Mathematical modeling of gravitational effects on the circulation: importance of the time course of venous pooling and blood volume changes in the lungs

Abstract

A dip in blood pressure (BP) in response to head-up tilt (HUT) or active standing might be due to rapid pooling in the veins below the heart (preload) or muscle activation-induced drop in systemic vascular resistance (afterload). We hypothesized that, in the cardiovascular response to passive HUT, where, in contrast to active standing, little BP dip is observed, features affecting the preload play a key role. We developed a baroreflex model combined with a lumped-parameter model of the circulation, including viscoelastic stress-relaxation of the systemic veins. Cardiac contraction is modeled using the varying-elastance concept. Gravity affects not only the systemic, but also the pulmonary, circulation. In accordance with the experimental results, model simulations do not show a BP dip on HUT; the tilt-back response is also realistic. If it is assumed that venous capacities are steady-state values, the introduction of stress-relaxation initially reduces venous pooling. The resulting time course of venous pooling is comparable to measured impedance changes. When venous pressure-volume dynamics are neglected, rapid (completed within 30 s) venous pooling leads to a drop in BP. The direct effect of gravity on the pulmonary circulation influences the BP response in the first ϳ5 s after HUT and tilt back. In conclusion, the initial BP response to HUT is mainly determined by the response of the venous system. The time course of lower body pooling is essential in understanding the response to passive HUT. baroreflex; cardiovascular system; modeling; tilt table A MODELING APPROACH to the study of the blood circulation and its response to postural changes can provide insight into the underlying physiology. Existing models approximate certain aspects of the circulation and its response to orthostatic stress, but the transients seen on passive head-up tilt (HUT) have proven difficult to capture.Transient response of the cardiovascular system to active standing and passive HUT has been the focus of various studies (3,34,45,47,51,59). The steady-state response to active standing and HUT is usually comparable, but there is a difference in the blood pressure (BP) and heart rate (HR) responses in the first 30 s (Fig. 1). On passive HUT, Rossberg and Martinez (34) found initial increases in HR comparable to the response to active standing. In later studies, however, a BP dip on active standing was reduced (3, 47) or absent (45,51,59) in passive HUT responses. The initial BP dip on standing is thought to be due to a decrease in peripheral resistance resulting from the active part of the maneuver (45). Sprangers et al. (45) point out two mechanisms: 1) the central (autonomic) command that accompanies active muscle contraction and 2) the displacement of large amounts of venous blood to the right atrium by the massive muscle action induced by active standing, eliciting a cardiopulmonary (CP) reflex effect on the systemic circulation. In passive tilt, if abrupt muscle contraction in response to sudden tilt maneuvers is avoided, perip...

Hemodynamic performance of the Fontan circulation compared with a normal biventricular circulation: a computational model study

“…Accordingly, the body surface area was calculated to be 1.92 m 2 . The total vascular compliance (Ctol) and pulmonary vascular compliance (Cpul) were related to body weight, respectively, by Ctol ϭ 2.1 ϫ weight (90) and Cpul ϭ 0.408 ϫ weight (55). The distribution of vascular compliance along the pulmonary vascular system was set to be 0.08:0.65:0.27 for the artery, capillary, and venous beds (67) and that of resistance was set to be 0.46:0.34:0.20 (12).…”

Age-Related Normative Changes in Phasic Orthostatic Blood Pressure in a Large Population Study

“…32 Decreased muscle strength and cardiorespiratory deconditioning may also lead to changes in muscle vasodilation and muscle pump activity. 33,34 The normative Tables and Figures provide a set of reference values that may have clinical application in establishing diagnostic criteria, determining the severity of impairment, and treatment titration.…”

Mechanical and physiological effect of partial pressure suit: Experiment and numerical study

“…Melek et al [19] established a second order discrete time model to predict blood pressure dynamics during pitch and roll rotation, and a fuzzy logic model to predict the cardiovascular dynamic changes of head-up/head-down and transitional motions. A lumped circuit model of the pressure sensory baroreflex by Van Heusden et al [20] studied the viscoelastic stress-relaxation of the systemic veins. Dyachenko et al [21] developed a model of gravity-dependent effects in the cardiovascular system using numerical integration procedures of a few lumped parameters.…”