Haemodynamic responses to nonhypotensive central hypovolaemia induced by lower body negative pressure in men and women

Abstract

Haemodynamic responses to low levels of lower body negative pressure (LBNP) were investigated in two groups of healthy, normotensive volunteers (8 men and 8 women) during two repeated experimental runs on two occasions, the latter determined by the different phases of the menstrual cycle in the women. The data consisted of systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean blood pressure (MBP), pulse rate (fc), forearm blood flow (FBF) and forearm vascular conductance (FC). The resting cardiovascular status was similar in men and women, except that women had a significantly higher fc than men. LBNP (1.3, 2.7 and 4 kPa) had no significant effect on any BP variable or on fc. However, FBF and FC were reduced at all levels of LBNP. Significant overshoots in FBF and FC were seen in all subjects following the release of LBNP of 2.7 and 4 kPa and, in most cases, after release of LBNP of 1.3 kPa. There were no significant gender differences in any of the responses to LBNP. Furthermore, none of the cardiovascular variables measured showed significant differences between the follicular and luteal phases of the menstrual cycle in women, either at rest or during exposure to LBNP, and the responses in the men on the two occasions were not different. These findings indicate that gender differences in responses to LBNP hypothesized previously are not apparent during and after exposure to low levels of LBNP.

Phase of the menstrual cycle does not affect orthostatic tolerance in healthy women

“…However, it is known that syncope is more common in warm environments when individuals are vasodilated, and it has been shown recently that orthostatic tolerance during whole body heating does vary with the phase of the menstrual cycle [28]. The present study confirms previous reports whereby the vascular responses, at least in response to orthostasis, are unaffected by phase of the menstrual cycle [38,30].…”

“…The absence of any influence of the phase of the cycle on the reflex vascular responses, although unexpected in terms of the hormonal changes, is broadly in agreement with the findings of others, who observed no effect of phase of the menstrual cycle on the responses to lower body suction [21,28,38], to quiet standing [24] or to other physiological stimuli [2,5,34]. However, it is suggested that there are alterations in orthostatic tolerance associated with the menstrual cycle when additional vasodilator stress is applied, such as whole body heating [28].…”

Cardiovascular and autonomic responses to lower body negative pressure

“…We did not assess blood volume, but others have noted women to have lower absolute plasma [13] or blood [3] volumes. Physically smaller people typically have smaller blood volumes than larger people and, as in all similar studies [3,[7][8][9]15], the women in the present study were smaller than the men.…”

“…However, the gender differences in these orthostatic stressors are quite variable. Depending on the magnitude of a given level of absolute submaximal LBNP, women have demonstrated smaller [5,6], similar [4,10] or larger [6] total peripheral resistance; similar [4,8,13] or greater forearm vasoconstriction [3,10,11], and similar [4,13,15] or potentiated [3,5,6,10,12,15] chronotropic responses. The mechanisms underlying these differences remain uncertain.…”

“…This may be based on the observations that disproportionately more women are diagnosed with clinically significant orthostatic intolerance [1] and there are gender differences in the cardiovascular responses to head-up tilt, standing or submaximal lower body negative pressure (LBNP [2][3][4][5][6][7][8][9][10][11][12][13][14][15]). However, the gender differences in these orthostatic stressors are quite variable.…”