By Eric Walter, MD, MSc

Pulmonary and Critical Care Medicine, Northwest Permanente and Kaiser Sunnyside Medical Center, Portland, OR

Dr. Walter reports no financial relationships relevant to this field of study.

SYNOPSIS: In a meta-analysis of 23 clinical trials, passive leg raise was shown to be an excellent predictor of fluid responsiveness.

SOURCE: Cherpanath TG, Hirsch A, Geerts BF, et al. Predicting fluid responsiveness by passive leg raising: A systematic review and meta-analysis of 23 clinical trials. Crit Care Med 2016:44:981-991.

Fluid resuscitation is a fundamental procedure in critical care medicine. At first, this appears easy: If blood pressure is low, give more fluids. Time has taught clinicians that this is not such a simple decision. In some patients, more fluids are lifesaving. For others, fluid resuscitation increases morbidity and mortality. Determining who will respond to fluids is now one of the principle questions in critical care. Passive leg raise (PLR) is a simple bedside test used to assess fluid responsiveness. The patient raises both straightened legs 45 degrees and holds the pose for approximately one minute. PLR acts as a reversible fluid bolus, rapidly, yet transiently increasing preload and cardiac output.

PLR has been well studied in small clinical trials. Cherpanath et al summarized the predictive value of PLR in various clinical situations. They presented a meta-analysis of 23 studies that compared PLR to a true fluid challenge (the gold standard). Studies defined a positive response to a fluid challenge differently, but in general an increase of more than 15% in blood flow was defined as a positive response. Techniques used to measure blood flow included esophageal doppler, transthoracic echocardiography, calibrated pulse contour analysis, and bioreactance. The meta-analysis included 1,013 patients, although individual studies were small (17-102 patients). The majority of patients were septic (57%) and required vasopressor support (56%).

The pooled sensitivity for PLR was 86% (95% confidence interval [CI], 79-92%) with a specificity of 92% (95% CI, 88-96%). The area under the receiver operating curve was 0.95. PLR diagnostic performance did not differ between spontaneously and mechanically ventilated patients. Using changes in pulse pressure as a measure of fluid responsiveness was less predictive (sensitivity 58% [95% CI, 44-71%]; specificity 83% [95% CI, 68-92%]) than the use of flow variables such as cardiac output, stroke volume, or aortic blood flow. Researchers could not assess the utility of PLR in patients in atrial fibrillation because most patients were in normal sinus rhythm.

COMMENTARY

PLR works by creating an “auto transfusion” that reversibly moves an estimated 250-350 mL of blood into the chest cavity. Cardiac preload increases, and if patients are fluid responsive, an increase in cardiac output occurs. Clinicians observe the maximal effect in about one minute and it disappears when the legs are lowered. The reported sensitivity of 86% and specificity of 92% is remarkable (almost too good to be true). Furthermore, PLR is easy, requires minimal training, and has few complications. This sounds like the perfect test. So should medical professionals use PLR as part of a routine clinical exam?

Maybe, but several questions remain. First, for most of these studies the assessment of whether a patient responded to PLR required measurements of cardiac output was not easily available to many ICU clinicians. Although echocardiography is used more commonly in the ICU, few clinicians will have the expertise to measure cardiac output, let alone differentiate between a change in cardiac output of 15%. Esophageal doppler and bioreactance are not commonly available. Pulse contour analysis is more widely available but requires equipment and arterial access.

Second, what does fluid responsive really mean? This was defined as an increase in cardiac output of ≥ 15%. Is this a clinically relevant definition? The goal of fluid resuscitation is not just to improve cardiac output but to improve perfusion and ultimately patient outcomes. The lessons learned from the pulmonary artery catheter remind one that simply knowing how to measure physiologic variables does not automatically translate into better care. Researchers must conduct more studies showing that a PLR-informed fluid strategy leads to better outcomes compared to other approaches.