Regional & Spinal Anesthesia

Regional anaesthesia looks like a collection of unrelated procedures with their own landmarks, doses and complications. It is not. Every technique in the chapter answers the same three questions, and once those are held in view the material collapses into something small enough to reason about.

The first question is where the needle stops. Subarachnoid, epidural, plexus, individual nerve, fascial plane. That single fact determines onset speed, drug volume, block quality and almost every complication.

The second question is how much drug the space needs. A space bathed directly in cerebrospinal fluid needs almost nothing. A space where the drug must diffuse through fat, dura and connective tissue needs ten to twenty times as much.

The third question is what happens if the drug goes somewhere else. Nearly every serious complication of regional anaesthesia is drug in the wrong compartment: too high in the subarachnoid space, in a vein, or in the subarachnoid space when the epidural space was intended.

Hold those three and the rest is detail.

1. Why Regional Anaesthesia Is Chosen

Regional anaesthesia blocks conduction in nerves supplying the surgical field while the patient remains conscious, or lightly sedated, with an intact airway and intact protective reflexes.

The advantages are concrete rather than aesthetic. The airway is not instrumented, so the risks of intubation and of aspiration during induction are avoided. Blood loss in hip and knee surgery is reduced. Postoperative nausea is far less frequent than after general anaesthesia.

The analgesic advantage extends past the operation. A block that outlasts the incision reduces opioid requirement in the first postoperative day, which reduces sedation, ileus and respiratory depression in exactly the patients least able to tolerate them.

There is a further benefit in obstetrics that has nothing to do with the mother's comfort alone. Regional technique for caesarean section avoids general anaesthesia in a population with a difficult airway rate several times that of the general surgical population.

The limitations are equally concrete. A block can fail or be incomplete, the duration is fixed once the drug is injected unless a catheter is placed, and a conscious patient may find a long operation intolerable even with a perfect block.

Absolute contraindications are few and worth knowing exactly: patient refusal, infection at the injection site, raised intracranial pressure, and uncorrected coagulopathy or therapeutic anticoagulation. Severe hypovolaemia and fixed cardiac output states such as critical aortic stenosis are relative but practically prohibitive for spinal anaesthesia.

2. Local Anaesthetic Pharmacology

Local anaesthetics block voltage-gated sodium channels from the inside of the axon, preventing the depolarisation that propagates an action potential.

The molecule must cross the membrane in its uncharged form and then act in its charged form. This single fact explains the most examined clinical observation in the chapter: local anaesthetics fail in infected tissue.

Inflamed tissue is acidic. A lower pH pushes the equilibrium towards the charged species, leaving less uncharged drug available to cross the nerve membrane, so less drug reaches the site of action. Injecting into an abscess therefore produces poor block, which is also why abscesses are drained under general anaesthesia or with a field block placed in healthy tissue outside the inflamed area.

Three physicochemical properties determine three clinical behaviours, and they map cleanly.

The two chemical classes differ in metabolism and in allergy. Esters such as procaine, chloroprocaine and tetracaine are hydrolysed by plasma cholinesterase to para-aminobenzoic acid, which is the reason true allergy is far commoner with esters.

Amides such as lignocaine, bupivacaine, ropivacaine and levobupivacaine are metabolised in the liver, so their duration is prolonged in hepatic disease and in low cardiac output states. The mnemonic that amides contain a second letter "i" in the drug name is crude but reliable.

Maximum safe doses are examined directly and are worth committing to memory rather than deriving. Lignocaine is 3 mg/kg plain and 7 mg/kg with adrenaline. Bupivacaine is 2 mg/kg, and adrenaline raises this only modestly to about 3 mg/kg.

Adrenaline is added for three reasons at once. It causes vasoconstriction that slows systemic absorption, which prolongs the block and raises the maximum safe dose, and it serves as an intravascular marker since a sudden tachycardia after a test dose suggests the needle is in a vein.

