Limitations of antenatal fetal heart rate monitors G. S. Dawes, DM, FRS, M. Moulden, and C. W. G. Redman, FRCP Oxford, England Erroneous or doubtful decelerations in fetal heart rate traces were present in 111 of 1000 consecutive antenatal clinical records obtained by monitors with autocorrelation. The inCidence was 20% in fetuses n
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Fig. 1. Examples of computer printouts of FHR traces at 27 weeks' (upper left) and 41 weeks' gestation (upper right) showing erroneous decelerations. Genuine accelerations ( ~ ) or decelerations ( t ) are identified by arrows above or below the trace, respectively. The conventional strip chart of part of the 27week record is shown below to illustrate the erroneous deceleration.
an error algorithm." Valid fetal pulse intervals were averaged over V16 minute and stored, and a baseline was fltted.' Signal loss was defined as the proportion of V16-minute epochs for which no valid FHR values were available. The mean record length was 27.5 minutes; mean signal loss was 4.7%. The records were filed on disk and were reanalyzed repeatedly to optimize error detection. Variations in heart rate from the baseline were detected as accelerations (> 10 beats/min and 15 seconds) or decelerations (> 10 beats/ min and 1 minute, or >20
Limitations of FHR monitors
Volume 162 Number 1
beats/min and 30 seconds). The area of decelerations was measured as the number of lost beats, that is, the sum of the differences between heart rate and baseline rate, calculated epoch by epoch, and divided by 16. Long-term variation in FHR was calculated as the mean minute range, that is, the mean difference between miminum and maximum pulse intervals during each minute unless, as during a prolonged acceleration, all pulse intervals were less than baseline when the range was measured from the baseline. Decelerations were separately categorized and excluded from the calculation of long-term variation. Episodes of high or low FHR variation were identified as 5 of 6 consecutive minutes in each of which the range of pulse interval was >32 msec or 50% should be rejected, and those with 20% to 50% signal loss should be drawn to the observer's attention, as in Fig. 2. Accelerations. The same problem in interpretation occurs in relation to accelerations. Rapid accelerations (> 10 bpm and 15 seconds), with an abrupt (1/16 minutes) rise and decline were relatively few (12 in the 1000 records). Accelerations with >50% signal loss were present in 104 records, but comprised only 2.9% of all accelerations. Relationship of errors to overall signal loss and gestational age. It was to be expected that rejected dubious decelerations (~50% signal loss during the event) would be associated with records that had high signal loss overall; that was indeed so. The same relationship was also valid in cases of erroneous decelerations. Signal loss is higher at earlier gestational ages,5 and the incidence of records with errors of all kinds is expected to be greater in immature fetuses. Table II shows that the incidence is greater (20%) at
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32
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200
160
-* 140
F-
100
100 ~o
~-l-I--+-H--I-H-+--
Fig. 1. Examples of computer printouts of FHR traces at 27 weeks' (upper left) and 41 weeks' gestation (upper right) showing erroneous decelerations. Genuine accelerations ( ~ ) or decelerations ( t ) are identified by arrows above or below the trace, respectively. The conventional strip chart of part of the 27week record is shown below to illustrate the erroneous deceleration.
an error algorithm." Valid fetal pulse intervals were averaged over V16 minute and stored, and a baseline was fltted.' Signal loss was defined as the proportion of V16-minute epochs for which no valid FHR values were available. The mean record length was 27.5 minutes; mean signal loss was 4.7%. The records were filed on disk and were reanalyzed repeatedly to optimize error detection. Variations in heart rate from the baseline were detected as accelerations (> 10 beats/min and 15 seconds) or decelerations (> 10 beats/ min and 1 minute, or >20
Limitations of FHR monitors
Volume 162 Number 1
beats/min and 30 seconds). The area of decelerations was measured as the number of lost beats, that is, the sum of the differences between heart rate and baseline rate, calculated epoch by epoch, and divided by 16. Long-term variation in FHR was calculated as the mean minute range, that is, the mean difference between miminum and maximum pulse intervals during each minute unless, as during a prolonged acceleration, all pulse intervals were less than baseline when the range was measured from the baseline. Decelerations were separately categorized and excluded from the calculation of long-term variation. Episodes of high or low FHR variation were identified as 5 of 6 consecutive minutes in each of which the range of pulse interval was >32 msec or 50% should be rejected, and those with 20% to 50% signal loss should be drawn to the observer's attention, as in Fig. 2. Accelerations. The same problem in interpretation occurs in relation to accelerations. Rapid accelerations (> 10 bpm and 15 seconds), with an abrupt (1/16 minutes) rise and decline were relatively few (12 in the 1000 records). Accelerations with >50% signal loss were present in 104 records, but comprised only 2.9% of all accelerations. Relationship of errors to overall signal loss and gestational age. It was to be expected that rejected dubious decelerations (~50% signal loss during the event) would be associated with records that had high signal loss overall; that was indeed so. The same relationship was also valid in cases of erroneous decelerations. Signal loss is higher at earlier gestational ages,5 and the incidence of records with errors of all kinds is expected to be greater in immature fetuses. Table II shows that the incidence is greater (20%) at
