After the above mentioned catheter is obtained, the patient is prepped and draped in normal sterile fashion and the catheter is inserted. Typically, plain water-based lubrication can be utilized for insertion. In certain circumstances, such as patients with urethral/pelvic pain, significant anxiety, or a history of difficult catheterization, 5–10 mL of 2 % lidocaine jelly may be injected into the urethra and allowed to anesthetize the urethra for up to 5 min prior to catheter insertion. After placement, any residual urine within the bladder is then allowed to drain until the bladder is empty. The catheter is then secured by paper tape placed on the dorsal aspect of the penis and wrapped around the catheter in men and on the medial thigh in women. In patients with catheterizable channels, the transurethral catheter is placed through the channel and taped to the abdominal wall. The pressure transducer is then taped to the patient’s side at the level of the superior edge of the pubic symphysis, even with the height of the bladder. The atmospheric cap on the opposite end of the transducer is removed then opening the transducer to the atmosphere. Using a three-way stopcock, the transducer is flushed and zeroed using the computer software (Fig. 2). The atmospheric cap is then replaced. The stopcock is then rotated and the connection tubing between the transducer and catheter is flushed to remove any air bubbles. The tubing is then connected to the catheter. The baseline intravesical pressure is assessed to confirm appropriate placement (typically 0–5 cm H₂O). A cough is used to confirm good signal transmission.

Regardless of the type of catheter placed, zeroing of the urethral catheter to the atmosphere prior to initiation of any study with any type of catheter is paramount and strongly recommended in the ICS guidelines. More recently, there has been increasing difficulty in identifying the upper border of the pubic symphysis with the ongoing obesity epidemic occurring worldwide. In light of this, many urodynamics labs have begun to zero the pressure transducer once within the patient’s bladder. This is strongly opposed by the ICS. This does not allow for a resting intravesical and abdominal pressure to be obtained, which can be variable in patients. Further, intracorporal zeroing may cause the abdominal pressure to become zero during the voiding phase as the pelvic floor relaxes, which can skew the results of the detrusor pressure [7].

After placement of the urethral catheter, the rectal catheter can then be placed. The catheter is well lubricated, inserted into the anus, and advanced ~4 cm. The abdominal pressure transducer is part of a multi-transducer apparatus, along with the intravesical transducer, and thus has already been placed at the time of urethral catheter placement. As with the urethral catheter, appropriate positioning of the transducer at the level of the superior edge of the pubic symphysis is important to obtaining an accurate reading. The abdominal pressure transducer is then flushed and zeroed in the same manner as the intravesical pressure transducer. As has been previously stated, zeroing the catheter to the atmosphere and not the intracorporeal baseline pressure is critical to accurate measurements. The connection tubing is then flushed to remove any air bubbles and connected to the catheter. The balloon of the rectal catheter is then inflated until a good pressure waveform is present on the urodynamics computer software, with a maximum of 2 mL of sterile saline being instilled. The balloon ensures that the catheter opening remains unobstructed by surrounding feces, which can affect the pressure sensation. Overfilling of the balloon can lead to elastic distention of the balloon surrounding the pressure transducer and thus result in a falsely elevated baseline pressure measurement. The baseline intravesical pressure is assessed to confirm appropriate placement (typically 0–5 cm H₂O). A cough or Valsalva can be used to confirm good signal transmission. The catheter is secured in placed using paper tape on the posteromedial aspect of the right thigh.

Numerous types of rectal catheters are commercially available, similar to the options for ureteral catheters. Catheters of various lumens (1–3), sizes (from 4.5 to 21 Fr), and lengths can be purchased. At our institution, we manufacture our own rectal catheters for urodynamic testing (Fig. 3). These are produced by first cutting the fingers off of a silicone glove. These silicone pieces are placed over the cut end of 14 Fr IV tubing to act as a balloon. A 12 Fr Foley catheter is then cut into cross-sectional rings, and using a hemostat, these rings are placed over the balloon and advanced 2–3 cm from the cut end of the IV tubing. This provides a watertight seal to the balloon. This catheter is attached to a three-way stopcock which can be connected to the pressure transducer. These institution-produced rectal catheters have proven to be effective both with regards to pressure measurements and cost (estimated cost of each assembled catheter is $1).

