Clinical Note

Physicians now understand the primacy of venous pressure in cardiac output control. As a result, a central venous catheter is almost always placed in the critically ill patient to monitor venous pressure and guide the physician in patient management. For example, if cardiac output and venous pressure fall, fluids are added to raise the blood volume. If, on the other hand, venous pressure rises as cardiac output falls, then the cause is low contractility and a cardiac stimulant would be indicated.

VR, VP2

Venous Pressure (mm Hg)

FIGURE 7 Effect of hypervolemia on the cardiovascular state as depicted on a simultaneous plot of the cardiac and vascular function curves.

VR, VP2

Venous Pressure (mm Hg)

FIGURE 7 Effect of hypervolemia on the cardiovascular state as depicted on a simultaneous plot of the cardiac and vascular function curves.

hemorrhage. Cardiac output is carefully controlled in the body primarily in association with blood pressure control. The body adjusts both blood volume and venous tone as an integral part of this control system. The former is associated with long-term control, primarily by the kidney over hours to days, whereas the latter can be adjusted by the central nervous system in a moment-to-moment fashion. These control systems are explained in detail in Chapter 15.

Effect of Changing Arteriolar Tone

Arteriolar constriction or relaxation has a different effect on the vascular function curve from that seen with changing blood volume. At zero cardiac output, altering the TPR will not affect the mean circulatory filling pressure because the volume of blood contained within the arterioles is negligible. However, except when cardiac output is zero, an increase in arteriolar constriction increases the pressure difference required to drive the cardiac output across the TPR. To illustrate this, let us return to our original example in which the heart has been replaced by a mechanical pump. If the output of the heart-lung machine is held constant, say, at 4 L/min, a sudden increase in TPR will temporarily decrease blood flow across the microcirculation. The mismatch between cardiac output and flow through the microcirculation will cause blood to accumulate in the arterial compartment until arterial pressure becomes high enough to force the entire cardiac output across the increased TPR. The additional blood in the arterial compartment came from the venous compartment. Therefore, increasing the TPR will cause venous pressure to be lower for any cardiac output, causing a counterclockwise rotation of the vascular function curve about the mean circulatory filling pressure, as is shown in Fig. 8. Arteriolar dilation decreases the TPR and has the opposite effect, a clockwise rotation.

Increasing the TPR will cause the equilibrium point to move toward reduced cardiac output and venous pressure (A to B in Fig. 8). Decreasing the TPR will likewise increase the cardiac output and the venous pressure (A to C in Fig. 8). But there is more to the story. Because changes in TPR have a marked effect on arterial blood pressure, they also shift the cardiac function curve. In Chapter 13, we learned that end-systolic volume is directly related to the arterial pressure. As a result, stroke volume decreases when arterial pressure increases. We therefore have to draw a new cardiac function curve down and to the right (equivalent to that seen with reduced contractility) when TPR is increased, as shown in Fig. 8. This shift of the ventricular function curve further exaggerates the effect on cardiac output beyond what would be expected by the change in venous pressure alone. Therefore, the equilibrium point actually moves from A to D (in Fig. 8) after an increase in TPR. Decreasing the TPR has the opposite effect and increases stroke volume, moving the cardiac function curve up and to the left, so the equilibrium point actually moves from A to E. Unlike the effect on cardiac output, the change in venous pressure resulting from a change in TPR will be ambiguous because both curves are altered. Depending on the exact characteristics of the individual, venous pressure may either rise or fall with a change in TPR.

Decreased

FIGURE 8 Effects of changes in total peripheral resistance on the vascular and cardiac function curves.

Venous Pressure (mm Hg)

FIGURE 8 Effects of changes in total peripheral resistance on the vascular and cardiac function curves.

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