Assessing Obstetric Variables and Classifying the Risk
Last updated: 10-May-2016 09:01 AM
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Although a valid indication for expediting the birth may exist, a number of other factors that may influence the outcome must be assessed and addressed before vacuum extraction is attempted. These include:
- dilatation of the cervix;
- condition of the fetus and mother,
- uterine contractions;
- duration of the second stage;
- station, position and moulding of the head;
- recognition of cephalopelvic disproportion;
- experience of the operator.
Dilatation of the cervix
Vacuum extraction attempted before the cervix has reached complete dilatation is in general a more complicated procedure associated with higher risk of injury to the fetus and mother
and should be regarded as a contraindication in standard obstetric practice. The difficulty arises because the presenting part is usually stationed in the mid pelvis, the occiput is frequently posterior or transverse and the incompletely dilated cervix often hinders correct application of the cup over the flexion point. Furthermore, the cervix may become trapped under the cup when the vacuum is induced.
Practice point: Caesarean section should be the preferred method of delivery when the cervix is incompletely dilated.
Condition of the fetus and mother
The vacuum extractor is sometimes regarded as being too slow to use when the condition of the fetus suddenly deteriorates
but in experienced hands the decision-to-delivery interval should be no greater for vacuum extraction than for forceps delivery
. However, the device should not be employed as a ‘rescue tool’ and, if the operator is not experienced or if difficulty is anticipated, an alternative method of delivery - either forceps or caesarean section - may be more appropriate. Continuous fetal monitoring in conjunction with normal fetal scalp blood pH or lactate estimation may be of assistance in allowing labour to continue until spontaneous delivery occurs or the head descends to a lower station, making vacuum extraction safer and easier.
The greater the maternal expulsive effort, the less traction force will be required to assist the delivery. The mother’s ability to push effectively will depend to some extent on her physical and emotional state. Physical exhaustion may occur at any stage during active labour but is more likely to be encountered during the expulsive phase of the second stage when the extra effort of pushing is added to the stress of contractions
. Furthermore, extension of the ‘normal’ duration of the second stage and widespread use of epidural analgesia may also contribute to maternal exhaustion and a reduction in the woman’s ability to push. In addition, a mother’s morale and emotional state may influence the extent to which she will become involved in the birth.
Practice point: When a decision has been made to expedite a birth with the vacuum extractor, the attendant should explain the reasons for the procedure to the mother and emphasise the need for her cooperation and active involvement.
In medical conditions where excessive maternal exertion is contraindicated during the expulsion phase, forceps are usually preferred since they are less dependent on maternal effort. However, the vacuum extractor can be employed in such situations if the fetal head is allowed time to descend to the pelvic floor, using oxytocin infusion if necessary to maintain normal uterine action, and epidural analgesia to reduce any urge to push prematurely.
Practice point: Steady traction synchronous with contractions and a vacuum cup application that promotes flexion of the fetal head will almost always result in delivery of the infant without undue effort on the part of the mother.
Successful vacuum extraction relies on effective uterine contractions and maternal expulsive effort. Oxytocin introduced at the onset of the second stage of labour has been shown to shorten the duration. In a nulliparous woman with inefficient uterine action and a healthy fetus, contractions should be stimulated with oxytocin to achieve a response that approximates a normal contraction pattern. On the other hand, in multiparous women, inefficient uterine action is less common and caution is required before introducing oxytocin to increase uterine activity because of the risks of hyperstimulation of the uterus.
Practice point: When a decision has been made to proceed with a vacuum delivery, the operator should consider introducing oxytocin to improve the uterine contractions if they are observed to be weak or infrequent. This is especially important if maternal expulsive powers are reduced either through exhaustion or epidural analgesia.
