Minute-to-Minute Urine Flow Rate Variability: A New Renal Physiology Variable

BACKGROUND: Urine output is a surrogate for tissue perfusion and is typically measured at 1-hour intervals. Because small urine volumes are difficult to measure in urine collection bags, considerable over- or underestimation is common. To overcome these shortcomings, digital urine meters were developed. Because these monitors measure urine volume in 1-minute intervals, they provide minute-to-minute measurements of the urine flow rate (UFR). In a previous study, we observed that the minute-to-minute variability in the UFR disappeared during hypovolemia. The aim of this study was to describe the minute-to-minute variability in the UFR as a new physiological variable and to show its relationship to blood volume depletion. METHODS: Seven adult pigs were used in this study. The UFR, minute-to-minute UFR, mean arterial blood pressure, heart rate, and base excess were measured at euvolemia and during gradual hemorrhaging (10%, 20%, and 30% of estimated blood volume). Variance and wavelet spectral analysis were used to measure the disappearance of the minute-to-minute UFR variability. RESULTS: The UFR decreased from 2.2 +/- 0.2 to 1.0 +/- 0.1 mL/min after a 10% estimated blood volume loss (+/- 1 SE, n = 7, P = 0.0348). The variance in the minute-to-minute UFR decreased from 1.4 +/- 0.3 to 0.4 +/- 0.1 mL/min (+/- 1 SE, n = 7, P = 0.046). CONCLUSIONS: The UFR and its minute-to-minute variability decrease during hemorrhaging. The variability in the UFR may be useful as an aid for the diagnosis of hypovolemia. (Anesth Analg 2012;115:843-7)