2 MATHEMATICAL-MODELS EXPLAIN THE VARIATION IN CYSTOMETROGRAMS OF OBSTRUCTED URINARY BLADDERS

Overdistension of the urinary bladder, secondary to outlet obstruction , causes cellular changes in the bladder wall, including hypertrophy o f the smooth muscle cells, which increase bladder mass. To investigate the effects of increased mass on the cystometrogram (CMG), we have de veloped two mathematical models. In the first model, we assume that ma ss is added such that the largest bladder volume at zero transmural pr essure, the zero pressure volume (ZPV), is constant. It predicts incre ased pressures and decreased compliance in the CMG. In the second mode l, we assume that both mass and ZPV increase proportionally. It predic ts unchanged pressures, increased compliance, and increased capacity i n the CMG. These results allow us to divide animal experiments in the literature into two groups. Cystometrograms performed on animals that have had outlet obstruction induced by a cuff method, inducing a small increase in mass, belong to the first group: hypertrophy with no chan ge in ZPV. Cystometrograms performed on animals that have had outlet o bstruction induced by a ligature method, inducing a large increase in mass, belong to the second group: hypertrophy with increased ZPV. We c onclude that increased ZPV results from a more severe obstruction whic h is indicated by the increased capacity and compliance. Copyright (C) 1996 Elsevier Science Ltd.