Key Point

The critical care field has misconceptions surrounding the idea of isotonic rehydration and the ways to measure responses to fluid rescucitation.

HONOLULU—As is the case in other specialties, critical care has its share of misconceptions. For one, the notion of isotonic rehydration is present in the setting of acute fluid resuscitation, pointed out John A. Kellum, MD, at the Society of Critical Care Medicine’s 37th Critical Care Congress. However, despite the numerous incorrect references in the literature to sodium chloride infusion for the rehydration of patients with acute kidney injury, “you cannot have hydration with sodium chloride resuscitation,” noted Dr. Kellum, Professor in the Department of Critical Care Medicine at the University of Pittsburgh. It is important to recognize that practitioners often use the term rehydration when they actually mean volume expansion, he stressed.

WHAT FLUIDS ARE BENEFICIAL FOR VOLUME EXPANSION?

Certain fluids appear to be better suited for volume expansion than others in patients with acute kidney injury. For example, some research has suggested that there is a lower incidence of contrast-induced nephropathy (as measured with creatinine levels) in these patients when they receive sodium bicarbonate versus sodium chloride.

Several years ago, studies showed that sodium chloride resulted in a delay in urination compared with lactated Ringer’s solution, which has a very different chloride concentration. Does that difference “have anything to do with why sodium bicarbonate, which has a low chloride concentration compared with isotonic saline, may be protective for contrast and why lactated Ringer’s is associated with a faster time to urination?” Dr. Kellum asked. This question remains a mystery, he said, but the answer may also be related to how quickly a particular solution expands the intravascular space.

“For example, it is well known that saline has a much smaller effect on the intravascular space 90 minutes after fluid administration compared with starch or 5% albumin solution,” he stated. Thus, if expansion of the intravascular space is the goal of renal protection, then it seems logical that colloid solutions would be more beneficial than crystalloids. However, the only available data suggest the opposite. “So we have a lot of work to do in sorting this out,” acknowledged Dr. Kellum.

ESTIMATING FLUID RESPONSE

With regard to clinical indicators of the response to fluid resuscitation, many clinicians believe that left- and right-ventricular preload are interchangeable. But they clearly are not the same, as shown by a poor correlation between the pulmonary artery wedge pressure (PAWP) and central venous pressure (CVP). The correlation does improve with volume loading but even in that instance the correlation only barely achieves significance.

Moreover, PAWP and CVP poorly predict whether cardiac output will increase in response to fluid challenge. “This is why most of us recommend that you either provide a fluid challenge and then carefully measure the response or use a technique that allows you to estimate fluid responsiveness,” said Dr. Kellum. One technique that works reasonably well is measurement of systolic pulse pressure variation during simultaneous positive pressure ventilation. Although this technique often requires pharmacologic paralysis of the patient to eliminate the effect of spontaneous respiration, it can provide a good breath-to-breath assessment of fluid responsiveness.

High pulse pressure variation (above 13%) is associated with a high like­lihood of further improvement in cardiac output following fluid resuscitation. Lower pulse pressure variation indicates a greater chance of fluid nonresponse. Some data suggest that fluid resuscitation should be avoided in those likely to be nonresponders because it may actually cause harm, Dr. Kellum cautioned.

Regardless of if patients are in a euvolemic, hypovolemic, or hypervolemic state, their responses to fluid expansion vary. “There is a difference between preload and preload response. If you want to know how a patient will respond to a fluid challenge, usually the most effective technique is to give that fluid challenge,” he concluded. However, “pulse pressure variation seems to be a pretty useful tool to predict that response.”

—Timothy Begany

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