Role of active changes in venous capacity by the carotid baroreflex: analysis with a mathematical model

To elucidate the role of venous capacity active changes in short-term cardiovascular homeostasis, a mathematical model of the carotid-sinus baroreflex system has been developed. In the model the cardiovascular system is represented as the series arrangement of six lumped compartments, which synthesize the fundamental hemodynamic properties of the systemic arterial, systemic venous, pulmonary arterial, and pulmonary venous circulations as well as of the left and right cardiac volumes. Cardiac outputs from the left and right ventricles are computed as a function of both downstream arterial pressure (afterload) and upstream atrial pressure (preload). Four distinct feedback regulatory mechanisms, working on systemic arterial resistance, heart rate, systemic venous unstressed volume, and systemic venous compliance, are assumed to operate on the cardiovascular system in response to carotid sinus pressure changes. All model parameters, both in the cardiovascular system and in feedback regulatory mechanisms, have been assigned on the basis of physiological data now available. The model is used here to simulate the pattern of the main hemodynamic quantities in the short time period (1-2 min) after acute carotid sinus activation in vagotomized subjects. Simulation results indicate that the model can reproduce experimental data quite well, with reference both to open-loop experiments and to an acute blood hemorrhage performed in closed-loop conditions. Moreover, computer simulations indicate that active changes in venous unstressed volume are of primary importance in regulating cardiac output and systemic arterial pressure during activation of the carotid sinus baroreflex.

How Does the Vein Wall Respond to Pressure?

Regulation of systemic venous capacity includes 1) myogenic control, 2) adaptive increases in gain of intrinsic control and the passive capacity to long-term physiological elevations in pressure, 3) chronic increases in wall thickness and reductions in mechanical stresses when exposed to high pressures, and 4) endothelium-derived relaxing factors.

Carotid baroreceptor control of right atrial mechanics in dogs

To investigate the influence of the carotid arterial baroreceptors on right atrial mechanics, the carotid sinus region was isolated surgically in eight dogs prepared acutely under pentobarbital. Right atrial pressure and conductance volume were measured with a strain-gauge tip catheter and a conductance catheter, respectively. Reduction of carotid sinus pressure from 225 to 50 mmHg elicited significant increases in the a wave in right atrial pressure, in atrial stroke volume, in atrial stroke work (2.5-fold), and in atrial stroke power (4-fold). Mean central venous pressure and atrial volume at the onset of each beat did not change. These responses were unchanged after bilateral cervical vagotomy. Head-up tilt was applied at carotid sinus pressures less than or equal to 150 mmHg in four dogs to oppose any contribution of decreased systemic venous capacity to the responses through increased atrial filling. Tilt did not change atrial stroke work or atrial filling during late ventricular systole before vagotomy but inhibited these variables significantly after vagotomy. The slope of the relationship between right atrial stroke work and atrial volume at the onset of contraction increased significantly with reduction of carotid sinus pressure. This response was unaffected by either vagotomy or tilt. Carotid arterial hypotension appears to augment right atrial stroke work and stroke volume through an increase in atrial contractility. A decrease in venous capacity may contribute to this response especially after vagotomy.

Plethysmographic Measurement of Venous Flow Resistance and Venous Capacity in Humans with Primary Lymphoedema

Venous flow resistance ( Rv) and venous capacity ( C10) was measured by strain-gauge plethysmography in patients with primary lympoedema of the legs. In the calf, Rv was significantly higher than normal and increased with the clinical severity of the oedema, while C10 was significantly lower than normal. Potential causes are discussed. No indications were found that the specificity of Rv, when used as a diagnostic criterion for recent deep vein thrombosis, is influenced by primary lymphoedema.

Plethysmographic Measurement of Venous Flow Resistance and Venous Capacity in Humans with Deep Venous Thrombosis

Venous flow resistance ( Rv) and venous capacity ( C10)2 was studied in humans with deep vein thrombosis (DVT) using strain-gauge plethysmography. Rv was found to be significantly higher than normal while C10 was significantly lower, notably in measurements at the calf-level. In a follow-up study it was demonstrated that Rv usually decreased sharply within the first few months after the acute event while C10 increased. Rv usually returned to (high-level) non-thrombotic values, while C10 returned to (low-level) normal values. The sensitivity of Rv when used as a diagnostic criterion for recent proximal DVT was assessed retrospectively. For proximal DVT, the sensitivity of Rv-prox (measured with strain-gauges around the calf) was 95% (95% confidence limits 83–99%) and for distal DVT, the sensitivity of Rv-dist (measured with strain-gauges around the foot) was 36% (95% confidence limits 13–65%). The specificity for Rv was 100% (95% confidence limits Rv-prox: 90–100%; Rv-dist: 59–100%). Influencing factors for sensitivity and specifity are discussed.

Venous Flow Resistance and Venous Capacity in Humans with Primary Varicosis of the Long Saphenous Vein

Venous flow resistance ( Rv) and venous capacity ( C10) was measured by strain-gauge plethysmography in patients with primary truncal varicosis of the long saphenous vein (LSV). Rv tended to be lower than normal, notably in measurements taken at the foot, but significant differences were hardly demonstrable. C10 in the calf and the foot were significantly higher than normal. Physical and morphological abnormalties of the LSV with relevance to Rv and C10 are discussed. It was demonstrated that the contribution of the LSV to venous haemodynamics amounts to about 10%. It is considered unlikely that the sensitivity of Rv, when used as a diagnostic criterion for recent deep vein thrombosis, is influenced by the LSV.

Plethysmographic Measurement of Venous Flow Resistance and Venous Capacity in Human Legs. Part Two: Normal Values Related to Age, Sex and Site of Measurement

This report presents normal values of venous flow resistance ( Rv) and venous capacity (C10) as measured in human limbs by strain-gauge plethysmography. No age dependency was found for either parameter. Rv was lower in males than in females and lower in left legs than in right legs. C10 was higher in males than in females and higher in left than in right legs. The left-right and sex dependency of the parameters cannot be explained by differences in limb volume. Correlations between selected parameters are presented.

Plethysmographic Measurement of Venous Flow Resistance and Venous Capacity in the Human Leg. Part One: Method

This report presents a study of the methodological aspects of the measurement of venous flow resistance ( Rv) and venous capacity (C10) in human legs as measured by strain-gauge plethysmography. The measurements are reproduceable within 10%, in subjects who are well-acclimatized to a room temperature of 28–30 °C. In measurements at the calf, extension of the knee joint may increase Rv by hundreds of percents, while high passive calf muscle tension may be associated with a decrease in Rv and an increase in C10. Inadequate acclimatization may increase Rv and decrease C10, notably in measurements at the foot. The determinants for Rv and C10 are discussed.