BLOOD-PRESSURE VERSUS BLOOD-FLOW

The pressure-volume diagram of the left ventricle is fundamental to un derstanding the mechanics of contraction and its modification by disea se, drugs and anaesthetics. For monitoring patients perioperatively, w e advocate variables derived from the pressure-volume diagram: cardiac output to monitor circulatory failure, pulmonary capillary wedge pres sure to monitor the acute state of left ventricular filling and echoca rdiography to determine the chronicity and severity of chronic heart f ailure. Use of the pressure-volume diagram shows that whenever arteria l 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 da maged or failing, this adverse effect of raising arterial pressure cau ses further deterioration of circulatory failure and flow. In view of the prime need of the body tissues for blood flow, together with oxyge n 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 mai ntaining the circulation . If circulatory failure continues in the pre sence of a high filling pressure of the left ventricle (wedge pressure ), drugs combining positively inotropic and vasodilator properties are advocated.