How does venous return effect stroke volume

Unstressed volume may be recruited into stressed volume via changes in the capacitance of the vessel beds, a protective mechanism for cardiac output in hypovolemia. Blood pressure at zero blood flow was examined long before Guyton. MCFP is the pressure in the whole circulation at zero blood flow after pressure equilibration in the entire vascular bed, including the heart chambers and the lung 39 and represents the elastic recoil of the whole vasculature as a function of total blood volume and the overall vascular compliance 3 , Mean systemic filling pressure is the elastic recoil pressure of the systemic vasculature, excluding the volume and compliances of the heart and lung 40 , This parameter focuses on the return function of the systemic circuit, which is most relevant for the description of altered vascular states and the clinical applications of the concept 19 , As MSFP excludes the central part of the circulation, it is not only dependent on the blood volume described above , but also on volume shifts from the central i.

MSFP is a function of systemic vascular compliance and blood volume 40 , 41 , not vice versa, i. Stressed and unstressed blood volume can be estimated from a step change in MSFP caused by volume infusion or bleeding and measurements of blood volume Figure 2 24 , 41 , There is considerable confusion about MSFP and MCFP in the literature 38 and the exclusion of the pulmonary vasculature is a source of criticism for the concept of venous return A distinction of MCFP and MSFP may clinically not be important because they are very similar in value and difficult to estimate or differentiate exactly.

Still, when discussing the effects of intrathoracic pressures, lung inflation may shift part of the pulmonary and cardiac blood volume towards the systemic circulation, thereby increasing MSFP while keeping MCFP constant 43 , With regards to the systemic circulation and its role for various disease states in critical care, we rely on the description of MSFP and formula 1 can be rewritten.

The upstream pressure MSFP may be interpreted in two ways. The first interpretation sees MSFP as the pivot pressure of the systemic circulation 3 , 39 , the second as the averaged systemic pressure weighted by vessel compliances, thereby representing the systemic stressed blood volume 37 , 41 , In any case, the pressure gradient necessary for blood flow is created by the heart lowering the RAP If a circulation is restarted from this equilibrated standstill pressure MSFP, volume is redistributed by the heart according to the compliances of the various vascular beds around this pivot pressure Figure 3.

It is easily illustrated that the pumping of the heart shifts volume according to the compliances of the vascular segments around this pivot pressure. The pivot also offers a run-off pressure behind the arteriolar system and capillary vessels, which explains how blood flow can continue behind arteriolar beds that show critical closing pressures and vascular waterfalls 21 , Critics argue that a pressure defined for circulatory standstill cannot be present within an ongoing circulation and its exact location in the vasculature would be unknown.

Further, such pressures could by no means drive flow at steady state 13 , 14 , 48 , 49 , because emptying would decrease the pressure without constant refill. These critics ignore that the emptying mechanics including a highly elastic venous reservoir, are central for the achievable flow 44 , In addition, since stressed volume is present during ongoing circulation, so must be its related pressure The large compliance of the veins will keep MSFP constant, because it will damp the effect of a stroke volume on the pressure 35 , So, the function of the heart can be seen as continuous restoration of stressed volume in the circuit 20 , 40 , There is experimental evidence 51 that the splanchnic region may operate on pressures close to MSFP.

The splanchnic vascular beds have the theoretical prerequisites of low resistances and high capacitance We favour the interpretation of MSFP as averaged pressure in the systemic vasculature, weighted by the compliances of the individual segments 20 , 40 , This interpretation is useful when dealing with systemic stressed blood volume and its influence on cardiac output 37 and allows for the explanation of volume shifts from central to peripheral circulations 43 , 44 , which will have an important role in the second part of this article.

Accordingly, the RVR should also be interpreted as the resistance encountered by the average systemic circulatory element 46 , excluding lung and heart.

