Technical note The measurement of residual urine volume by electrical impedance in man* Keywords--Residual urine volume, Electrical impedance Introduction
DENNISTON and BAKER (1974) 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 stasis. They did not, however, report any actual experience with the method in conscious man.
A knowledge of the residual urine volume is of importance in the investigation of patients suffering from disturbance of micturition and little information is available concerning normal values for residual urine volumes. ROWAN et al. (1972) have described a dilution technique using a radioactive isotope, but this method is not direct reading. The electrical-impedance approach offers the possibility of a direct, non-invasive, method. This makes it very attractive, in principle, for use in the routine clinical assessment of outpatients. For this reason, it was decided to investigate the method proposed by DENNISTON and BAKER (1974) in regard to its applicability both in conscious patients and those under a general anaesthetic. It was further decided to fill the bladder with physiolgical saline since this fluid is routinely used in bladder volume investigations.
the midline. The other was mounted on the perineum or over the third sacral vertebra. The cardiograph provided a digital display of the electrical impedance in ohms and tenths of ohms. Results
Table 1 shows the impedance changes found when the bladders of two patients had first been emptied and then filled with known volumes of saline. The male patient (S.G.T.) was sitting upright on a commode, whilst the female patient (H.M.) was lying supine on an examination couch.
Table 1
Materials and methods
The informed consent for the procedure was obtained from male and female patients attending the Incontinence Unit of the Shaftesbury Hospital. In all the patients, the bladder was catheterised with a No. 8 Charri~re catheter to facilitate the emptying and filling of the bladder with urine or sterile physiological saline. Saline could be pumped into the bladder via the catheter at a flow rate of 5 0 m l / m i n by means of a Holter type 175 peristaltic pump. A model 304 Impedance Cardiograph by Instrumentation for Medicine Inc. was used to measure the abdominal electrical impedance. The cardiograph supplied a constant current of approximately 4 mA r.m.s, to a pair of disposable aluminium band electrodes mounted on adhesive tape (Electrode Tape M6001, 3M Company). One band electrode was placed around the thorax just below the level of the xiphisternum, whilst the other consisted of two halves placed one around the top of each thigh and connected together by a length of wire with crocodile clips at each end. A signal at 100kHz proportional to the abdominal impedance was picked up from a pair of Beckman Biopotential Skin Electrodes. One of these was always mounted on the skin just below the umbilicus in
Volume in bladder
Impedance
ml 0 50 100 150 200 250 300 350 400 450 500 550 600 650
7.2 7.1 7.0 6.8 6.7 6.6 6.5 6.3 6.2 6.2 6-1 6-0 5.9 5.8
After micturition when the bladder was checked to be empty, the impedance was 6.9~ H.M. (female) Umbilicus and 3rd sacral vertebra Volume in bladder ml 0 50 100
150 200 250 300 350
Impedance 2.7 2.6 2.5 2.4 2.3 2.2 2.2 2.1
After micturition when the bladder was checked to be empty the impedance was 2 . 4 ~
"Received 20th December 1973
Medical and Biological Engineering
Patient S.G.T. (male) Electrodes Umbilicus and perineum
March 1975
307
Similar changes were obtained in eight other patients. Three of these were anaesthetised with halothane and in two out of the three when the bladder was emptied the impedance returned to the initial value found when the bladder was empty. The introduction of a metal cystoscope into the bladder lowered the impedance. With the conscious patients it was observed that, if the patient changed his or her position on the commode, the impedance changed to a new value. This also occurred if the patient laughed. In two patients a regular respiratory swing of plus or minus 0" 1 f2 was observed. The measurement of the abdominal impedance did not disturb the patients at all. Sometimes the perineal electrode became wet and then erratic readings were obtained. This problem was overcome by the use of the electrode over the third sacral vertebra. In eight out of ten adult patients, an impedance change of approximately 0" 1 ~ per 100 ml of saline was found. This is in agreement with the results of DENNISTON and BAKER (1974). AS the bladder was filled so the impedance decreased. As the cardiograph's display could only be read to the nearest 0.1 f2, the impedance change per 100 ml could not be found more accurately. One female patient (M.W.) did not produce a significant change in impedance when the bladder was filled with saline. For umbilical and sacral electrodes the impedance was only 1' 4 ~2 with the bladder empty, 1" 4 f~ with 150 ml in the bladder and 1.5 f~ with the bladder containing 300 ml. Another female patient, who was obese, had an impedance of 19'7 f~ measured between umbilical and perineal electrodes.
