Haemodynamic structure of the venous return changes in action of pressor neurogenic and humoral stimuli upon the circulation system

In cats, constancy of blood flow changes was found in the anterior vena cava under the effect of pressor neurogenic and humoral stimuli. The latter proved to be more efficient in their effect. In the pressor systemic responses, changes of the blood flow in the anterior vena cava yielded the greates contribution.     

Changes in the major circulation vessel capacity and their role in forming the venous return shifts under the effect of catecholamines

The study relates to characteristics of the major circulation vessel capacity and their part in forming the venous return shifts under the effect of catecholamines. In anesthetized cats, using the developed technique of controlled experiment enabling to stabilize the blood flow in the circulation arterial segment, fulfilling of pressor responses to i. v. administration of noradrenaline, adrenaline was found to increase the venous return, on the average, by 40% by means of changes in the major circulation vessel capacity (its venous segment, mainly). About 5% of the blood volume seems to become mobilized in the animal organism.     

Numerical simulation of blood flow and pressure drop in the pulmonary arterial and venous circulation

A novel multiscale mathematical and computational model of the pulmonary circulation is presented and used to analyse both arterial and venous pressure and flow. This work is a major advance over previous studies by Olufsen et al. (Ann Biomed Eng 28:1281–1299, 2012) which only considered the arterial circulation. For the first three generations of vessels within the pulmonary circulation, geometry is specified from patient-specific measurements obtained using magnetic resonance imaging (MRI). Blood flow and pressure in the larger arteries and veins are predicted using a nonlinear, cross-sectional-area-averaged system of equations for a Newtonian fluid in an elastic tube. Inflow into the main pulmonary artery is obtained from MRI measurements, while pressure entering the left atrium from the main pulmonary vein is kept constant at the normal mean value of 2 mmHg. Each terminal vessel in the network of ‘large’ arteries is connected to its corresponding terminal vein via a network of vessels representing the vascular bed of smaller arteries and veins. We develop and implement an algorithm to calculate the admittance of each vascular bed, using bifurcating structured trees and recursion. The structured-tree models take into account the geometry and material properties of the ‘smaller’ arteries and veins of radii \(\ge \) 50  \(\upmu \) m. We study the effects on flow and pressure associated with three classes of pulmonary hypertension expressed via stiffening of larger and smaller vessels, and vascular rarefaction. The results of simulating these pathological conditions are in agreement with clinical observations, showing that the model has potential for assisting with diagnosis and treatment for circulatory diseases within the lung.     

Independent influence of blood flow rate and mixed venous PO2 on shunt fraction

We have tested the independent and combined effects of changes in mixed venous PO2 (PvO2) and blood flow (QT) on shunt fraction (Qs/QT) in isolated blood-perfused canine left lower lobes with edema. The lobes were ventilated with pure O2. Inflow (Pi) and outflow (Po) pressures always exceeded lobar alveolar pressure. PvO2 was varied by means of a clinical bubble oxygenator with appropriate mixtures of O2 and N2. QT was varied by changes in Pi and Po with care not to produce changes in lobar weight. Changes in QT did not influence Qs/QT. Increasing PvO2 from 40 +/- 6 to 88.4 +/- 40 Torr at constant QT significantly increased Qs/QT from 5.5 +/- 2.0 to 15.6 +/- 7.0%. Combined increases in QT and PvO2 from 66.4 +/- 2.7 to 135.6 +/- 21.5 ml/min and from 38.8 +/- 1.3 to 61.8 +/- 2.2 Torr, respectively, also produced a significant increase in Qs/QT from 7.33 +/- 2.27 to 15.43 +/- 4.45%. However, this combined change was explained exclusively by changes in PvO2. We therefore concluded that, under the conditions of our experiment, changes in PvO2 influence Qs/QT, and this may account for apparent dependence of Qs/QT on cardiac output in pulmonary edema.     

Hemodynamic mechanisms of blood flow regulation in the vena cava anterior

A similarity of the blood flow shifts in response to every single pressor or depressor stimulus in the v. cava anterior in cats and a possibility of dissimilar changes of capacity of the vein's vascular basin under the effect of one and the same stimulus, are revealed. This dissimilarity is mainly due to a skin-muscular component of this vascular basin. An increase in the central venous pressure from 0 to 40 mm of water limits the extent of shifts in the capacity function of the v. cava anterior's basin. The blood flow changes in the v. cava anterior is mainly due to a relationship among regional components of the vessels' total peripheral resistance. A potential possible contribution of the v. cava anterior basin vessels capacitance's shifts constitutes 17.5%, whereas under the effect of neurogenic stimuli in the increase of the blood flow in the v. cava anterior may be completely (up to 100%) maintained by a drop of the capacitance of the vein's vascular basin.     

Instaneous changes in the radial artery blood flow under different physiological conditions

Using a Doppler pulse flowmeter we measured the blood flow in the radial artery at rest and during physical exercise and various other stimuli (arithmetical calculations, electrical stimulation, deep inspiration). The mean resting flow in the radial artery was 0.66 ml/s. Every stimulus was instantaneously followed by a drop in the blood flow to a minimum value; there was no significant differences between these values. The results demonstrate that the new, non-invasive apparatus can be used to study quick changes in the blood flow not detected by routine non-invasive methods.     

