How do we evaluate cardiac output?

Factors associated with both venous return and cardiac function determine a patient’s cardiac output (Q). Their respective curves (shown here) help to simplify complex hemodynamic systems.

Venous Return – The Guyton Curve

In the 1950’s, Arthur Guyton demonstrated the determinants of venous return graphically in what are now called Guyton curves or venous return curves. The Guyton curve demonstrates the relationship between venous return (Flow) on the Y-axis and PRA on the X-axis. The slope of this curve represents the resistance to venous return.  Understanding these curves and how manipulations to Pms, PRA, or Rv influences them is important to fully understanding hemodynamic physiology.   Let’s explore the curves a bit further.

Hagen-Poiseuille Law

Blood flow from the reservoir toward the heart is passively driven by the pressure gradient between Pms and the downstream right atrial pressure (PRA) according to the principles of the Hagen-Poiseuille law which states that flow is determined by the pressure gradient across the length of a tube divided by the resistance (R) within the tube.  If we apply this formula to our model, venous return from the reservoir toward the heart is determined by the gradient between the upstream Pms and the downstream PRA, divided by the resistance to flow within the venous circuit.

Stressed And Unstressed Volume

Blood volume in the reservoir is classified in two ways, stressed and unstressed volume. Similar to a deflated balloon, the unstressed volume is the blood volume required to fill (but not distend) the walls of vessels within the circulatory system.  Unstressed volume does not support blood flow back to the heart.  Stressed volume is the volume that distends vessel walls once the vessel's lumen is filled.  The distending pressure created by stressed volume in the reservoir is a primary determinant of blood flow towards the heart.  The pressure the stressed volume exerts on the venular walls is called the mean systemic filling pressure (Pmfs). The mean circulatory filling pressure represents the mean blood pressure within the whole systemic circulation.

So, where on Guyton’s curve is the Pmfs located? In a theoretical scenario where there is no flow, both the right atrial pressure (RAP) and the mean systemic filling pressure (Pmsf) will equilibrate at the same value. This point corresponds to the intercept of the venous return curve on the pressure axis; essentially where venous return equals zero.

Clinical correlation: On manipulating the Pmsf

Right Atrial Pressure

Right Atrial Pressure (PRA) or Central Venous Pressure (CVP) is a fascinating parameter with significant physiological meaning. It is readily measurable in any patient with a central venous catheter. Yet, its clinical utility has been frequently questioned, often unfairly. Much of this criticism arises from a misunderstanding of what CVP truly represents.

In simple terms, CVP is not an isolated variable. Rather, it is the point of equilibrium between two curves: