The principle is to ascend the Frank-Starling curve for each individual by obtaining the optimal filling pressure associated with the best afterload ( Fig 1). It is vital to achieve the optimal filling pressure, thus ensuring an adequate blood volume which will provide complete perfusion for all organs and adequate cardiac filling. The simplest method of assessing the intravascular fluid volume in the perioperative period is by using a central venous pressure line and fluid challenge. With a central venous pressure of 6 to 8 mmHg, set to zero at the mid-axillary line, a fluid challenge (bolus doses of 100 or 200 ml of intravenous fluid) is given (and if necessary repeated) until the central venous pressure maintains a sustained rise in response to the challenge.
Fig. 1 Summary of the components of management and their effects on cardiac output.
In sicker patients the pulmonary artery catheter allows a more sophisticated version of the fluid challenge to be performed. The patient is given intravenous fluid until the pulmonary artery occlusion pressure is adequate (10-14 mmHg). The patient is then given fluid challenges in bolus doses of 100- to 200-ml aliquots of intravenous fluid until the doses result in an increase in pulmonary artery pressure without a corresponding increase in cardiac output.
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