Objective
Tocodynamometry (Toco; strain gauge technology) provides contraction frequency and approximate duration of labor contractions but suffers frequent signal dropout, necessitating repositioning by a nurse, and may fail in obese patients. The alternative invasive intrauterine pressure catheter (IUPC) is more reliable and adds contraction pressure information but requires ruptured membranes and introduces small risks of infection and abruption. Electrohysterography (EHG) reports the electrical activity of the uterus through electrodes placed on the maternal abdomen. This study compared all 3 methods of contraction detection simultaneously in laboring women.
Study Design
Upon consent, laboring women were monitored simultaneously with Toco, EHG, and IUPC. Contraction curves were generated in real-time for the EHG, and all 3 curves were stored electronically. A contraction detection algorithm was used to compare frequency and timing between methods. Seventy-three subjects were enrolled in the study; 14 were excluded due to hardware failure of 1 or more of the devices (n = 12) or inadequate data collection duration (n = 2).
Results
In comparison with the gold-standard IUPC, EHG performed significantly better than Toco with regard to the Contractions Consistency Index (CCI). The mean CCI for EHG was 0.88 ± 0.17 compared with 0.69 ± 0.27 for Toco ( P < .0001). In contrast to Toco, EHG was not significantly affected by obesity.
Conclusion
Toco does not correlate well with the gold-standard IUPC and fails more frequently in obese patients. EHG provides a reliable noninvasive alternative, regardless of body habitus.
Electronic fetal monitoring is used to assess both uterine activity (frequency of contractions) and fetal well-being (fetal heart rate pattern, especially in relation to contractions). The former is typically assessed noninvasively with a tocodynamometer (Toco): a strain gauge positioned over the maternal fundus, which responds to changes in uterine tension transmitted to the abdomen. The device identifies the frequency of contractions, but not their intensity, and suffers both from misalignment following maternal movement and technical limitations in obese parturients. Currently the only alternative for Toco failure is the invasive intrauterine pressure catheter (IUPC), which carries inherent risks, most prominently infection.
In a comparison of Toco monitoring in obese (body mass index [BMI] >35 kg/m 2 ) and nonobese (BMI 20-25 kg/m 2 ) women, Ray et al describe a 30% rate of “difficult monitoring” in the obese group (vs 0% in the nonobese), requiring a 26% rate of internal monitoring (vs 0% in the nonobese). Vanner and Gardosi similarly reported poor-quality Toco during more than one quarter of the monitoring time in 36% of obese parturients (compared with 16% in nonobese).
Even in the nonobese, Toco suffers frequent failures. Bakker et al described some period of inadequate registration (no tracing or unreliable pattern because of inadequate calibration) in 98% of 41 labors, for an average of 35% of stage 1 duration and 33% of stage 2. They also reported inadequate registration in 60% of 151 patients monitored with IUPC, for 28% of stage 1 and 30% of stage 2 durations.
Electrohysterography (EHG), the uterine electromyography, uses a different modality for monitoring uterine activity. Similar to observing an electrocardiograph rather than intracardiac pressures, EHG reports the electrical activity of the uterine muscle. This noninvasive monitor utilizes surface electrodes applied to the maternal abdomen and a high-frequency, low-noise amplifier. Although prior studies have compared EHG with IUPC or Toco, to our knowledge this is the first study to compare EHG with both standard technologies in real time during active labor.
Materials and Methods
Adult patients admitted to labor and delivery in active labor with a singleton fetus in cephalic presentation and without bleeding, uterine scar, or contraindication to IUPC placement were eligible for inclusion. The study protocol was approved by our institutional review board, and all subjects provided written, informed consent. Following rupture of membranes (either spontaneous or artificial by obstetric indication), the EHG monitor was applied. In addition, if an IUPC was not already present, one was placed by the obstetrician. Data from the IUPC, Toco (using a second electronic fetal monitoring unit), and EHG were collected simultaneously via a laptop computer. The patients’ nurses were blinded to all but the IUPC output for uterine activity monitoring and interventions.
While a research data collector was available, monitoring continued through vaginal delivery or movement to the operating room for cesarean delivery.
Following skin preparation by gentle rubbing with an abrasive gel, 6 3 cm 2 Ag/AgCl 2 electrodes (Ambu, Glen Burnie, MD) were attached to the maternal abdomen ( Figure 1 ). The electrodes were connected to the amplifier in a monopolar fashion with common reference and common mode rejection leads on the left side of the patient’s abdomen, to reduce 60 Hz environmental noise. Electrode positions were modified slightly for each patient, as required by the location of the tocodynamometer and ultrasound fetal heart rate monitor. Impedance of each electrode was measured (as compared with the reference) (General Devices EIM-105 Prep-Check; General Devices, Ridgefield, NJ). Skin preparation was repeated as needed at each site until the measured impedance was below 10 kΩ where possible.
The recorded signals were fed to a 4-channel, high-resolution, low-noise unipolar amplifier. All 4 signals were measured with respect to a reference electrode. The amplifier design used driven right-leg circuitry to reduce common mode noise between the patient and the amplifier common. The amplifier 3 dB bandwidth was 0.05-250 Hz.
Data from each patient included a uterine activity channel from 2 maternal-fetal monitors (Corometrics; GE Medical Systems, Waukesha, WI) sampled at 8 Hz with an 8 bit resolution. These cardiotocographs reported the Toco- and IUPC-derived contraction curves. The data also included output from 4 abdominal EHG channels sampled at 500 Hz with a 24 bit resolution.
To produce the EHG contraction curve, the 4 EHG signals were downsampled at 20 Hz, band pass filtered between 0.2 and 1 Hz to eliminate low- and high-frequency noise while preserving the main contraction power, and combined based on their signal-to-noise ratio. The output was then downsampled at 8 Hz and normalized to scale the signal from 0 to 100 units. All 3 uterine activity curves were displayed in real time to the data collector and stored electronically for subsequent analysis.
Contraction location was statistically computed. Contractions were rejected if duration was less than 45 seconds or longer than 180 seconds, amplitude less than 10 mm Hg (IUPC) or less than 10 units (Toco and EHG), and amplitude less than 40% or greater than 350% of the median of the last 10 contractions. To evaluate contraction consistency, we used a modification of the Contractions Consistency Index (CCI) defined by Jezewski et al to compare EHG and Toco each to the gold standard, IUPC:
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