British Journal of Obstetrics and Gynaecology June 1976. Vol83. pp 454-459
THE EFFECT OF MEMBRANE RUPTURE ON FETAL HEART RATE IN INDUCED LABOUR BY
P . J . STEER,House OfJicer* D . J . LITTLE,House Oficer
N. L. LEWIS,Medical Student MARYC. M. E. KELLY,Medical Student AND
R . W. BEARD,Professor Department of Obstetrics and Gynaecology St Mary’s Hospital Medical School, Praed Street, London W2 Summary The influence of the state of the membranes on the fetal heart rate was studied in a group of twelve women whose membranes were electively ruptured for oxytocin induction of labour. The results have been compared with a group of eight women of similar gestation and parity whose membranes were kept intact. While membranes were ruptured, there was an increase in the frequency and amplitude of early decelerations throughout labour. The significance and practical implications of the results are discussed.
IT has been shown that decelerations in fetal heart rate during labour are more common when membranes are ruptured than when they are intact (Caldeyro-Barcia et al, 1972; Gabert and Stenchever, 1973). The Montevideo group have suggested that this is evidence of fetal asphyxia and that the membranes should, if possible, be left intact (Schwarcz et al, 1973). To investigate this hypothesis further a study was made of two matched groups of patients who had labour induced with an intravenous oxytocin infusion. In one group the membranes were left intact and in the other artificial rupture of membranes was performed at the onset of induction. The resulting labours were studied with respect to their length, pattern of uterine activity, the incidence and magnitude of decel-
erations, the duration of the second stage, method of delivery and the Apgar score. PATIENTS AND METHODS Twelve patients were studied with ruptured membranes and eight with intact membranes (Steer et al, 1975). The reasons for induction were postmaturity (1 1 patients), very mild pre-eclampsia (4 patients), small antepartum haemorrhage (1 patient) or a minimal medical indication (4 patients), such as maternal age or the presence of a cardiac murmer. There was no significant difference between the two groups in respect of maturity, age, Bishop score at induction, head descent, or parity (Steer et al, 1975). Random selection was achieved in 17 patients by alternately rupturing or keeping membranes intact. In three patients the membranes ruptured accidentally during the insertion
* Present appointment : Research Assistant. 454
FETAL HEART RATE IN INDUCED L,AROUR
of the catheter used to record intrauterine pressure. Subsequent analysis showed that uterine action in these patients was indistinguishable from that in patients who had their membranes ruptured intentionally and they were therefore included in this group. Oxytocin was administered by infusion pump (Rocket of London) or IVAC drip counter. It was commenced at 2 mU/minute and increased by 2 mU/minute every 15 minutes until there were at least three contractions every 10 minutes. Intrauterine pressure was recorded by an openended catheter connected to a transducer (Csapo, 1970) operating with either the Hewlett Packard 8020A or the Sonicaid FM2. These two monitors were also used to record the fetal heart rate, using an ultrasound transducer to detect the heart beat. By constant attention to the direction of the beam, a good heart rate trace was obtained for over 90 per cent of recording time. In a proportion of patients with ruptured membranes, we compared the ultrasound recordings with traces obtained via a scalp electrode ECG. In terms of the statistical analysis described below, there was no significant difference in decelerations as recorded by ECG or ultrasound.
