The central venous compartment corresponds to the volume enclosed by the right atrium and the great veins in the thorax. Central venous pressure (CVP) is the intravascular pressure in the great thoracic veins, measured relative to atmospheric pressure. It is conventionally measured at the right atrium-superior vena cava junction and provides an estimate of the right atrial pressure.
CVP is primarily influenced by blood volume and compliance in the central venous compartment. The interaction of cardiac function and the physiologic components that influence venous return to the heart determine the final CVP.
Multiple factors influence CVP measurements, including total blood volume, blood volume distribution between vascular compartments, cardiac inotropic state, right ventricle compliance, and an imbalance between cardiac output and venous return. Intrathoracic pressure changes also affect CVP, particularly in consideration of mechanical respiratory support with positive end-expiratory pressure (PEEP).
A normal CVP waveform consists of five phasic events: three peaks (a, c, v) and two descents (x, y) (Figure 2-1). The most prominent wave is the a wave, resulting from atrial contraction following the ECG P wave at the end-diastole (ventricular diastole). Atrial pressure decreases following the a wave, as the atrium relaxes. This decline in atrial pressure is interrupted by the c wave at the beginning of ventricular systole. This wave is a transient increase in atrial pressure produced by isovolumic right ventricular contraction. The ventricular contration closes the tricuspic valve and displaces it toward the right atrium in early systole, producing an increase in atrial pressure. As an early systolic event, the c wave must follow onset of the QRS or R wave on the ECG.
CVP tracing. (Modified with permission from Mark JB. Central venous pressure monitoring: clinical insights beyond the numbers. J Cardiothorac Vasc Anesth. 1991;5:163–173.)
Atrial pressure continues its decline during ventricular systole as a consequence of continued atrial relaxation and changes in atrial geometry produced by ventricular contraction and ejection. This is the x descent or systolic collapse in atrial pressure. The x descent is considered to be the systolic decline, or collapse, in atrial pressure. The x descent can be divided in two segments: x before the c wave and x′ after the c wave. The last atrial pressure peak is the v wave, caused by venous filling of the right atrium during late systole while the tricuspid valve remains closed. The v wave peaks just after the ECG T wave. Atrial pressure then decreases as the tricuspid valve opens and blood flows from atrium to ventricule. This is the y descent or diastolic collapse in atrial pressure (Table 2-1).
TABLE 2-1Central Venous Pressure Waveform Phasic Events and Corresponding Description