Using rectal pressure measurements to estimate intraabdominal pressure has long been considered the gold standard [12]. The rectum is an optimal point of measurement given its low baseline pressures (4 cm H₂O) and good wall compliance, thus minimizing any rectal generated pressures from being transmitted [13]. More recently, intra-vaginal pressure measurements have also been investigated as an alternative given the greater comfort of a vaginal catheter in women. Results have been largely supportive of accurate pressure transmission [14, 15] with the exception of one study which found a statistically significant difference in the vaginal and rectal pressures in specific situations (changes in positioning and filling) [16]. While significant, the mean pressure difference was only 3.0–3.33 cm H₂O, and thus the clinical applicability of an intra-vaginal means of pressure measurement seems reasonable. The International Continence Society continues to recommend a rectal balloon catheter in the measurement of the abdominal pressure.

The detrusor pressure (Pdet) is a calculated value based upon the pressures sensed from the intravesical (Pves) and rectal (Pabd) catheters. The concept of detrusor pressure stems from the fact that the intravesical pressure is a combination of the pressure generated by the detrusor muscle of the bladder/bladder wall, as well as transmitted abdominal pressure created by abdominal musculature. Pressure generated by the detrusor muscle or bladder wall can be isolated by subtracting the abdominal pressure as measured in a different location, in this case the rectum: Pdet = Pves − Pabd.

During filling, a number of provocative maneuvers can be performed to expose an abnormality. The most common maneuver utilized is a cough, which is used to elicit stress urinary incontinence (SUI). Diagnosis of stress urinary incontinence by means of symptomology and an office cough stress test has been shown to have poor sensitivity [42]. Formal cystometry has demonstrated both good sensitivity (81–91 %) and specificity (80–100 %) for SUI [43–45], though the evidence of an abnormal urethral pressure profile in not often present [45, 46]. Another often used maneuver is the Valsalva technique, again in an attempt to induce SUI. This will be further explored later in the leak point pressure section. Handwashing [47, 48] and heel bouncing [48, 49] has also been well documented as effective aggravating factors in the evaluation of detrusor overactivity.

Patient positioning is another important consideration, particularly in the evaluation of detrusor overactivity (DO). It is well documented that DO is more commonly elicited with standing [50–53] and sitting [49, 54], compared to a supine position. Ramsden et al. refuted these findings but did not consider rectal pressure in evaluating detrusor pressure and included pediatric patients, which likely skewed their results [55]. Other studies have found that DO can be elicited when changing positions with a full bladder or during filling bladder [56–58]. A recent review article by Al-Hayek suggested that all patients be filled sitting or standing, so as to most accurately reproduce normal filling habits and have the best opportunity to elicit detrusor overactivity [59]. Figure 5 demonstrates an example of DO elicited during testing. Our institution utilizes cough, Valsalva, and handwashing as provocative measures. With regard to patient positioning, as filling is taking place, the fluoroscopy table is sequentially tilted until the patient is in a standing position. Sitting and walking in place are also used in select patient populations, such as post-prostatectomy patients, to assess SUI and DO.

Administration of provocative medications have also been reported during or just prior to cystometry. Potassium chloride instillation prior to urodynamic studies has previously been reported as a means to identifying IC/BPS. This was first reported by Parsons et al. in 1998 and specifically looked to detect abnormal epithelial permeability, as is common in IC/BPS [60]. Multiple other studies confirmed that these instillations could reproduce the common symptoms of IC/BPS [61, 62], namely frequency, urgency, nocturia, and bladder pain. Unfortunately, even in the original study, 26 % of patients who met all National Institute for Diabetes and Digestive and Kidney Diseases criteria for IC had a negative potassium sensitivity test. Further studies revealed only minimal improvement with known treatments [63]. Increased urothelial permeability was also found to be prevalent in detrusor overactivity and thus not specific to IC/BPS [64]. For these reasons, potassium sensitivity testing is not recommended in the American Urological Association Interstitial Cystitis/Bladder Pain Syndrome Guidelines [65].

Bethanechol, a muscarinic agonist, has also been investigated in the evaluation of detrusor underactivity. Use of bethanechol to aid with postoperative urinary retention was reported as early as the 1940s. Later, the bethanechol supersensitivity test was described as part of the evaluation of a neurogenic bladder [66–68]. The test involved the instillation of 100 cc of fluid intravesically followed by administration of 2.5 mg of subcutaneous bethanechol. Repetition of fluid instillation and bethanechol administration would occur every 10 min up to three times. A positive test was indicated by an increase in the detrusor pressure by at least 15 cm H₂O at 30 min. This reflected a neurologic etiology to the detrusor dysfunction instead of a myogenic cause. Further evaluation found only moderate sensitivity (76 %) and poor specificity (50 %) of this test [69], as well as inconsistent efficacy in the treatment of an underactive bladder [70–72]. As the results of this testing would not change the management of the patient (i.e., the need for intermittent catheterization), the use of bethanechol for bladder stimulation is no long recommended.

