Association of gender with cardiovascular and autonomic responses to central hypovolemia

Abstract

Introduction: Lower body negative pressure (LBNP) eliminates the impact of weight-bearing muscles on venous return, as well as the vestibular component of cardiovascular and autonomic responses. We evaluated the hemodynamic and autonomic responses to central hypovolemia, induced by LBNP in both males and females.

Methodology: A total of 44 participants recruited in the study. However, 9 participants did not complete the study protocol. Data from the remaining 35 participants were analysed, 18 males (25.28 ± 3.61 years, 181.50 ± 7.43 cm height, 74.22 ± 9.16 kg weight) and 17 females (22.41 ± 2.73 years, 167.41 ± 6.29 cm height, 59.06 ± 6.91 kg weight). During the experimental protocol, participants underwent three phases, which included 30 min of supine rest, four 4 min intervals of stepwise increases in LBNP from -10 mmHg to -40 mmHg, and 5 min of supine recovery. Throughout the protocol, hemodynamic variables such as blood pressure, heart rate, stroke index, cardiac index, and total peripheral resistance index were continuously monitored. Autonomic variables were calculated from heart rate variability measures, using low and high-frequency spectra, as indicators of sympathetic and parasympathetic activity, respectively.

Results: At rest, males exhibited higher systolic (118.56 ± 9.59 mmHg and 110.03 ± 10.88 mmHg, p < 0.05) and mean arterial (89.70 ± 6.86 and 82.65 ± 9.78, p < 0.05) blood pressure as compared to females. Different levels of LBNP altered hemodynamic variables in both males and females: heart rate [F(1,16) = 677.46, p < 0.001], [F(1,16) = 550.87, p < 0.001]; systolic blood pressures [F(1,14) = 3,186.77, p < 0.001], [F(1,17) = 1,345.61, p < 0.001]; diastolic blood pressure [F(1,16) = 1,669.458, p < 0.001], [F(1,16) = 1,127.656, p < 0.001]; mean arterial pressures [F(1,16) = 2,330.44, p < 0.001], [F(1,16) = 1,815.68, p < 0.001], respectively. The increment in heart rates during LBNP was significantly different between both males and females (p = 0.025). The low and high-frequency powers were significantly different for males and females (p = 0.002 and p = 0.001, respectively), with the females having a higher increase in low-frequency spectral power.

Conclusions and future directions: Cardiovascular activity and autonomic function at rest are influenced by gender. During LBNP application, hemodynamic and autonomic responses differed between genders. These gender-based differences in responses during central hypovolemia could potentially be attributed to the lower sympathetic activity in females. With an increasing number of female crew members in space missions, it is important to understand the role sex-steroid hormones play in the regulation of cardiovascular and autonomic activity, at rest and during LBNP.

Keywords: autonomic; gender; heart rate variability; hemodynamics; lower body negative pressure; sex; sympathetic; vascular.

Figure 1

Study protocol and time points for data analysis. The study protocol shows that each LBNP level lasted 4-minute. The time points used for data analysis are also shown: each selected time point (1–10) had a duration of 10 s: the last 10 s of supine rest (baseline), at each LBNP level, the first 10 s (start) and the last 10 s, and the last 10 s of post LBNP application (recovery).

Figure 2

Hemodynamic responses to LBNP in (male (n = 18) vs. female (n = 17)). Data shown are relative changes (percentage) to the baseline: (A) Heart rate, (B) systolic pressure; (C) diastolic pressure and (D) mean arterial pressure. “Ø” significant differences between baseline and start of −30 mmHg; “¥” significant differences between baseline and end of −30 mmHg; “‡” significant differences between baseline and start of −40 mmHg; “#” significant differences between baseline and end of −40 mmHg (p < 0.05); “*” and “**” significant differences between males and females responses to LBNP (p < 0.05) and (p < 0.001), respectively.

Figure 3

Cardiovascular compensatory responses to LBNP [male (n = 18) vs. female (n = 17)] in (A) stroke index, (B) cardiac index and (C) total peripheral resistance index (TPRI). Data are displayed as mean ± SD. “†” significant differences between baseline and end of −20 mmHg; “Ø” significant differences between baseline and start of −30 mmHg; “¥” significant differences between baseline and end of −30 mmHg; “‡” significant differences between baseline and start of −40 mmHg; “#” significant differences between baseline and end of −40 mmHg (p < 0.05). “**” significant differences between males and females responses to LBNP (p < 0.001) respectively.

Figure 4

Relative changes in autonomic responses to LBNP [male (n = 18) vs. female (n = 17)] as compared to baseline. (A) low-frequency band power; (B) high-frequency band power.“¥” significant differences between baseline and end of −30 mmHg; “‡” significant differences between baseline and start of −40 mmHg; “#” significant differences between baseline and end of −40 mmHg (p < 0.05); “**” significant differences between males and females responses to LBNP (p < 0.001).