In Journal of Perinatal Medicine.

In previous papers [1, 6] it was reported that the rupture of membranes facilitates the production of type 1 dips in the fetal heart rate (FHR) tracing. This Statement is based on the effect of amniotomy on FHR records [6] and also on the quantitative study of a group of 26 monitored labors induced with oxytocin [1], in which membranes were artificially ruptured when cervical dilatation was between 4 and 6 cm : The incidence of type 1 dips was significantly higher after amniotomy than before this procedure. The type 1 dips recorded after rupture of the membranes were attributed to the uneven compression and deformation of the fetal head, or to occlusion of the umbilical vessels during uterine contractions [1, 6, 19]. It was postulated that both effects of uterine contractions were facilitated by the rupture of the bag of waters and corresponding loss of amniotic fluid around the fetal head or the umbilical cord. The design of this previous study [1] did not allow the analysis of the effects of amniotomy independently from the influence of the engagement of the fetal head in the pelvis. The head was usually not engaged from the beginning of labor until the time of amniotomy, whereas it was engaged during most of the period elapsing after rupture of membranes until delivery.

Since engagement increases the pressure received by the fetal head during uterine contractions [11, 19], it may facilitate the production of type 1 dips. The present study was designed prospectively to allow the analysis of the effects of amniotomy independently from the influence of the engagement of the fetal head. Further Information which will be provided by this design, is the FHR pattern throughout labor with intact membranes. At present, this is not available since most obstetricians rupture the membranes artificially before cervical dilatation has reached 6 cm. Furthermore, most of the FHR tracings are obtained during labor by means of the fetal scalp electrode which cannot be inserted when membranes are intact.

Methods

1.1 Design of the study.

In one group of labors the bag o£ waters was not ruptured artificially, remaining intact until full cervical dilatation had been reached¹.

¹ Those labors in which the membranes ruptured spontaneously before the end of the first stage, were excluded from the analysis.

In the second group of labors, amniotomy was performed early during the first stage when cervical dilatation was between 4 and 5 cm (“early amniotomy”). This design allows the comparison of the incidence of type I dips between both groups of labors (intact or ruptured membranes) in two conditions : a) before the fetal head becomes engaged, and b) after head engagement. It also enables to study the influence of the engagement of the head a) within the group of labors with intact membranes, and b) within the group of labors with ruptured membranes.

1.2 Rupture of the membranes

The 37 labors are classed in two groups.

Group 1. In 17 labors, artificial rupture of membranes was performed when cervical dilatation was between 4 and 5 cm “early amniotomy”).

Group 2. In the remaining 20 patients early amniotomy was not performed and the membranes remained intact at least until f ll cervical dilatation had been reached; during the second stage of labor the membranes either ruptured spontaneously or were ruptured by amniotomy². In two labors of this group the head and shoulders of the fetus were delivered with unruptured membranes. The decision to perform or not early amniotomy was made at random.

² In group 2, the contractions and dips recorded after rupture of membranes in the second stage were not included in this study.

1.3 Monitoring of labor

Intrauterine pressure was recorded in all 37 labors during the whole period of study with a catheter introduced via the vagina and cervix into the Uterus between the ovular membrane and the uterine wall.

Fetal heart rate (FHR) was recorded simultaneously with intrauterine pressure. When the membranes were intact the signal triggering the tachometer was obtained with the

Doppler principle applied to a beam of ultrasounds. After amniotomy, an electrode was inserted into the fetal scalp and the fetal ECG was the trigger signal.

When in a given section of the record the FHR was illegible or unreliable, the whole section was excluded from the study and the corresponding uterine contractions, even if distinctly recorded, were not counted.

Clinical material

Thirty-seven pregnant women without known complications of pregnancy, with a single living fetus in cephalic presentation and without cephalopelvic disproportion, were studied. Labor started spontaneously at term and progressed without need of any medication (oxytocin, analgesics, etc.). The women were allowed to change their position (supine or lateral) s they wished.

Parity. Seven primigravidae and 10 multiparae constitute the group with early amniotomy. The group with unruptured membranes included 7 primigravidae and 13 multigravidae.

Loop of cord around fetal neck. Such a loop was found in 5 fetuses of the group with early amniotomy and in eleven fetuses of that with intact membranes.