Duration of the second stage
A prolonged first stage of labour may sometimes result in physical and emotional exhaustion of the mother and threaten the well-being of the fetus
. Should vacuum extraction be required subsequently to assist the delivery, the procedure may be more hazardous for the baby if maternal participation is reduced through exhaustion. For these reasons, the prevention or early detection of prolonged labour should be a major objective for all birth attendants. The cervicograph, which records cervical dilatation against time is a practical method for detecting abnormal progress in labour. The visual impact of the graph crossing the action line provides a timely warning that progress is too slow. Poor progress in labour is most commonly due to ineffective uterine action, a condition that nearly always responds to augmentation with oxytocin
Although the precise onset and actual length of the second stage of labour is usually not known, the normal duration has been traditionally stated to be somewhere between 1 and 2 hours for a nulliparous woman and between 30 minutes and 1 hour for a multipara
. More recently it has been suggested that prolongation of the second stage should not be regarded as a reason in itself for intervention provided the mother and baby’s condition are satisfactory and there is progressive descent of the presenting part. Thus with the change in the accepted ‘normal’ duration of the second stage has come a recommendation that the definitions for prolonged second stage of labour be modified along the following lines
- In a nulliparous woman without epidural analgesia, failure to progress over two hours or, if she has received regional analgesia, over three hours.
- In a multiparous woman without epidural analgesia, failure to progress over one hour or, if she has received regional analgesia, over two hours.
In addition, O’Driscoll proposed that the second stage of labour should be considered in two distinct phases for the purposes of management
. The first phase extends from the time of full dilatation of the cervix until the fetal head reaches the pelvic floor and may be regarded as an extension of the first stage. The second phase extends from the time the head reaches the pelvic floor until the baby is born.
Although a more conservative approach to delay in the second stage of labour may result in some babies being born spontaneously who might otherwise have had instrumental deliveries, those who eventually do require assistance may form a higher risk group for vacuum extraction because the obstetric conditions may be less favourable for instrumental delivery as a result of the self selection process.
Practice point: For this reason, when the second stage is prolonged, assessment of the condition of the fetus and mother, rate of progress of labour and suitability for vacuum extraction may require considerable obstetric judgement and should be performed only by experienced attendants.
Station and level of the fetal head
Station may be assessed by the traditional method of relating the lowermost portion of the presenting part to the plane of the ischial spines (
). By this method, engagement of the head is considered to have occurred when the presenting part is stationed at or below the ischial spines, for at this level the biparietal diameter of the head is presumed to have passed through the plane of the inlet of the pelvis.
The distance in centimetres between the cranial presenting part and the plane of the ischial spines is estimated by clinical examination. When the presenting part is at the level of the ischial spines, it is designated as being at zero station. If the head is above the spines, the station is allocated a minus value and, if it is below the spines, a plus value. Extensive moulding of the head and a large caput may confound the assessor into thinking that the head has descended deeper into the pelvic cavity than is actually the case. Care must be taken not to overestimate descent because adverse outcomes are more likely to be associated with mid or high pelvic extractions
. For this reason an attempt has been made to classify station of the head more precisely in relation to the ischial spines
- Mid pelvis: fetal cranium is 0cm (at the level of the spines) or +1cm below the ischial spines
- Low pelvis: fetal cranium is +2cm or +3cm below spines. Low fetal stations are subdivided into (a) sagittal suture rotated <45˚ and (b) sagittal suture rotated >45˚ from the vertical
- Outlet pelvis: fetal scalp is visible at or outside the introitus and rotation does not exceed 45˚.
Instrumental deliveries have been classified according to the above criteria into mid cavity, low and outlet procedures. Visibility of the scalp at the introitus is a simple and practical way of demarcating an outlet station from the higher classifications.
Level of the head
devised an alternative method for assessing engagement by estimating the proportion of the head expressed in fifths that is palpable abdominally (
Practice point: Engagement is considered to have occurred when not more than one-fifth of the head is palpable suprapubically.
Although the method is simple to perform, there is still a problem with interobserver variation and a tendency to overestimate descent. Reliability of the technique may be improved by defining the clinical characteristics that are palpable at the designated levels of the head (
Practice point: The problem of making a wrong diagnosis of engagement may be reduced by considering both the number of fifths of head palpable abdominally with the findings obtained by vaginal examination. Bimanual palpation may reveal that more head is palpable above the symphysis pubis than is detected by abdominal palpation alone.
