Fluid therapy

No formula exists to predict how much fluid should be given to expand plasma volume in critically ill patients. Figy.re.3 shows the effect of increasing extracellular fluid volume on blood volume. The maximal blood volume that occurs for a given extracellular volume is a function of the existing microvascular pressures, the compliance of the interstitium, and the permeability of the microvascular barrier. When these factors are increased, a lower blood volume will always result for a given extracellular fluid volume (black broken curve). When all factors are decreased, a larger blood volume results (green broken curve). A higher compliance of the blood vessels will result in a larger blood volume relative to the extracellular fluid volume. The effective microvascular pressure can be estimated in most organs ( Taylor..,, 1996) and plasma oncotic pressure can be routinely measured in intensive care units, and so they can help to guide fluid replacement therapy. If, with a given mode of fluid replacement, the measured microvascular and plasma oncotic pressures are normal but the tissues are edematous, particularly in the lung, there is no doubt that the microvascular barrier has been damaged and fluid will leak out into the organ's interstitium. Effective patient therapy will help to stabilize cellular membrances, allowing for adequate resuscitation.

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Fig. 3 Relationship between extracellular fluid volume and blood volume: green solid curve, normal conditions; green broken curve, decreased microvascular pressures, decreased interstitial compliance, and less permeable challenge; black broken curve, increased microvascular pressures, increased interstitial compliance, and more permeable challenge. (Reproduced with permission from Guyton.eta.l; (19Z51)

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