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Blood pressure versus blood flow.

The pressure-volume diagram of the left ventricle is fundamental to understanding the mechanics of contraction and its modification by disease, drugs and anaesthetics. For monitoring patients perioperatively, we advocate variables derived from the pressure-volume diagram: cardiac output to monitor circulatory failure, pulmonary capillary wedge pressure to monitor the acute state of left ventricular filling and echocardiography to determine the chronicity and severity of chronic heart failure. Use of the pressure-volume diagram shows that whenever arterial pressure is raised by the use of vasoconstrictor agents, the flow of blood to the tissues is impaired. In patients in whom the heart is damage or failing, this adverse effect of raising arterial pressure causes further deterioration of circulatory failure and flow. In view of the prime need of the body tissues for blood flow, together with oxygen and substrates, it is advocated that vasoconstricting agents should not be used to maintain arterial pressure in the perioperative period. Plasma expansion and control of posture are preferred methods for maintaining the circulation. If circulatory failure continues in the presence of a high filling pressure of the left ventricle (wedge pressure), drugs combining positively inotropic and vasodilator properties are advocated.