Assessing significance of reduced plasma proteins
Plasma albumin levels are almost invariably reduced in critically ill patients. Causes include interstitial leakage, failed synthesis and increased metabolism. However, the same group of patients often have raised levels of acute phase proteins which contribute to COP. Since there is no evidence that correction of plasma albumin levels is beneficial, many clinicians correct plasma volume deficit with artificial colloid. These will contribute to COP while also reducing hepatic albumin synthesis. If COP is maintained >20mmHg it is likely that reduced plasma albumin levels are of no significance.
It has been suggested that a difference between COP and pulmonary artery wedge pressure >6mmHg minimises the risk of pulmonary oedema. However, in the face of severe capillary leak it is unlikely that pulmonary oedema can be avoided if plasma volumes are to be maintained compatible with circulatory adequacy. Conversely, a normal COP would not necessarily prevent pulmonary oedema in severe capillary leak; the contribution of COP to fluid dynamics in this situation is much reduced.
It is difficult not to support the use of colloid fluids in hypo-oncotic patients. In patients with renal failure the repeated use of colloid fluid may lead to a hyperoncotic state. This is associated with tissue dehydration and failure of glomerular filtration (thus prolonging the renal failure). Measurement of a high COP in patients who have been treated with artificial colloids should direct the use of crystalloid fluids. It is important to note that excessive diuresis may also lead to a hyper-oncotic state for which crystalloid replacement may be necessary.
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