cerebral circulations using transcranial Doppler ultrasound during LBNP stress (-50 mmHg) in three body positions (45°HUT, supine, 45°HDT). PETCO2 was measured continuously and maintained at constant levels during LBNP through coached breathing. Our main findings were that (a) steady-state tilt had no effect on CBV responses during LBNP in both the MCA (P = 0.077) and PCA (P = 0.583), and (b) despite controlling for PETCO2, both the MCAv and PCAv decreased by the same magnitude during LBNP in HUT (P = 0.348), supine (P = 0.694), and HDT (P = 0.407). Here, we demonstrate that there are no differences in anterior and posterior circulations in response to LBNP in different body positions.

Keywords: Central hypovolemia; cerebral blood velocity; head‐down tilt; head‐up tilt; lower body negative pressure.

Figure 1

Schematic of a participant positioned in 45°HDT in the custom‐built integrated tilt and LBNP apparatus. The participant was instrumented with a mouthpiece and nose clip (to sample P_ETCO ₂), Finometer (for monitoring beat‐by‐beat blood pressure), ECG leads (for monitoring HR), and TCD (for monitoring MCAv and PCAv). The LBNP chamber was secured to a custom‐built frame on an axle, and was connected to an industrial shop‐vacuum to generate negative pressure, and a digital manometer was used to monitor pressure within the chamber. The participant was secured inside the chamber with ankle restraints, and they were sealed within the chamber by means of a kayak spray skirt. ECG, electrocardiogram; HR, heart rate; LBNP, lower body negative pressure; PCA, posterior cerebral artery.

Figure 2

Absolute and relative MCAv and PCAv responses to LBNP in each body position. (▲) represents MCAv data, (∆) represents PCAv data, (•) represents P_ETCO ₂. Absolute MCAv and PCAv during baseline (BL) and at 20%, 40%, 60%, 80%, and 100% of the LBNP protocol in 45°HUT (Panel A), supine (Panel B), and 45°HDT (Panel C) positions. Relative MCAv and PCAv during BL and at 20%, 40%, 60%, 80%, and 100% of the LBNP protocol in 45°HUT (Panel D), supine (Panel E), and 45°HDT (Panel F) positions. P_ETCO ₂ during BL and at 20%, 40%, 60%, 80%, and 100% of the LBNP protocol in 45°HUT (Panel G), supine (Panel H), and 45°HDT (Panel I) positions. Mean data ± SEM is represented at each data point. No statistical comparisons between BL and percent of LBNP protocol are shown in the figure, these comparisons can be found in Table 2. Brackets between MCAv and PCAv data represents main effect (P < 0.05) between the two vessels. *P < 0.05, between baseline and percent of the LBNP protocol. **P < 0.05, between 100% of LBNP protocol and 20%, 40%, 60% of LBNP protocol. ***P < 0.05, between 100% of LBNP protocol and 40%, and 60% of LBNP protocol. NSD, no significant differences detected; HUT, head‐up tilt; LBNP, lower body negative pressure; PCA, posterior cerebral artery.

Figure 3

Absolute MCAv and PCAv responses to LBNP in each body position when P_ETCO ₂ was appropriately controlled. (▲) represents MCAv data, (∆) represents PCAv data, (•) represents P_ETCO ₂. Absolute MCAv and PCAv during baseline (BL) and at 20%, 40%, 60%, 80%, and 100% of LBNP protocol in 45°HUT (n = 8; Panel A), supine (n = 7; Panel B), and 45°HDT (n = 13; Panel C). Relative MCAv and PCAv during BL and at 20%, 40%, 60%, 80%, and 100% of LBNP protocol in 45°HUT (n = 8; Panel D), supine (n = 7; Panel E), and 45°HDT (n = 13; Panel F). P_ETCO ₂ during BL and at 20%, 40%, 60%, 80%, and 100% of LBNP protocol in 45°HUT (n = 8; Panel G), supine (n = 7; Panel H), and 45° HDT (n = 13; Panel I). Mean data ± SEM is represented at each data point. Brackets between MCAv and PCAv data represent main effect (P < 0.05) between the two vessels. *P < 0.05, between baseline and percent of LBNP protocol. **P < 0.05, between 100% of LBNP protocol and 20%, 40%, 60% of LBNP protocol. ***P < 0.05, between 100% of LBNP protocol and 40%, and 60% of LBNP protocol. ****P < 0.05, between 100% of LBNP protocol and 60% of LBNP protocol. NSD, no significant differences detected; LBNP, lower body negative pressure; PCA, posterior cerebral artery.