The traditional teaching that adrenaline must never be used in end-arterial territories such as digits has been substantially revised for elective digital blocks with commercial low-concentration adrenaline, but the caution remains standard examination material and remains sensible in compromised digits.

Ropivacaine and levobupivacaine exist for one reason: cardiotoxicity. Racemic bupivacaine binds cardiac sodium channels avidly and dissociates from them slowly, which is why bupivacaine cardiac arrest is notoriously resistant to resuscitation. The single-enantiomer agents retain most of the sensory block with a wider cardiac safety margin.

3. Spinal Anaesthesia

In spinal, or subarachnoid, block a small volume of local anaesthetic is injected directly into cerebrospinal fluid, where it bathes the nerve roots without any diffusion barrier.

This is why the drug volume is so small and the onset so fast. Two to three millilitres of 0.5 per cent bupivacaine produce a dense surgical block within five to ten minutes, compared with fifteen to twenty millilitres and twenty minutes for an epidural covering the same dermatomes.

The needle is inserted below the termination of the spinal cord, which lies at L1 to L2 in the adult and lower, at about L3, in the neonate. The L3-L4 interspace is used, identified from Tuffier's line joining the iliac crests.

The layers traversed from skin inwards are skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura and arachnoid. The endpoint is unambiguous: free flow of clear cerebrospinal fluid.

Spread within the cerebrospinal fluid is what determines block height, and it is governed principally by baricity and patient position, with the injected dose mattering more than the volume or the concentration.

Baricity is the density of the solution relative to cerebrospinal fluid. Hyperbaric solutions, made by adding dextrose, sink with gravity, so the block follows position and is predictable and controllable. This predictability is why hyperbaric bupivacaine is the standard agent.

Isobaric solutions stay approximately where they are placed and are less affected by position. Hypobaric solutions rise, and are used rarely and deliberately, for example in a lateral position for hip surgery.

Height matters because of what lies at each level. T10 covers hip surgery and transurethral resection. T6 to T8 covers lower abdominal surgery. T4, the nipple line, is required for caesarean section because peritoneal traction is otherwise felt.

The autonomic block extends roughly two segments above the sensory block, and the motor block roughly two segments below it. The autonomic component is what causes the haemodynamic consequences, and it is always higher than the level the patient reports.

Sympathetic blockade produces the two predictable cardiovascular effects. Vasodilatation below the block level causes hypotension by reducing venous return and systemic vascular resistance. A block above T4 also blocks the cardiac accelerator fibres from T1 to T4, producing bradycardia rather than the compensatory tachycardia that would otherwise occur.

Management follows the mechanism: fluid preload or co-load, left uterine displacement in the pregnant patient, and vasopressors. Phenylephrine is preferred in obstetrics because it restores resistance without the fetal acidosis associated with ephedrine, which crosses the placenta and stimulates fetal metabolism.

4. Epidural Anaesthesia

The epidural space is a potential space outside the dura containing fat, lymphatics and a rich venous plexus. Drug injected here must diffuse across the dura to reach the nerve roots, which changes everything about the technique.

Larger volume, slower onset, segmental block. Fifteen to twenty millilitres are needed, onset takes fifteen to twenty minutes, and the block spreads from the injection level in both directions rather than filling everything below a level.

Identification uses loss of resistance to saline or air as the needle passes through the dense ligamentum flavum into the space, most often with a Tuohy needle whose curved tip directs a catheter cranially.

The catheter is the point. A catheter allows the block to be topped up, extended, prolonged for days, and titrated to a level, none of which a single-shot spinal permits. This is why epidurals dominate labour analgesia and postoperative analgesia for thoracic and major abdominal surgery.

A test dose is given before the full dose because the epidural space contains veins and lies immediately outside the dura. A small dose of local anaesthetic with adrenaline distinguishes both misplacements: subarachnoid placement produces a rapid dense block, intravascular placement produces tachycardia.

Combined spinal-epidural technique takes the speed and density of the spinal and adds the catheter of the epidural, and is used where an operation may outlast a single-shot spinal or where postoperative analgesia is planned.

