Hemodynamic effects of intermittent compression as a countermeasure to orthostatic stress

Abstract

Upright posture can be associated with symptoms of dizziness, light-headedness and blurred vision, which are fundamentally caused by impaired cerebral hemodynamics. As gravity acts along the body’s longitudinal axis, pooling of blood in the dependent vasculature of the lower body results in a decrease in venous return and compromises the heart's ability to perfuse the brain. The improvement of venous return has been a therapeutic focus to increase tolerance of the upright position. The use of static external compression to limit venous pooling while standing has yielded mixed results. Intermittent external compression of the lower legs provides an alternative strategy to redirect blood from the extremities back to the heart. The main objective of this thesis was to assess how the application of intermittent compression of the lower legs affected the local, central and cerebral hemodynamic responses to orthostatic stress. To accomplish this, a preliminary method comparison study was conducted to address potential shortcomings in the commonly used Modelflow method to accurately estimate dynamic fluctuations in stroke volume during orthostatic stress. The results of this method comparison study indicated that the Modelflow method could not accurately estimate stroke volume in the conditions elicited by orthostatic stress. Intermittent compression of the lower legs attenuated the orthostatic-induced reductions in blood pressure, cerebral blood flow velocity and indices of cerebral oxygenation, while enhancing blood flow to the legs. Thus, the use of intermittent compression of the lower legs as a countermeasure to orthostatic stress may improve the tolerance of certain conditions in which brain blood flow is compromised.