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

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Urine output is a time-honored measure of the patient’s effective blood volume (EBV) and a surrogate for tissue perfusion. Urine output is typically measured at one-hour intervals and expressed in milliliters per hour (ml/h). Because small volumes are difficult to measure, initial information becomes available only 20-30 minutes after catheter insertion by extrapolating to one full hour. This extrapolation can result in considerable over- or underestimation.

Lately digital urine-meters were developed as a nursing tool to reduce urine output reading errors reaching 30%. 1 During the performance validation process of one model (URINFOTM, FlowSense Medical, Misgav, Israel) we realized that since the monitor measures urine volume in one-minute increments, it actually provides real-time continuous minute-to-minute urine flow measurement.

In a previous study, we evaluated 1-minute-urine urine flow rate (UFR) change in response to hemorrhage.2 During the study we noticed that during euvolemic conditions UFR is not constant i.e. there is minute-to-minute variability. Even more striking was the observation that this variability disappears as hypovolemia ensues.2 (Fig. 1). The aim of this study was to describe this new physiological phenomenon (UFR’s minute-to-minute variability) and its relation to EBV depletion.

Methods

The Hebrew University Animal Care and Use Committee approved the protocol (MD-07-10924-2), and each study was supervised by a veterinary surgeon. Each trial was conducted following a week of acclimation under veterinary care.

Seven adult female pigs, weighing 50±10 kg, were fasted overnight prior to protocol initiation with unlimited water access. Induction of anesthesia was achieved with intramuscular injection ...

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