Variable intrauterine pressure catheter (IUPC)tracings with two catheters

OBJECTIVE: Accurate intrauterine pressure catheter (IUPC) readings are critical for the management of complicated labor. Two basic IUPC designs are currently available for assessment of intrauterine pressure in labor, one in which both the pressure sensor and transducer are at the catheter tip and intrauterine (Intran Plus IUP-400, Utah Medical Products, Utah) or one in which the transducer is extrauterine (Koala Balloon IPC 5000E, Clinical Innovations, Utah). These two IUPC models were tested in a controlled artificial amniotic sac simulation under various unobstructed and obstructed conditions to assess their accuracy. STUDY DESIGN: Both models were placed in duplicate in a liquid filled pressure vessel and subjected to no, partially, or fully obstructed scenarios utilizing direct or partial occlusion with pig uterus around the distal catheter tip while applying simulated contractions. RESULTS: Unobstructed IUPC pressure readings were slightly, but not significantly different (within 5 mm Hg of reference baseline) for both catheters. However partial obstruction generated significant variability, with elevated baselines (up to 25 mm Hg), lower peak amplitudes (up to 30 mm Hg) and lower total contraction strengths (up to 45% reduction) for the external transducer device only. Re-zeroing the external transducer catheter did not fully correct the false readings and resulted in a lowering of both the baseline and peak amplitude below reference values. The internal transducer did not develop any significant variations at any partial obstruction. However, total obstruction of the internal transducer tip created an elevated baseline. CONCLUSION: The two IUPCs reacted differently in models of partial to total intrauterine obstruction with the internal transducer device recording pressure more consistently. External transducer tracings were more susceptible to variations (up to 60%) in simulated intrauterine compressions, and were not corrected by recommended “re-zero” maneuvers. INTRODUCTION Intrauterine pressure catheters have evolved from plastic, saline-filled tubes connected to an external pressure transducer dome to disposable units which offer greater convenience and sterility advantages. At no point in this evolution have intrauterine pressure catheters been trouble-free. Abnormal wave forms have long been understood to represent problems with catheter placement. Two types of catheters are in common usage. One of these (Koala, Clinical Innovations, Inc., Murray, UT) uses a column of air to connect a balloon in the amniotic cavity to an external pressure transducer. The other (Intran Plus, Utah Medical Products, Midvale, UT) places the transducer directly in the uterus. A previous study (Dowdle, J Reprod Med 2003;48:501505) concluded that under conditions of active labor, the two catheter types recorded similar mean baseline uterine tone, peak pressures, contraction frequency and duration. Performance of these two catheter types under conditions where amniotic fluid communication with the sensing distal tip was impaired by tissue obstruction was not compared. The purpose of this study was to simulate conditions of partial obstruction of the catheter tip in an artificial environment to determine the performance characteristics of these two catheter types under adverse conditions of limited communication with amniotic fluid. MATERIALS AND METHODS No human or living animal subjects were used in any of these experiments. Contractions were simulated in a pressure vessel filled with saline and cycled at pressures which were consistent with contractions of the human uterus. The pressure vessel was fitted with a calibrated reference transducer and four ports for test catheters. Two Intran and two Koala catheters were used simultaneously in each test. Partial obstruction was induced using three methods; rubber stoppers, latex tubing, or pig uteri. The data from the catheters was recorded digitally, and presented in graphic form. The test equipment was provided by John Raynes Engineering, UT. The test chamber and the materials used for simulated obstruction are shown in the photographs on the right. RESULTS Both catheters produced similar curves under optimal test conditions in a full liquid environment. When obstructed, the air balloon catheters produced much more erratic pressure tracings than did the transducer tipped device. Some representative results are shown in graphic form on this poster (at right). Most typical of the performance of the balloon tip catheter is elevation of the baseline pressure and diminution of the contraction strength and ΔP (change in pressure [contraction intensity]) of the contraction. Attempting to correct for the variation in balloon-tip catheter by re-zeroing according to the manufacturer’s instructions did not correct the false readings. The internal transducer device was more consistent at all levels of obstruction until total obstruction occurred. In this respect, obstructive interference of the internal transducer device appeared to be an all or none phenomenon. CONCLUSIONS 1. The two IUPC designs reacted differently under test conditions designed to simulate human uterine contractions when the catheters were subjected to partial obstruction. Such obstruction may occur clinically when the IUPC tip becomes lodged between the fetus and the uterine wall. 2. Under conditions of partial obstruction, the balloon tip catheter is more prone to be erratic. This errant behavior manifests as elevated baselines, decreased contraction maximum pressures and incorrectly decreased ΔP (contraction intensity). 3. Under certain clinical conditions, this errant behavior could result in diagnostic error that could affect an intervention decision. Examples include the use of increased resting tone to diagnose abruption of the placenta and the use of Montevideo Units to assess the adequacy of uterine contractions and govern the administration of oxytocin. 4. When the obstruction is removed from the transducer-tipped catheter, the baseline and contraction amplitude return to normal. This was not true for the balloon-tipped catheter. Obstruction Materials 1/2” rubber stopper 21/2” pig uteri Latex rubber sleeve Latex rubber tubing Intran placement Koala placement Pressure tank with Intran & Koala catheters covered with pig uteri and latex tubing obstruction.