Respiration.
Epidural analgesia prevents transient hyperventilation during, and hypoventilation between, uterine contractions, so that maternal PaCO2 remains 28-32mmHg and PaO2 about 100mmHg (1). The incidence of hypoxaemia during first or second stage labour is 3 times greater with no analgesia, or 1.4 times greater with intramuscular pethidine, than when epidural analgesia is provided by bupivacaine 0.125% without fentanyl (2) (Chapter 87). Maternal hypoxia during labour has not correlated with non reassuring cardiotocography . Some studies have (3) and others (4, 5) have not correlated maternal hypoxia with low neonatal Apgar scores (Table 35.1) or abnormal umbilical cord blood gases.
Reduced Neuroendocrine Effects of Pain
The relief of pain and associated anxiety with continuous epidural analgesia reduces maternal catecholamine, beta endorphins, ACTH and cortisol. (6, 7, 8, 9) Total work of labour, maternal metabolism and oxygen consumption are reduced. Maternal and fetal acidosis are reduced (10) though active pushing during second stage sustains some metabolic acidosis whether or not epidural analgesia is administered.
Reduced Cardiovascular Effects of Pain
Epidural analgesia eliminates the increase in cardiac output, heart rate and blood pressure caused by pain.
Uterine Activity
Catecholamine alpha receptor stimulation causes uterine hypertonus, and beta receptor stimulation decreases uterine tone and contractility. Epidural analgesia without adrenaline may
(a) reduce effect of circulating catecholamines causing uterine hypo and hyper activity, and
(b) change incoordinate uterine activity to a normal labour pattern.
Allowing an epidural to wear off during second stage increases circulating catecholamines, leading to impairment of uterine activity and a longer second stage. (11)
Effect of analgesia on duration of 2nd stage of labour and mode of delivery
Studies of the effects of epidural analgesia can be classified into those in which a particular epidural drug or administration
1. Slows labour, increasing the rate of instrumental deliveries (12, 13, 14, 15, 16, 17, 18, 19) or has no effect (14, 20, 21, 22, 23, 24, 25, 26), and
2. Improves the progress of labour (11, 22, 25, 27, 28, 29)
Why do studies give conflicting results ?
There are many unanswered questions with regard to effects of epidural analgesia on second stage of labour and mode of delivery. When designing a study, particular attention should be paid to the following areas:
1. Patient matching:
Patients studied are not always comparable to controls in proportion of primigravidae / multigravidae, age, pre-existing medical or obstetric complication, maternal height, fetal size. When groups not receiving epidurals are compared with those undergoing epidural analgesia, the process of randomization should occur before analgesia is requested.
2. Conduct of labour:
Labours are not always comparable to controls. Onset of labour before or after 37 weeks, duration of active stage, posture of mother during second stage (30), timing and dose of oxytocin, prostacyclin, rupture of membranes all affect duration of second stage. Similar definitions are required between study and control groups of beginning of second stage (full cervical dilatation, or appearance at introitus of presenting part).
3. Obstetric Management:
The extent of labour monitoring, indications for augmentation and rates of instrumental and operative delivery are sometimes not specified or are not supervised by someone other than the provider of epidural analgesia.
4. Anaesthetic management:
Anaesthetic end-points and outcomes are not always consistent. Investigators should report the incidence of spontaneous vaginal delivery, quality of analgesia in both first and second stage, degree of motor block, rate of perineal trauma (31) and extent of maternal expulsive efforts. Studies do not always specify adverse effects on gastric emptying, fetal acidosis or respiratory depression. Among a large group of patients with a uniform sensory level, epidural analgesia will have different effects on labour haemodynamics.
5. Statistical power:
The size of groups may not be large enough to detect significant differences (32).
The mechanisms of prolongation of labour by epidural analgesia:
I. Somatic motor blockade increases dystocia.
Removal of the urge to push and perineal muscle relaxation may slow internal rotation and descent of the fetal head.
A. The use of dilute concentrations of local anaesthesia with/without opioids provides less motor blockade.
B. Some local anaesthetics (Chapter 4) preferentially block motor over sensory fibres (Chapter 88) (33).
C. Opioids alone (epidural or parenteral) reduce somatic block but provide inadequate maternal analgesia, increase gastric stasis and fetal depression. Epidural opioids increase duration of active phase of stage 1 (34).
D. Epidural clonidine and fentanyl, when used alone, preserve motor function but analgesia is inadequate (35).
E. Predictably, intrathecal bupivacaine in addition to or instead of, epidural bupivacaine reduces the required dose for labour analgesia but does not reduce the degree of motor blockade or the duration of the second stage (36).
II. Epidural blockade of pelvic autonomic nerves may abolish the increment in oxytocin levels that occurs normally between full cervical dilatation and crowning of the fetal head (37). Oxytocin is claimed to be crucial to correct rotation and descent of fetal head but does not correlate with uterine contractility or spontaneous delivery. Plasma oxytocin levels have questionable clinical significance (38).
III. Aortocaval compression, not epidural blockade, reduces uterine perfusion and contractility even in the absence of demonstrable hypotension (39).
IV. If the intravenous fluid load prior to an epidural is administered as a bolus, rather than at a maintenance infusion rate, a 20 minute decrease in uterine activity results (22). The bolus transiently inhibits posterior pituitary production of antidiuretic hormone and perhaps of oxytocin.
V. Medico legal climate (40).
Legal notions regarding the appropriate duration of the various stages of labour may influence obstetric management.
VI. An arbitrary time limit, not maternal or fetal well being, may be used to terminate labour and justify operative delivery.
VII. Epidurals (41) account for only 8.3% of total variability in the duration of second stage of labour. 75% of variability in the length of second stage cannot be accounted for (Figure 33.3). It is no wonder that an improvement in spontaneous delivery rate has not been shown despite absence of motor block in majority of parturients (32, 42, 43, 44). While the need for instrumental vaginal delivery will decline as motor power is retained, this effect can exert only a minor influence until all other factors influencing second stage duration are better understood.
Summary
Labour is unpredictable and influenced by many factors. Anaesthetic technique can affect the course of fetal descent and delivery but judicious obstetric and anaesthetic management provide safe maternal and fetal care without excessive prolongation of labour.
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