5. Spinal Compared With Epidural

The comparison is examined more often than either technique alone, because the differences all follow from one anatomical fact.

The two risks trade against each other in a way worth stating explicitly. The spinal punctures the dura deliberately with a fine needle, so headache risk is low but not zero. The epidural should not puncture the dura at all, but if the large Tuohy needle does, headache risk is very high.

Similarly, systemic toxicity is essentially a non-issue for spinal because the dose is tiny, and a genuine hazard for epidural because the dose is large and the space is full of veins.

6. Complications of Neuraxial Blockade

Hypotension is the commonest, follows directly from sympathetic blockade, and is anticipated rather than treated as a surprise.

Total spinal is the emergency. It occurs when local anaesthetic intended for the epidural space enters the subarachnoid space, or when a spinal block ascends far higher than intended, blocking the cervical roots and the brainstem.

The presentation is progressive and unmistakable: rising sensory level, arm weakness, difficulty breathing and speaking, bradycardia, profound hypotension, then apnoea and loss of consciousness.

Management is entirely supportive and must be immediate. Secure the airway and ventilate, support the circulation with fluids and vasopressors, treat bradycardia with atropine, and sedate the patient, since a conscious patient who is paralysed and being ventilated is experiencing exactly what awareness under paralysis feels like.

Urinary retention follows blockade of the sacral parasympathetic outflow and is common enough that catheterisation is routine for longer blocks.

Vertebral canal haematoma is rare and catastrophic. It presents with back pain and progressive neurological deficit, and it requires urgent magnetic resonance imaging and decompression, ideally within eight hours, because delay converts a recoverable deficit into a permanent one. The risk is concentrated almost entirely in anticoagulated patients.

Epidural abscess presents more slowly, over days, with fever, back pain and then deficit, and is the reason for strict asepsis and for avoiding neuraxial technique through infected skin.

Transient neurological symptoms describe buttock and leg pain appearing after resolution of a spinal block, classically associated with lignocaine and the lithotomy position, self-limiting over days, and distinct from true neurological injury because there is no objective deficit.

Cauda equina syndrome, by contrast, is a genuine injury with saddle anaesthesia, bowel and bladder dysfunction and lower limb weakness, historically associated with high concentrations of local anaesthetic maldistributed through microcatheters.

7. Post-Dural Puncture Headache

Cerebrospinal fluid leaks through the dural hole faster than it is produced. Intracranial pressure falls, the brain sags, and traction on pain-sensitive meninges and vessels produces headache, with compensatory vasodilatation contributing.

The mechanism explains every feature of the presentation. The headache is postural, worse on sitting or standing and relieved by lying flat, because the sag is gravity dependent. It is frontal or occipital, and it is frequently accompanied by neck stiffness, photophobia, tinnitus and occasionally diplopia from sixth nerve traction.

Onset is typically twenty-four to forty-eight hours after the puncture, not immediately, which is a useful discriminator from other causes of postoperative headache.

Needle design is the dominant modifiable risk factor. Cutting needles such as the Quincke split dural fibres cleanly, leaving a hole that stays open. Pencil-point needles such as the Whitacre and Sprotte spread the fibres apart and produce a ragged tear whose inflammatory response seals it more effectively.

The numbers make the point. Incidence is under two per cent with fine-gauge pencil-point spinal needles, but rises to thirty to forty per cent after inadvertent dural puncture with a large-bore Tuohy epidural needle.

Other risk factors are young age, female sex, pregnancy, low body mass index, and a previous history of the same complication.

Conservative management includes bed rest for symptom relief, hydration, simple analgesia and caffeine, but none of these alter the natural history and most are supportive only.

The definitive treatment is the epidural blood patch. Fifteen to twenty millilitres of the patient's own blood are injected into the epidural space at or below the level of the puncture, where it clots and seals the leak while simultaneously raising epidural pressure and restoring cerebrospinal fluid pressure.

