Effect of urethral compliance on the steady state p-Q relationships assessed with a mechanical analog of the male lower urinary tract

Abstract

Aims: Using a lumped parameter theoretical model of bladder outlet function, we previously explored the relationship between the tube law of the urethra and the pressure-flow characteristics during micturition [Mijailovich et al., 2004]. To validate this theoretical model, we constructed a mechanical analog of the male lower urinary tract that incorporated elements simulating all relevant parameters of the theoretical model. In addition, we determined the effect of alterations in compliance of the flow-controlling zone on these relationships.

Methods: In the mechanical analog, the bladder neck and urethra were represented by a thin-walled conduit made of latex rubber and the prostate with a fluid filled cuff of variable compliance encircling the conduit. We measured in the same system steady-state pressure-flow (p-Q) and pressure-area (p-A) relationships of the flow-controlling zone. The effects of bladder outlet obstruction and prostatic compliance on these relationships were simulated by varying cuff pressure and capacitative coupling of the cuff, respectively.

Results: We demonstrated two previously described flow regimes-critical for low Q, and subcritical for higher Q. In the critical flow regime, the cross-sectional area of the collapsible conduit downstream of the cuff became narrow up to a site where the area suddenly expanded (elastic jump). Pressure losses across the cuff region decreased with increasing Q as the elastic jump approached the cuff, and the jump vanished when Q became subcritical. By altering prostatic cuff compliance and cuff pressure, we showed that an increase in opening pressures was associated with a steeper p-Q relationship.

Conclusions: Using this mechanical analog, the measurements of p-A and p-Q relationships under various experimental conduit conditions validated our previous theoretical model of the male lower urinary tract. Both the experimental measurements and our previous theoretical model predictions indicate that an increase in opening pressure is associated with an increase in the slope of the p-Q relationship which becomes steeper with decreased cuff compliance. These data are in accordance with urodynamic findings in patients with benign prostatic hypertrophy (BPH) and suggest that a reduction in prostatic compliance exacerbates the severity of obstruction.