Historically, the first urodynamic parameter measured to characterize stress incontinence was the resting urethral pressure profile (UPP), which was based on the idea that continence is maintained as long as urethral pressure exceeds intravesical pressure. It was thought that measuring the pressure exerted by the urethra in relation to the intravesical pressure would predict both presence and severity of stress incontinence.
Although this is conceptually reasonable, the pressure measurements of both symptomatic incontinent patients and controls overlap so widely that no conclusions can be made by the absolute measurements. Furthermore, the measured values vary widely depending on the study conditions. Despite numerous efforts to derive additional urodynamic parameters based on UPP measurements, none of these have been demonstrated to correlate reliably with the presence of stress incontinence. In the following sections, we outline the components of urethral pressure profilometry and discuss the techniques and limitations of these measurements.
Relationship Between UPP and Urethral Anatomy
Multiple studies of the structural and functional composition of the female urethra have been performed. Both the intrinsic urethral components and surrounding structures have been identified as contributors to urethral continence. The current understanding of urethral function is complex but essentially reflects a combination of sphincteric components and structural support of the bladder neck and urethra.
Components of urethral closure have been described by Rud et al. in a systematic study of the effects of muscular and vascular contributions to UPP (5). The conclusion of this work was that approximately one-third of the urethral closure is dependent on striated sphincter and pelvic floor musculature, one-third is caused by blood flow, and the final one-third is related to periurethral and connective tissues. Ultimately, the exact anatomy and function of the female urethra has yet to be fully elucidated, and measurements of urethral pressure must be considered a summation of a number of complex interactions.
UPP and Urethral Closure Pressure Profile
The UPP is the direct measurement of urethral pressure at rest. Urethral pressure measurement was first described by Bonney in 1923 as the pressure required to infuse fluid retrograde into the urethra as measured by an external manometer (6). Although the measurement techniques have evolved significantly from fluid perfusion catheters to membrane, microtip, and fiber-optic catheters, the underlying concept remains unchanged; the static urethral pressure is measured throughout its length, and a topographical pressure curve is generated. Several parameters can be measured from this curve; the International Continence Society has recommended a standard nomenclature (7). Maximum urethral closure pressure (MUCP) is the difference of maximum urethral pressure (MUP) and bladder pressure (MUCP = MUP – Pves) and correlates to the junction of the striated and smooth muscle sphincters. Functional profile length is the total length at which urethral pressure exceeds bladder pressure and represents the total length of urethra that contributes to continence.
The urethral closure pressure profile (UCPP) involves the same techniques of urethral pressure measurement but further delineates intra-abdominal and detrusor pressures. A total of six measurements is required: perineal electromyography, intravesical pressure Pves, intra-abdominal pressure Pabd, intraurethral pressure Pura, true detrusor pressure Pdet, and urethral closure pressure (UCP). Many of the parameters measured in the UCPP have been studied regarding both prediction of stress incontinence and the response to surgical intervention, without reliable results. Whether measuring MUCP or total functional length, the overlap of continent and incontinent subjects precludes using these parameters to stratify patients in the workup for stress incontinence. The only parameter that has been shown to correlate with operative result is an MUCP less than 20 cm H2O, which is considered to be indicative of intrinsic sphincter deficiency.
Weber reviewed the published literature on UPP and female SUI in 2001 and concluded that, although the terms of UPP are consistently defined, the techniques used to measure UPP are not standardized, and results are therefore variable. In addition, patients with an MUCP less than 20 cm H2O have been shown to have higher failure rates after suspension procedures, but as Weber pointed out, these are based on retrospective studies only (8).
Dynamic UPP: Cough and Valsalva
Similar to the VLPP, dynamic UCPPs are an attempt to measure the urethral response to stress. This is thought to more appropriately characterize the presence and severity of stress incontinence than the static measurement of resting UPP and MUCP.
There are two types of dynamic UPP: Valsalva and cough. Valsalva UPP is measured by manually withdrawing the urethral pressure transducing catheter while the patient sustains maximum Valsalva effort. Cough UPP is measured with the patient coughing every 2–3 s while the catheter is manually withdrawn. During both tests, the urethral meatus is monitored for leakage. The pressure is measured in the same way as static UPPs are obtained.
Pressure Transmission Ratio
More recently, the pressure transmission ratio (PTR) has been described (9). In this measurement, the functional urethral length is divided into quarters, and the PTR is calculated for each quarter using the equation (∆ Pura / ∆ Pves) X 100%. Lower PTRs are consistent with SUI, although significant overlap exists between continent and incontinent patients, and no set cutoff has been determined.
Techniques of Profile Measurement
Whether measuring resting UPP, UCP, or dynamic UPP, the study conditions are similar. Patients are typically studied while seated with a full bladder in a urodynamic chair. A specialized catheter (most commonly a microtip catheter) is inserted intravesically with a transducer measuring the intravesical pressure and an additional transducer located 6 cm more proximally measuring the urethral pressure. An additional pressure probe is inserted either intravaginally or rectally to measure Pabd, and external electrodes are applied to record electromyographic activity. Similar to VLPP measurement, there are many variables that affect the results of a UPP study. The following discussion is a summary of the important variables.
UPP was first described using perfusion catheters in which the pressure measured reflected the pressure required to lift the urethral wall from the catheter side holes.
Membrane catheters function by the same mechanism, but the catheter tip is surrounded by a balloon or membrane to prevent loss of infusion fluid during measurement, a shortcoming that was identified in the perfusion catheters. Microtip catheters are the most commonly used at present, and pressure is measured by a pressure-sensitive transducer that allows for dynamic pressure measurement. Finally, fiber-optic catheters similarly measure the urethral pressure in a direct manner. As described in the evaluation of LPPs, the catheter size has been shown to affect the results of UPP measurements, with largerbore catheters resulting in higher UPP pressure measurements.
The UPP can be measured with the patient in the supine, seated, or standing position.
At present, the only conclusion obtained from many studies comparing UPP measurements in different patient positions is that position affects the results of the UPP in some patients but not in a predictable or reproducible manner. From supine to seated to standing measurements, some studies have reported increasing, decreasing, or no change, respectively, in UPP measurement (8).
CATHETER WITHDRAWAL SPEED AND ORIENTATION
Commonly, the withdrawal speed used is 1–2 mm/s. Orientation of the catheter transducer can change the results of the test. With the transducer at the 12 o’clock position, the pressure recorded will be elevated, whereas at the 6 o’clock position it will be decreased. Therefore, the UPP is measured with the transducer oriented at a lateral position (6 or 9 o’clock).
ZEROING OF PRESSURE SENSORS
As with LPP measurement, the pressure recorded will depend on whether the transducer is zeroed to the patient or to the atmosphere. It is important to consider the starting point for accurate interpretation.
Interpreting UPP measurements for stress-incontinent patients is problematic. Lack of standardization in technique and the number of measurement variables contribute to the significant overlap of measurements in stress-incontinent patients and controls.
Stratification of severity and type of incontinence has only been described for the low-pressure urethra with a MUCP less than 20 cm H2O, which has been related to type III or intrinsic sphincter deficiency. At present, despite wide usage and much effort to describe derivations of urethral pressure profilometry, it is not a standardized, reproducible, or predictive test of SUI. The main utility of this test may be to follow individual patient outcomes throughout treatment.
Kelly M. Maxwell, MD, and J. Quentin Clemens, MD, MSCI