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Uterine Baseline Accurate measurements of uterine activity are a valuable adjunct to the management of parturition. Uterine activity can be measured by external tocodynamometers or by intrauterine pressure catheters. Tocodynamometers adequately detect uterine contraction frequency and duration, but do not record accurate uterine pressures—resting tone and contraction pressure. Intrauterine pressure catheters (IUPCs) accurately reflect the uterine pressure in waveforms recorded on the fetal strip. The area under the curve reveals the strength of the |
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contraction and is used in the management of normal and abnormal labor, especially in the face of oxytocin induction and/or augmentation. In the past, intrauterine pressure was evaluated by using resistive strain gauges, hydraulically coupled to external, fluid-filled domes, with catheters placed inside the uterus1. More recently, transducer-tipped/sensor-tipped catheters were introduced, which simplified the set-up and maintenance of IUPCs2,3,4. These ‘new’ devices no longer required priming the system prior to use or constantly moving the external transducer up and down an IV pole according to the patient’s position. As a result, the use of IUPCs to manage labor increased tremendously. Since
the change from fluid-filled catheters to sensor-tipped catheters, it has
been reported in the literature that the uterine baseline appears to be
elevated5.
This artificial elevation of the uterine resting tone is felt to be due to
something called the ‘hydrostatic pressure’ effect. Before this
can be completely understood, one must understand what components make up
the waveform recorded by IUPCs. The uterine pressure wave is made up of three components: [1] the contractile pressure of the myometrium (0-100mmHg), [2] the elastic recoil of the uterus—which is caused by distension of the uterus by the fetus (7-15mmHg) and [3] the hydrostatic pressure (5-10mmHg)6,7. With the fluid-filled catheters and the mobile transducer placed outside of the body, the hydrostatic pressure, or the pressure created by the height of the water column (consider the uterus as a column of water) could be eliminated. The external strain gauge transducer was simply moved up or down in relation to the patient’s position until a baseline of 0-5mmHg was displayed on the monitor. This effectively alleviated the hydrostatic water pressure, but could not totally alleviate the ‘resting tone’ or elastic recoil of the uterus and of course, had no effect on the strength of the contraction. However, with sensor-tipped catheters, because the sensing membrane or transducer |
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(depending on the catheter) is inside the uterus, the hydrostatic pressure cannot be alleviated. In fact, the lower the catheter is placed in the uterus, the higher the hydrostatic pressure effect. An example of this is swimming underwater, the deeper you go, the more pressure you feel on your eardrums because the height of the water column above you is getting larger. Thus, using the previously mentioned ‘normal’ values of the elastic recoil of the uterus (7-15mmHg) and the hydrostatic pressure (5-10mmHg), it is conceivable to have a uterine resting tone of 12 to 25mmHg. |
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This
has caused concern in some because the traditional definition of the
‘hypertonic’ uterus was 25mmHg. However, this definition is based
loosely on original data using the fluid-filled catheters. In fact,
many experts of fetal monitoring concentrate more on the overall contractile
wave pattern and the response of the fetal heart rate, i.e.,
‘hyperstimulation syndrome’, as opposed to placing a particular value on
the contractile strength. Thus, it is important to take into account
the type of IUPC being used (fluid-filled v. sensor-tipped) when evaluating
the baseline. Remember, a sensor-tipped catheter includes both the
elastic recoil of the uterus, as well as the hydrostatic pressure and thus,
as a result may show slightly higher uterine baselines as compared to the
original fluid-filled catheters. |
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User-error can also cause artificially elevated baselines. If the catheter is inadvertently placed outside the membranes--in between the decidua of the endometrial lining and the chorion of the amniotic membrane, extraovular—the baseline can be artificially elevated8. The Koala® IUPC, unlike the other sensor-tipped catheters on the market, has a clear lumen which assists in the confirmation of placement at the time of insertion. In addition, if the Koala® IUPC is attached to the cable prior to insertion--leading to an ‘over-charging’ of the membrane sensor, the baseline may be also |
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artificially elevated. If this occurs, one only has to disconnect the catheter from the reusable cable, re-zero the monitor (count five seconds) and then reconnect the catheter to the cable. This essentially discharges the sensing membrane, recalibrates ‘zero’ to atmospheric pressure, and re-charges the membrane with the appropriate amount of ambient air. Fetal position and maternal position can also alter the uterine baseline, although only slightly (5-7mmHg). If the catheter is placed underneath the fetus or the patient is lying on one side and the catheter is positioned on that dependent side, an increase in the baseline may be seen. To alleviate this, one can advance or retract the catheter slightly or move the patient from one side to the other and observe any change in the baseline (it may take several minutes for the baseline to stabilize). If a change is appreciated document the change according to position in the medical record for future reference. Finally, it is important to remember when evaluating the recording of any IUPC, the clinical assessment not be forgotten. Palpating the abdomen between and during contractions, taking into account maternal body habitus, and whether or not an amnioinfusion has been started, will assist in accurately assessing transient uterine baselines. In summary, accurate measurements of uterine activity are a valuable adjunct to the management of parturition. Thus, it is important to understand the components that make up the uterine waveform. Remember that with the change in technology from fluid-filled catheters to sensor/transducer-tipped catheters the uterine baseline is slightly ‘elevated’ due to the hydrostatic effect. The normal uterine baseline is 12-25mmHg7. This should be taken into account when evaluating the wave pattern and diagnosing hypertonic uterine activity. In addition, user error can lead to elevated uterine baselines, thus if observed a check of the set-up and initial placement of the catheter should be reviewed. |
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1Dowdle M. Comparison of two intrauterine pressure catheters during labor. J Reprod Med 2003; 48:501-5. 2Devoe LD, Gardner P, Dear C et al. Monitoring intrauterine pressure during active labor. A prospective comparison of two methods. J Reprod Med 1989; 34:811-814. 3Arulkumaran S, Yang M, Yee Tieca C, et al. Reliability of intrauterine pressure measurements. Obstet Gynecol 1991;78:800-2. 4Dowdle, Mark. Evaluating a new intrauterine pressure catheter. J Reprod Med, 1997; 42;8:506-13. 5Ross M, Walton J. Artificially elevated basal uterine tonus resulting from measurement of hydrostatic pressure by transducer-tipped intrauterine catheters. J Perinat 1994; XIV:408-10. 6Caldeyro R, Alvarez H, Reynolds S. A better understanding of uterine contractility through simultaneous recording with an internal and a seven-channel external method. Surg Gynecol Obstet 1950;91:641-50. 7Murray, M, Urbanski P. Essentials in Fetal Monitoring. Learning Resources International, Inc. 2000. 8Arulkumaran C, Yank M, Steer P, Ratnam S. Intrauterine pressure: comparison of extra vs intra amniotic methods using a transducer tipped catheter. Asia Oceania J Obstet Gynecol 1994; 20(1):35-8. |
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For more information on uterine baselines and IUPCs, please view our online educational video entitled Essentials of IUP Monitoring at: www.clinicalinnovations.com/koala_video.htm or, get your free DVD copy today by emailing us at |
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Visit us at CREOG & APGO Booth # 210! |
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