LAUSR

Dear Editor, We are grateful to Dr. Peter Rosier for his comments. We would like to start clarifying that we defined detrusor contraction (DET‐cont) as the increase in detrusor pressure at maximum flow rate (pdetQmax) ≥10 cmH2O above baseline. Using this definition, 22.6% of women voided without DET‐cont. This somewhat arbitrary definition in no way means that those women really urinated without a detrusor contraction, but only that they did not have an increase of their detrusor pressure ≥10 cmH2O above baseline at maximum flow rate (Qmax). For example, how could we say that a woman who presented a 9 cmH2O rise above baseline in detrusor pressure at Qmax plus any recorded urinary flow rate voided without detrusor contraction? By no means. This concept was indirectly affirmed using the word “measurable” detrusor contraction throughout the article, and we think was thus understood by the Reviewers and the Editor of the journal. This is why to our understanding and perhaps for most of these experts' journal readers, Dr Rosier's complaint only seems to be a misinterpretation. However, Dr. Rosier's interest to emphasize the importance of knowing about the Hill equation and the bladder output relation (BOR) to properly evaluate detrusor contraction strength is very valuable. As one of us has summarized in a previous review article, the detrusor muscle follows the Hill equation, which is an essential relation between the force generated by a contracting muscle and its shortening velocity. For a given muscle length and degree of activation, a shortening speed of zero will produce that force attains its isometric value. As the speed of shortening increases, the force falls and reaches zero at a maximum shortening velocity typical of that muscle. For a hollow organ like the bladder, the Hill equation can be converted to BOR, relating detrusor pressure to urinary flow rate. For a determined detrusor contraction strength, if there is no bladder outlet obstruction (BOO), the pressure required to propel urine through the urethra is low meanwhile the flow rate is high. On the contrary, if there is BOO, the pressure needed is high and the flow rate is low. If BOO develops gradually, voiding conditions change progressively from low pressure and high flow to high pressure and low flow, with the same detrusor contraction strength. Considering this, we recognize that Dr. Rosier's analogy of biking uphill (high bladder outlet resistance) and downhill (low bladder outlet resistance) is very good. Nevertheless, we need to clarify that “a woman voiding with a pdetQmax of 2 cmH2O and a Qmax of 30ml/s” and “a man with a (grade 4 linPURR) bladder outflow obstruction: a pdetQmax of 102 cmH2O and a Qmax of 10ml/s” certainly do not have the same detrusor contraction strength, as Mr. Rosier has affirmed. One way to actually measure detrusor contraction strength is using the BOR curve to obtain the projected isovolumetric pressure. By a mechanic stop test during voiding (in men by a penile compression and in women by pulling a balloon catheter against the bladder neck) the urinary flow rate is brought to zero, increasing the detrusor pressure to its isovolumetric value. The bladder contractility index (pdetQmax+ 5 Qmax) is derived from the “contractility” groups described by Prof. Schäfer in men (which are simplified BOR curves using straight lines with a fixed slope of 5 cmH2O/ml per second). 5,6 This parameter tend to greatly overestimate isovolumetric pressures in women. A more reliable estimate of detrusor contraction strength in women has been called projected isovolumetric pressure 1 (PIP1), and is based on the following formula: PIP1 = pdetQmax+Qmax. 7 In the cases presented by Dr. Rosier, the women had a much lower projected isovolumetric pressure (32 cmH2O) than the man (152 cmH2O). Finally, we must again disagree with Dr. Rosier when he state that “voiding without detrusor contraction is however impossible.” One example would be a woman who voids without any increase in detrusor pressure, that uses abdominal straining to urinate and whose “stop test” (to measure her isovolumetric detrusor pressure) does not show any increase in detrusor pressure.