Purpose Several studies have suggested that clinical indicators of a patient's fluid status that are frequently used in practice are highly variable and not useful in the differentiation of these patients. In this study, we examine the potential use of ultrasound measurements of soft tissue and interstitial fluid thickness for the detection of conditions of mild to moderate dehydration. Materials and methods This study involved healthy subjects in which a mild dehydration was a consequence of tilt bed rest (average loss of plasma volume of *10%). The soft tissue and interstitial thickness superficial to the calcaneous and immediately below the lateral malleolus were measured in the supine position using high frequency ultrasound both before and after the treatment. Also the echocardiographic measurements of orthostatic induced changes in stroke volume were obtained. Results In the eight healthy subjects studied, there was an average of a 13% diminution in both the ankle and pretibial soft tissue and interstitial thickness after tilt bed rest (p = 0.05) that coincided with mildly decreased orthostatic induced changes in stroke volume. Conclusions Traditional tilt testing has been a less than satisfactory technique for determining the hydration status of patients. This study looks at the possibility of using ultrasound as a method for the objective measurement of tissue dehydration. The method was able to detect significant changes in tissue interstitial fluid under conditions of mild dehydration. Since the technique obtains measurements in the supine position, it also obviates creating orthostatic symptoms in restrictive clinical environments.

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