The goal of modern epidural analgesia in labour is optimum sensory blockade without motor blockade. Parturients ideally are aware of contractions and descent of the fetal head, feel the urge to push and do so effectively, and feel the baby's expulsion - but without pain at any stage. It is fortunate for parturients and epiduralists alike that most of the pain of the first stage of labour is conducted along sympathetic nerves, which enter the spinal cord from T10 to L1. These nerves are small-diameter, unmyelinated C fibres, which are readily blocked with small volumes of dilute local anaesthetic (LA). It is therefore theoretically possible to obtain complete relief from this most severe and (for some) distressing pain without using the concentrated solutions which lead to motor blockade. Motor block is considered undesirable because of the possible increase in the need for assisted deliveries (1), loss of pelvic floor tone (which may interfere with fetal head rotation) (2) and impaired ability to ambulate. Russell (3) has described a method of assessing motor blockade in labour.
Attempts have been made to obtain analgesia without using LA, but the results of intrathecal opioids alone are disappointing (4). Apart from inadequate pain relief, there is a significant incidence of nausea, vomiting and pruritus. Data concerning combined spinal-epidural techniques (using spinal analgesia for the initial dose) in respect of labour outcome are awaited. As low-dose LA (e.g 0.125% bupivacaine alone) is generally inadequate for epidural analgesia (5), the addition of adjuvants is necessary. Fentanyl achieved initial popularity following work which demonstrated its efficacy in improving perineal analgesia (6), while pethidine and the expensive sufentanil have been used extensively in different parts of the world. More recently, clonidine has been tried with good results, although sedation is significant (7).
Critics of epidural analgesia suggest that there is an increased rate of intervention, particularly caesarean section (8), although evidence is conflicting. Chestnut's work suggests that infusions of 0.125% bupivacaine beyond 8cm cervical dilatation in primigravid women improve analgesia compared to placebo but at the expense of a prolonged second stage and increased instrumental deliveries (9). Importantly, caesarean rates are unchanged. Surprisingly, very low dose bupivacaine-fentanyl infusions which are maintained until full dilatation do not reduce the rate of intervention, even though motor block is reduced. Analgesia is, however, considerably improved (10), indicating that the practice of withholding analgesia in second stage is to be condemned. Active management, using oxytocin infusions, reduces the instrumental delivery rate (11). "Catastrophe" modelling techniques suggest that changes in epidural practice from high-dose to low-dose LA-opioid result in fewer interventions, despite increased use of epidurals in labour (12).
Low-dose patient controlled epidural analgesia (PCEA) may improve the rate of spontaneous vaginal deliveries (13). It certainly minimises drug consumption when compared to infusions and intermittent bolus as techniques for maintaining analgesia (14, 15) and may, by empowering the parturient, introduce an important factor in the debate over how best to manage the second stage (pain from which involves the larger pudendal nerve S2-S4 (Figure 21.1)): the woman's choice. The upright posture for delivery is historically important, and maternal satisfaction is enhanced if mobility in labour is possible (16). Excessive motor block makes both of these difficult. The challenge for obstetric anesthesiologists is to produce consistently a degree of sensory blockade that increases the likelihood of spontaneous delivery by minimising motor blockade and maximising expulsive powers. Optimum analgesia may enable parturients to push more effectively if they are neither distressed nor distracted by pain.
References:
1. Thorburn J, Moir DD Extradural analgesia: the influence of volume and concentration of bupivacaine on the mode of delivery, analgesic efficacy, and motor block. Br.J. Anaesth. 1981; 53:933
2. Walton P Anaesthesia 1984, p 218
3. Russell R Int J Obstet Anesth 1992 p 230
4. Caldwell L Reg Anesth 1994 p 2
5. Yau G Anaesthesia 1990 p 1020
6. Reynolds F Anaesthesia 1989, p 341
7. O'Meara M BJA 1993; 71: p 651
8. Thorp J Am J Obstet Gynecol 1993, p 851
9. Chestnut D Anesthesiology 1987; 66: p 774
10. Chestnut D Anesthesiology 1990; 72; p 613
11. CarIi F Int J Obstet Anesth 1993, p 15
12. Naulty J Anesthesiology 1988 69: 3A, A660
13. Ferrante M Anesth Analg 1991; 73: p 547
14. Gambling D Can J Anaesth 1988, p 249
15. Gambling D Anesth Analg 1990; 70: p 256
16. Murphey J BMJ 1991, p 564.