Duration of labor. The time elapsing in each labor from the moment when cervical dilatation reached 5 cm until delivery of the fetus is illustrated in Fig. 1

.

 

Uterine contractions. No significant difference was found between both groups of labors either in the intensity or in the frequency of uterine contractions [2].

Characteristics of newborn. The birth weight, cephalic perimeter and AP GAR scores at 1 and 5 minutes are illustrated for each neonate in Figs. 2, 3 and 4.

 

 

 

 

There is no difference between both groups of labors in the distribution of parity, duration of labor and characteristics of the newborn. The incidence of cord loop was higher in the group with intact membranes than in that with early amniotomy.

Results

3.1 Type 2 dips (late decelerations).

The overall incidence of type 2 dips in both groups of labors was very low in agreement with the vigorous condition of the newborns and the absence of complications of pregnancy and labor in these women. Less than 2% of the uterine contractions recorded in the 37 labors produced type 2 dips. No significant difference was found in this aspect between the group of labors with intact membranes and that with early amniotomy. Type 2 Dips will not be further analyzed in this paper.

3.2 Type 1 dips³.

The incidence of type I dips was influenced by the Status of the membranes and the Station of the fetal head. When the membranes remained intact until a few minutes before delivery (Figs. 5 and 6) the incidence of type I dips was null or very low.

³ Type 1 dips [6] include FHR patterns commonly attributed to the compression of the fetal head and/or the umbilical cord [10].

 

 

 

When the rupture of membranes occurred with full cervical dilatation and the head was engaged (Fig. 7) a very high percentage of uterine contractions produced type 1 dips after the rupture.

 

Type 1 dips usually appeared after early amniotomy but initially their incidence was low (Fig. 8-A). As labor progressed and the head engaged in the pelvis the incidence of type I dips increased and in some cases came close to 100% in advanced labor (Fig. 8-B).

 

 

 

3.2.1 Quantitative study of the incidence of type 1 dips

For any given group of labors the incidence of type 1 dips was calculated as follows. The number of uterine contractions recorded during a given period of labor in all the women of the group was added. The same sort of pooling was made for all the type I dips recorded. The percentage of contractions producing dips in the pool indicated the incidence of dips for the group. This method was employed to calculate the incidence of type 1 dips in the groups of labors illustrated in Figs. 9,10,11 and 12.

3.2.2 Method for statistical analysis

The non parametric MANN-WHITNEY U test [22] was employed to determine the significance of the differences found between the incidence of dips in different groups o£ labors (Figs. 9,10, 11 and 12). This method takes into account the number of labors in the compared groups and also the number of uterine contractions studied.

3.2.3 Factors analyzed

The influence of three factors on the incidence of type 1 dips was analyzed quantitatively :

1. Status of the membranes (intact or ruptured)
2. Station of the fetal head (before or after engagement)
3. Loop of cord around the fetal neck, trunk or limb (present or absent).

3.2.3.1 Influence of the Status of membranes (intact or ruptured)

Study made disregarding Station of the fetal head. The section of the FHR records analyzed in this study corresponded to the first stage of labor, starting when cervical dilatation was 4-5 cm and finishing when full dilatation was reached (Fig. 9).

 

 

In 20 labors in which the membranes remained intact, the 881 contractions which were recorded during the above mentioned period produced 26 dips. The incidence of dips for this group of labors was 2.95%.

In 17 labors with early amniotomy 745 contractions were recorded in the study period and produced 156 dips. The incidence of dips for this group was 20.95%.

The incidence of type 1 dips was significantly lower (p<0.001) in the group of labors with intact membranes (2.95%) than in that with ruptured membranes (20.95%).

Study made taking into account the Station of fetal head

Before engagement of the fetal head (Station to -1). Fig. 10-A shows that in 20 labors with intact membranes the incidence of dips (4.39%) was significantly lower than in 17 labors with early amniotomy (incidence of dips = 15.62%).

 

After engagement of the fetal head (Station 0 to +4). Fig. 10-B shows that in the group of 16 labors with early amniotomy the incidence of dips (45.68%) was significantly higher than in the group of 9 labors in which, after head engagement, the membranes still remained intact (incidence of dips = 9.24%).