More recently, edrophonium, a short-acting cholinergic agonist, has been investigated as a new provocative agent for bladder overactivity, specifically aimed for those who do not have overactivity symptoms. One small study demonstrated a response in 78 % of patients with overactive symptoms at baseline, of whom 64 % had baseline normal cystometry results [73]. Unfortunately, this study suffered from methodologic flaws (zeroing intracorporealy) and failed to show consistent subjective or objective responses to the edrophonium. No provocative medications are used at our institution.

The ability of the bladder to store urine is constantly assessed during cystometry. One means of assessing bladder storage is to ask the patient about their sensation of filling. More specifically, the volumes at which a patient reports his/her “first sense of bladder filling,” “first desire to void,” and “strong desire to void” can greatly help to characterize the patient’s bladder. Further, assessing when a patient has a “strong desire to void” or significant urgency that does not abate typically defines a patient’s maximal cystometric capacity.

Outside of simple bladder volume sensation, objective measures of the ability of the bladder to store urine can be expressed by means of compliance. Compliance is the opposite of stiffness and indicates the amount of bladder wall elasticity. This is calculated as C = ΔV/ΔPdet.

More formally, bladder compliance is defined as the “change in volume relative to the corresponding change in the intravesical pressure” and is measured in units of mL/cm H ₂ O [74]. The initial proposed cutoff for low compliance was 5 mL/cm H ₂ O but further study demonstrated a wide range of normal among healthy subjects, likely due to the diversity in bladder volumes and most investigators currently consider normal compliance to be greater than 12.5 mL/cm H ₂ O [75]. Harris et al. later demonstrated that 40 mL/cm H₂O was a reasonable cutoff as the lower limit of normal [76]. This value also corresponded with a report by McGuire regarding evidence of upper tract deterioration with urethral opening pressures above 40 cm H₂O [77]. Figure 6 demonstrates a poorly compliant bladder with high pressures which would predispose a patient to obstructive nephropathy. With these findings, some have suggested that detrusor pressure may be a more useful tool for evaluating compliance changes than an actual calculated compliance value.

Leak point pressure is the pressure at which a patient experiences urinary loss in the absence of a detrusor contraction during urodynamic testing. There are two different leak point pressures that are routinely measured: the abdominal (ALPP) and the detrusor (DLPP). The abdominal leak point pressure (ALPP), is the intravesical pressure at which urine leaks due to an elevated abdominal pressure. This is ostensibly a measure of the patient’s sphincteric strength and can be used to demonstrate SUI or intrinsic sphincter deficiency. Attempts have been made to use this measure to define intrinsic sphincter deficiency as an ALPP less than 60 cm H₂O [78]. However, given that urethral hypermobility is also typically present, it is difficult to make this value an absolute cutoff. The ALPP is commonly assessed via either performance of the Valsalva maneuver (VLPP) or with a cough (CLPP). Studies have demonstrated that both maneuvers are effective in eliciting SUI but have different effects on the urethra [79]. Figure 7 demonstrates an example of a positive VLPP assessment. It has also been shown that while CLPP is typically larger than the VLPP, VLPP demonstrates less variability in provoking SUI [80, 81]. The presence of a urethral catheter has been shown to have an obstructive effect, thus leading to an exaggerated ALPP value [81]. This has thus made placement of a rectal catheter even more important. The detrusor leak point pressure (DLPP) is defined by Abrams as the “lowest detrusor pressure at which urine leakage occurs in the absence of either a detrusor contraction or increased abdominal pressure” [36]. This parameter is an especially important element of evaluation in those with neurogenic bladder given the known possibility for renal dysfunction and upper tract compromise with detrusor pressures >40 cm H₂O [82]. Other factors that may affect these results include the presence of pelvic organ prolapse, which can create a reservoir effect and affect abdominal pressure transmission to the urethra, patient positioning (as discussed above), and bladder volume. The recommended volume to perform this testing is half the normal cystometric capacity, typically 200–300 mL, though some studies have suggested that this capacity is too large and that testing should begin at smaller volumes (~150 mL) [83]. Other investigators utilize a voiding diary to establish the volume at which patients should undergo this testing and given the wide variations in bladder capacity, the optimal value should be determined based upon a patient’s voiding diary. Videourodynamics can also be used to assess the DLPP and ALPP. Our institution utilizes the Valsalva maneuver at a bladder volume of 200 mL and repeats this testing at 300 mL under fluoroscopy in the standing position. In addition, if there is evidence on the pre-cystometry physical exam or during fluoroscopic evaluation of filling of pelvic organ prolapse, cystometry is repeated after performance of the voiding phase with vaginal packing in place to further characterize bladder function with proper anatomic positioning of the pelvic organs.