Classifying vacuum deliveries according to station and level of the head and visibility of the scalp
As with forceps deliveries, vacuum extraction should be classified according to the station and level of the fetal head in the pelvis
. Thus, a vacuum delivery is regarded as mid-cavity when the leading part of the fetal cranium is at station +0cm, or +1cm (i.e. at or just beyond the ischial spines) and low-cavity when the leading part is at +2cm or +3cm. Low-cavity procedures are subdivided into two groups, rotational and nonrotational, depending on whether the sagittal suture is rotated more or less than 45˚ from the vertical respectively. A delivery is classified as outlet when fetal scalp is visible at the introitus, the head has reached the pelvic floor and the sagittal suture is rotated less than 45˚ from the vertical. For practical purposes a useful demarcation is to observe whether caput is visible in the introitus between uterine contractions (
Practice point: If fetal scalp is not visible the head may be significantly rotated (>45˚) resulting in the flexion point shifting away from the introitus. In these circumstances a manoeuvrable vacuum cup that incorporates Bird’s posterior design feature should be used for the procedure.
Moulding of the head
Moulding should be regarded by birth attendants as a measure of the compression to which the fetal head has been subjected during labour
. A method of assessing moulding has been proposed that involves scoring the degree of overlap of cranial bones at the coronal, sagittal and lambdoid suture lines thus allowing comparison of results from one vaginal examination to another
. A simplication of this method is shown in
Practice point: If the parietal bones are touching but not overlapping at the sagittal suture, moulding is recorded as slight (+); if the parietal bones are overlapping but can be reduced to the normal position by finger pressure, moulding is moderate (++); and if the overlapping of the bones cannot easily be reduced, moulding is extensive (+++).
The degree of moulding should always be assessed prior to attempting vacuum-assisted delivery. If significant moulding is present, additional force generated by applying the vacuum extractor incorrectly or without first correcting the fetal malposition and attitude may increase the risk of intracranial injury
Recognition of cephalopelvic disproportion and obstructed labour
Delay or failure to progress in the second stage of labour may be due to inefficient uterine action or cephalopelvic disproportion
. Disproportion may be described as true disproportion, when even the most favourable diameters of the presenting part are too big to pass through the pelvis, or relative disproportion, caused by the larger presenting diameters of the head that are commonly associated with transverse and posterior positions of the occiput. The distinction between the two types of disproportion may be impossible to make but should be attempted because correction of the malposition in the case of relative disproportion, either by enhancing uterine contractions with oxytocin or by manipulating the fetal head with the vacuum extractor, may allow safe vaginal delivery of the baby. Unfortunately, there is no reliable test that will diagnose cephalopelvic disproportion with certainty before the onset of labour. It may be suspected, however, if there is a history of previous difficult labours or when delay occurs in the late active phase or during the second stage of labour despite strong contractions and from the findings on clinical examination.
Extensive or increasing moulding of the head and a high presenting part that fails to descend despite strong uterine contractions especially if the fetal head is in occipitoanterior position are clinical findings suggestive of cephalopelvic disproportion
Relative disproportion may be diagnosed if the head has engaged and if the position is occipitoposterior or transverse and the capacity of the pelvis is assessed to be adequate for the size of the fetus. If steps are not taken to relieve disproportion, labour may become obstructed and threaten the well-being of both mother and baby. The primigravid uterus responds to obstruction with exhaustion and diminiution of uterine contractions whereas the multiparous uterus will often increase its activity in an attempt to overcome the obstruction and may culminate in a rupture.
Clinical signs suggestive of obstructed labour include
- tenderness and ‘ballooning’ of the lower uterine segment,
- formation of a uterine retraction ring,
- presence of oedema of the cervix and vulva,
- bloodstained urine
- bleeding from the vagina.
Since the diagnosis of cephalopelvic disproportion, in most cases, cannot be established unless uterine contractions are adequate, and because spontaneous rupture of the nulliparous uterus is rare, oxytocin stimulation should be tried in all nulliparae whenever there is delay in labour, provided there are no other contraindications to its use
. In the multipara, on the other hand, oxytocin should be used cautiously and only under close supervision because of the additional risk of hyperstimulation and uterine rupture. If delay in labour has advanced to the stage of obstruction, caesarean section should be the method of choice for the delivery. In areas where it is practised, symphysiotomy may offer an alternative in some cases of obstructed labour