Estimates with extrapolation methods for critically ill patients with beating hearts range from 18 mmHg up to 33 mmHg These values are much higher than from animal experiments with controlled conditions, were values below 10 mmHg are found 41 , MSFP seems to be constant between species RAP at the intersection of the Starling curve with the venous return curve represents the equilibrium point at which a given cardiac function and vascular circuit can work Figure 1 6 , If RAP, measured towards atmosphere, falls because of lower intrathoracic pressure, like during spontaneous inspiration, both venous return and transmural RAP will increase The downstream role of RAP is central for heart lung interactions 18 , Patients undergoing surgery or in critical illness situations may require higher than normal CI and it may be more appropriate to aim for optimal rather than normal CI.

Grossman S. Porter R. See also www. Stroke Volume Stroke Volume SV is the volume of blood in millilitres ejected from the each ventricle due to the contraction of the heart muscle which compresses these ventricles.

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. There is much confusion about the pressure gradient that determines venous return largely because of different conceptual models that are used to describe venous return. Furthermore, although transient differences occur between the flow of blood leaving cardiac output and entering the heart venous return , these differences when they occur cause adjustments that rapidly return in a new steady-state in which cardiac output flow out equals venous return flow in.

Transient changes in venous return can occur in response to several factors as listed below:. Cardiovascular Physiology Concepts Richard E.

Klabunde, PhD. Limits to flow around the system are produced by the diastolic volume capacity of the ventricles, the flow limitation to venous drainage that occurs when the pressure inside the floppy veins is less than the pressure outside the vessels, and the time limits imposed by time constants of drainage on the movement of the volume wave due to the fixed cycle time determined by heart rate.

These mechanical factors can have a much larger impact than actual changes in blood volume. Finally, clinical responses to treatments can only be in the realm of the physiologically possible. National Center for Biotechnology Information , U.

Journal List Crit Care v. Crit Care. Published online Sep Author information Copyright and License information Disclaimer. Magder, Email: ac. Corresponding author. This article has been corrected. See Crit Care. This article has been cited by other articles in PMC. Abstract Volume infusions are one of the commonest clinical interventions in critically ill patients yet the relationship of volume to cardiac output is not well understood. Keywords: Cardiac output, Venous return, Compliance, Capacitance, Circulatory filling pressure, Mean systemic filling pressure, Time constants, Stressed volume.

Background Ernest Starling [ 1 ] recognized at the turn of the last century that the heart can only pump out what comes back to it. Constant volume A central axiom in the circulation is that vascular volume is constant under steady state conditions.

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. Open in a separate window. Capacitance In electrical models capacitance is the equivalent of compliance but in hydraulic models capacitance means something very different [ 17 — 21 ]. Resistance Resistance accounts for the frictional loss of energy as blood flows through the vasculature and is calculated from the difference between the inflow pressure and the outflow pressure divided by the flow.

Limits of cardiac and return functions Venous return becomes limited when the pressure inside the great veins is less than the pressure outside their walls because the floppy walls of veins collapse and produce what is called a vascular waterfall or a flow limitation Fig. Cardiac contractions and flow of blood Blood flows down an energy gradient, which generally means that blood flows from an area of high pressure to an area with a lower pressure.

Time constants The rhythmic pulsations produced by the time-varying elastances of the ventricles produce important limitations to blood flow. Pulmonary compliance and volume shifts between systemic and pulmonary circuits So far in this review cardiac function has been considered as one unit starting from the right atrium and exiting from the aortic valve.

Krogh model So far in this review the systemic vascular compliance has been lumped into one region with a large compliance. Implications of the physiology for clinical interventions Volume therapy The existence of unstressed volume and the ability to adjust stressed volume by changes in capacitance introduces a role for volume infusions that is not simply to increase cardiac output but rather to ensure reserves.

Adrenergic drugs The study on the effect of the baroreceptor response to hypotension discussed above [ 9 ] gives insight into the response of the peripheral circulation to infusions of norepinephrine. Conclusions The circulation starts with a potential energy which is due to the stretching of the elastic walls of all its components by the volume it contains even when there is no blood flow.

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