Slower lower limb blood pooling in young women with orthostatic intolerance

Abstract

New Findings r What is the central question of this study?Orthostatic stress is mostly caused by venous blood pooling in the lower limbs. Venous distension elicits sympathetic responses, and increased distension speed enhances the cardiovascular response. We examine whether lower limb blood pooling rate during lower body negative pressure is linked to orthostatic intolerance. r What is the main finding and its importance?A similar amount of blood was pooled in the lower limb, but at a slower rate in women who developed signs of orthostatic intolerance. The difference in blood pooling rate increased with orthostatic stress and was most prominent at a presyncope-inducing level of lower body negative pressure. The findings have implications for the pathophysiology as well as treatment of orthostatic intolerance.Vasovagal syncope is common in young women, but its aetiology remains elusive. Orthostatic stress-induced lower limb blood pooling is linked with central hypovolaemia and baroreceptor unloading. Venous distension in the arm elicits a sympathetic response, which is enhanced with more rapid distension. Our aim was to study both the amount and the speed of lower limb pooling during orthostatic stress and its effects on compensatory mechanisms to maintain cardiovascular homeostasis in women with orthostatic intolerance. Twenty-seven healthy women, aged 20-27 years, were subjected to a lower body negative pressure (LBNP) of 11-44 mmHg. Five women developed symptoms of vasovagal syncope (orthostatic intolerant) and were compared with the remaining women, who tolerated LBNP well (orthostatic tolerant). Lower limb blood pooling, blood flow and compensatory mobilization of venous capacitance blood were measured. Lower body negative pressure induced equal lower limb blood pooling in both groups, but at a slower rate in orthostatic intolerant women (e.g. time to 50% of total blood pooling, orthostatic intolerant 44 ± 7 s and orthostatic tolerant 26 ± 2 s; P < 0.001). At presyncope-inducing LBNP, the mobilization of venous capacitance blood was both reduced (P < 0.05) and much slower in orthostatic intolerant women (P = 0.0007). Orthostatic intolerant women elicited blunted arterial vasoconstriction at low-grade LBNP, activating only cardiopulmonary baroreceptors, while orthostatic tolerant women responded with apparent vasoconstriction (P < 0.0001). In conclusion, slower lower limb blood pooling could contribute to orthostatic intolerance in women. Mobilization of venous capacitance blood from the peripheral to the central circulation was both slower and decreased; furthermore, reduced cardiopulmonary baroreceptor sensitivity

Cardiovascular regulation in women with vasovagal syncope : With special reference to the venous system

“…Although the time to develop 50% of the capacitance response (poolingtime) was similar in controls and VVS, the groups presented with dichotomous correlation between poolingtime and LBNP-induced cardiovascular responses as well as orthostatic tolerance. A recent study found that the time for lower limb blood pooling was much shorter in hemodynamic stable healthy women compared to otherwise healthy women experiencing vasovagal reactions during moderate levels of LBNP (77). In corroboration, we found that shorter poolingtime correlated with higher LBNP-tolerance in healthy controls ( figure 16A).…”

“…Furthermore, studies in healthy subjects without a history of VVS have suggested that not just the pooled volume, but also the rate by which the hypovolemic stimulus is instituted can affect the responses to orthostatic stress (27,77). Despite a similar capacitance response, Lindenberger and Länne (77) found a much slower lower limb blood pooling in otherwise healthy women experiencing vasovagal reactions during moderate levels of LBNP compared to hemodynamic stable women.…”

“…This results in an increase in the transmural pressure over the vessel wall, followed by a vessel dilatation and a concomitant blood pooling (100). The LBNP method is a commonly used model to study cardiovascular responses to hemorrhage and orthostatic stress (46,77,99,101). In comparison with other techniques used to model orthostatic stress, such as HUT, the LBNP method has some major advantages.…”

“…LBNP induce central hypovolemia with concomitant unloading of baroreceptor and is a widely used technique to simulate and quantify orthostatic stress and tolerance (46,77,101). There are nonetheless some differences between LBNP and standing (or HUT which is used in clinical settings).…”

“…Although blood pooling in the lower body is essential for the decrease in venous return to come about in the first place, our data, and others (75), indicate that increased blood pooling in lower limbs cannot explain the increased susceptibility to orthostatic intolerance in VVS. Cardiovascular response to orthostatic stress has also been shown to be affected by the speed of the induced stimulus (27,77,78). Although the time to develop 50% of the capacitance response (poolingtime) was similar in controls and VVS, the groups presented with dichotomous correlation between poolingtime and LBNP-induced cardiovascular responses as well as orthostatic tolerance.…”