The results illustrated in Fig. 10 indicate that the presence of intact membranes reduced the incidence of type 1 dips both before and after engagement of the fetal head. Furthermore, this protective effect of intact membranes was more marked after the fetal head had become engaged (Fig. 10-B) than before engagement (Fig. 10-A).

3.2.3.2 Influence of the Station of the fetal head (before or after engagement)

With early ruptured membranes. Fig. 11-B shows that after engagement of the fetal head the incidence of dips (45.68%) was significantly greater than before engagement (15.62%). This result indicates that, with ruptured membranes, engagement of the fetal head markedly facilitated the production of type 1 dips.

 

The 17 labors shown in Fig. 11-B before engagement were the same 17 labors with ruptured membranes shown in Fig. 9 In Fig. 11-B, after engagement only 16 labors were analyzed since in one the FHR record became illegible in that period.

With intact membranes. Figure 11-A shows that the incidence of dips was 4.39% before engagement of the head and that it increased to 9.24% after engagement. This difference in the incidence of dips before and after engagement is not significant, in keeping with the above postulated protecting effect of the intact membranes.

The group with intact membranes in Fig. 11-A included the same 20 labors presented in Fig. 9.

All 29 labors were analyzed for the period before engagement in Fig. 11-A. Only 9 of 20 labors could be analyzed in the period after engagement (Fig. 11-A) since in the other 11 labors the membranes ruptured during that period.

3.2.3.3 Influence of a loop of cord around the fetal neck (present or absent) (Fig. 12).

The influence of cord loops was analyzed in the section of FHR records corresponding to the first stage of labor, starting when cervical dilatation was 4-5 cm and ending when full dilatation had been reached. The Station of the head was not taken into account in the study.

With intact membranes the incidence of dips was very low (Fig. 12-A) ; there is very little and not significant difference between the group of 9 labors without cord loop (incidence of dips =

2.06 %) and the group of 11 labors with cord loop (incidence of dips = 3.66 %).

 

With ruptured membranes (Fig. 12-B) the incidence of dips was higher in the group of 5 labors with cord loop (incidence of dips 24.82 %) than in that of 12 labors without cord loop (incidence of dips 20.06%). The difference is not significant (p>0.05). This result should be considered with caution since there were only 5 labors in the group with cord loop.

According to the results illustrated in Fig. 12 the presence or absence of cord loop has no significant influence on the incidence of type 1 dips.

This Statement holds true both in labors with intact membranes (Fig. 12-A) and for labors with ruptured membranes (Fig. 12-B). The present results, obtained in labors which started spontaneously and progressed without oxytocin, differ from those obtained by the same authors [1] in a larger group of labors induced with oxytocin infusion. In that study, after amniotomy the incidence of type 1 dips was significantly higher in the group of 13 labors with cord loop (incidence of dips = 39.16%) than in that of 13 labors without cord loop (incidence of dips = 28.18%).

Discussion

The results presented in this paper indicate that in uncomplicated pregnancies in which labor started spontaneously at term and progressed without medication, as long as the ovular membranes remained intact, uterine contractions usually did not produce dips (transient decelerations) in the FHR tracing. Two factors increased the incidence of type 1 dips :

a) the rupture of the ovular membranes ;

b) the engagement of the fetal head beyond Station 0.

The combined action of both factors caused the highest incidence of type 1 dips.

BERDAGUER et al [2] have shown that there is no significant difference in the intensity of uterine contractions between the group of labors with early amniotomy and that with intact membranes. The higher incidence of type 1 dips in the first group cannot thus be explained by stronger uterine contractions. A different mechanism of action such as a protective effect of the intact bag of waters may be postulated.

Type 1 dips are the result of stimulation of the fetal vagus [13] which may be caused by the uneven compression of the fetal head with the consequent cephalic deformation [19] or by umbilical occlusion [1, 6]. Both effects of uterine contractions are facilitated by the rupture of the bag of waters [1, 6, 11, 19] and as is shown in this paper, by engagement of the fetal head.

In animals it has been demonstrated that during fetal head compression, cerebral blood flow and oxygen availability to the brain diminish [14] whereas intracranial pressure increases. At the same time, a slowing in fefal EEG was observed [12].

In human fetuses, similar changes in the EEG were recorded during labor with ruptured membranes [7] ; these EEG changes coincided with the peak of uterine contractions that caused type 1 dips in FHR tracing.

