Top: Water charged catheter with appropriate orientation of catheter. Lumen transmits pressure to the transducer. Middle: Occlusion of the catheter lumen by urothelium precludes pressure transmission. Bottom: Air-charged catheter is omnidirectional, transmitting pressure regardless of the balloon orientation with the bladder sidewall
Several parameters are measured during complex cystometry, and all are potential sources of artifacts.
This is measured via a catheter that is typically inserted per urethra into the bladder and used to infuse fluid into and measure pressure within the bladder during the urodynamic study. The catheter is typically of relatively small diameter, approximately 7 F, to minimize obstruction of the bladder outlet during the voiding phase of the study.
This parameter is measured by a catheter that lacks an infusion port and is typically inserted into the rectum to measure abdominal pressures via a fluid filled or air-charged balloon. Some practitioners prefer to insert this into the upper vaginal vault to capture abdominal pressure readings. The International Continence Society recommends use of a rectal balloon catheter with the balloon only filled to 10–20 % of its unstretched capacity . This can help prevent pressure artifacts arising from contact between the catheter opening and the cavity wall. The Pabd catheter measures the total abdominal pressure including that due to intra-abdominal contents.
This represents the intrinsic pressure generated by the detrusor, determined by subtracting Pabd from Pves. This value reflects static pressure from the compliance of the bladder wall and contractions of the detrusor muscle.
Along with pressure readings recorded over time, the electrical activity of the perineal musculature is recorded as a part of most pressure-flow urodynamic studies. This electromyographic, or EMG, portion of the study allows the examiner an insight into the activity of the pelvic floor and striated sphincter during the filling and voiding phases of micturition. While needle electrodes might give a more accurate representation of muscle activity, they are impractical due to invasiveness and patient discomfort. As a rule, patch electrodes are positioned over the perineum to record EMG activity. These surface electrodes are noninvasive, rarely cause any discomfort for the patient, but do introduce sources of artifact.
Uroflowmetry and Voided Volume
Gravimetric uroflow machines utilize a load cell, essentially a scale, or a hydrostatic transducer to record the weight of voided fluid over time and in total. A second type of system utilizes a spinning disc rotated at a constant rate. Flow rates are determined by measuring the increased electrical energy required to maintain the constant rotational velocity of the disc as it is slowed by contact with the voided fluid.
The rate of filling is usually standardized for each lab. A typical medium filling rate for an adult is around 50–70 mL per minute. Rates for pediatric patients may be based on age and estimated bladder capacity.
Potential Sources of Study Artifacts: Recognition and Trouble-Shooting
Typical “medium” and “high” filling rates are usually much higher than what would be considered physiologic, and can lead to changes in bladder behavior manifested by increases in detrusor pressure and sensory sensitivity.
When infusion at a standard rate triggers immediate increases in detrusor pressure either due to decreased compliance or detrusor overactivity, a reduction in the filling rate may allow the bladder to accommodate a larger volume. The goal should be to attain a filling volume similar to typical voiding volumes seen in the patient’s voiding diary. At times the infusion rate may be adjusted based on the capacity and irritability of the individual’s bladder. For instance, when filling a very capacious low-pressure bladder, one may choose to increase infusion to a rapid rate of 100 mL per minute in order to expedite completion of the study.
Mismatch of Signals
One of the more common artifacts seen during performance of urodynamic studies is mismatch of signals. During voluntary abdominal straining or cough maneuvers, both Pves and Pabd should demonstrate nearly mirror-image increases in pressure. If the Pves and Pabd readings do not track each other (Fig. 6.2) the examiner should be alerted that the pressures read by the transducers may not accurately reflect those within the bladder or the abdomen.
