Value of urodynamic pressure-flow studies in evaluation of bladder outlet obstruction has been recognized. Voiding during these studies is influenced by transurethral catheter, which is used for measurement of vesical pressure. We have investigated the influence of 7 F (2.3 mm) catheter on flow rate during pressure-flow study as a potential cause of misclassification.
Patients and methods: Data of free urinary flow and pressure flow from 111 men with symptomatic benign prostatic hyperplasia were analyzed. Inclusion criteria for analysis: age over 45 years, total International Prostate Symptom Score over 8, maximum flow rate in range of 4-20 ml/s, total voided volume of 100 ml or greater.
Results: Of all patients, means of maximum free and pressure-flow rate were 9.8 and 9.0 ml/s (p=0.01) with mean voided volume 199 and 212 ml (p=0.03) respectively. Maximum flow rate decreased in 56.8%, increased in 41.4% and was stable in 1.8% of cases. The difference ranged from -8.5 to +10.2 ml/s ('+' is indicated when maximum rate of free flow is higher). In the group of obstructed subjects mean maximum flow rates were respectively 8.8 ml/s and 7.9 ml/s (p=0.01). There was no significant difference in maximum flow rate within the group of unobstructed/equivocal subjects. More pronounced mean 1.3 ml/s difference in maximum flow rate was observed also in subgroup of patients with prostate volumes over 60 cc (p=0.01).
Conclusions: Catheter of 7 F (2.3 mm) generally slightly diminishes maximum flow rate. Overdiagnosis of obstruction is more likely if considering the effect of catheter and vesical pressure. Misclassification of subject is possible in case of mild obstruction so such cases should be interpreted with caution. In the case of big difference in maximum flow rate it is necessary to take into account the free flow.