Cardiovascular System

Basic Physiologic Principles

Achieving adequate oxygen delivery to meet tissue metabolic demands is the primary objective to ensure smooth recovery from cardiac surgery. As systemic blood pressure does not always equal satisfactory organ perfusion, it is imperative to understand the physiologic principles of hemodynamics and tissue oxygenation.

Mean systemic blood pressure (MAP) is the product of cardiac output (CO) and systemic vascular resistance (SVR) added to the central venous pressure (CVP). As CVP is normally low, it is commonly omitted when using the formula for clinical purposes. Cardiac output equals stroke volume (SV) times heart rate (HR). The stroke volume is determined by preload, contractility, and afterload, which themselves are affected by ventricular compliance, interventricular dependence, and cardiac rhythm, among other components (see FIGURE 1). Rhythm disturbances, intrinsic and extrinsic metabolic factors, and postoperative pacing affect heart rate. The cardiac index (CI) is calculated to adjust for the individual patient, and can be derived by dividing CO by the body surface area (BSA). The normal values of the hemodynamic parameters mentioned are seen in Figure 2.

Figure 1.

Components determining mean arterial pressure (MAP). Qt = cardiac output, SVR = systemic vascular resistance; SV = stroke volume; HR = heart rate; AVC = arterial vascular compliance

Figure 2.

Hemodynamic formulas and normal values. CVP = central venous pressure; Pra = right atrial pressure