Discussion 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. Thus, unfortunately, the method proposed by DENNISTON and
308
BAKERcannot be recommended for the routine determination of residual urine volumes. The patients must also remain in exactly the same position; otherwise the value of the impedance may be altered. In the anaesthetised dog, DENNISTONand BAKERfound resting values of the abdominal impedance in the range 27-31 ~2. In adult male and female patients, our values were generally much lower, typically 6-13 f2 for electrodes in the umbilical and perineal positions and 1' 3 to 2.7 f2 for the umbilical and sacral positions. These values are too low for the accuracy of reading obtainable with the I F M impedance cardiograph which is designed to work with thoracic impedances of the order of 20-30 fL A greater accuracy could be obtained by the use of a digital voltmeter having a four-significant-figure display, but there seems little point in this when the method does not reliably measure the residual urine volumes.
Acknowledgment--The I F M impedance cardiograph was provided by means of a grant from the Vandervell Foundation.
References ROWAN, D., MILLAR, W. T. and ALEXANDER,S. (1972) A n investigation of bladder-function by pressure-flow studies Bio-Med. Eng. 7. 304-308. DENNISTON, J. C. and BAKER, t . E. (1975) Measurement of urinary bladder emptying using electrical impedance Med. Biol. Eng. 13, 305-306. P. T. DOYLE
Institute of Urology University of London The Shaftesbury Hospital London WC2, England D. W. HILL
Research Department of Anaesthetics Royal College of Surgeons of England and Institute of Basic Medical Sciences London WC2, England
Medical and Biological Engineering
March 1975
DENNISTON and BAKER (1974) 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 stasis. They did not, however, report any actual experience with the method in conscious man.
A knowledge of the residual urine volume is of importance in the investigation of patients suffering from disturbance of micturition and little information is available concerning normal values for residual urine volumes. ROWAN et al. (1972) have described a dilution technique using a radioactive isotope, but this method is not direct reading. The electrical-impedance approach offers the possibility of a direct, non-invasive, method. This makes it very attractive, in principle, for use in the routine clinical assessment of outpatients. For this reason, it was decided to investigate the method proposed by DENNISTON and BAKER (1974) in regard to its applicability both in conscious patients and those under a general anaesthetic. It was further decided to fill the bladder with physiolgical saline since this fluid is routinely used in bladder volume investigations.
the midline. The other was mounted on the perineum or over the third sacral vertebra. The cardiograph provided a digital display of the electrical impedance in ohms and tenths of ohms. Results
Table 1 shows the impedance changes found when the bladders of two patients had first been emptied and then filled with known volumes of saline. The male patient (S.G.T.) was sitting upright on a commode, whilst the female patient (H.M.) was lying supine on an examination couch.
