Are conventional pressure‐flow measurements dependent upon filled volume?

OBJECTIVE To determine, in a prospective study, whether detrusor pressure (pdet.Qmax) and maximum urinary flow rate (Qmax) measurements obtained after filling to maximum cystometric capacity (MCC) differ from those obtained with filling restricted to average voided volume (Vvoid), as standard protocols for pressure flow studies (PFS) mandate bladder filling until the subject has a strong desire to void, which aids standardization but further divorces the test from real-life experience. PATIENTS AND METHODS After calculating the appropriate sample size, 84 patients attending for PFS with an adequately completed 3-day frequency-volume chart were recruited. Each underwent two consecutive PFS with filling to MCC and average Vvoid in a random order, and measurements of pdet.Qmax and Qmax were compared. For men, the agreement for a diagnosis of obstruction between the tests was also assessed. RESULTS Complete data were obtained from 76 (90%) of the patients, with a mean (range) age of 64 (20–94) years. The mean (sd) difference between MCC and average Vvoid was 134 (113) mL (P < 0.01). There were no significant differences between estimates of Qmax, at − 0.1 (3) mL/s (P = 0.75), and of pdet.Qmax, at − 1 (13) cmH2O (P = 0.91), obtained within each patient. For men there was 91% agreement (32 of 35) in the classification of obstruction. CONCLUSIONS Restriction of filling to the average Vvoid during PFS allows a closer approximation to normal voiding and results in no clinically relevant change to the value of standard pressure-flow measurements or alters individual classification of obstruction.