Relief is often immediate, and reported success after a single patch ranges from about sixty-five to over ninety per cent, with the great majority of the remainder responding to a second patch.

8. Neuraxial Blockade and Anticoagulation

This is the area where a procedural decision most directly produces a permanent injury, which is why timing intervals are examined so precisely.

The logic is uniform across every drug. Wait long enough after the last dose for the anticoagulant effect to have substantially worn off before puncturing near the epidural venous plexus, and wait long enough after the block or after catheter removal before restarting.

Catheter removal is treated with the same caution as insertion, because withdrawing a catheter can traumatise a vein exactly as inserting a needle can. This is the single most commonly missed point.

Aspirin alone does not contraindicate neuraxial blockade, and non-steroidal anti-inflammatory drugs alone do not either. Combination therapy is the danger, and aspirin plus a thienopyridine plus prophylactic heparin is a genuinely high-risk combination.

The American Society of Regional Anesthesia guidance on antithrombotic therapy, whose fifth edition appeared in 2025, remains the reference standard for these intervals, and Indian practice follows it.

Whatever the interval used, the block does not end the vigilance. Regular neurological observation after neuraxial blockade exists specifically to detect haematoma while it is still reversible, and any deficit that is deeper or longer-lasting than the block should be is imaged urgently rather than watched.

9. Local Anaesthetic Systemic Toxicity

Systemic toxicity follows the plasma concentration, not the site of injection, and rises with vascularity of the site, total dose and speed of absorption.

Central nervous system toxicity appears before cardiovascular toxicity, because the brain is more sensitive, and this ordering is the reason early symptoms are worth teaching to patients receiving large-volume blocks.

The sequence is progressive: perioral tingling and metallic taste, then tinnitus and visual disturbance, then agitation and muscle twitching, then convulsions, then coma. Cardiovascular collapse follows only at higher concentrations.

Bupivacaine breaks this rule, and dangerously so. It can produce cardiovascular collapse with little or no central warning, and because it dissociates slowly from cardiac sodium channels, arrhythmias are refractory and resuscitation is prolonged.

Prevention is more effective than treatment: calculate the maximum dose by weight, aspirate before each incremental injection, inject in divided doses rather than as a bolus, and use ultrasound guidance to keep the needle tip away from vessels.

Treatment has a specific antidote, which is unusual in anaesthesia. Stop injecting, call for help, secure the airway with 100 per cent oxygen, control seizures with a benzodiazepine, and give intravenous lipid emulsion.

The lipid emulsion regimen is examined. Twenty per cent lipid emulsion is given as a bolus of 1.5 mL/kg over two to three minutes, or 100 mL for patients over seventy kilograms, followed by an infusion, with the bolus repeated or the infusion doubled if the response is inadequate, to an upper limit of about 12 mL/kg.

The proposed mechanism is a lipid sink that partitions the highly lipid-soluble drug out of cardiac tissue, together with a direct metabolic effect on the myocardium.

Two modifications to standard resuscitation matter. Propofol is not a substitute for lipid emulsion, since its lipid content is far too low and its cardiovascular depression is unhelpful. And adrenaline doses should be reduced, with vasopressin avoided, since large doses impair the effectiveness of lipid rescue.

Cardiac arrest from bupivacaine may require prolonged resuscitation, and cardiopulmonary bypass has been used successfully where available, so the standard duration of a resuscitation attempt does not apply.

10. Peripheral Nerve Blocks

Peripheral blocks anaesthetise a defined territory without any sympathetic blockade of the trunk, so they avoid the hypotension that limits neuraxial technique in the frail or cardiac patient.

Ultrasound guidance changed the field, replacing paraesthesia-seeking and blind landmark technique with direct visualisation of the nerve, the needle and the spread of local anaesthetic. It improves success rates, reduces the volume needed, and reduces vascular puncture.

Nerve stimulation remains a useful adjunct, since a motor response at a low current confirms proximity, but it is no longer the primary method in most centres.

