Urodynamic measure of urethral cross-sectional area: application for obstructive uropathy

A conductance formula was developed based on pressure (P) and flow (Q) data collected from rigid tubes with a cross-sectional area in the range of the flow controlling zone of the urethra. Curve fitting of data resulted in an exponential formula and a simple proportionality constant related tube of differing cross-sectional area. The resulting formula estimates cross-sectional area based on P and Q data and is in the form of a conductance ratio because Q is in the numerator and P is in the denominator. The formula is AEFm = 3.5 Q/P0.58, where AEFm is the area equivalent factor-male and Q is measured in ml/sec and P is cm H2O. To further introduce the estimate of cross-sectional area, urodynamics were retrospectively reviewed from 19 males over the age of 50 with complaints of prostatism. The AEFm was plotted on standard urodynamic records much like P and Q data. The onset of voiding was characterized by a rapid increase in urethral area. Maximal cross-sectional area obtained at maximal flow was 2.9 +/- 1.4 mm2. The maximal urethral area per unit of applied detrusor pressure, an estimate of urethral compliance, was 0.06 +/- 0.6 mm2/cm H2O. This latter measure may help to compare patients with widely different detrusor pressures. The urodynamic data from two young adult males is also presented for comparison. Their average maximal urethral cross-sectional area was 7.1 mm2. Their average area per unit of applied detrusor pressure was 0.15 mm2/cm H2O. These larger values contrast sharply with the BPH group. The AEFm clearly shows urethral function in terms of the principal factor regulating the urine flow rate, the urethral cross-sectional area.