A review published in Annals of Intensive Care examines two physiologic concepts that help explain how blood returns to the heart and how clinicians interpret venous pressures in critically ill patients: mean circulatory filling pressure and mean systemic filling pressure.

In the review, Sheldon Magder, MD, of McGill University Health Centre in Montreal, Canada, and colleagues discuss the physiology underlying these pressures and evaluate their potential relevance for clinical care. The authors conclude that although the concepts are central to understanding venous return, directly measuring these pressures rarely provides more practical information than standard hemodynamic monitoring.

Mean circulatory filling pressure describes the pressure that would exist throughout the vascular system if the heart stopped and blood flow ceased. Under those conditions, pressures equalize across the circulation. However, blood volume does not distribute evenly because different vascular regions have different levels of compliance.

Systemic veins and venules account for the majority of vascular compliance—approximately 70%—and therefore play the dominant role in determining circulatory filling pressure. Because the venous system is highly compliant, it serves as the primary reservoir for blood within the circulation.

When blood flow resumes, the pressure driving venous return toward the right side of the heart is referred to as mean systemic filling pressure. Under typical physiologic conditions, the difference between mean circulatory filling pressure and mean systemic filling pressure is small because the venous system dominates overall vascular compliance.

Mean systemic filling pressure has two important physiologic roles. First, it represents the upstream pressure that drives blood back to the right atrium, influencing venous return and ultimately cardiac output. Second, it functions as the downstream pressure for capillary drainage. If this pressure becomes too high, capillary hydrostatic pressure may rise and increase the risk of tissue edema.

Studies examining venous return physiology suggest that the pressure gradient between mean systemic filling pressure and right atrial pressure is typically small—generally about 3 to 6 mmHg. Because of this relationship, mean systemic filling pressure can often be approximated by adding several millimeters of mercury to a carefully measured right atrial pressure.

The authors also describe physiologic factors that can influence the relationship between circulatory filling pressures. Changes in cardiac function, vascular resistance, and blood flow distribution may alter how pressure is distributed within the circulation. For example, reduced ventricular performance or increased systemic vascular resistance may lower mean systemic filling pressure relative to mean circulatory filling pressure, whereas conditions that accelerate systemic venous filling—such as exercise or sepsis—may increase it.

Although researchers have proposed several techniques to estimate mean systemic filling pressure at the bedside, including respiratory maneuvers and mathematical modeling approaches, the authors suggest that these methods offer limited additional clinical value in most situations. In practice, careful measurement of right atrial pressure and cardiac output typically provides more actionable information for guiding hemodynamic management.

Because the pressure gradient driving venous return is relatively small, modest increases in right atrial pressure can reduce venous return and cardiac output. The authors note that when right atrial pressure rises above approximately 8 to 10 mmHg, clinicians should be cautious with additional fluid administration because higher pressures may contribute to venous congestion and impaired organ function.

Overall, the review concludes that mean circulatory filling pressure represents the potential energy stored within the vascular system, while mean systemic filling pressure—and particularly its relationship to right atrial pressure—offers the more clinically relevant framework for interpreting venous return in critically ill patients.

The authors declared having no competing interests.

Source: Annals of Intensive Care