The upper limb is supplied by the brachial plexus, and the approach is chosen by the site of surgery rather than by preference.

Interscalene block causes ipsilateral hemidiaphragmatic paresis in a very high proportion of cases through spread to the phrenic nerve, which is why it is avoided in patients with significant respiratory disease or contralateral phrenic palsy.

Lower limb surgery requires blockade of both the lumbar and sacral plexus contributions, most often a femoral or adductor canal block combined with a sciatic block. The adductor canal block is preferred after knee arthroplasty because it preserves quadriceps strength and therefore allows earlier mobilisation.

Fascial plane blocks deposit large volumes of dilute local anaesthetic in a plane between muscles, where it spreads to reach multiple small nerves. The transversus abdominis plane block for abdominal wall analgesia and the erector spinae plane block for thoracic and abdominal analgesia are the two most widely used.

Intravenous regional anaesthesia, or Bier's block, uses a different principle entirely: a tourniquet isolates the limb, and local anaesthetic injected intravenously diffuses out to the nerves. The tourniquet is the entire safety mechanism, so it must not be released before at least twenty minutes have elapsed, and bupivacaine must never be used because sudden release would deliver a cardiotoxic bolus.

11. Obstetric and Paediatric Considerations

Pregnancy changes both the anatomy and the drug requirement, and the changes act in the same direction.

Engorged epidural veins reduce the volume of the epidural and subarachnoid spaces, so the same dose spreads further. Progesterone additionally increases neural sensitivity to local anaesthetic. The dose requirement therefore falls by roughly a third.

Aortocaval compression by the gravid uterus in the supine position reduces venous return, and combined with sympathetic blockade this produces severe hypotension. Left uterine displacement, by wedge or table tilt, is mandatory rather than optional.

Spinal anaesthesia is the default for elective caesarean section, requiring a block to T4, with phenylephrine as the vasopressor of choice and fluid co-loading at the time of injection.

Labour epidural analgesia is titrated with dilute local anaesthetic and an opioid, aiming for analgesia without dense motor block, so that the mother can move and push.

In children, caudal block through the sacral hiatus is the commonest regional technique, used for sub-umbilical surgery. It is usually performed under general anaesthesia, which is accepted practice in paediatrics precisely because a moving child is more dangerous than an asleep one.

Local anaesthetic toxicity risk is higher in neonates and infants because of reduced protein binding, lower alpha-1 acid glycoprotein levels and immature hepatic metabolism, so weight-based maximum doses are calculated with particular care.

12. Worked Examples

Example 1. A patient develops bradycardia and hypotension five minutes after a spinal for caesarean section, then reports difficulty breathing and tingling in the hands. What is happening and what is done?

The block is ascending too high. Hand tingling indicates a level at C8 to T1, and dyspnoea indicates intercostal paralysis, with the diaphragm still working through C3 to C5.

The bradycardia confirms it: blockade above T4 has reached the cardiac accelerator fibres, so the normal compensatory tachycardia is abolished.

This is a developing total spinal. Management is anticipatory rather than reactive. Give 100 per cent oxygen, be prepared to intubate and ventilate, give fluids and a vasopressor, and treat bradycardia with atropine.

Ensure left uterine displacement, since aortocaval compression is contributing. Do not tilt the patient head-down with a hyperbaric solution, since that will make the ascent worse.

Example 2. A labouring woman receives an epidural. Two days later she has a severe frontal headache, relieved by lying flat. Explain and manage.

Post-dural puncture headache after inadvertent dural puncture by the Tuohy needle. The large bore of the needle explains the severity and the high incidence, which reaches thirty to forty per cent after such a puncture.

The postural character is diagnostic and follows directly from the mechanism: cerebrospinal fluid leaks out, intracranial pressure falls, and the brain sags under gravity, so upright posture worsens it.

