Volume and its relationship to cardiac output and venous return.
Cardiac output is the product of stroke volume and heart rate. need for, or adequacy of, therapeutic interventions and provides a more detailed picture of the patient's cardiovascular status. Cardiac output may be reduced by poor venous return and end-diastolic These relationships are expressed by the Fick equation. Learning objectives • Define Cardiac Output and Venous Return. . The vascular function (venous return) curve depicts the relationship. Goals. • To recognize that cardiac output varies directly with heart rate and stroke volume. . What's the relationship between venous return and stroke volume?.
These concepts have been reviewed previously [ 67 ] but in this paper the emphasis will be on the role of volume as a determinant of cardiac output because so much of the management of critically ill patients revolves around infusions of volume. Constant volume A central axiom in the circulation is that vascular volume is constant under steady state conditions. This volume stretches the elastic walls of the vasculature structure and creates an elastic recoil force that is present even when there is no flow but is also a key determinant of flow [ 48 ].
The potential energy of this elastic recoil becomes evident when there is no flow in the circulation and large veins are opened to atmospheric pressure. Vascular volume empties from the veins even without cardiac contractions. The heart adds a pulsatile component to this static potential energy which redistributes the volume according to the compliances and resistances entering and draining each elastic compartment of the circulation.
It may sound obvious that vascular volume is constant but this point is often neglected.Venous return cardiac output curves
A key example is the use of electrical circuitry to model circulatory flow. In hydrodynamics, the study of flowing liquids such as in the circulation, pressure is the equivalent of voltage, cardiac output in liters per minute is the equivalent of current, and resistance is the frictional loss of energy due to the interactions of the layers of the moving fluid with the vessel wall.
In electrical models voltage is determined by the decrease in charge from a fixed source, such as a wall socket or battery to a ground value. An increase in resistance or change in voltage across the system changes the amount of electrons in the system. In the vasculature this change in number of electrons is the equivalent of a change in volume. In contrast to an electrical system, the vascular volume is constant in the circulation and the pressure drop, the equivalent of the voltage gradient, changes with changes in resistance or volume.
The output from the heart shifts volume to the arterial compartment and creates an arterial pressure which is dependent upon the total vascular resistance. It is thus the volume per time, i. Compliance Compliance is a measure of the distensibility of a spherical structure and is determined by the change in volume for a change in pressure.
Volume and its relationship to cardiac output and venous return.
A simple example is inflation of a balloon with a known volume and then measuring the change in pressure across the wall. It may seem surprising that this static property is a key determinant of flow, which is a dynamic state.
The importance of compliance is that the elastic recoil force created by stretching the walls of vascular structures creates a potential force that can drive flow when the downstream pressure is lower. Second, compliance is necessary to allow pulsatile flow through a closed circuit Fig.
Cardiac contractions create a volume wave that moves through the vasculature. The walls of vessels must be able to stretch in order to transiently take up the volume.
The pressure created by the stretch of vascular walls moves the volume on to the next vascular segment with a lower pressure. If vascular walls were all very stiff, pressure generated by a pump at one end would be instantaneously transmitted throughout the vasculature. Balance is achieved, in large part, by the Frank—Starling mechanism.
For example, if systemic venous return is suddenly increased e. The left ventricle experiences an increase in pulmonary venous return, which in turn increases left ventricular preload and stroke volume by the Frank—Starling mechanism.
In this way, an increase in venous return can lead to a matched increase in cardiac output.
Hemodynamically, venous return VR to the heart from the venous vascular beds is determined by a pressure gradient venous pressure - right atrial pressure and venous resistance RV. Therefore, increases in venous pressure or decreases in right atrial pressure or venous resistance will lead to an increase in venous return, except when changes are brought about by altered body posture.
Although the above relationship is true for the hemodynamic factors that determine the flow of blood from the veins back to the heart, it is important not to lose sight of the fact that blood flow through the entire systemic circulation represents both the cardiac output and the venous return, which are equal in the steady-state because the circulatory system is closed. Therefore, one could just as well say that venous return is determined by the mean aortic pressure minus the mean right atrial pressure, divided by the resistance of the entire systemic circulation i.
However, as noted above it is clear that, equally, cardiac output must dictate venous return since over any period of time both must necessarily be equal.
Venous return curve - Wikipedia
Similarly, the concept of mean systemic filling pressure, the hypothetical driving pressure for venous return, is difficult to localise and impossible to measure in the physiological state. Furthermore, the Ohmic formulation used to describe venous return ignores the critical venous parameter, capacitance. It is confusion about these terms that has led some physiologists to suggest that the emphasis on 'venous return' be turned instead to more measurable and direct influences on cardiac output such as end diastolic pressure and volume which can be causally related to cardiac output and through which the influences of volume status, venous capacitance, ventricular compliance and venodilating therapies can be understood.
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- Venous return curve
- Volume and its relationship to cardiac output and venous return
Rhythmical contraction of limb muscles as occurs during normal locomotory activity walking, running, swimming promotes venous return by the muscle pump mechanism. Sympathetic activation of veins decreases venous compliance, increases venomotor tone, increases central venous pressure and promotes venous return indirectly by augmenting cardiac output through the Frank-Starling mechanism, which increases the total blood flow through the circulatory system.
During inspiration, the intrathoracic pressure is negative suction of air into the lungsand abdominal pressure is positive compression of abdominal organs by diaphragm. This makes a pressure gradient between the infra- and supradiaphragmatic parts of v.
An increase in the resistance of the vena cava, as occurs when the thoracic vena cava becomes compressed during a Valsalva maneuver or during late pregnancy, decreases return.