The plasma compartment is usually subdivided into forward and backward components. The forward compartment consists of the blood on its way from the heart to the tissues and the backward compartment consists of blood on its way back from the tissues to the heart. Changes in the forward compartment are reflected in changes in pulse, blood pressure and tissue perfusion. Changes in the backward compartment are reflected in changes in the venous pressure. In the critically ill patient continuous blood pressure monitoring is valuable and is best accomplished by placement of an arterial line. Venous pressure may be measured by placement of a central venous line. Pressures in the pulmonary circulation and cardiac output may be measured by placement of a pulmonary artery catheter (Swan-Ganz). Outcome studies, however, have shown that placement of a pulmonary artery catheter may worsen the patients outlook and the decision to choose invasive monitoring is taken carefully.
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