Ultrasound: A Valuable Translational Tool to Measure Postvoid Residual in Awake Rats?

BACKGROUND: Ultrasound is generally used to measure postvoid residual (PVR) in daily clinical practice for a basic assessment of voiding dysfunction. In animal research, however, PVR is measured mostly by expelling the urine with gentle squeezing of the bladder. OBJECTIVE: To assess the translational value of measuring PVR by ultrasound in awake rats with the aim of obtaining directly comparable data sets in patients and rodent models. DESIGN, SETTING, AND PARTICIPANTS: A prospective animal study was conducted in 10 rats with large, incomplete thoracic spinal cord injury resulting in severe bladder impairment. Lower urinary tract function was assessed by urodynamics with implanted bladder catheter and external urethral sphincter electrodes, allowing for repeated measurements over time. Immediately after the last micturition cycle in the urodynamic investigation, PVR was first assessed by ultrasound using a 7.5 MHz linear probe and then by manually expelling the urine via gentle pressure on the abdomen. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: PVR was measured by ultrasound and by manually expelling the urine. Paired t test was used to analyze the difference between the two measurements 1 and 2 wk after spinal cord injury. RESULTS AND LIMITATIONS: PVR assessed by ultrasound was equal to and not statistically different from the volumes obtained by manual expulsion in intact rats, both before injury and during the first 2 wk after spinal cord injury (intact: 0.16 ± 0.07 vs 0.14 ± 0.09 ml, p = 0.08; week 1: 1.67 ± 0.53 vs 1.71 ± 0.55 ml, p = 0.67; week 2: 1.16 ± 0.35 vs 0.98 ± 0.43 ml, p = 0.11). The main limitation of ultrasound for measuring PVR is the restricted availability of ultrasound machines in animal research laboratories. CONCLUSIONS: Ultrasound is a valuable translational tool to measure PVR in awake rats reflecting the situation in humans. PATIENT SUMMARY: We measured postvoid residual by ultrasound in awake rats, analogous to clinical examination in humans. Ultrasonography provided similar values to the generally used manual bladder expulsion.