Obstetric anaesthetists, in routinely extending neuraxial analgesia up to the level of T4, are accustomed to dealing with regional anaesthetic blocks that other practitioners would regard as excessively high. It is inevitable that occasionally the block will extend beyond the anticipated area, either because of accidental subarachnoid or subdural administration or merely because of the unpredictability of spread in some individuals. Although many such blocks may be quite benign and not cause any cardiovascular or respiratory embarrassment, it is important that they are detected in order to pick up misplacement of the local anaesthetic, which may cause more serious problems later.
‘Total spinal block’ is strictly defined as a spinal block that results in unconsciousness and central depression of respiratory and myocardial activity, accompanied by massive vasodilatation. Since the same may also result from epidural and subdural blocks, and one should not wait until unconsciousness before acting, the terms ‘high regional block’ or ‘extensive regional block’ are preferred. A practical definition of these terms would be a regional block that results in the need for tracheal intubation or other airway intervention. The reported incidence of such blocks is between 1 in ~2000 and 1 in ~13,000, probably reflecting differences in definitions used in the studies from which these figures arise.
Problems and special considerations
The effect and spread of local anaesthetic drugs is enhanced in pregnancy, and this should be borne in mind when planning doses for a spinal or epidural block.
An apparently fixed spinal block may extend further if the patient is moved, even 30 minutes or more after the local anaesthetic has been administered. This particularly applies to rotation through the fully supine position from one side to the other and may be due to dural compression resulting from dilatation of the epidural veins, which act as a collateral circulation during aortocaval compression.
Early features of extensive block include weakness or tingling of the upper arms and shoulders, breathing difficulties, slurred speech and sedation. Symptoms and signs may develop late and insidiously.
Hypotension may be severe and may be associated with reduced placental perfusion and fetal hypoxia or ischaemia. Urgent delivery may be necessary both to relieve maternal hypotension and to protect the fetus.
Airway management following total spinal block is made more difficult in pregnancy because of the increased risk of aspiration and the difficulty in maintaining a clear airway without tracheal intubation.
A woman may find the experience of a high regional block very frightening, despite the anaesthetist’s reassurance that her breathing is adequate and she is safe. It is important to be aware of this, and to reassure her continuously; offering general anaesthesia should even be considered for extreme distress.
Epidural analgesia and anaesthesia
Relatively large doses of local anaesthetic drugs are used which, if they find their way into the wrong compartment, can cause a dangerously extensive block.
Prevention is the key, and this is achieved by maintaining a high index of suspicion and regarding every dose of local anaesthetic as subarachnoid until proven otherwise. The potential problems are best discussed under the following headings:
• Epidural analgesia. A test dose suitable for distinguishing subarachnoid placement should be used after the epidural catheter is inserted, and the effect should be assessed before further local anaesthetic is given. Each epidural dose should be given sufficiently slowly to allow detection of a spinal block before it spreads to a dangerously high level; doses should be administered at intervals of 5 minutes or longer, with the mother moving between increments. These precautions should be used with every dose in labour, since catheter migration has been known to occur between doses. The use of low- dose local anaesthetic-opioid mixtures reduces the risk to the mother if accidentally given intrathecally; the local anaesthetic concentration should be the lowest for the effect required.
• Epidural top-up for instrumental or caesarean delivery. Volumes of up to 20 ml concentrated solution may be injected over 3 minutes, the risk of extensive block being weighed against the need for rapid extension for surgery. Alternatively, 3-4 ml of the top-up solution can be given as a test dose, 3-5 minutes before the main dose. It has been suggested that the top-up can safely be given in the labour room and the patient transferred to theatre while the block is extending, although this is controversial, since the ability to monitor and/or resuscitate may not be ideal before or during transit. It is essential that the anaesthetist is by the patient at all times and ensures adequate monitoring and lateral tilt. Regular testing of the block is mandatory.
• Epidural after dural puncture with the Tuohy needle. If an epidural catheter has been re-sited following accidental dural puncture, the risk of high block is increased, both because the local anaesthetic can leak through the puncture and because the catheter can migrate. Epidural doses/infusions should be reduced and given by an anaesthetist.
