Urology & Urogynecology > Education and Reference

Troubleshooting pressure channels during a urodynamic study can be frustrating for both the novice and the experienced urodynamicist. Despite the obvious benefits of years of experience ("been there, done that"), the novice can gain a quick advantage by learning to incorporate quick, pro-active troubleshooting techniques into every step of the set-up and connection process.

BEFORE STARTING A STUDY AND BALANCING (ZEROING) TRANSDUCERS:
1. Learn about the principles of hydrostatics.

The assembly of a transducer (w/ or w/o dome), connecting tubing and catheter filled with fluid creates a measurement device that is governed by the principles of hydrostatics. The increment of pressure measurement used in urodynamics is cmH2O. It is recommended that pressures be balanced (zeroed) to atmospheric pressure with the tip of the water column (usually tip of connecting tubing) level with the transducer or dome and open to air, not yet connected to the patient. When the tubing is later connected to a catheter, the tip of the catheter becomes the end of the water column. Because the fluid in the tubing (water column) has weight (cmH2O), it is important to maintain the leveling of the tip of the water column (symphysis pubis) to the transducer (dome) throughout a urodynamic study. For example, if the tip of the column (symphysis pubis) is 10cm higher than the transducer, the weight of that 10cm of water in the tubing will be added to the pressure coming from the bladder/abdomen/urethra.

2. Always prepare Pressure Transducers according to recommended procedures, making sure that there are no air bubbles in the transducer chamber, dome or connecting tubings.

• Air bubbles cause damping of the pressure waveform, especially noticeable during provocative maneuvers such as cough and valsalva. When a pressure response is "damped" the channel is not displaying increases and decreases in pressures with the rapidity or amplitude expected (Figure A. next page). Peaks of coughs will often be rounded instead of sharp spikes (Figure B.).
• Air bubbles can also cause "air-lock" in the pressure measurement system. This is manifested on the pressure channel during the release of a provocative maneuver. For example, in a perfectly prepared system, when the patient coughs, the down-slope of the spike should be as vertical as the upslope and return rapidly to its original baseline. An air bubble in the system will cause a delay in the return to baseline and the pressure may not return (ever) to the original resting pressure value (Figure C).
• An air bubble sitting directly on the transducer membrane can result in no transmission of pressure.

NOTE: On occasion, the bladder pressure channel may fail to display a cough spike after connection, even when all other aspects of the pves channel indicate it is working fine. If there was a urine return before connection, the channel is displaying a resting pves that seems appropriate, and if the initial connection troubleshooting indicates that the equipment-end is working, consider the possibility that the lumen opening ("eye") of the catheter is just pressed against the wall of the empty bladder. Typically a quick flush with 10-20cc will be of no help in this situation. Most of the time, proceeding with filling until there is 50 to 75cc in the bladder is all that is required. At that point, the detrusor wall will have moved away from the catheter and not block the "eyes" with a cough or strain.

3. Never reprocess disposable or "Single Use" transducers or domes. Exposure to sterilizing or disinfecting agents can damage the sensitive membranes in these devices, causing them to measure pressures inaccurately.

BEFORE CONNECTING TO PRESSURE CATHETERS: 1. Always flush the pressure measurement lumen(s) (pves/pura) of the BLADDER catheter before connecting to transducers. As the catheter is inserted, the lumen openings scoop up lubricant, loose cells and other debris and cannot possibly transmit pressures maximally if connected without flushing. 2. Be sure that the ABDOMINAL pressure (balloon) catheter has been prepared and inserted according to recommended procedure, with special attention to ensure that: The catheter and balloon have been pre-flushed with water or saline to purge air. The catheter is inserted far enough that the balloon is well beyond the anal sphincter and taped securely from or across the anus (according to the recommended procedure). The balloon is refilled with the recommended amount of fluid after insertion.

NOTE: Under-filling causes damping (Figure B on previous page). Over-filling the balloon on the abdominal pressure catheter can cause an excessively high up-spike (over- sensitivity) and a negative rebound wave at the end of the down-spike (illustration to right).

** NOTE: Feces in the rectal vault can cause excessively high abdominal pressure readings, no pressure readings or sudden loss of pressure.

AS THE CATHETERS ARE BEING CONNECTED TO TRANSDUCERS/TUBINGS
1. After system has run through transducer balancing process, look at the numbers flashing under each channel label (pves, pabd, pdet, Q, CVol, etc) and make sure that each is showing a zero. Digital systems will usually fluctuate between -1 and +1 and this is o.k.

• If the channels have not zeroed, End or Abort the study and start over.

2. After confirming that all channels have properly balanced (zeroed), begin connecting the pressure catheters to the APIS tubings that are attached to each transducer.

• As you pick up each APIS, your eyes should move over to the computer screen and confirm that there is a positive pressure deflection as the tubing is raised and moved to the patient. This is a very quick troubleshooting step that will show you that the equipment is working properly.
• If the deflections on channels are appropriate and pressures are within reasonable limits after connection, have the patient cough to confirm good transmission through entire water column.
• If there is no deflection, or if the deflection does not appear to reflect the motion properly (i.e. if the tubing tip is raised 10cm above the transducer, and the pressure channel doesn't go up to 10cm of H2O), DON'T MAKE THE CATHETER CONNECTION, THERE IS A PROBLEM! Proceed to Step #3.

3. If there is not proper deflection of the tubing, begin hardware troubleshooting. Failure to register any pressure at all is typically caused by the demise of the pressure transducer. It is an excellent policy to keep backup transducers on hand. However, other potential problems can be detected by checking the following:

• Is the transducer securely connected into the interface cable?
• Are there any stopcocks in the pressure system that are turned "Off" where they should be open?
• Are there any noticeable kinks in the tubing?
• Are there caps on the ends of any tubings? The caps that come on the tip of the APIS tubing must NOT be replaced after the tubing is filled with sterile solution as they produce a high-pressure seal that will interfere with the atmospheric balancing process.
• Is the interface cable securely connected into the base of the Patient Unit?
• Is the large cable running from the Patient Unit to the back of the CPU secured tightly?

CAUTION: NEVER disconnect or reconnect the Patient Unit Cable if it has come loose, without first powering down the computer. Reconnecting with electrical current running in the system will cause damage to the Patient Unit and some of the boards in the computer. Before disconnecting and/or reconnecting any equipment cables, end the study, close all software programs and shutdown the computer first.
4. If all above troubleshooting does not help you detect the cause for a pressure problem, abort the study and restart the system. This will reinitiate all peripheral equipment communications and may rectify the situation.