Splanchnic Circulation

A cardiovascular mathematical model of graded head-up tilt

A lumped parameter model of the cardiovascular system has been developed and optimized using experimental data obtained from 13 healthy subjects during graded head-up tilt (HUT) from the supine position to . The model includes descriptions of the left and right heart, direct ventricular interaction through the septum and pericardium, the systemic and pulmonary circulations, nonlinear pressure volume relationship of the lower body compartment, arterial and cardiopulmonary baroreceptors, as well as autoregulatory mechanisms. A number of important features, including the separate effects of arterial and cardiopulmonary baroreflexes, and autoregulation in the lower body, as well as diastolic ventricular interaction through the pericardium have been included and tested for their significance. Furthermore, the individual effect of parameter associated with heart failure, including LV and RV contractility, baseline systemic vascular resistance, pulmonary vascular resistance, total blood volume, LV diastolic stiffness and reflex gain on HUT response have also been investigated. Our fitted model compares favorably with our experimental measurements and published literature at a range of tilt angles, in terms of both global and regional hemodynamic variables. Compared to the normal condition, a simulated congestive heart failure condition produced a blunted response to HUT with regards to the percentage changes in cardiac output, stroke volume, end diastolic volume and effector response (i.e., heart contractility, venous unstressed volume, systemic vascular resistance and heart rate) with progressive tilting.

Theoretical considerations in the dynamic closed-loop baroreflex and autoregulatory control of total peripheral resistance

The most important goal of this study is to enhance our understanding of the crucial functional relationships that determine the behavior of the systemic circulation and its underlying physiological regulatory mechanisms with minimal modeling. To the present day, much has been said about the indirect hydraulic effects of right atrial pressure (PRA) via cardiac output (CO) on arterial pressure (Pa) through the heart and pulmonary circulation or the direct regulatory effects of PRAon Pathrough the cardiopulmonary baroreflex; however, very little attention has been given to the hydraulic influence that PRAexerts directly through the systemic circulation. The experimental data reported by Guyton et al. in 1957 demonstrated that steady-state PRAand the rate at which blood passes through the systemic circulation are locked in a functional relationship independent of any consequence of altered PRAon cardiac function. With this in mind, we emphasize the analytic algebraic analysis of the systemic circulation composed of arteries, veins, and its underlying physiological regulatory mechanisms of baroreflex and autoregulatory modulation of total peripheral resistance (TPR), where the behavior of the system can be analytically synthesized from an understanding of its minimal elements. As a result of this analysis, we present a novel mathematical method to determine short-term TPR fluctuations, which accounts for the entirety of observed Pafluctuations, and propose a new cardiovascular system identification method to delineate the actual actions of the physiological mechanisms responsible for the dynamic couplings between CO, Pa, PRA, and TPR in an individual subject.

Test of dynamic closed-loop baroreflex and autoregulatory control of total peripheral resistance in intact and conscious sheep

This is the first study able to examine and delineate the actual actions of the physiological mechanisms responsible for the dynamic couplings between cardiac output (CO), arterial pressure (Pa), right atrial pressure (PRA), and total peripheral resistance (TPR) in an individual subject without altering the underlying regulatory mechanisms. Eight conscious male sheep were used, where both types of baroreceptors were independently exposed to simultaneous beat-to-beat pressure perturbations under intact closed-loop conditions while CO, Pa, PRA, and TPR were measured. We applied the cardiovascular system identification method proposed in a companion paper ( 4 ) to quantitatively characterize the dynamic closed-loop transfer relations CO→Pa, PRA→Pa, Pa→TPR, and PRA→TPR from the measured signals. To validate the dynamic properties of the estimated transfer relations, the essential parts of the linear dynamics of the model were independently and comprehensively evaluated via error model cross-validation, and the overall model's steady-state behavior was compared with a separate random effects regression approach. In addition to numerous physiological findings, we found that the cardiovascular system identification results were exceptionally consistent with the analytically derived solutions previously discussed in Ref. 4 . In conclusion, this study presents the first time validation of a cardiovascular system identification method by means of experimentally acquired animal data in the intact and conscious animal and offers a set of powerful quantitative tools essential to advancing our knowledge of cardiovascular regulatory physiology.

Assessment of the human splanchnic venous volume-pressure relation using radionuclide plethysmography. Effect of nitroglycerin

BACKGROUND

No method exists to assess human splanchnic venous function, the most important region in terms of vascular capacity.

METHODS AND RESULTS

We studied 25 stable patients without heart failure or hypertension to develop a method to assess the splanchnic venous volume-pressure (V-P) relation and to determine the effect of nitroglycerin (GTN). We used blood pool scintigraphy to assess changes in regional splanchnic vascular volume (SVV) and low levels of continuous positive airway pressure (CPAP) to passively alter venous pressure and thus, SVV. We postulated that the relation between SVV and the CPAP used would reflect the capacitance of the splanchnic venous bed and that changes in the position of this relation would provide a relative measurement of any change in capacitance. In 12 patients (group 1), the splanchnic vascular V-P curves were recorded before and 2, 9, and 20 minutes after 0.6 mg sublingual GTN; in eight patients (group 2), recordings were made at similar times before and after sublingual administration of placebo; in five patients (group 3), the hemodynamic effects of CPAP were assessed by means of right and left cardiac catheterization. Right atrial and femoral venous pressures increased (p < 0.001) and cardiac output fell (p < 0.05) during CPAP. There was an apparently linear relation between CPAP and SVV (r = 0.74-0.98); SVV increased an average of 7.4 +/- 2.2% (p < 0.001) by 12 cm H2O CPAP: The splanchnic vascular V-P curves were reproducible with minimal variability in SVV (+/- 2%, p > 0.2) in group 2. After administration of GTN, the splanchnic vascular V-P curve shifted away from the pressure axis in a parallel fashion by an average of 9.4 +/- 5.4% (p < 0.001).

CONCLUSIONS

We have developed a reproducible noninvasive technique that may be used to assess human splanchnic venous V-P relations. We have demonstrated for the first time in humans that GTN exerts its dilatory effect by increasing the unstressed splanchnic venous volume.