Cardiovascular Physiology Concepts
                                    Richard E. Klabunde, Ph.D.

Cardiac and Systemic Vascular Function Curves

Guyton and colleagues in the 1950s and 1960s conducted extensive animal experimentation studying the interrelationships between cardiac function and systemic vascular function. These elegant studies led to a model of these relationships that could be graphically represented by plotting both cardiac function and systemic vascular function curves on the same graph. This analysis is very helpful in understanding how changes in cardiac function affect venous pressures, and how changes in arterial and venous resistance, and blood volume affect venous pressure and cardiac output. To examine these interactions, the two component curves will first be described individually, then they will be combined to show how changes in one affects the other.

Cardiac Function Curves

Cardiac function curves (sometimes called cardiac output curves) are essentially Frank-Starling curves, but differ in that cardiac output instead of ventricular stroke volume is plotted against changes in venous pressure (usually right atrial pressure, P_RA). If in a controlled experimental model right atrial pressure is varied (independent variable) and the cardiac output measured (dependent variable), one will find that as P_RA is increased, the cardiac output (CO) increases. When the mean P_RA is about 0 mmHg (note that P_RA normally fluctuates with atrial contraction and respiration), the cardiac output in an adult human is about 5 L/min. Because of the steepness of the cardiac function curve, very small changes in P_RA (just a few mmHg) can lead to large changes in CO.

Similar to Frank-Starling curves, there is no single cardiac function curve. Instead, there is a family of curves that can shift upward when cardiac performance is enhanced or shift downward when cardiac performance is depressed. Performance is enhanced by increased inotropy, increased heart rate, and reduced afterload. Performance is depressed by decreased inotropy, decreased heart rate, and by increased afterload.

Systemic Vascular Function Curves

Systemic vascular function curves (sometimes called venous return curves) are generated by measuring P_RA (dependent variable) as CO (independent variable) changes. Note that the independent and dependent variables are reversed for these curves compared to the cardiac function curves. Experimentally, if cardiac output is stopped, aortic pressure falls and P_RA increases to a common value of about 8 mmHg (if the baroreceptor reflex is blocked). This pressure, which is recorded shortly after the heart is stopped, is called the mean circulatory filling pressure (P_mc). If the heart is restarted, then P_RA decreases as the CO increases. As the P_RA starts to fall below zero, the CO begins to level off because the vena cava collapses, thus limiting venous return to the heart.

There is no single systemic vascular function curve, but instead there is a family of curves that are determined by the blood volume (Vol), venous compliance (C_V; inverse of venous tone) and systemic vascular resistance (SVR; primarily arterial resistance). If, for example, blood volume is increased due to renal retention of sodium and water, or venous compliance is decreased due to sympathetic activation of the veins (Panel A), there is a parallel shift to the right in the vascular function curve, which leads to an increase in the P_mc when the heart is stopped. The opposite shift occurs with decreased blood volume or increased venous compliance. If SVR is increased (Panel B) by administering an arterial vasoconstrictor drug, the slope of the systemic vascular function curve decreases, but there is little or no change in the P_mc. The opposite occurs with a decrease in SVR. If, for example, both arteries and veins are constricted during sympathetic activation, then the curve will shift to the right as shown in Panel C (increased P_mc due to decreased C_V) and the slope will decrease due to the increase in SVR.

Coupling of Cardiac and Vascular Function

When the cardiac and vascular function curves are plotted together in the same graph, there is a unique intercept between the two curves (see point A in left panel of figure below). This intercept represents the steady-state operating point that defines the cardiac output and right atrial pressure for these particular physiological conditions. In this example, the CO is 5 L/min at a P_RA of 0 mmHg.

If the heart were stimulated, the cardiac function curve would shift up and to the left; however, there would only be a very small increase in CO because decreasing the P_RA below zero causes venous collapse, which impedes venous return and hence filling of the ventricle.

If cardiac function is depressed (e.g., by decreasing inotropy) as shown in the right panel of the figure, the cardiac function curve shifts down and to the right, and the intercept will change from Point A to B. This shows that depressing the heart leads to an increase in P_RA and venous pressures along with the decrease in CO. If this depressed cardiac function is also accompanied by an increase in blood volume, venous constriction (decreased venous compliance, C_V) and arterial constriction (increased SVR), the systemic function curve will shift to the right and have a reduced slope. The new operating point (C) represents this condition. Notice that these systemic vascular function changes help to partially restore CO despite the depressed cardiac function curve, although at the expense of a further increase in P_RA and venous pressures.

RK Revised 03/31/08

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.

© 1999-2008 Richard E. Klabunde, all rights reserved.