The pressure within a collapsed tube, with special reference to urethral pressure.

A definition is proposed for tube pressure (the pressure within a collapsible tube in the collapsed state, e.g. the urethral pressure or the pressure in the anal or oesophageal sphincter) which conforms to the common sense view that it is a measure of the ability of the tube to prevent leakage. It is not dependent on the orientation of a measuring probe in the tube. The orientation effects which have previously been reported are due to attempted bending of a stiff measuring catheter by internally and externally applied forces. A satisfactory way of measuring the tube pressure, by slow perfusion through a thin, flexible catheter with an end hole, and of testing whether it is a meaningful concept in a given case, is proposed. It is illustrated by measurements of urethral pressure in 42 continent and incontinent females. No cases of urinary incontinence due to poor urethral sealing ability are observed, indicating that tube pressure is a meaningful concept. The theory of viscosity-limited flow through a compressive zone in a collapsible tube, on which the perfusion method of measuring tube pressure is based, is developed.