Participation of ATP-sensitive potassium channel in autoregulation of coronary blood flow under the condition of limited motor activity

The aim of study involved detection of the effect of the K(ATP)-channel blocker glibenclamide on autoregulation of coronary flow, the expression of reactive hyperemia, the value of coronary dilatation reserve, and the myocardial contractile function in isolated rat hearts after a 6-hour immobilization stress. The experiments have been performed on 64 isolated rat hearts (female): into the cavity of left ventricle, a latex balloon connected with electromanometer has been introduced. Every experiment consisted of 2 stages. The heart has been perfused by Krebs-Henseleite solution in the first stage, and in the second stage--by the same solution with glibenclamide (1 mkM) or its combination with verapamile (10(-)6 M) or saponin (44 mcg/ml of coronary flow within 2 minutes) added to it. During the experiment, the perfusion pressure has been elevated step by step from 40 to 120 mm Hg with 20 mm Hg steps (coronary autoregulation). In rats after immobilization, the glibenclamide effect on cardiac vessel tone and expression of maximal hyperemic coronary flow (in contrast to its influence on myocardial contractile function) is lower than in control and depends on endotheliocyte presence which suggests an important role of endothelium in maintaining cardiac vessel smooth cell activity of K(ATP)-channels inhibited under the stress condition. After immobilization stress, the role of endothelium in the reactive hyperemia origin was enhanced, that of the K(ATP)-channels was reduced. The general activity of both mechanisms of tone regulation of cardiac vessels remains the same as in control. This suggests that the K(ATP)-channels as nitric monoxide and eicozanoids are the local system of myogenic tone regulation of the rat cardiac vessels; that the rat immobilization inhibits the activity KATp-channel's smooth cells of coronary vessels and creates a marked dependence of their activity on endothelium presence.     

Influence of venous emptying on the reactive hyperemic blood flow response

Background

Previous research indicates that venous emptying serves as a stimulus for vasodilation in the human forearm. This suggests the importance of recognizing the potential influence of venous volume on reactive hyperemic blood flow (RHBF) following occlusion. The purpose of this study was to examine the influence of venous emptying on forearm vascular function.

Methods

Forearm RHBF, venous capacitance and venous outflow were examined in 35 individuals (age = 22 ± 2 years), using mercury in-Silastic strain gauge plethysmography, at rest and following five minutes of upper arm occlusion using standard procedures (Control). In addition, the same measures were obtained following five minutes of upper arm occlusion preceded by two minutes of passive arm elevation (Pre-elevation).

Results

Average resting arterial inflow was 2.42 ± 1.11 ml·100 ml^-1·min^-1. RHBF and venous capacitance were significantly greater during Pre-elevation compared to Control (RHBF; Pre-elevation: 23.76 ± 5.95 ml·100 ml^-1 ·min^-1 vs. Control: 19.33 ± 4.50; p = 0.001), (venous capacitance; Pre-elevation: 2.74 ± 0.89 % vs. Control: 2.19 ± 0.97, p = 0.001). Venous outflow did not differ between the two conditions.

Conclusion

Venous emptying prior to upper arm occlusion results in a significant greater RHBF response and venous capacitance. Recognition of the influence of venous volume on RHBF is particularly important in studies focusing on arterial inflow, and also provides further evidence for the interplay between the venous and arterial system.

     

Numerical models of auto-regulation and blood flow in the cerebral circulation

A two-dimensional time-dependent computational fluid dynamics model of the Circle of Willis has been developed. To simulate, not only the peripheral resistance of the cerebrovascular tree but also its auto-regulation function, a new "active" boundary condition has been defined and developed using control theory to provide a model of the feedback mechanism. The model was then used to simulate different common abnormalities of the Circle of Willis while a pressure drop, simulating a rapid compression of the right internal carotid artery, was imposed. Test results using a simple tube compared excellently with experiment. The total time-dependent flux for each efferent artery was tabulated and showed the important relationship between geometrical variations in the Circle of Willis and the auto-regulation of blood flow by vascular vaso-dilation and contraction. From this study, it was found that the worst case seemed to be that of a missing or dysfunctional right A1 segment of the anterior cerebral artery. The use of valid physiological models of the peripheral resistance allows for more realistic models of the blood flow in the Circle whilst allowing an easy extension to 3D patient specific simulations.     

Measurement of inferior vena cava diameter for evaluation of venous return in subjects on day 10 of a bed-rest experiment.

We evaluated the usefulness of measurements of the inferior vena cava (IVC) diameters on abdominal echograms as an indicator of changes of venous return in subjects with orthostatic intolerance (OI) induced by simulated microgravity. We performed a standing test and recorded the IVC diameters on abdominal echograms in 12 subjects placed on a 20-day 6 degrees head-down-tilt bed-rest experiment. We found that different patterns of changes in IVC diameter occurred in the standing test on day 10 of the experiment; in five subjects with a marginal decrease in pulse pressure, IVC diameters in the upright position were markedly decreased compared with those in the supine position. In five subjects with feelings of discomfort, the IVC diameters in the upright position distended or did not decrease from those in the supine position. These results suggested that the changes in IVC diameter on the standing test indicated the presence of various types of hemodynamic responses of OI caused by simulated microgravity. In this study, we also evaluated changes in body-water compartments by conducting multifrequency bioelectrical impedance analysis. Longitudinal data analysis showed that the total body-water-to-fat-free mass and extracellular fluid-to-fat-free mass ratios decreased during the experimental period and recovered thereafter, and that the ratio of intracellular fluid to fat-free mass decreased during the experiment. No significant difference in changes in body-water compartments was seen among subjects with different patterns of changes in IVC diameters. Measurement of IVC diameter was useful to estimate hemodynamic changes in subjects with OI.