Several mechanisms have been proposed to explain the association of epidural analgesia with instrumental delivery.
These mechanisms generally become manifest in the second stage of labour (from full dilatation of the cervix to delivery) and include:
1. blockade of the motor units of sacral nerves resulting from local anaesthetic agents administered during the first stage of labour. This effect may be cumulative and persist into the second stage (3). The resulting weakness of pelvic floor muscles reduces the effectiveness of maternal pushing and the involuntary bearing down reflex;
2. blockade of sensory fibres disrupting the coordination between maternal pushing and uterine contractions;
3. diminution of a neuro-endocrine reflex, in which distension of the vaginal wall causes an augmented secretion of oxytocin from the posterior pituitary. Evidence to support this includes an observed reduction in the force of uterine contractions and oxytocin levels during second stage of labour during epidural analgesia (4, 5);
4. depression of uterine contractility by adrenaline-containing local anaesthetic solutions by a direct effect similar to that of beta-agonist tocolytic agents used to inhibit premature labour (6).
In contrast to the above, there is evidence to show that epidural analgesia may accelerate an already prolonged and exhausting labour and reduce the need for delivery by caesarean section for failure to progress. The provision of effective analgesia reduces the inhibitory effect of endogenous maternal catecholamines on uterine contractility, attenuates the maternal acidosis and permits the mother to tolerate augmentation with oxytocin (7, 8, 9) (Chapter 33).
Evidence in the literature does not consistently support either view. Clarification of the issue has been difficult for a number of reasons:
1. Whilst the collective experience is great, the techniques used to achieve epidural analgesia over the last four decades have not been uniform. For example, the techniques vary in choice of agent, drug concentration, administration during the second stage and addition of an opioid. Over the same period of time, obstetric practice has evolved. Prolongation of the second stage beyond 2 hours in the absence of fetal distress is no longer an indication for forceps delivery (10). The definition of 'forceps delivery' has been revised to distinguish between true mid-cavity and low forceps (11). A tendency to avoid forceps delivery in favour of caesarean section has lowered the rate of instrumental delivery independent of anaesthetic practice (12). Finally, the inability to control for institutional or individual practice persists.
2. Many of the studies to date have been criticised for bias because allocation to the analgesic groups has been determined by maternal request for epidural analgesia rather than true randomization (13). For example, a parturient with a minor degree of cephalo-pelvic disproportion or occipito-posterior presentation is more likely to experience severe pain and therefore more likely to request epidural analgesia. Such patients are also more likely to require instrumental delivery. Two studies have attempted to eliminate this bias, one by randomizing study groups (13), the other by restricting the study group to those with occipito-anterior presentations (14). In both of these studies, epidural analgesia was associated with an increased instrumental delivery rate.
3. Early reports describing the association between instrumental delivery and poor neonatal outcome failed to differentiate between the different instrumental techniques used. It is now recognised that mid-cavity, rotational forceps deliveries are strongly associated with increased perinatal mortality whereas there is no such association with the use of low- and outlet-forceps, or Ventouse vacuum extraction.
Despite these difficulties, three firm conclusions can be drawn about the effects of epidural analgesia on the progress of labour:
1. Overall, epidurals 'per se' are associated with an increased incidence of instrumental deliveries but progressively more dilute concentrations of local anaesthetic agents reduce this increased incidence. The dose-sparing effect of opioid-local anaesthetic combinations further adds to this reduction.
2. Epidurals are associated with a prolongation of the second stage of labour. This is not associated with neonatal morbidity in an properly managed labour.
3. Low (as opposed to high) concentration local anaesthetic solutions appear to reduce the increased incidence of mid-cavity rotation forceps delivery.
The arguments for these conclusions are expanded below:
1. Progressively more dilute concentrations of local anaesthetic agents reduce the increased incidence of instrumental deliveries observed with epidurals.
The increasing trend towards the use of bupivacaine in low concentrations reflects a perception that in doing so there will be less motor block and that there will be maximisation of its property of differential sensory blockade thus providing analgesia without motor blockade, a lower instrumental delivery rate, and greater maternal satisfaction (15). This has been borne out by the observation that 0.5% bupivacaine is associated with high rates of malposition (3 times control rate) and forceps delivery (5 times control rate) (16).