Method offetal heart rate analysis and terminology The change in fetal heart rate most commonly seen is the ‘early deceleration’. It has the following characteristics: the onset of the deceleration is usually at the start of a contraction; recovery of the fetal heart rate occurs by the end of the contraction; and the amplitude of the deceleration is relatively small, less than 40 beats per minute (Beard et ai, 1971). The next change in order of incidence is the ‘variable deceleration’. It appears at the onset, or slightly later, of the contraction. It has an irregular shape (hence ‘variable’) and its amplitude exceeds 40 beats per minute. Recovery may be slightly delayed after the end of the contraction, but when it occurs it is usually rapid. Almost all the decelerations occurring in our study were of these two types. They had a characteristic ‘V’ shape and the following method was therefore used to characterize them. Three measurements of fetal heart rate were made in relation to the peak of every single contraction throughout labour (Fig. 1). Firstly,
455
the heart rate one minute before the peak of the contraction was noted (baseline rate, X). Secondly, the heart rate one minute after the peak of the contraction was noted (recovery rate, Z). Thirdly, the lowest rate reached during the two minute period between X and Z was noted (deceleration rate, Y). From these basic values, two derived values were calculated. Firstly, the ‘deceleration index’ or X-Y, representing the amplitude of the deceleration. Secondly, the ‘recovery index’ or X-Z, representing any late component of the deceleration which was still persisting one minute after the peak of the contraction. Using this method of analysis, two approaches were possible. Firstly, the indices could be used to study the fluctuations in deceleration between one contraction and the next, for example, relating them to the size of the contraction. Secondly, they could be used to study the longer term fluctuations in deceleration, for example, the difference between the average decelerations at the onset of labour and the second stage. It was decided to concentrate on the second approach, and to this end each labour was divided into half-hour periods and the three base values (X, Y, Z) and the two derived values (X-Z, X-Y) were expressed as the mean and standard deviation for each period. In fact, unless there was a specific disturbing factor such as the administration of an epidural anaesthetic, short term fluctuations weie relatively small. Typical examples of the variation for different magnitudes of deceleration are shown in Table I. TABLE I Deceleration index Mean values and variancefor diferent magnitudes of early deceleration Mean value Per half hour
SD
Variance @er cent)
8.7
3.2
37
Mild early decelerations 17.9
7.3
41
No decelerationsnormal fetal heart rate variability only
Marked earlylvariable decelerations
53.0
20.4
38.5
456
STEER, LITTLE, LEWIS, KELLY AND BEARD
FH f 160
Y ------
I
I
111
I INTRA UTERINE PRESSURE
I I 1 minute after peak
before peak
TIME
x = mean baseline y = mean deceleration z= mean recovery (?baseline) FIG.1 Calculation of the deceleration and recovery indices.
ClassiJication of the stage of labour It has been shown that induced labour can be conveniently considered as having three distinct phases (Steer et al, 1975). The ‘incremental phase’ is one in which there is a progressive increase in uterine activity in response to oxytocin infusion followed by the ‘stable phase’ during which uterine activity is relatively constant and largely independent of changes in the rate of oxytocin infusion. Lastly, the final phase when maternal ‘bearing down’ occurs, which is approximately equivalent to the second stage. This method of analysis facilitates the comparison of labours of different lengths and has therefore been adopted in this study.
RESULTS The mean (&SD) duration of labour was significantly shorter when elective rupture of membranes was performed, 5.54&2.53 hours, as compared with 9 * 3 1 &3 70 hours when membranes were left intact (pt0.005). The state of the membranes did not significantly influence the duration of the second stage, the mean (*SD) with spontaneous rupture being 29f22 minutes and with elective rupture 46f37 minutes (p
THE EFFECT OF MEMBRANE RUPTURE ON FETAL HEART RATE IN INDUCED LABOUR BY
P . J . STEER,House OfJicer* D . J . LITTLE,House Oficer
N. L. LEWIS,Medical Student MARYC. M. E. KELLY,Medical Student AND
R . W. BEARD,Professor Department of Obstetrics and Gynaecology St Mary’s Hospital Medical School, Praed Street, London W2 Summary The influence of the state of the membranes on the fetal heart rate was studied in a group of twelve women whose membranes were electively ruptured for oxytocin induction of labour. The results have been compared with a group of eight women of similar gestation and parity whose membranes were kept intact. While membranes were ruptured, there was an increase in the frequency and amplitude of early decelerations throughout labour. The significance and practical implications of the results are discussed.
IT has been shown that decelerations in fetal heart rate during labour are more common when membranes are ruptured than when they are intact (Caldeyro-Barcia et al, 1972; Gabert and Stenchever, 1973). The Montevideo group have suggested that this is evidence of fetal asphyxia and that the membranes should, if possible, be left intact (Schwarcz et al, 1973). To investigate this hypothesis further a study was made of two matched groups of patients who had labour induced with an intravenous oxytocin infusion. In one group the membranes were left intact and in the other artificial rupture of membranes was performed at the onset of induction. The resulting labours were studied with respect to their length, pattern of uterine activity, the incidence and magnitude of decel-
erations, the duration of the second stage, method of delivery and the Apgar score. PATIENTS AND METHODS Twelve patients were studied with ruptured membranes and eight with intact membranes (Steer et al, 1975). The reasons for induction were postmaturity (1 1 patients), very mild pre-eclampsia (4 patients), small antepartum haemorrhage (1 patient) or a minimal medical indication (4 patients), such as maternal age or the presence of a cardiac murmer. There was no significant difference between the two groups in respect of maturity, age, Bishop score at induction, head descent, or parity (Steer et al, 1975). Random selection was achieved in 17 patients by alternately rupturing or keeping membranes intact. In three patients the membranes ruptured accidentally during the insertion
* Present appointment : Research Assistant. 454
FETAL HEART RATE IN INDUCED L,AROUR
of the catheter used to record intrauterine pressure. Subsequent analysis showed that uterine action in these patients was indistinguishable from that in patients who had their membranes ruptured intentionally and they were therefore included in this group. Oxytocin was administered by infusion pump (Rocket of London) or IVAC drip counter. It was commenced at 2 mU/minute and increased by 2 mU/minute every 15 minutes until there were at least three contractions every 10 minutes. Intrauterine pressure was recorded by an openended catheter connected to a transducer (Csapo, 1970) operating with either the Hewlett Packard 8020A or the Sonicaid FM2. These two monitors were also used to record the fetal heart rate, using an ultrasound transducer to detect the heart beat. By constant attention to the direction of the beam, a good heart rate trace was obtained for over 90 per cent of recording time. In a proportion of patients with ruptured membranes, we compared the ultrasound recordings with traces obtained via a scalp electrode ECG. In terms of the statistical analysis described below, there was no significant difference in decelerations as recorded by ECG or ultrasound.