Signal mismatch. The Pabd catheter stops transducing (red arrow) and a cough maneuver demonstrates brisk pressure increases in Pves, but not in Pabd (black arrow). The Pabd issue is corrected, and repeat cough maneuver demonstrates mirrored pressure increases in Pves and Pabd (green arrow)
One of the more common issues seen with mismatch of signal is catheter misplacement and resultant dampening of the signal from the transducer. A common finding encountered is the placement of the catheter tip against or within a fold of the wall of the bladder. In such a situation, the sensor port or balloon is not exposed to the lumen and therefore will not accurately transmit the lumen pressure. As the bladder fills, distention of the bladder wall may expose the tip of the catheter, suddenly revealing a different pressure or a dramatic change in the sensitivity to variations in pressure.
Stool in the rectal vault may impact readings from the Pabd catheter. This is a common issue in the population undergoing urodynamics due to the prevalence of constipation. Fecal material engulfing the tip of the Pabd catheter may cause considerable signal dampening and thus prevent accurate pressure measurements.
Likewise, air bubbles or kinks in the tubing, incorrectly positioned stopcocks in the lines of water-filled systems, or failure to charge an air-charged catheter may dampen or obscure the transmission of pressures. At times one may observe decreasing pressures, along with mismatch, raising the possibility of a poor connection and resultant leak somewhere in the system. Observing a gradual increase in resting pressures may suggest migration of a catheter into the region of the urethral or anal sphincter. Should continued catheter migration occur beyond the sphincter, resting pressure may decrease accompanied by pressure transmission attenuation. Rarely one may encounter a catheter with a defect rendering it unusable for a study.
Signal transduction should always be confirmed at the start of the pressure-flow urodynamic study by observing symmetric changes in Pves and Pabd with a cough or Valsalva maneuver. Observation of signal mismatch should be followed by a systematic investigation of its cause, correction, and resumption of the study. A thorough check of the tubing, connections, stopcocks, and transducers may reveal the source of the artifact. Air within a line or transducer of a water charged system, or a partially closed stopcock or a loose connection point can drastically alter and dampen transmission of pressure changes. Flushing out the water-filled system and placing all stopcocks fully in the appropriate positions should remedy the situation. Assure tight connections between the catheters and lines, and that the tubing is connected to the correct port of the catheter. Double check that the air-charged catheter is in the charged position.
Once the tubing and connections have been checked, the investigator should consider problems with the position of the catheter. Visual inspection may reveal a catheter that has clearly migrated out of position. If utilizing radiography during the study, this may be an adjunct to establishing catheter position. Excessive advancement of catheters may result in kinking and subsequent obstruction and dampening of pressure transduction. Correction of these issues may be accomplished by gently changing the position of the catheter while monitoring the pressure reading during cough and strain maneuvers. The catheter should be secured when appropriate readings are obtained. The infusion of a small amount of fluid into the bladder may make it easier to attain an interface between the catheter tip and intravesical fluid.
Mismatch caused by advancing the Pabd catheter into a fecal bolus or within a fold in the rectal wall may be remedied by repositioning or fecal evacuation. Asking the patient to attempt to have a bowel movement prior to the study may prevent issues with stool in the rectal vault. Patients on a bowel regimen can time their treatments in order to enter the urodynamics laboratory with an empty rectum. Occasionally more aggressive interventions, such as enemas, laxatives, and manual disimpaction are required.
If all investigations and repositioning attempts are unsuccessful, consideration should be given to replacement of the malfunctioning catheter.
On occasion pressure readings will begin to shift, or “drift,” after zeroing as the study ensues. This results in an inaccurate calculation of detrusor pressure (Pdet).
Changes in the pressure within the rectum caused by peristalsis may also cause shifts in Pabd (Fig. 6.3). If the lead was zeroed during a period of increased tone, subsequent relaxation during the study will cause a decrease in Pabd and a corresponding increase in the Pdet reading that does not reflect a true increase in bladder pressure. If Pabd is zeroed at low rectal pressure, a peristaltic increase in rectal pressure during the study results in increased Pabd readings with corresponding spurious decrease in Pdet. Pdet pressure readings therefore may even be deflected into the negative range, which is not physiologically possible.