During the perioperative period a number of changes occur which alter the normal physiological responses of the cardiorespiratory system. The most obvious of these is the surgery itself, but the physiological changes caused by positive-pressure ventilation, reduction in functional residual capacity, and reduced muscle tone all tend to reduce tissue perfusion. In addition, many drugs given during the perioperative period have the potential to reduce tissue perfusion, redistribute blood flow, and override normal compensatory mechanisms. Conversely, postoperative stress responses, the maintenance of normothermia, the effects of starvation and altered requirement for specific organ function, and the inflammatory response secondary to the trauma of surgery, followed by healing, may all have metabolic consequences requiring an increase in tissue perfusion. By monitoring and, if necessary, manipulating the circulation with intravenous fluid therapy, the goal will be to restore, optimize, and maintain the cardiovascular function in order to prevent tissue hypoperfusion and organ failure.

If the balance of the supply and demand is not maintained, hypoxic tissue damage may result. This is particularly important in those patients with pre-existing organ dysfunction where very mild changes in perfusion cause significant damage to the organ. Clinical trials have shown that if tissue perfusion is maintained during the perioperative period, mortality and morbidity may be reduced (Shoemaker§!§/ 1988; B.o,y.d...§.ta./: 1993).

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