FEDRICK and BUTLER [8] found that the incidence of cerebral birth trauma (subdural hemorrhage associated with lacerations of the falx, tentorium or great cerebral vein) increased with the length of time between rupture of membranes and delivery.

Marked cephalic deformation and severe brain damage in the newborn demonstrated by necropsy, has been attributed by P.SCHWARTZ [21] to the absence of an intact bag of waters during labor.

The results from the first 215 labors analyzed in the Latin American Collaborative Study [20] indicate that the head of neonates born from labors with intact membranes show less overlapping of parietal bones and a lower incidence of caput succedaneum than neonates born of labors in which early amniotomy was performed.

MULLER et al [15] found that incidence of mental retardation was significantly lower in children born from labors in which membranes were not ruptured until delivery than in children in whose labors membranes were ruptured some hours before delivery.

In view of the findings reported in this paper and those of the bibliography herein reviewed it seems advisable to re-evaluate the common obstetrical practice of rupturing membranes artificially early in the first stage of labor as a routine procedure. Particularly since the view duration of labor [3, 17, 23] has not been confirmed by other investigators [9, 18, 20].

Even if early amniotomy would actually shorten the duration of labor, the benefits for the child are questionable. NISWANDER and GORDON [16] have reported that in multiparae the lowest incidence of children with neurological abnormalities at one year of age, corresponds to labors in which the duration of the first stage ranged between 11 and 15 hours. When the duration of the first stage was less than 11 hours, or more than 15, the incidence of neurological abnormalities at one year of age increased.

Nowadays the graphic recording of FHR may be obtained by external methods (ultrasounds, phonocardiography) not requiring amniotomy.

Monitoring of the fetus during labor can be performed with intact membranes. However if suspicious signs of fetal anoxia and acidosis (type 2 dips, variable dips or type 1 dips lasting more than 30-40 seconds) should appear in the FHR tracing, the artificial rupture of membranes would be justified to obtain samples of fetal scalp blood ; after amniotomy an electrode can be inserted into the fetal scalp for easier and better recording of FHR.

More research work should be performed to establish which are, if any, the long-term consequences on the central nervous system of the child, of the uneven compression and deformation of the head, which apparently is facilitated by the rupture of membranes [1, 6, 19, 20, 21]. Similarly, to verify the consequences of temporary occlusion of umbilical vessels during uterine contraction, which could also be facilitated after amniotomy, particularly in those fetuses with a loop of cord around the fetal neck or body [1, 6].

Finally, BROTANEK et al [4, 5] have shown that amniotomy produced a long lasting reduction of uterine blood flow. All these effects should increase the incidence of type 2 dips in FHR.

However, since the women studied in the present paper had no pregnancy complications, which could potentiate the reduction of uterine blood flow caused by amniotomy, the incidence of type 2 dips was equally low (less than 2 %), both in the group of labors with intact membranes and with early amniotomy. This ischemic effect of amniotomy on IVS blood flow may produce clear fetal consequences in pregnancies affected by diseases which chronically reduce IVS blood flow (such as toxemia, etc.).

Résumé

La fréquence cardiaque fœtale au cours du travail d’accouchement avec les membranes intactes et rompues.

II est déjà connu que la production de Dips Type 1 par les contractions utérines est significativement plus haute après amniotomie qu’avant la même. Les dips type 1 sont attribués a l’inégale compression de la tête fœtale ou a l’occlusion des vaisseaux ombilicaux pendant les contractions utérines.

Le but de ce travail c’est d’etablir le patron de FCF (Fréquence Cardiaque Fœtale) au cours de l’accouchement, quand l’amnios reste intact jusqu’a la dilatation complète du col. Aussi, de faire la comparaison entre la présence de dips type 1 au cours du travail (avec une dilatation cervicale plus grande que 5 cm) entre les femmes qui ont leurs membranes intactes et des autres auxquelles on les a rompues.

L’étude est faite sur 37 femmes enceintes qui n’ont pas eu des complications pendant la grossesse, qui ont commencé spontanément le travail a terme sans avoir besoin d’oxytocine, d’analgésie ou d’autres médicaments.

