Chapter 8: Hemodynamic Interactions

OBJECTIVES

The student understands how central venous pressure can be used to assess circulatory status and how venous return, cardiac output, and central venous pressure are interrelated:

• Describes the overall arrangement of the systemic circulation and identifies the primary functional properties of each of its major components.

• Defines mean circulatory filling pressure and states the primary factors that determine it.

• Defines venous return and explains how it is distinguished from cardiac output.

• States the reason why cardiac output and venous return must be equal in the steady state.

• Lists the factors that control venous return.

• Describes the relationship between central venous pressure and venous return and draws the normal venous return curve.

• Defines peripheral venous pressure.

• Lists the factors that determine peripheral venous pressure.

• Predicts the shifts in the venous return curve that occur with altered blood volume and altered venous tone.

• Describes how the output of the left heart pump is matched to that of the right heart pump.

• Draws the normal venous return and cardiac output curves on a graph and describes the significance of the point of curve intersection.

• Predicts how normal venous return, cardiac output, and central venous pressure will be altered with any given combination of changes in cardiac sympathetic tone, peripheral venous sympathetic tone, or circulating blood volume.

• Identifies possible conditions that result in abnormally high or low central venous pressure.

INTRODUCTION

In previous chapters, we have primarily described how individual components in the cardiovascular system work. That is, we have tried to establish their fundamental individual “rules of operation.” (For example, a basic rule for the heart is CO = SV × HR, and a basic rule for any vessel is $Q˙=ΔP/R$.) Such individual rules must be obeyed in all situations including those that exist within the intact cardiovascular system. However, in the intact cardiovascular system, the individual components are interconnected. An abnormal operation of any one component necessarily causes “ripple-effect” changes throughout the entire system that may seem abnormal. Such interactions are the subject of this chapter. They are of special importance to the clinician who must be able to distinguish between primary abnormalities and secondary consequences.

KEY SYSTEM COMPONENTS

As illustrated in Figure 8–1, the cardiovascular system is a closed hydraulic circuit that includes the heart, arteries, arterioles, capillaries, and veins.¹ The venous side of this system is often conceptually separated into 2 different compartments: (1) a large and diverse peripheral section (the peripheral venous compartment) and (2) a smaller intrathoracic section that includes the vena cavae and the right atrium (the central venous compartment). Each of the segments of this circuit has a distinctly different role to play in the overall operation of the system because of inherent differences in anatomical volume, resistance to flow, and compliance that are summarized in Table 8–1.