Method offetal heart rate analysis and terminology The change in fetal heart rate most commonly seen is the ‘early deceleration’. It has the following characteristics: the onset of the deceleration is usually at the start of a contraction; recovery of the fetal heart rate occurs by the end of the contraction; and the amplitude of the deceleration is relatively small, less than 40 beats per minute (Beard et ai, 1971). The next change in order of incidence is the ‘variable deceleration’. It appears at the onset, or slightly later, of the contraction. It has an irregular shape (hence ‘variable’) and its amplitude exceeds 40 beats per minute. Recovery may be slightly delayed after the end of the contraction, but when it occurs it is usually rapid. Almost all the decelerations occurring in our study were of these two types. They had a characteristic ‘V’ shape and the following method was therefore used to characterize them. Three measurements of fetal heart rate were made in relation to the peak of every single contraction throughout labour (Fig. 1). Firstly,
455
the heart rate one minute before the peak of the contraction was noted (baseline rate, X). Secondly, the heart rate one minute after the peak of the contraction was noted (recovery rate, Z). Thirdly, the lowest rate reached during the two minute period between X and Z was noted (deceleration rate, Y). From these basic values, two derived values were calculated. Firstly, the ‘deceleration index’ or X-Y, representing the amplitude of the deceleration. Secondly, the ‘recovery index’ or X-Z, representing any late component of the deceleration which was still persisting one minute after the peak of the contraction. Using this method of analysis, two approaches were possible. Firstly, the indices could be used to study the fluctuations in deceleration between one contraction and the next, for example, relating them to the size of the contraction. Secondly, they could be used to study the longer term fluctuations in deceleration, for example, the difference between the average decelerations at the onset of labour and the second stage. It was decided to concentrate on the second approach, and to this end each labour was divided into half-hour periods and the three base values (X, Y, Z) and the two derived values (X-Z, X-Y) were expressed as the mean and standard deviation for each period. In fact, unless there was a specific disturbing factor such as the administration of an epidural anaesthetic, short term fluctuations weie relatively small. Typical examples of the variation for different magnitudes of deceleration are shown in Table I. TABLE I Deceleration index Mean values and variancefor diferent magnitudes of early deceleration Mean value Per half hour
SD
Variance @er cent)
8.7
3.2
37
Mild early decelerations 17.9
7.3
41
No decelerationsnormal fetal heart rate variability only
Marked earlylvariable decelerations
53.0
20.4
38.5
456
STEER, LITTLE, LEWIS, KELLY AND BEARD
FH f 160
Y ------
I
I
111
I INTRA UTERINE PRESSURE
I I 1 minute after peak
before peak
TIME
x = mean baseline y = mean deceleration z= mean recovery (?baseline) FIG.1 Calculation of the deceleration and recovery indices.
ClassiJication of the stage of labour It has been shown that induced labour can be conveniently considered as having three distinct phases (Steer et al, 1975). The ‘incremental phase’ is one in which there is a progressive increase in uterine activity in response to oxytocin infusion followed by the ‘stable phase’ during which uterine activity is relatively constant and largely independent of changes in the rate of oxytocin infusion. Lastly, the final phase when maternal ‘bearing down’ occurs, which is approximately equivalent to the second stage. This method of analysis facilitates the comparison of labours of different lengths and has therefore been adopted in this study.
RESULTS The mean (&SD) duration of labour was significantly shorter when elective rupture of membranes was performed, 5.54&2.53 hours, as compared with 9 * 3 1 &3 70 hours when membranes were left intact (pt0.005). The state of the membranes did not significantly influence the duration of the second stage, the mean (*SD) with spontaneous rupture being 29f22 minutes and with elective rupture 46f37 minutes (p
