29. LABOUR ANALGESIA

It is estimated that about two thirds of normal healthy pregnant women suffer severe intolerable pain during labour and only 2% describe little or no discomfort. It is always the mother’s decision as to whether she will have any treatment for labour pain, but this can only be done in an informed fashion if she is educated about her pain control options.

The pain of childbirth is often rated by women as being the most painful experience of their lives. It is frequently severe but due to the large emotional experience of pain, each woman's experience of labour pain is unique. Analgesic options must therefore be varied to allow for such a wide variation in the pain experienced.

The most appropriate time to discuss the options for pain relief is before the woman goes into labour. There needs to be a degree of flexibility so that as the painful experience of labour progresses, the woman is allowed to exercise further options.

Physiological Effect of Labour Pain 

As with any sort of acute pain, a stress response is mounted to severe pain in labour. The woman will experience anxiety and fear; she may become pale and sweaty and hyperventilate. The hyperventilation can lead to giddiness, fatigue and circumoral tingling as well as to uterine vasoconstriction in response to a low carbon dioxide concentration. The autonomic response to pain will lead to an increase in the cardiac workload with tachycardia and vasoconstriction. Adrenaline release leads to hypertension and acidosis. There is delayed gastric emptying that may lead to nausea and vomiting. The progress of labour may be impaired due to severe pain as a result of inefficient contractions.

Relieving Labour Pain

Labour pain may be thought of as two different sorts of pain. The first stage of labour involves uterine contractions, and cervical dilatation and effacement. This causes autonomically mediated pain, which is poorly localised and often referred to the back, abdomen and upper thighs. The nerve impulses are transmitted to the spinal cord via visceral afferents (C and A-delta fibres) entering from T10 to L1 spinal segments. The second stage of labour is defined as the period after complete cervical dilatation until delivery of the foetus. The pain of second stage is due to stretching of the vagina and perineum and is somatic. It is better localised and is transmitted via the pudendal nerves to the spinal cord (S2 to S4).

There are many different ways of treating labour pain. The relief of labour pain ranges from the non-pharmacological to systemic opioids to regional anaesthesia.

Non-pharmacological methods are generally learned beforehand during antenatal classes and have a large role to play in the early part of labour and in conjunction with pharmacological methods. They include: psychological preparation of the parturient and her partner, having a support person present throughout labour, positioning and movement, relaxation and breathing techniques, massage, heat and cold, imagery, hypnosis and transcutaneous electrical nerve stimulation (TENS).

Relaxation and breathing techniques.

The term “psychoprophylaxis” means to prevent pain though psychological methods, and this will require a combination of antenatal instruction and the use of coping methods during labour. The basis of psychoprophylaxis is the belief that pain of labour can be suppressed by reorganisation of cerebral cortical activity. The expectant mother is taught to respond to the beginning of a contraction by immediately taking a deep “cleansing breath”, gently exhaling, and then breathing in a shallow pattern until the contraction ends as well as focusing on a specific object. It is claimed that by using this technique mothers experience 30% less pain in labour and that the incidence of forceps delivery is reduced.

Positioning and movement.

Pain relief requirements may be decreased again by up to 30% if the mother is mobile during labour. Changing to a more comfortable position may be of great benefit as long as lying flat on the back (aorto-caval compression) is avoided.

Heat, cold, showering and massage.

Are all harmless techniques that may provide additional comfort.

Hypnosis

It is claimed that the hypnotic trance achieves analgesia, shortens labour and that the acid-base status of the neonate is better at birth. In reality only about 25% of patients in labour, with the hypnotherapist present, can be hypnotised so that pain appreciation is adequately reduced. Usually hypnotic conditioning begins with sessions obtaining a greater degree of trance until a level of analgesia is acquired. The failure rate for self-hypnosis by pre-hypnotic suggestion is very high. Hypnosis is not without complications. Side-effects include anxiety, and even frank psychosis.

Acupuncture.

The success rate of acupuncture is relatively low i.e. less than 25%.

Transcutaneous electrical Nerve Stimulation (TENS).

The gate theory of pain proposes that stimulation of large myelinated A-ß nerve fibres will close the gate (i.e. increase the pain modulating function of the substantia gelatinosa). Pain sensation from A-delta and C nerve fibres may thus be altered or blocked. TENS is thought to affect A-ß fibres (although others suggest that the endogenous opioid system is responsible for TENS). Regardless of the aetiology, TENS has been reported to produce pain relief in 20 to 25% of mothers and to be of slight benefit in up to 60%.

Nitrous Oxide.

Nitrous oxide is an analgesic. The exact mechanism of action is unknown. About 50% of women find it effective for labour. For it to achieve its peak analgesic effects, it is necessary to start breathing it 45 seconds before a contraction, which is very difficult to time. Its onset of action is 15 seconds and the elimination is rapid as it is not very soluble in blood. A concentration of 50% is required to produce worthwhile analgesia. The side-effects include a feeling of disorientation or confusion and possibly nausea. Because it is completely eliminated via the lungs without being metabolised, there are no effects on the foetus. Unfortunately it is difficult to time effectively when in labour and so about 30% of women have no relief from nitrous oxide.

