Epidural Management and Choice of Drugs

Single injection versus catheter techniques

Single shot epidurals, without the use of a catheter, is still widely used in various settings, and is effective in providing intraoperative anaesthesia and analgesia in the immediate postoperative period. The major disadvantages of single shot epidurals are 1) the duration of postoperative analgesia is limited to the duration of action of the drug given and cannot be topped up, and 2) the risk involved in injecting a full "anaesthetic" dose of local anaesthetic into the epidural space without a test dose and without the ability to give slow increments. This means that the risks of inadvertent high block, total spinal and local anaesthetic toxicity (see below) are much greater. For this reason it is difficult to justify the use of single shot techniques under any circumstances, and especially by inexperienced practitioners.

Once a catheter is placed, the filter and its connector are attached to the proximal end of the catheter. At this point, a test dose of local anaesthetic is injected to ensure that the catheter is not in fact in the subarachnoid space. A small dose, e.g. 0.5% bupivacaine 3.5ml, bearing in mind the volume of the filter, which is about 1ml, is injected and the response noted over the next few minutes. This dose, if injected into the subarachnoid space, will cause complete surgical anaesthesia below the level of injection, and will be accompanied by the drop in blood pressure usually seen in spinal anaesthesia. It is unlikely to cause significant sensory block or hypotension if correctly injected into the epidural space. Following the test dose, the procedure for the administration of further local anaesthetic will depend on the purpose of the epidural. The important principle is that any bolus injection of local anaesthetic should be given incrementally, and the response carefully monitored, so that the practitioner can react promptly to any adverse reaction. Once a satisfactory block is established, whether for surgical anaesthesia, analgesia in labour or any other indication, the block can be maintained either by intermittent bolus administration of local anaesthetic (with or without opioids) or as a continuous infusion, if the necessary equipment is available.

Examples of procedures, techniques and drug choice
	Labour analgesia	LSCS	Hip / knee surgery	Laparotomy under general anaesthetic	Thoracotomy or fractured ribs
Level of Insertion	L2-L4	L2-4	L2-4	T8-10	At relevant interspace usually T5-7
Recommended height of block	T8-9	T6-7	T10	Upper abdo T7-8, lower abdo T10	Relevant area
Density of block	Minimal motor	Motor and sensory	Motor and sensor	Sensory + minimal motor	Sensory + minimal motor
Choice of Local Anaesthetic	0.1-0.25% bupivacaine	Lignocaine 2% + adrenaline 15-20mls or bupivacaine 0.5%	Bupivacaine 0.5%	Bupivacaine 0.25%-0.5% in theatre	Bupivacaine 0.25%-0.5% in theatre or to establish block
Choice of Opioid	Fentanyl 50mcg	Fentanyl 100mcg	Morphine 1-2mg or diamorphine 2-3mg	Morphine 1-2mg or diamorphine 2-3mg	Morphine 1-2mg or diamorphine 2-3mg
Infusion	Bupivacaine 0.1% + fentanyl 2mcg/ml	Postoperative bupivacaine 0.166% + diamorphine 0.1mg/ml	Not usually necessary	Postoperative bupivacaine 0.166% + diamorphine 0.1mg/ml	Postoperative bupivacaine 0.166% + diamorphine 0.1mg/ml
Rate of infusion	0-12mls/hour	0-8mls/hour	-	0-12mls/hour	0-8mls/hour
Note: 0.166% bupivacaine is made by diluting 10mls of 0.5% with 20mls saline

Choice of drugs

The choice of drugs administered epidurally depends on the indication for the epidural:

• Surgical anaesthesia - requires dense sensory block and usually moderate to dense motor block. To achieve this, concentrated local anaesthetic preparations are required. The most commonly used local anaesthetics in this setting are 2% lignocaine 10-20ml (with or without adrenaline 1:200 000) or 0.5% bupivacaine 10-20ml. The latter has a longer duration of action, but a slower onset time, compared with lignocaine.
• For analgesia during labour, 0.1-0.25% bupivacaine 5-10ml is more popular, as it produces less motor block.
• Postoperative analgesia, weaker concentrations of bupivacaine, e.g. 0.1-0.166% with or without added low dose opioids, by bolus, continuous infusion or PCEA (patient controlled epidural analgesia) has been shown to be safe and efficient when given by via a syringe pump.

Opioids in the epidural space

The addition of opioids to local anaesthetic solutions has gained popularity; as the opioids have a synergistic effect by acting directly on opioid receptors in the spinal cord. Various opioids, such as morphine (2-5mg), fentanyl (50-100mcg) and diamorphine (2-4mg), have been used successfully both alone and in combination with local anaesthetic drugs, during labour, for intraoperative use and for postoperative analgesia. The combination of low-concentration local anaesthetic and low-concentration mixtures of opioids, administered by slow infusion rather than as intermittent boluses, has, in particular, been shown to be very effective in the management of postoperative pain.

The amount of opioid, e.g. diamorphine in the examples above, should be reduced where there is an increased risk of respiratory depression, i.e. the elderly, the very frail or in patients with significant chronic obstructive airway disease.