Conservative measures give symptomatic relief only. The definitive treatment is an epidural blood patch, injecting fifteen to twenty millilitres of the patient's own blood at or below the level of the puncture, which seals the leak and restores pressure.

A crucial caution: in a postpartum woman, a headache that is not postural, or that is accompanied by fever, seizures, focal deficit or hypertension, is not this diagnosis, and pre-eclampsia and cerebral venous thrombosis must be excluded.

Example 3. Ten minutes after a supraclavicular block a patient reports a metallic taste and tinnitus, then convulses. What is the treatment sequence?

Local anaesthetic systemic toxicity from vascular absorption or unrecognised intravascular injection.

Stop injecting immediately and call for help. Secure the airway and give 100 per cent oxygen, since hypoxia and acidosis both worsen toxicity substantially.

Control the seizure with a benzodiazepine in preference to propofol, because propofol contains far too little lipid to be therapeutic and depresses the cardiovascular system further.

Give 20 per cent lipid emulsion: a bolus of 1.5 mL/kg over two to three minutes, or 100 mL if over seventy kilograms, followed by an infusion, repeated if the response is inadequate.

If cardiac arrest supervenes, modify standard resuscitation by reducing adrenaline doses and avoiding vasopressin, and continue for a prolonged period, since bupivacaine dissociates slowly from cardiac sodium channels and recovery can be late.

Example 4. A patient for hip surgery received prophylactic enoxaparin at 10 pm. Surgery is at 8 am. Can a spinal be performed?

Yes. Ten hours have not quite elapsed at 8 am, so the block is delayed to 10 am to satisfy the twelve-hour interval after a prophylactic dose of low molecular weight heparin.

The interval exists because puncturing the epidural venous plexus while the anticoagulant is active risks a vertebral canal haematoma, which produces permanent paraplegia unless it is decompressed within roughly eight hours.

The next dose is withheld until at least four hours after the block, and if a catheter is placed, removal is treated with the same caution as insertion, since withdrawal can traumatise a vein just as insertion can.

Postoperative neurological observation is not optional. Any deficit deeper or more prolonged than the expected block requires urgent magnetic resonance imaging, not observation.

Example 5. Why does a dental block fail in an abscessed tooth, and what is done instead?

Because local anaesthetics must cross the nerve membrane in the uncharged form and act from within in the charged form, and inflamed tissue is acidic.

The lower pH shifts the equilibrium towards the ionised species, so less uncharged drug is available to diffuse across the membrane and less reaches the sodium channel. The block is weak or absent regardless of how much is injected.

Increasing the dose does not solve it and risks systemic toxicity, since the acidic tissue is also hyperaemic and absorption is rapid.

The solutions are to inject proximally into healthy tissue, using a regional nerve block placed outside the inflamed field, or to use general anaesthesia, and to drain the pus, since removing the inflammation restores the tissue pH.

Summary

Regional anaesthesia is three questions: where the needle stops, how much drug that space needs, and what happens if the drug is somewhere else.

Local anaesthetics block sodium channels, crossing the membrane uncharged and acting charged, which is why they fail in acidic infected tissue. pKa sets onset, lipid solubility sets potency, protein binding sets duration.

Spinal injects a tiny dose into cerebrospinal fluid for a fast dense block whose height depends on baricity and position. Epidural injects a large dose outside the dura for a slower segmental block that a catheter can extend and prolong.

Hypotension follows sympathetic blockade, and bradycardia is added once the block passes T4. Total spinal is the emergency, treated entirely by supporting airway and circulation, with sedation, until it recedes.

Post-dural puncture headache is postural, commonest after a large needle punctures the dura, and definitively treated by epidural blood patch.

Anticoagulation intervals exist to prevent vertebral canal haematoma, and catheter removal counts as a puncture.

Systemic toxicity gives central symptoms before cardiovascular collapse, except with bupivacaine, and lipid emulsion is the specific treatment.

Peripheral blocks avoid trunk sympathetic blockade, and each approach carries a characteristic complication that follows from what lies next to the target.