Dans 17 femmes les membranes sont rompues artificiellement au moment que la dilatation du cervix est arrivée a 5 cm (amniotomie précoce). Dans les autres 20 femmes les membranes restaient intactes jusqu’à la dilatation complète du cervix. La décision de rompre les membranes est faite au hasard. Dans tous les 37 accouchements on a registre la pression amniotique et la fréquence cardiaque fœtale. Quand les membranes sont intactes, la FCF est enregistrée en utilisant les ultrasons.

II n’y a eu aucune différence significative entre les deux groupes dans la duration du travail et le poids de naissance, le périmètre de la tête et le score d’Apgar des nouveau-nés (Figs. 2, 3, 4).

L’incidence des dips type 1 dans un groupe est indiquée par le percentage des contractions uterines qui produisent des dips type 1. Pour ce calcul toutes les contractions enregistrées dans un groupe sont sommées ensemble. La signification des différences dans l’incidence des dips type 1 entre les deux groupes est déterminée par un test statistique non paramétrique.

Quand les membranes restent intactes jusqu’a quelques minutes avant l’accouchement l’incidence des dips type 1 est basse ou presque nulle (Figs. 5 et 6).

Si la rupture se produit tardivement, quand la tête fœtale est déjà engagée, on enregistre une grande quantité de dips type 1 après la rupture (Fig. 7). Après l’amniotomie précoce, faite avant l’engagement de la tête, l’incidence des dips type 1 est basse (Fig. 8-A) ; quand la tête s’engage l’incidence augmente (Fig. 8-B).

L’influence des trois facteurs sur l’incidence des dips type 1 est analysée quantitativement :

1) Etat des membranes (intactes ou rompues),

2) Situation de la tête fœtale (avant ou après l’engagement),

et 3) circulaire du cordon ombilical autour du col (présent ou absent).

Pendant la période de dilatation cervicale (Fig. 9) l’incidence des dips type 1 est significativement plus basse dans le groupe des travaux avec des membranes intactes (3%) que dans le groupe avec amniotomie précoce (21%). Ce résultat est d’accord avec l’hypothèse que la poche des eaux protège la tête fœtale de la compression reçue pendant les contractions utérines.

On rencontre un résultat semblable aussi en étudiant l’incidence des dips type 1 dans la période avant l’engagement de la tête (Fig. 10-A) qu’après cet engagement (Fig. 10-B). Dans les deux conditions l’incidence de dips type 1 est significativement plus basse dans les travaux avec des membranes intactes que dans les travaux avec une amniotomie précoce. Ce fait suggère que l’effet protecteur de la poche des eaux est présent avant et après l’engagement de la tête.

Aux travaux avec les membranes rompues, l’incidence est significativement plus haute après que la tête est engagée qu’avant cet engagement (Fig. 11-B). Cela indique qu’en absence de la poche des eaux l’engagement favorise la production des dips type 1. Une même comparaison faite dans le groupe avec les membranes intactes ne démontre pas de différence significative avant et après l’engagement de la tête. Ce fait suggère que la protection donnée par les membranes intactes est présente même quand la tête est engagée (Fig. 11-A).

On n’a pas rencontré d’influence significative du circulaire du cordon ombilical autour du col sur l’incidence des dips type 1 dans les deux groupes étudies (Fig. 12).

L’incidence des dips type 2 (ralentissement tardif, [10]) est très basse (moins que 2%) dans les deux groupes, d’accord avec la tres bonne condition de tous les nouveau-nés. Les dips type 2 ne seront plus analysés dans ce travail.

L’évidence obtenue de l’observation clinique [1, 6,13,19, 20] indique que les dips type 1 peuvent être associes a la compression non-uniforme et a la déformation de la tête fœtale, laquelle peut produire des altérations a l’EEG [7], des lésions cérébrales [21] et des séquelles neurologiques [15].

Des recherches faites dans des animaux [12,14] démontrent que la compression de la tête fœtale réduit le flux du sang et l’apport d’oxygène au cerveau, lesquels sont les responsables des altérations de l’EEG. Aussi, cette compression augmente le tonus du vague avec la conséquente chute de la FCF.

Tous ces facteurs nous font conseiller la reconsidération de la pratique obstétricale, très commune, de rompre les membranes artificiellement tôt dans la période de dilatation cervicale, comme une méthode de routine dans la conduite du travail d’accouchement