Pmfs can be altered in two distinct ways:

  1. The total volume of the reservoir could be changed (Fig. A and B). This occurs with the addition or removal of fluids.

Fig. A: Assuming favorable venous compliance, the addition of intravascular volume leads to an increase in Pmsf by expanding the stressed volume compartment.

Fig. A: Assuming favorable venous compliance, the addition of intravascular volume leads to an increase in Pmsf by expanding the stressed volume compartment.

Fig. B: Pmsf shifts rightward, thereby increasing the venous return (VR) gradient.

Fig. B: Pmsf shifts rightward, thereby increasing the venous return (VR) gradient.

This illustrates the effect a fluid bolus has on venous hemodynamics. When coupled with the Starling curve, if the patient has good cardiovascular performance and is positioned on the ascending limb of the curve, administering a fluid bolus expands the stressed volume compartment, increases Pmsf, and enhances the venous return gradient. As a result, cardiac output will undoubtedly increase.

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  1. The compliance (or diameter) of the reservoir can be altered in such a way that the proportion of stressed to unstressed volume is shifted (Fig. C). Systemic vasodilatory states, changes in neural tone, catecholamine responses, or the use of exogenous vasoactive medications all have the potential to modify vascular compliance. Let’s delve deeper into how a venular vasopressor can enhance venous return in the setting of a highly compliant system.

    Fig. C: A systemic vasodilatory state increases overall venous compliance, shifting the balance toward unstressed volume despite no net loss of fluid. When a vasopressor with venular tropism is administered, venous compliance decreases, converting unstressed volume into stressed volume and thereby increasing mean systemic filling pressure (Pmsf).

    Fig. C: A systemic vasodilatory state increases overall venous compliance, shifting the balance toward unstressed volume despite no net loss of fluid. When a vasopressor with venular tropism is administered, venous compliance decreases, converting unstressed volume into stressed volume and thereby increasing mean systemic filling pressure (Pmsf).

    Interestingly, profound inflammatory states may impair cardiovascular performance, even when cardiac output appears relatively high. This apparent paradox is often explained by a significant reduction in systemic vascular resistance. In such scenarios, the use of a vasopressor with β₁-adrenergic properties (such as norepinephrine or epinephrine) can provide a beneficial positive inotropic effect. When combined with a reduction in venular compliance, the result is a synergistic enhancement of venous return, cardiac contractility, and ultimately, a net increase in cardiac output.

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Further reading

Hamzaoui O, Jozwiak M, Geffriaud T, Sztrymf B, Prat D, Jacobs F, Monnet X, Trouiller P, Richard C, Teboul JL. Norepinephrine exerts an inotropic effect during the early phase of human septic shock. Br J Anaesth. 2018 Mar;120(3):517-524. doi: 10.1016/j.bja.2017.11.065. Epub 2017 Nov 21. PMID: 29452808.