PAIN RELIEF IN LABOUR - REVIEW ARTICLE

Professor A Rudra,
Calcutta National Memorial Hospital, India

Introduction		Epidural and subarachnoid administration of local anaesthetics & opioids for labour analgesia
Physiology of pain in labour		Local anaesthetics.
Neural pathway of pain		Opioids
The stress response to pain in labour		Patient-controlled epidural analgesia and combined spinal-epidural analgesia in labour
Severity of labour pain		Non-narcotic analgesic techniques for labour
Role of the Anaesthesiologist		Effects of pain-relief on labour, mother and foetus/neonate
History of pain relief		Effect of epidural analgesia on labour.
Psychological methods of pain relief		Effects on mother
Physical methods of pain relief		Effects on foetus/neonate
Inhalational analgesia		Drug interactions
Systemic opioid analgesia		Conclusion
Patient-controlled analgesia		References
Perineal infiltration, pudendal nerve block and paracervical nerve block

Introduction

Giving birth is a painful process. This applies to all social and ethnic groups and has probably been so since mankind walked upright. It is very difficult to measure pain which is recognised via the signals carried through the nervous system and the woman's intellectual response to the stimulus.

Physiology of pain in labour

Labour pain is the result of many complex interactions, physiological and psychological, excitatory as well as inhibitory. Pain during the first stage of labour is due to distention of the lower uterine segment, mechanical dilatation of the cervix and lastly due to stretching of excitatory nociceptive afferents resulting from the contraction of the uterine muscles¹. The severity of pain parallels with the duration and intensity of contraction².

In the second stage additional factors, such as traction and pressure on the parietal peritoneum, uterine ligaments, urethra, bladder, rectum, lumbosacral plexus, fascia and muscles of the pelvic floor increase the intensity of pain.

Neural pathway of pain

The uterus and cervix are supplied by afferents accompanying sympathetic nerves in the uterine and cervical plexuses, the inferior, middle and superior hypogastric plexuses and the aortic plexus. The small unmyelinated 'C' visceral fibres³ transmit nociception through lumbar and lower thoracic sympathetic chains to the posterior nerve roots of the 10th, 11th and 12th thoracic and also to 1st lumbar nerves to synapse in the dorsal horn⁴. The chemical mediators involved are bradykinin, leukotrienes, prostaglandins, serotonin, substance P and lactic acid⁵. As the labour progresses severe pain is referred to the dermatomes supplied by T10 and L1.

In the second stage, the direct pressure by the presenting part on the lumbosacral plexus causes neuropathic pain. Stretching of the vagina and perineum results in stimulation of the pudendal nerve (S2,3,4) via fine, myelinated, rapidly transmitting 'A delta' fibres³. From these areas, the impulses pass to dorsal horn cells and finally to the brain via the spino-thalamic tract.

The stress response to pain in labour

Segmental and supra-segmental reflex-responses from the pain of labour may affect respiratory, cardiovascular, gastro-intestinal, urinary and neuro-endocrine functions.

Respiratory - Pain in labour initiates hyperventilation leading to maternal hypocarbia, respiratory alkalosis and subsequent compensatory metabolic acidosis. The oxygen dissociation curve is shifted to the left and thus reduces tissue oxygen transfer, which is already compromised by the increased oxygen consumption associated with labour⁶.

Cardiovascular - Labour results in a progressive increase in maternal cardiac output, primarily due to an increase in stroke volume, and, to a lesser extent, maternal heart rate. The greatest increase in cardiac output occurs immediately after delivery, from the increased venous return associated with relief of venocaval compression and the autotransfusion resulting from uterine involution.

Hormonal - Stimulation of pain results in the release of beta-endorphine and ACTH from the anterior pituitary. Associated anxiety also initiates further pituitary response⁷.

Pain also stimulates the increased release of both adrenaline and noradrenaline from the adrenal medulla which may lead to a progressive rise in peripheral resistance and cardiac output. Excessive, sympathetic activity may result in incoordinate uterine action, prolonged labour and abnormal fetal heart-rate patterns. Activation of the autonomic nervous system also delays gastric emptying and reduces intestinal peristalsis.

Metabolic - Maternal: During labour, glucagon, growth hormone, renin and ADH level increases while insulin and testosterone level decreases⁷. Circulating free fatty acids and lactate also increase with a peak level at the time of delivery. Fetal : Maternal catecholamines secreted as a result of labour pain may cause fetal acidosis due to low placental blood flow⁸.

Severity of labour pain

The severity of labour pain varies greatly among women in labour. If women are asked during or shortly after birth to score their labour pain most rate it as severe while few mention little or no pain^9,10. Using the McGill pain questionnaire, Melzack et al in Montreal, Canada, found that labour pain usually rated a high score particularly among primiparae, those with a history of dysmenorrhoea and those belonging to low socio-economic status⁹.

