The measurement of residual urine volume by electrical impedance in man

J. C. Denniston and L. E. Baker have reported a linear relationship between the electrical impedance at 100 kHz measured between electrodes placed close to the umbilicus and on the pubis in anaesthetised dogs and the volume of urine present in the bladder. They pointed out that this simple, non-invasive, technique for the monitoring of the urinary bladder content would be of value in post-surgical patients and in paraplegics as an index of renal function and urinary statis. They did not, however, report any actual experience with the method in conscious man. For this reason, it was decided to investigate the method proposed by Denniston and Baker in regard to its applicability both in conscious patients those under a general anaesthetic. It was further decided to fill the bladder with physiological saline since this fluid is routinely used in bladder volume investigations. When the bladder was being filled, a linear decrease in impedance with bladder volume was nearly always observed. However, when the bladder was emptied in all conscious patients and one out of three anaesthetised patients, the final value of the abdominal impedance did not correspond with the measured residual volume. The present authors therefore conclude that the method proposed by Denniston and Baker cannot be recommended for the routine determination of residual urine volumes. The patients must also remain in exactly the same position; otherwise the value of impedance may be altered.