Assessing the response to a fluid challenge

The response of CVP or PAWP should be monitored during a fluid challenge ( Fig 2). Fluid challenges should be repeated when the response suggests continuing hypovolemia. However, if such monitoring is not available it is reasonable to assess the clinical response to up to two fluid challenges (200 ml each).

Fig. 2 Algorithm detailing the use of dynamic CVP responses to guide fluid therapy.

The change in CVP after a 200-ml fluid challenge depends on the starting blood volume. A rise of 3 mmHg in CVP represents a significant increase and is probably indicative of an adequate circulating volume. However, a positive response may sometimes occur in the vasoconstricted patient with a lower blood volume. It is important to assess the clinical response in addition; if it is inadequate, it is appropriate to monitor stroke volume and PAWP before further fluid challenges or considering further circulatory support.

In the inadequately filled left ventricle a fluid challenge will increase the stroke volume ( Fig, 3). Failure to increase the stroke volume with a fluid challenge may represent an inadequate challenge, particularly if the PAWP fails to rise significantly (3 mmHg). This indicates that cardiac filling was inadequate and the fluid challenge should be repeated. Such a response may also be seen in right heart failure, pericardial tamponade, and mitral stenosis. It may be important to monitor stroke volume rather than cardiac output during a fluid challenge. If the heart rate falls appropriately in response to a fluid challenge, the cardiac output may not increase despite an increase in stroke volume.

Fig. 3 The response of stroke volume and PAWP to small-volume fluid challenges. In the hypovolemic patient an increase in stroke volume with no significant rise in PAWP would be expected. In the optimally filled patient a rise in PAWP with no significant rise in stroke volume would be expected.

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