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Electronic Function of IUPCs
IUPCs measure intrauterine pressure by converting the pressure in the uterus to an electronic signal that can be displayed on a fetal monitor. All IUPC systems employ a pressure transducer with a wheat stone bridge (see Figure 1) to convert the uterine pressures to electronic signals. How the pressure transducer is used in the IUPC system and how the environment affects it differs by the type of IUPC being used.

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Figure 1

Pressure Transducer - how does the zero button work?

Zero buttons on IUPC systems merely short the system. To eliminate zeroing inaccuracies and offset always make sure the pressure transducer is exposed to atmosphere when zeroing the monitor and avoid using “zero buttons.”

  

 
Early IUPC systems used a water filled catheter to transmit the uterine pressure to the transducer located outside the patient. These systems kept the transducer and electronics outside the uterine environment, but they were cumbersome to set-up and were subject to leveling inaccuracies. The generation of catheters following, put the transducer at the tip of the catheter where it was inserted into the uterus. This simplified catheter set-up  eliminated leveling concerns, but subjected the electronic transducer to the uterine environment. The latest technology uses a membrane sensor at the tip of the catheter which communicates with the pressure transducer outside the uterus. This technology provides the benefits of direct pressure measurement without subjecting the electronics to the uterine environment. The sections that follow explain how the uterine environment affects the electronic transducer. These effects can influence clinical intrauterine pressure readings significantly.

Zeroing
Pressure catheters must be set to zero or calibrated with the fetal monitor and the atmospheric pressure. This process ensures that no electronic variability from the catheter system or the fetal monitor is included in clinical readings. This is done with any IUPC system by exposing the transducer to atmosphere and zeroing the monitor.

With transducer-tipped systems zeroing must be done before the catheter is inserted. Once the transducer is in the uterus it cannot be exposed to atmosphere and cannot be zeroed. Although a “zeroing” button is included in the reusable cable or catheter connector these buttons simply short the system. (See figure 1). Zeroing the monitor while pressing this button introduces error into the measurement by ignoring the actual offset of the transducer. Offset is the degree that a transducer is off from true zero voltage. In effect, the “zeroing” button ignores the real transducer offset (assuring it to be negligible), which means pressure readings can be off by ±3 mm Hg (within specification) or more depending on the quality or problems with the transducer.

Offset can be eliminated from clinical readings in systems where the transducer is not inserted into the uterus because the transducer can be exposed to atmosphere at anytime for system zeroing. 		 .
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Thermal Effects
Temperature changes or thermal effects are a known variable that affect intrauterine pressure readings. Temperature changes affect electronic pressure transducers and when catheters in which the pressure transducer is mounted in the tip of the catheter are inserted into the uterus, the change from room temperature to body temperature affects the offset or baseline of the pressure measurement. (See figure 2) This means that the offset or baseline can drift up or down by 4.5 mm Hg in going from a room temperature of 22 ºC to body temperature of 37 ºC. The difficulty is that the direction and magnitude of the drift are unknown. Another thermal effect is that of a change in sensitivity or how accurately the catheter reads uterine contractions due to temperature changes. The practical meaning of this is that at 100 mm Hg, the pressure reading can be off (high or low) by 1.5 mm Hg. Zeroing the catheter once it has come to temperature cannot eliminate thermal sensitivity effects.  These effects apply to transducers that are within specifications.  Out of specification transducers show larger errors.

Systems with the pressure transducer mounted in the reusable cable or otherwise outside the patient are thermally stable because the sensor is always at room temperature.


Figure 2

Effect of temperature on internal pressure transducers

Figure two depicts the possible effect on clinical readings of warming a pressure transducer from room to body temperature. To eliminate thermal errors from IUP measurements, use a system where the pressure transducer is not inserted into the patient.

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Conclusion
In review, the offset of a transducer is how far off it is from zero at room temperature. This offset and the sensitivity of readings are affected by inserting the transducer into the uterus and warming it to body temperature. While offset and thermal effects separately are relatively small, when added together they can effect the accuracy of clinical readings significantly. In practice these effects could make a uterine baseline that is 22 mm Hg read 26 mm Hg or a baseline that is 33 mm Hg read 26 mm Hg. It is reassuring to know that the Koala IUPC has a distinct advantage in keeping the electronics outside the patient and free of these effects. When combined with true zero and a clear lumen for flashback visualization, the low thermal effects for the Koala IUPC make it a clear choice for better measurements.


AIM Technology
 The Koala technology consists of a membrane sensor at the tip that communicates pressures through a micro column of air. This technology is similar to non-invasive blood pressure monitors and provides many benefits because the electronics are kept outside the uterus.

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True Zero in Utero
 Koala is the only sensor-tipped catheter available that provides true zero in utero. By disconnecting and checking for zero on the monitor, clinicians can instantly validate the zero status of both the transducer and the monitor. This allows complete electronic trouble-shooting for enhanced reliability and reduced IPC replacement.

Click for Statistical Analysis & Technical Description


True Zero In Utero

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This Month's Winners
This month's winner - US Olympic Beret
Diane Baskin, Seattle, WA
Cynthia Bonney, Anchorage, AK
Laraine Guyette, Denver, CO
Prize: US Olympic Beret

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