Caution should be exercised when morphine is administered epidurally, as it is associated with delayed respiratory depression. This is thought to be as a result of its low lipid solubility, which means that instead of binding to opioid receptors in the spinal cord, some of the drug remains in solution in the CSF, and the circulation of CSF transports the remaining drug to the brainstem where it acts on the respiratory centre. This may occur many hours (up to 24 hours) after morphine has been administered epidurally.

Opioids have also been used on their own in the epidural space. Pethidine (meperidine) 25-75mg, in particular, has a structure similar to local anaesthetics and is effective in providing surgical anaesthesia and postoperative analgesia.

All opioids given by this route have the potential to cause respiratory depression, and this should be borne in mind when the patient is discharged from the care of the anaesthetist. Patients should be managed postoperatively in an area with a high nurse-to-patient ratio, and should be monitored carefully with special attention to their respiratory rate and level of consciousness. Epidural opioids should be avoided where there are inadequate resources for such careful monitoring. Other drugs used successfully via the epidural route include ketamine and alpha-2 receptor blockers such as clonidine. 

Complications and Side Effects

Serious complications may occur with epidural anaesthesia. Facilities for resuscitation should always be available whenever epidural anaesthesia is performed.

Hypotension has been discussed and is the commonest side effect of successful therapeutic blockade for procedures above the umbilicus. It is especially common in pregnancy, both in labour and when used for Caesarean Section, and should be corrected promptly using fluid and vasopressors. The presenting symptom of hypotension is often nausea, which may occur before a change in blood pressure has even been detected.

Inadvertent high epidural block due to an excessively large dose of local anaesthetic in the epidural space may present with hypotension, nausea, sensory loss or paraesthesia of high thoracic or even cervical nerve roots (arms), or difficulty breathing due to blockade of nerve supply to the intercostal muscles. These symptoms can be very distressing to the patient and in the most severe cases may require induction of general anaesthesia with securing of the airway, while treating hypotension. If the patient has a clear airway and is breathing adequately they should be reassured and any hypotension immediately treated. Difficulty in talking (small tidal volumes due to phrenic block) and drowsiness are signs that the block is becoming excessively high and should be managed as an emergency - see total spinal.

Local anaesthetic toxicity can also occur as a result of an excessive dose of local anaesthetic in the epidural space. Even a moderate dose of local anaesthetic, when injected directly into a blood vessel, can cause toxicity. This is especially possible when an epidural catheter is inadvertently advanced into one of the many epidural veins. It is therefore vital to aspirate from the epidural catheter prior to injecting local anaesthetic. Symptoms usually follow a sequence of light-headedness, tinnitus, circumoral tingling or numbness and a feeling of anxiety or "impending doom", followed by confusion, tremor, convulsions, coma and cardio-respiratory arrest. It is important to recognise these symptoms early, and discontinue the further administration of local anaesthetic drugs. Treatment should be supportive, with the use of sedative/anticonvulsants (thiopentone, diazepam) where necessary, and cardiopulmonary resuscitation if required.

Total spinal is a rare complication occurring when the epidural needle, or epidural catheter, is advanced into the subarachnoid space without the operator's knowledge, and an "epidural dose" e.g. 10-20 ml of local anaesthetic is injected directly into the CSF. The result is profound hypotension, apnoea, unconsciousness and dilated pupils as a result of the action of local anaesthetic on the brainstem. The use of a test dose should prevent most cases of total spinal, but cases have been described where the epidural initially appeared to be correctly sited, but subsequent top-up doses caused the symptoms of total spinal. This has been ascribed to migration of the epidural catheter into the subarachnoid space, although the precise mechanism is uncertain.

Management of total spinal

• Airway - secure airway and administer 100% oxygen
• Breathing - ventilate by facemask and intubate.
• Circulation - treat with i/v fluids and vasopressor e.g. ephedrine 3-6mg or metaraminol 2mg increments or 0.5-1ml adrenaline 1:10 000 as required
• Continue to ventilate until the block wears off (2 - 4 hours)
• As the block recedes the patient will begin recovering consciousness followed by breathing and then movement of the arms and finally legs. Consider some sedation (diazepam 5 - 10mg i/v) when the patient begins to recover consciousness but is still intubated and requiring ventilation.

Accidental dural puncture is usually easily recognised by the immediate loss of CSF through the epidural needle. This complication occurs in 1-2% of epidural blocks, although it is more common in inexperienced hands. It leads to a high incidence of post dural puncture headache, which is severe and associated with a number of characteristic features. The headache is typically frontal, exacerbated by movement or sitting upright, associated with photophobia, nausea and vomiting, and relieved when lying flat. Young patients, especially obstetric patients, are more susceptible than the elderly. The headache is thought to be due to the leakage of CSF through the puncture site. Basic measures, such as simple analgesics, caffeine, bed rest, fluid rehydration and reassurance are indicated in the first instance, and are often sufficient to treat the headache. Where the headache is severe, or unresponsive to conservative measures, an epidural blood patch may be used to treat the headache. This procedure is effective in treating approximately 90% of post dural puncture headaches. If unsuccessful, the blood patch may be repeated, and the success rate increases to 96% on the second attempt. The blood injected into the epidural space is thought to seal the hole in the dura.