While a progressive reduction from a high concentration has been associated with a reduction in instrumental deliveries (3, 17, 19), the experience with bupivacaine 0.25% (14, 20, 21, 22, 23, 24) and 0.125% (13) has not clearly demonstrated a decreased instrumental delivery rate. In contrast, studies using very low concentration solutions of both bupivacaine 0.0625% (25) and lignocaine 0.75% (26) have shown that epidural analgesia may be continued throughout the second stage without an increase in the instrumental delivery rate.
In the case of lignocaine 0.075%, this has been associated with less effective analgesia (26). The addition of epidural opioids to local anaesthetic solutions appears to improve the analgesic efficacy of low dose bupivacaine regimens which hitherto had demonstrated a reduction in the forceps delivery rate, but which were limited by high failure rates with respect to pain relief (25, 26, 27, 32). The combination of bupivacaine 0.0625% with fentanyl 0.002% has been shown to decrease the higher instrument rate which is observed when bupivacaine 0.125% is used alone whilst maintaining the same quality of analgesia (27). This is consistent with the dose sparing effect of opioids described in other series (33, 34, 35, 36, 37) (Figure 102.1) (Chapter 60).
There is further support for the practice of minimizing motor blockade and preserving involuntary expulsive efforts by reducing the amount of local anaesthetic which reaches sacral segments of the spinal cord. This support is derived from studies which demonstrate a reduced forceps rate as a result of practicing segmental epidural blockade. This technique involves positioning the tip of the epidural catheter near the T11 - T12 segments during the first stage of labour, thereby avoiding the progressive effects of local anaesthesia on pelvic muscle tone. When full cervical dilation has been achieved, it may be repositioned to include sacral segments during second stage (7, 28, 29, 30).
Ropivacaine may be further able to reduce instrumental delivery rate.
The practice of discontinuing the epidural when the cervix is fully dilated is widespread in labour wards. It should be appreciated that this will not reliably eliminate the effects of first stage epidural analgesia. Several studies have described persistent perineal anaesthesia after saline was blindly substituted for epidural solution during second stage. The forceps rate is not reliably reduced. This may reflect progressive motor weakness occurring during first stage (22). Moreover, the percentage of women reporting inadequate analgesia is clearly increased ( 21, 22, 26, 31).
2. Epidurals are associated with a prolongation of the second stage of labour.
The effect of epidural analgesia on the duration of labour has been studied in many series. As with the mode of delivery, results are inconsistent. In general, there appears to be no clear effect on the duration of the first stage (8, 22). The second stage is more consistently prolonged in both primiparous and multiparous women (8, 13, 39, 29, 40), but this prolongation appears to be less marked when dilute solutions are used (17, 26, 22, 27).
Several authors have argued that a prolonged second stage is not associated with fetal heart rate abnormalities, low Apgar scores (Table 35.1), or low umbilical cord pHs, as long as electronic fetal heart rate monitoring is employed, and maternal analgesia and hydration are maintained (13, 40, 41, 42). In addition, there is support for observation of a latent period in early second stage to allow descent of the head before pushing is commenced, as opposed to pushing from the time of cervical dilatation. It has been argued that the instrumental delivery rate can be reduced by the acceptance of a longer second stage (24, 40, 42, 43).
3. Low concentration local anaesthetic solutions appear to reduce the incidence of mid-cavity rotation forceps delivery.
The increased perinatal mortality rate associated with instrumental delivery is largely attributable to intracranial hemorrhage or birth asphyxia and is often associated with the use of mid-cavity, rotational (eg. Kiellands) forceps (1, 2).
Premature infants are at particular risk as are term infants in the presence of undiagnosed cephalo-pelvic disproportion. In contrast, morbidity associated with low forceps or ventouse, vacuum extraction appears to be confined to facial bruising and cephalohaematoma, respectively (44).
Low concentration local anaesthetic solutions can reduce the forceps delivery rate (43). The equivocal results obtained for 0.25% bupivacaine are of interest: a high incidence of mid-cavity forceps deliveries in some series (14, 20), low forceps or vacuum extractions in others (21) or no observed increase in instrumental deliveries in others still (22, 23, 24).
Similarly, 0.125% bupivacaine has not reliably demonstrated a non-increased instrument rate (13, 17). However, a high incidence of mid-cavity forceps deliveries is not apparent, with instrumental deliveries largely representing low forceps or vacuum extractions.
A meta-analysis of the subject has been recently published by Halpern et al. (45).
References:
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11. American College of Obstetricians and Gynecologists Committee Opinion: Obstetric forceps, #59, 1988.
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