Table 1
Materials and methods
The informed consent for the procedure was obtained from male and female patients attending the Incontinence Unit of the Shaftesbury Hospital. In all the patients, the bladder was catheterised with a No. 8 Charri~re catheter to facilitate the emptying and filling of the bladder with urine or sterile physiological saline. Saline could be pumped into the bladder via the catheter at a flow rate of 5 0 m l / m i n by means of a Holter type 175 peristaltic pump. A model 304 Impedance Cardiograph by Instrumentation for Medicine Inc. was used to measure the abdominal electrical impedance. The cardiograph supplied a constant current of approximately 4 mA r.m.s, to a pair of disposable aluminium band electrodes mounted on adhesive tape (Electrode Tape M6001, 3M Company). One band electrode was placed around the thorax just below the level of the xiphisternum, whilst the other consisted of two halves placed one around the top of each thigh and connected together by a length of wire with crocodile clips at each end. A signal at 100kHz proportional to the abdominal impedance was picked up from a pair of Beckman Biopotential Skin Electrodes. One of these was always mounted on the skin just below the umbilicus in
Volume in bladder
Impedance
ml 0 50 100 150 200 250 300 350 400 450 500 550 600 650
7.2 7.1 7.0 6.8 6.7 6.6 6.5 6.3 6.2 6.2 6-1 6-0 5.9 5.8
After micturition when the bladder was checked to be empty, the impedance was 6.9~ H.M. (female) Umbilicus and 3rd sacral vertebra Volume in bladder ml 0 50 100
150 200 250 300 350
Impedance 2.7 2.6 2.5 2.4 2.3 2.2 2.2 2.1
After micturition when the bladder was checked to be empty the impedance was 2 . 4 ~
"Received 20th December 1973
Medical and Biological Engineering
Patient S.G.T. (male) Electrodes Umbilicus and perineum
March 1975
307
Similar changes were obtained in eight other patients. Three of these were anaesthetised with halothane and in two out of the three when the bladder was emptied the impedance returned to the initial value found when the bladder was empty. The introduction of a metal cystoscope into the bladder lowered the impedance. With the conscious patients it was observed that, if the patient changed his or her position on the commode, the impedance changed to a new value. This also occurred if the patient laughed. In two patients a regular respiratory swing of plus or minus 0" 1 f2 was observed. The measurement of the abdominal impedance did not disturb the patients at all. Sometimes the perineal electrode became wet and then erratic readings were obtained. This problem was overcome by the use of the electrode over the third sacral vertebra. In eight out of ten adult patients, an impedance change of approximately 0" 1 ~ per 100 ml of saline was found. This is in agreement with the results of DENNISTON and BAKER (1974). AS the bladder was filled so the impedance decreased. As the cardiograph's display could only be read to the nearest 0.1 f2, the impedance change per 100 ml could not be found more accurately. One female patient (M.W.) did not produce a significant change in impedance when the bladder was filled with saline. For umbilical and sacral electrodes the impedance was only 1' 4 ~2 with the bladder empty, 1" 4 f~ with 150 ml in the bladder and 1.5 f~ with the bladder containing 300 ml. Another female patient, who was obese, had an impedance of 19'7 f~ measured between umbilical and perineal electrodes.
Discussion 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. Thus, unfortunately, the method proposed by DENNISTON and
308
BAKERcannot be recommended for the routine determination of residual urine volumes. The patients must also remain in exactly the same position; otherwise the value of the impedance may be altered. In the anaesthetised dog, DENNISTONand BAKERfound resting values of the abdominal impedance in the range 27-31 ~2. In adult male and female patients, our values were generally much lower, typically 6-13 f2 for electrodes in the umbilical and perineal positions and 1' 3 to 2.7 f2 for the umbilical and sacral positions. These values are too low for the accuracy of reading obtainable with the I F M impedance cardiograph which is designed to work with thoracic impedances of the order of 20-30 fL A greater accuracy could be obtained by the use of a digital voltmeter having a four-significant-figure display, but there seems little point in this when the method does not reliably measure the residual urine volumes.
Acknowledgment--The I F M impedance cardiograph was provided by means of a grant from the Vandervell Foundation.
References ROWAN, D., MILLAR, W. T. and ALEXANDER,S. (1972) A n investigation of bladder-function by pressure-flow studies Bio-Med. Eng. 7. 304-308. DENNISTON, J. C. and BAKER, t . E. (1975) Measurement of urinary bladder emptying using electrical impedance Med. Biol. Eng. 13, 305-306. P. T. DOYLE
Institute of Urology University of London The Shaftesbury Hospital London WC2, England D. W. HILL
Research Department of Anaesthetics Royal College of Surgeons of England and Institute of Basic Medical Sciences London WC2, England
Medical and Biological Engineering
March 1975