Principles of pain relief

The essentials of obstetric pain relief are:

• Simplicity
• Safety
• Preservation of fetal homeostasis

Women who are given any form of analgesia should be monitored closely. After spinal or epidural anaesthesia they should be monitored with frequent measurements of blood pressure, level of consciousness and maternal oxygen saturation by pulse oximetry.

Role of the Anaesthesiologist

Anaesthesiologists do have a role antenatally. They should be ready to answer the mothers' questions about the methods of analgesia. It is important that women with serious underlying chronic disease should be assessed antenatally by the anaesthesiologist to adopt a management plan before the onset of labour. Good communication between obstetrician, physician, haematologist and any other relevant specialist can help the anaesthesiologist in the management of high-risk pregnancy.

History of pain relief

Ancient methods of pain relief included various plant-derived sedatives, acupuncture and physical methods such as binding.

• In 1847 James Young Simpson administered the first obstetric general anaesthetic using ether.
• In 1853 John Snow delivered Queen Victoria's eighth child under chloroform.
• In 1881 Stanislav Klikovitch described the use of nitrous oxide for labour in Russia.
• In 1902 morphine and hyoscine was first used in labour. Pethidine was first used in 1940.
• In 1931 Eugen Bogdan Aburel, Romanian obstetrician, described continous caudal plus lumboaortic plexus blocks in labour.
• In 1945 Curtis Mendelson described the syndrome of acid aspiration under general anaesthesia for caesarean section.
• In 1949 Cleland described continuous lumbar epidural block in labour.
• In 1958 Ferdinand Lamaze published his book suggesting that pain was a conditioned reflex triggered by uterine contractions, and that psychoprophylaxis could reduce pain.
• In 1961 Brian Sellick described cricoid pressure as a means of preventing gastric aspiration.

Psychological methods of pain relief

Methods of psychological analgesia can be divided into three broad categories :

• Natural child birth - the Read method.
• Psychoprophylaxis - the Lamaze technique.
• Hypnosis

Each technique claims the elimination of pain without any harm to the mother, the baby or to the progress of labour and without the need for chemical analgesia. All require adequate antenatal preparation. Still most women experience severe labour pain⁹. Furthermore, psychological analgesia can place increased demand on the staff.

Support during labour

A friendly atmosphere in the labour room is preferable to help a woman to cope with pain. Homely surroundings help to allay anxiety and reduce the need for pharmacological analgesia.

• Hypnosis. Hypnosis (hypnos, sleep) can produce analgesia and amnesia during labour and delivery for some selected patients. Only about 25% of women however are suitable as deep trance hypnotic subjects. And the technique relies on extensive preparation.
• Bio-feedback. This is borderline between psychological and physical methods of analgesia. Relaxation is a major component of psychological preparation for child-birth and is claimed to relieve pain, reduce anxiety and speed labour.

Physical methods of pain relief

• Transcutenous Electrical Nerve Stimulation (TENS). TENS was introduced to relieve pain in childbirth in the early 1980s. Since then the use of TENS in labour has become increasingly popular as it is simple to use and is non-invasive. The mode of action depends on the two principal theories. One that A-fibres are stimulated by the electrical stimulation preventing the transmission of afferent noxious stimulus originating from C-fibres, the other that the electrical stimulus increases endorphines and enkephalins within the system. TENS electrodes are applied over the dermatomes supplied by T10 to L1. The TENS machine then gives a low background stimulus which can be augmented at the time of each contraction. It has been observed in clinical practice that TENS may provide limited pain relief during the first stage of labour. Meta-analysis of randomised controlled trials of TENS in labour does not, however, confirm its efficacy.
• Acupuncture. Mentioned in the literature in 581 B. C. and widely practiced in China. Acupuncture is not used for childbirth in China, however, and there are no acupuncture points described for pain relief in labour.
• Water (bath or shower). A bath or shower is relaxing and should be encouraged. There has been enthusiasm in some quarters to extend this to the delivery of the baby under water and many maternity units have the facility to offer water birth. However, while its use during the first stage of labour is not discouraged, very few units would encourage the use of the birthing pool for the delivery of the baby. At present there is little evidence to support the use of immersion in water during labour¹¹.

Inhalational analgesia

Several inhalational agents, both gaseous and volatile, have been used successfully in labour. The earliest to be used were ether, chloroform¹² and cyclopropane,¹³ followed by trichloroethylene and methoxyflurane.¹⁴ Enflurane, isoflurane and desflurane¹⁵ are more recent additions.

Analgesia during labour can be provided by the inhalational anaesthetic agents in subanaesthetic concentrations thus relieving pain whilst maintaining maternal consciousness and avoiding regurgitation or aspiration of stomach contents. In fact, the competence of the upper oesophageal sphincter is well maintained under light general anaesthesia, although lost under mild sedation with barbiturate or diazepam¹⁶. Inhalational agents readily cross the placenta and the concentration in foetal blood soon approaches that of the mother but, since these agents are excreted almost entirely through the lungs, they are readily excreted from the newborn.