• Subdural block. This is thought to occur in up to 1% of ‘epidurals’. It may occur when the epidural catheter is passed into the potential space between the dura mater and the arachnoid, probably after the needle has torn the dura. The block is characteristically slow in onset (20-30 minutes) and spreads cranially much higher than expected, often involving the lower cervical dermatomes. Extensive motor block is, however, uncommon, and hypotension is usually mild. The block tends to spare the lumbar and sacral segments and maybe patchy; consequently, pain relief is often poor. If analgesia is acceptable, it is tempting to leave the catheter in situ and to continue to use smaller doses. This technique is not recommended, because of the risk of a top-up rupturing the arachnoid, with subsequent development of an extensive subarachnoid block.
• Accidental subarachnoid block. This is rarer than subdural block, largely because the anaesthetist is usually alerted by the free flow of cerebrospinal fluid from the hub of the catheter. The consequences are far more hazardous, however, since the resulting block is very rapid in onset, has a considerable motor component, and is normally associated with severe hypotension.
Spinal anaesthesia
High blocks associated with spinal anaesthesia are related to greater spread rather than deposition of local anaesthetic into the wrong space. This may result from the use of hypobaric solutions, or compression of the dural sac from the outside as a result either of recent epidural top-up or of aortocaval compression, or it may represent an extreme of normal variation as anaesthetists have sought higher and higher blocks in order to avoid pain during surgery. The continuous presence of the anaesthetist and the immediate availability in the operating theatre of the necessary equipment and assistance ensure that further supportive measures are readily available if needed.
Prevention of excessive block is achieved by using the minimum necessary dose of local anaesthetic. Excessive barbotage should also be avoided. Maintenance of the natural kyphosis of the thoracic spine if in lateral tilt, or the use of pillows under the shoulders and head if in the full lateral position, will help prevent hyperbaric local anaesthetic spreading higher than the T4 dermatomes. Head-down tilt is very occasionally needed to encourage a recalcitrant block to spread high enough for surgery, but this should be used with great care and reversed as soon as the desired effect has been achieved. The same precautions apply if the mother is rolled through the full supine position as part of the positioning or if she is coughing or otherwise performing a Valsalva manoeuvre; these can result in sudden cranial spread of the block, and this can even happen at the end of a procedure when the block has been established for some time.
The ideal dose of spinal solution to use after a recent (failed) epidural top-up is uncertain. There have been reports of extensive blocks if normal spinal doses are used, presumably as a result of dural compression, but there have also been reports of normal responses or even inadequate anaesthesia if smaller doses are used.
Management options
It has been suggested that all women receiving regional anaesthesia for an operative procedure should be warned about the risk of a high block. Calm reassurance will be necessary for the woman and her partner in what may be a very distressing experience.
The basics of resuscitation (‘ABC’) should be remembered. Aortocaval compression should be prevented, and the full lateral position is best if cardiopulmonary resuscitation is not needed.
Oxygen should be given by facemask, and tracheal intubation should be performed early if a raising block progresses; waiting until the patient is unconscious may risk airway obstruction and/or aspiration of gastric contents. Careful use of a hypnotic agent (and neuromuscular blocking drug) will be needed for intubation in the presence of haemodynamic compromise.
Cardiovascular support includes intravenous fluids, vasopressors such as phenylephrine or ephedrine if bradycardic (adrenaline may be needed if hypotension is resistant) and cardiopulmonary resuscitation if cardiac arrest or severe myocardial depression is compromising cerebral oxygenation.
Rapid delivery of the fetus should be considered to protect it from hypotension and to relieve aortocaval compression.
Sedation and ventilation will need to continue until there is evidence that the block has regressed to a satisfactory level, i.e. haemodynamic stability with adequate spontaneous respiratory effort.
A post-incident debrief with the woman and her partner should be provided.
Key points
• All epidural doses should be divided into safe aliquots, if time permits.
• Subdural catheter placement is common and may progress to subarachnoid block.
• Spinal blocks can spread cranially even 30 minutes after administration.
• Careful and regular monitoring of the height of block is required after institution of spinal or epidural anaesthesia.
• Delivery of the fetus may protect it from ischaemia and may also benefit the mother.
Further reading
Jenkins JG. Some immediate serious complications of obstetric epidural analgesia and anaesthesia: a prospective study of 145,550 epidurals. Int J Obstet Anesth 2005; 14: 37-42.
Yentis SM, Dob DP. High regional blockade: the failed intubation of the new millennium? Int J Obstet Anesth 2001; 10: 159-61.