Inhalational analgesia

In the UK, 60–70% of labouring women seek to achieve analgesia by inhalation of a 50 : 50 mixture of nitrous oxide and oxygen (N₂O/O₂). Marketed as Entonox and Equanox, the gas mixture is supplied in cylinders with blue body and blue/white shoulders and is piped to delivery rooms in many hospitals.

The gas is self-administered by inspiration through a facemask or mouthpiece, which opens a demand valve. Diffusion from alveoli to pulmonary capillaries and delivery to the brain by the cardiac output is not instantaneous – inhalation should start as soon as a contraction begins so maximum effect is achieved at its peak (see Chapter 7, Box 7.3).

The drug is non-cumulative, and does not affect the fetus. N₂O/O₂ causes a variable degree of sedation. Some women appear to be dreaming or drunk; others become somnolent or even briefly unarousable. Hyperventilation with N₂O/O₂ can be followed by a short period of apnoea, therefore the woman should hold the mouthpiece or mask herself. If she loses consciousness, she will let go. A few breaths of air eliminate the N₂O and consciousness will invariably be regained soon. A number of studies have questioned the analgesic effect of N₂O/O₂. Pain is still perceived under the influence of the drug – it is merely rendered more bearable by the intoxicated state.

The risk of cross-contamination between women sharing breathing systems dictates that mouthpieces and masks should be disposable or sterilised between users. Either a new disposable breathing system should be used for each woman, or a disposable breathing system filter interposed between the tubing and mouthpiece/mask.

Parenteral opioids

Clinical studies of pain scores have cast doubt on the analgesic efficacy of opioids in labour. The drugs are certainly sedative, inducing a feeling of disorientation and thereby making pain more tolerable. All opioids can induce maternal and neonatal respiratory depression (decreased Apgar and neurobehavioural scores). Gastric emptying is inhibited, and the incidence of nausea and vomiting increased.

Midwives can prescribe and administer controlled drugs in accordance with locally agreed policies and procedures. In the UK, pethidine is the most widely used intramuscular opioid. A usual dose of 100 mg lasts around 3 hours. Plasma pethidine concentrations are maximal in the neonate when the mother has received the drug about 3 hours before delivery. It is therefore illogical to withhold the drug if delivery is imminent, for fear of causing neonatal respiratory depression.

Naloxone is a specific opioid antagonist. The neonatal dose is 10 μg/kg intramuscularly, repeated if necessary.

Comparisons of pethidine 100 mg with diamorphine 5 mg, meptazinol 100 mg and tramadol 100 mg have failed to demonstrate any convincing benefits. An antiemetic (e.g. cyclizine 50 mg or prochlorperazine 12.5 mg) should be given intramuscularly along with whatever the chosen opioid. Because of the additive risk of respiratory depression, intramuscular opioids should never be given in the event of inadequate regional analgesia without prior reassessment of the woman by an anaesthetist.

Transcutaneous electrical nerve stimulation

Electrical impulses applied to the skin via flexible carbon electrodes from a battery-powered stimulator modulate the transmission of pain by closing a ‘gate’ in the dorsal horn of the spinal cord. The effect is similar to massage of the lower back by a birthing partner.

Transcutaneous electrical nerve stimulation (TENS) is used in about 1 in 20 labours in the UK. The technique is completely free from adverse effects, and can diminish the need for other analgesic interventions.

A study comparing TENS and ‘sham’ TENS’ (TENS devices which appeared to be working but had been disabled) failed to demonstrate reduced pain scores in labour. After delivery, however, those women who had had working TENS retrospectively rated their analgesia more highly. More of these women stated they would choose TENS again in a future labour.

Regional analgesia for labour

Regional analgesia is the provision of pain relief by blockade of sensory nerves as they enter the spinal cord. Local anaesthetic can be introduced into the epidural or subarachnoid (intrathecal) space, or both.

The epidural space is identified by the loss of resistance to depression of a syringe plunger as a Tuohy needle is advanced (Figure 9.1) through the ligamentum flavum. A catheter is then threaded through the Tuohy needle (Figure 9.2) to facilitate top-ups or continuous infusion. The subarachnoid space (containing cerebrospinal fluid (CSF)) is a few millimetres deeper, inside the meninges. Needles used for spinal injection are much finer than Tuohy needles (Figure 9.3). A significant advance has been the development of ‘pencil point’ tips (Figure 9.4). Compared with standard cutting bevel or Quincke needle tips, the leak of CSF and likelihood of consequent headache are vastly reduced.

Figure 9.1 Advancement of Tuohy needle with loss of resistance to syringe plunger.

Figure 9.2 Tuohy needle advanced through the ligamentum flavum, with a catheter in the epidural space.

Figure 9.3 Epidural Tuohy (above) and spinal needles.

Figure 9.4 Pencil point (above) and cutting bevel tip spinal needles (courtesy of SIMS Portex Ltd).

A trained anaesthetist must always be immediately available whenever regional analgesia is provided. Regional techniques improperly administered can be as hazardous as any general anaesthetic. Resources that should be available on every labour ward in the event of complications (see below) which might arise during the provision of regional analgesia are listed in Box 9.1.

Compared with every other analgesic technique, pain relief from regional blockade is undoubtedly superior. In the UK, 90% of consultant obstetric units provide a 24-hour epidural service; the average epidural rate is 24%.