The efficacy of inhalational analgesia depends on the analgesic strength of the agent and on how quickly it reaches analgesic concentration after the start of inspiration. A rapid offset with complete elimination between contractions would prevent accumulation completely. Nitrous oxide is the best match in current use.

Various portable machines exist for administration of nitrous oxide blended with oxygen through an on-demand valve. Nitrous oxide concentrations can be varied from 0 to 75% in oxygen. For self-administration, a concentration above 50% nitrous oxide should not be allowed. Entonox, which is a mixture of 50% nitrous oxide and 50% oxygen is most commonly used.

• Ether has several side effects including potent emetic effects with an unpleasant pungent odour, irritant to the respiratory tract and explosive. Chloroform has a pleasant odour, is non-irritant, more potent and faster acting than ether but has undesirable, dose-related side effects, namely arrhythmias and liver damage.
• Methoxyflurane and trichloroethyline have been used for analgesia in labour, but have been withdrawn for other, nonobstetric, reasons.
• Enflurane and isoflurane have been given via a draw-over vaporiser in subanaesthetic concentrations to relieve pain in labour. The usual concentrations, in oxygen, of enflurane and isoflurane for self administration are 0.3-1% and 0.2-0.7%. Such concentrations will not change uterine contractility or responsiveness to oxytocin. The neonate is not affected by these analgesic concentrations of these inhalational agents. Enflurane, however, causes long-term drowsiness so was never popular. Both the agents are expensive and since neither shows a significant advantage over entonox in terms of analgesia they are unlikely to be widely used on their own.
• Desflurane is the newest volatile agent to be applied in labour. The chief advantage of this agent is rapid onset and offset of action, however it is expensive and since it has not been shown to provide superior analgesia to entonox, it is unlikely to become a popular agent for labour analgesia.

Systemic opioid analgesia

Opioids have been used for anaesthesia in labour for hundreds of years. However, it was not until the early twentieth century that techniques deliberately employing the analgesic effects of the opioids gained major attention. Unfortunately, dosage and effect are limited by maternal and neonatal side-effects, so that only moderate pain relief could be obtained with these drugs.

• Pethidine has become the most commonly used and widely investigated systemic opioid in labour. It is principally a muagonist but of a low potency. Administered as hydrochloride in a dose of 75-100mg intramuscularly it reduces labour pain by about 25%. Delayed gastric emptying is a prominent feature. Respiratory depression is not usually observed in women who receive pethidine, because contractions continue to be painful and to provoke hyperventilation. However hypoxic episodes have been observed probably associated with significant underventilation between contractions. The major metabolite, norpethidine, is itself active, and has convulsant properties. Thus, pethidine may be inadvisable for use in fulminating preeclampsia or eclampsia, particularly in repeated doses.
• Morphine fell from favour in the first half of the twentieth century, in part because of its association with "twilight sleep" and in part because of its addictive side effects.
• Meptazinol is a mixed opioid agonist/antagonist, act primarily at the kappa receptor. It is given in a dose of 100-150mg intramuscularly every 2-4 hours. In high doses it has dysphoric side effects and also produce nausea and vomiting. The antagonist properties of meptazinol may cause withdrawal in parturients dependent on mu-agonists. It has a reduced potential to cause respiratory depression.
• Buprenorphine is a partial agonist acting selectively at mureceptors. It is about 20 times as potent as morphine and has a high affinity for opioid receptors and slow dissociation from them. It has a capacity for self-antagonism, which tend to produce a biphasic time course of action. This may be observed for both analgesia and respiratory depression. It appears to have a long duration of action and though side effects are rare, when nausea and respiratory depression do occur they can be exceedingly persistent and difficult to reverse.
• Nalbuphine is a synthetic mixed mu-agonist/antagonist and a kappa-agonist. For analgesia in labour it is given in doses of 10-20mg intramuscularly. Maternal or foetal respiratory depression is less likely with nalbuphine due to the ceiling effect. The chief disadvantages of this drug are sedation and dysphoria.
• Fentanyl primarily acts on mu-receptors and is approximately 80-100 times as potent as morphine. It has a rapid onset action and shorter duration of action. The peak analgesic effect occurs within 5 minutes and the duration of effect is about 30 minutes after 1 mcg/kg administered intravenously. Fentanyl is principally bound to albumin which favours its transplacental transfer. For analgesia in labour 50-100mcg/hour is required, given in increments of 10mcg IV.
• Tramadol is a weak mu-agonist that has been prescribed in labour in doses of 50-100mg 4 hourly. The incidence of nausea is more common with tramadol than with pethidine or morphine¹⁷.
• Butorphanol is a synthetic narcotic given as a 1-2 mg dose which lasts 3 to 4 hours. Neonatal respiratory depression is reported to be less than with pethidine¹⁸.

Patient-controlled analgesia

Patient-controlled analgesia with intravenous administration of opioid analgesics was assessed for obstetric pain as early as 1970¹⁹. The patient's ability to control the analgesic administration may produce pharmacological as well as psychological benefits.