Opioids

Women in labour are commonly prescribed pethidine 1 to 1.5 mg/kg intramuscularly 4 hourly prn. This alone is effective in about 60% of patients. The dose is usually timed to be at least thee hours before delivery to avoid foetal respiratory depression. Patient controlled analgesia narcotics have also been used with patients receiving 15 to 25 µg bolus of fentanyl with a 5-minute lockout.

Epidural Analgesia

Epidural anaesthesia can provide complete analgesia for labour and delivery as well as for caesarean section; however, epidural anaesthesia requires a greater level of skill for the anaesthetist and nursing staff. Epidural anaesthesia may cause hypotension, delayed progress of labour and headache. Extremely rare complications include total spinal, epidural haematoma, epidural abscess and neurological damage.

Combined Spinal Epidural (CSE)

The indications for the use a combined spinal epidural include:

very early labour in women who wish to ambulate

late in labour for multiparous women

operative or instrumental delivery where epidural analgesia is
indicated postoperatively

EPIDURAL ANAESTHESIA FOR LABOUR

Epidurals are the most effective and consistently reliable way of relieving childbirth pain. An epidural will provide conduction anaesthesia of the spinal nerves and the spinal cord. (neuraxial block) The aim is to provide analgesia by blocking the A-delta and C fibres of the spinal segments involved in the transmission of labour pain. However, because spinal nerves transmit motor, autonomic and other sensory impulses, they will also be blocked if a large enough dose of local anaesthetic is applied to them.

Epidural Anaesthesia

The conduct of epidural analgesia for labour requires the operator to explain the procedure and gain consent for the procedure. A skilled assistant should be in attendance during the insertion and after the block has been established. The assistant should help to position the patient and perform 5 minutely observations of maternal blood pressure and heart rate, height of the block and foetal heart rate for 20 minutes after a top-up or the establishment of the epidural block. Where an epidural infusion is in use in labour and the block is stable, observations can be performed half-hourly with continuous CTG monitoring.

Intravenous access is established before the conduct of the epidural. A fluid bolus of at least 500 ml of crystalloid is given. Resuscitation drugs and equipment should be immediately available and checked.

After positioning the patient in the lateral or sitting position, the skin is prepared with antiseptic solution. The correct spinal level for epidural insertion is identified (usually L3/L4 or L4/L5) and local anaesthetic is infiltrated into the skin and subcutaneous tissues. An 18 or 16 guage Tuohy needle is inserted with the bevel directed cephalad. A loss of resistance technique is used to identify the epidural space and a 20 guage catheter is fed so that 3 to 4 cm remain in the epidural space. The catheter can then be tested with a 3 ml dose of local anaesthetic (generally 2% lignocaine) to ensure that it is correctly positioned. The total dose of local anaesthetic (for example 8 to 12 ml of 0.25% bupivacaine) is then given in increments until the correct block height is attained. (T10 upper level for first stage of labour) This may take up to 20 minutes with longer-acting local anaesthetics such as bupivacaine or ropivacaine. An infusion of weak local anaesthetic with opioid (for example 0.125% bupivacaine with fentanyl 2 ug/ml at 6 to 12 ml/h) is commenced to provide ongoing analgesia during the labour. Further top ups of the catheter may be given for breakthough pain.

Complications

The side-effects of the epidural depend largely on the dose of local anaesthetic used. A loss of sensation is inevitable and some degree of motor block can be expected. This generally means the patient cannot walk, will require a urinary catheter and may require a lift-out forceps delivery.

The autonomic blockade will produce vasodilatation and may create hypotension. If the block extends to the T1 to T4 fibres, then bradycardia may also occur. Shivering is very common. The cause is not clear, but there may be a degree of heat loss (although the women often do not complain of feeling cold) and it is more common with larger doses of local anaesthetic.

The complications of epidural analgesia range from the more common but mild to the rare and catastrophic.

Accidental dural puncture is usually recognized when it occurs by the free flow of CSF though the needle or catheter. The incidence is roughly 1 in 300 epidural insertions. When it is recognized, there are usually no serious complications. However, 80% of the women will develop a post dural puncture headache, some of which will require an epidural blood patch. If a large dose of local anaesthetic has been administered into the subarachnoid space, then this will cause a high spinal block and will lead to refractory hypotension and a loss of consciousness requiring intubation and ventilation until the block wears off.

Local anaesthetic toxicity is another potentially severe complication. If injected intravenously, the large dose used to establish an epidural block may cause fitting and loss of consciousness. If a large dose of bupivacaine is injected intravenously into an epidural vein, cardiac toxicity will occur.

Epidural infection leading to abscess or epidural haematomata will cause compression of the spinal cord leading to paraplegia if the mass is not compressed within 6 hours. This is rare and difficult to quantify. Neural injury due to parturition (obstetric palsy - often a foot drop or obturator nerve palsy from a difficult forceps delivery) occurs in one in 3000 deliveries. These are temporary and resolve within 6 weeks. Similarly, nerve root injury from needling of the epidural space may occur and are mostly temporary.

Backache occurs in up to 50% of women who have had a baby regardless of whether or not they have received an epidural. Most of this is related to changes in posture, relaxation of the pelvic joints and childbirth itself. Bruising and tenderness over the insertion site however is common.