Blockade of motor fibres causes relaxation of pelvic floor muscles and impairs expulsive efforts, therefore provision of adequate maternal analgesia with as little motor block as possible has emerged as a goal common to all regimens. Intermittent epidural boluses, continuous epidural infusions, and patient-controlled epidural analgesia all provide comparable pain relief and maternal satisfaction.

Unlike local anaesthetics, which prevent conduction of nerve impulses, opioids act on specific receptors in the spinal cord. Synergistic mixtures of local anaesthetic and opioids (usually fentanyl) have permitted significant reductions in the amount of local anaesthetic used. Side effects specific to the use of opioids are respiratory depression (in the most unlikely event that opioid spreads cephalad to reach the brainstem) and pruritus.

Combined spinal-epidural analgesia entails an initial subarachnoid injection of fentanyl mixed with a tiny amount of local anaesthetic. The resulting motor block is sufficiently minimal for women to retain sufficient muscle power to walk in labour. However, proprioception (information from joint receptors to maintain balance) can be impaired. In the absence of any proved advantage to mother or baby of walking in labour, some argue that it is hard to justify the risk of falling.

Signed consent is not necessary for regional analgesia in labour. Verbal consent is sufficient, and a brief record should be made in the notes of the risks/benefits that have been discussed.

Table 9.1 outlines the contraindications to regional analgesia, together with the associated risks.

Table 9.1 Contraindications to regional analgesia, with associated risks

A full blood count (FBC) is not required unless antepartum bleeding has occurred, anaemia is suspected, or there is evidence of pre-eclampsia. If the platelet count is normal, a coagulation screen is not necessary. If a pre-eclamptic woman’s overall condition is deteriorating an FBC should have been processed no more than 2 hours before a regional block is undertaken. If platelet count is 70–100 × 10⁹/l or if the trend is steadily downwards, a coagulation screen must be normal if spinal/epidural block is to be undertaken. Petechiae or platelet count <70 × 10⁹/l are indications for platelet transfusion. Aspirin therapy alone is not a contraindication to regional analgesia.

Low molecular weight heparins (LMWHs) pose the risk of vertebral canal haematoma and consequent spinal cord or nerve root compression. This risk has to be weighed against the risk of emergency general anaesthesia in a woman denied regional blockade. As far as possible, institution of regional blocks – and removal of epidural catheters – should be undertaken when anti-factor Xa activity is least. After delivery, continued vigilance for signs of haematoma is essential. If symptoms of leg weakness or numbness, back pain, or bowel/bladder dysfunction develop (onset can be delayed beyond 24 hours) neurological/neurosurgical advice should be sought without delay.

Although a vertebral canal haematoma or abscess is exceedingly rare, transient neuropathy has an incidence of about 1 in 2000. Women can be reassured that epidural analgesia does not cause new, long-term backache.

All women should be warned about the possibility of dural tap – the inadvertent tearing of the meninges by the Tuohy needle or epidural catheter, which should have an incidence of <1%. This complication can predispose to two potentially serious sequelae: total spinal anaesthesia, and post-dural puncture headache (PDPH). The vast majority of pregnant women who develop PDPH will require an epidural ‘blood patch’. Other points worthy of explanation before performing a regional block in labour are listed in Box 9.2.

The rate of cervical dilatation, duration of second stage and the instrumental delivery rate are similar when epidural analgesia is established before or after 4 cm cervical dilatation. Regional analgesia does not result in an increase in the overall caesarean section rate or the rates specific for dystocia or fetal distress. However, a significant proportion of those women who undergo caesarean section for dystocia will have had epidural analgesia for labour. Risk factors for dystocia produce painful labour and a desire for epidural analgesia.

It is well worth explaining in advance to women with risk factors for operative delivery that analgesia for labour can be converted to effective anaesthesia to facilitate forceps or caesarean delivery. Some women need time to consider the notion of being awake for a caesarean section.

Complications of regional analgesia

The two principal potential complications of epidural injection of local anaesthetic are local anaesthetic toxicity and total spinal anaesthesia. Aspirating the catheter to exclude intravenous or subarachnoid placement can prevent both. The delivery of local anaesthetic to the brain and heart by the bloodstream causes the symptoms and signs of toxicity (Table 9.2).

Table 9.2 Symptoms and signs of local anaesthetic toxicity

Symptoms	Signs
Numbness of tongue or lips	Slurring of speech
Tinnitus	Drowsiness
Light-headedness	Convulsions
Anxiety	Cardiorespiratory arrest

Epidural venous engorgement in the obstetric patient predisposes to blood vessel puncture in up to 1 in 5 epidurals. If blood is aspirated, flush with saline and withdraw the catheter by another 1 cm. If blood is still present, re-site at another interspace. Local anaesthetic toxicity is not an issue with spinals because the mass of drug injected is so much smaller.

Total spinal anaesthesia is the effect of excessive local anaesthetic within the subarachnoid space. If a dose intended for the epidural space is inadvertently delivered to the subarachnoid space, cephalad spread can cause respiratory arrest by blocking innervation of the diaphragm (C3–C5), and profound hypotension secondary to extensive sympathetic blockade.

A ‘total spinal’ can occur even when no CSF has been apparent on aspiration, and remains a possibility hours after initiation of an epidural block and following several top-ups. The risk of inadvertent subarachnoid block is not eliminated by epidural catheterisation at another interspace after the dura has been punctured, because the dural tear might still allow passage of local anaesthetic to the CSF.

In addition to local anaesthetic toxicity and total spinal anaesthesia, maternal collapse can occur as a result of:

• massive haemorrhage (which might be intrauterine or intraperitoneal)

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