Procedure for epidural blood patch

Indications

• Clinical diagnosis of post dural puncture headache.
• Sufficiently severe so as to be incapacitating.
• Unrelieved by 2-3 days of conservative management.

Contraindications

• Unexplained neurological symptoms
• Active neurological disease
• Localised sepsis in lumbar area
• Generalised sepsis
• Coagulopathy

Technique

• Obtain informed consent following full explanation of technique, potential hazards and anticipated success rate
• Move patient to fully equipped work area
• Two operators required, both taking full sterile precautions (gloves, gown, mask)
• Position patient in lateral position or sitting
• Operator 1: sterilise skin over back, drape and perform epidural puncture at the same level as previous puncture or one level below
• Operator 2: simultaneously sterilise skin over antecubital fossa, drape and perform venepuncture withdrawing 20ml of blood.
• Blood is handed to operator 1 who injects blood via epidural needle until either the patient complains of a tightness in the buttocks or lower back, or until 20ml is injected
• Inject remaining blood into blood culture bottles for culture and sensitivity
• Nurse patient supine for 1hour followed by careful mobilisation.

Epidural haematoma is a rare but potentially catastrophic complication of epidural anaesthesia. The epidural space is filled with a rich network of venous plexuses, and puncture of these veins, with bleeding into the confined epidural space, may lead to the rapid development of a haematoma which may lead to compression of the spinal cord, and can have disastrous consequences for the patient including paraplegia. For this reason, coagulopathy or therapeutic anticoagulation with heparin or oral anticoagulants has long been an absolute contraindication to epidural blockade.

Infection is another rare but potentially serious complication. Pathogenic organisms can be introduced into the epidural space if strict asepsis is not observed during the performance of the block. The commonest pathogens are Staphylococcus aureus and streptococci. Meningitis has been described, as has epidural abscess. In addition to the symptoms of spinal cord compression described above, the patient may exhibit signs of infection such as pyrexia and a raised white cell count. Once again, a high index of suspicion is needed, and surgical decompression of an abscess should be performed without delay.

Failure of block can occur as a result of many factors, the most important being the experience of the operator. False loss of resistance during performance of the block may lead to insertion of the epidural catheter into an area other than the epidural space, with failure to establish anaesthesia. Segmental sparing occurs occasionally for reasons that are unclear, but are assumed to be the result of anatomic variation of the epidural space, so that local anaesthetic fails to spread evenly throughout the space. The result is that some nerve roots are inadequately soaked with local anaesthetic, leaving the dermatomes of these nerve roots poorly anaesthetised. Unilateral blockade occurs occasionally, and this is thought to be the result of a septated epidural space, with failure of the local anaesthetic solution to spread to one half of the epidural space. Positioning the patient on his side with the unblocked side down is sometimes successful in allowing spread of the local anaesthetic to the dependent side, giving bilateral anaesthesia. 

Further reading:

1. Sharrock NE, Haas SB, Hargett MJ et al. Effects of epidural analgesia on the incidence of deep vein thrombosis after total knee arthroplasty. Journal of Bone and Joint Surgery American Volume 1991;73:502-6.
2. Dalldorf PG, Perkins FM, Totterman S, Pellegrini VD. Deep venous thrombosis following total hip arthroplasty. Effects of prolonged postoperative epidural analgesia. Journal of Arthroplasty 1994;9:611-6.
3. Perler BA, Christopherson R, Rosenfeld BA et al. The influence of anaesthetic method on infrainguinal bypass graft patency: a closer look. American Journal of Surgery 1995;61:784-9.
4. Bach S, Noreng MF, Tjellden NU. Phantom limb pain in amputees during the first 12 months following limb amputation, after preoperative lumbar epidural blockade. Pain 1988;33:297-301.
5. Yeager MP, Glass DD, Neff RK, Brinck-Johnsen T. Epidural anaesthesia and analgesia in high-risk surgical patients. Anesthesiology 1987;66:729-36.
6. Bromage PR. Continuous Epidural Analgesia. In "Epidural Analgesia" Bromage PR (ed) W.B. Saunders 1978 p. 237-8.
7. Mulroy MF. Epidural opioid delivery methods: bolus, continuous infusion, and patient-controlled epidural analgesia. Regional Anaesthesia 1996;21:100-4.
8. Ngan Kee WD. Epidural pethidine: pharmacology and clinical experience. Anaesthesia and Intensive Care 1998;26:247-55.
9. Horlocker TT, Wedel DJ. Spinal and epidural blockade and perioperative low molecular weight heparin: smooth sailing on the Titanic (editorial). Anesthesia and Analgesia 1998;86:1153-6.
10. Rodgers A, Walker N, Schug S et al. Reduction of postoperative mortality and morbidity with epidural or spinal anaesthesia: results from overview of randomised trials. British Medical Journal 2000;321:1493-7 

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