Total Spinal Anaesthesia - MD Anaesthesia Exam Answer

Definition

Aetiology / How It Occurs

1. Intentional spinal anaesthesia - inadvertent administration of a dose higher than intended
2. Epidural-to-intrathecal migration - the most common mechanism: accidental dural puncture during epidural placement, with epidural doses (5-10x the spinal dose) inadvertently injected intrathecally
3. Subdural injection - local anaesthetic in the subdural space can ascend intracranially (onset delayed 15-30 min, patchy block)
4. Caudal anaesthesia - accidental intrathecal injection of large caudal volumes
5. Retrobulbar/peribulbar block - rare, but reported (local anaesthetic tracks to brainstem via optic nerve sheath)

Pathophysiology

Risk Factors

• Obesity (increased intra-abdominal pressure compresses epidural veins, drives CSF cephalad)
• Short stature
• Pregnancy
• Increased intra-abdominal pressure (ascites, tumour)
• Spinal deformity (scoliosis, kyphosis)
• Extremes of age

• Large drug dose or volume
• Hyperbaric solution in head-down (Trendelenburg) position
• High level of injection
• Needle bevel directed cephalad
• Spinal anaesthesia attempted after a failed epidural (dural microperforations allow epidural drug to leak intrathecally)
• Epidural placed after accidental dural puncture at the same or different level

PREVENTION

1. Careful Patient Positioning

• Use isobaric solutions when possible, or control position meticulously with hyperbaric solutions
• Avoid excessive Trendelenburg after spinal injection
• Do not place patient in head-down position after a hyperbaric spinal injection

2. Appropriate Drug Dosing

• Use the minimum effective dose of local anaesthetic
• Avoid higher-than-required volumes for the planned procedure level
• Know the patient's individual risk factors (obesity, short stature, pregnancy) and adjust dose downward

3. Test Dose Before Epidural/Caudal Injection

• Administer 3 mL of 1.5% lidocaine + 15 mcg epinephrine (epidural test dose) before any epidural loading dose
• Intrathecal placement signals: rapid onset of lower limb warmth, weakness, sensory block within 2-5 minutes
• Intravascular placement signals: tachycardia >20 bpm increase within 30-60 seconds (epinephrine marker)

4. Aspiration Before Each Injection

• Aspirate the epidural catheter/needle for blood or CSF before every injection
• Even a properly placed catheter can migrate intravascularly or intrathecally over time - "every dose is a test dose" (Morgan & Mikhail)

5. Incremental/Fractionated Injection

• Never inject the full epidural volume in one bolus
• Administer in 3-5 mL increments, pausing 30-60 seconds between each to detect early signs of intrathecal spread
• Use the slowest injection rate possible

6. Use of Standardised Connectors

• ISO 80369-6 (NR-Fit) connectors make neuraxial and intravenous connections technically incompatible, preventing wrong-route drug administration

7. Situational Vigilance

• Be especially vigilant when performing spinal after a failed epidural attempt
• Reduce the spinal dose by 25-50% when there is known prior dural puncture
• Avoid large epidural top-up doses in labour when catheter position is uncertain

DIAGNOSIS

Timeline

• After spinal anaesthesia: typically within 5-10 minutes
• After inadvertent intrathecal epidural dose: rapid onset, within 2-5 minutes (because the full epidural volume enters the CSF at once)
• After subdural injection: delayed 15-30 minutes, patchy block

Symptoms and Signs (Ascending Pattern)

• Rapidly ascending sensory block beyond the intended level
• Bilateral upper limb tingling, paraesthesia, or weakness (lower cervical involvement)
• Nausea and anxiety
• Hypotension out of proportion to the block level

• Difficulty breathing, use of accessory muscles, then paradoxical breathing (intercostal loss)
• Inability to phonate or speak (intercostal + accessory muscle loss)
• Severe hypotension and bradycardia (cardiac accelerator block + sympathectomy)
• Weakness or paralysis of both arms

• Apnoea (diaphragmatic paralysis at C3-5)
• Loss of consciousness (brainstem/intracranial spread)
• Cardiac arrest (vagal predominance, severe hypotension, hypoxia)
• Fixed dilated pupils (brainstem involvement)

Investigations

• This is a bedside clinical diagnosis - no time for investigations
• ABG may show hypoxaemia and hypercapnia (if not immediately treated)
• SpO2 monitoring will show desaturation

MANAGEMENT

ABCDE Approach

A - Airway + B - Breathing

1. Reassure the conscious patient (psychological support is critical - patients may be paralysed but awake)
2. Apply high-flow oxygen (100% FiO2) via face mask immediately
3. If SpO2 falling or patient cannot protect airway - rapid sequence intubation (RSI)
   • Cricoid pressure, preoxygenation, and standard RSI drugs (propofol/ketamine + suxamethonium)
   • Intubation may be difficult in the obtunded, hypotensive patient with possible full stomach
4. Initiate mechanical ventilation - maintain normocapnia (PaCO2 35-45 mmHg)
5. Continue ventilation until the patient can perform a sustained head lift (adequate motor return)

C - Circulation

1. IV access - large-bore IV (if not already in situ), or central access if peripheral difficult
2. Fluid resuscitation - rapid IV crystalloid bolus (500-1000 mL) to counter venodilation and hypotension
3. Vasopressors:
   • Phenylephrine (50-100 mcg IV boluses) for isolated hypotension with normal/elevated heart rate (alpha-agonist)
   • Ephedrine (5-10 mg IV boluses) for hypotension with bradycardia (mixed alpha+beta action)
   • Noradrenaline infusion for refractory hypotension
4. Bradycardia:
   • Atropine 0.6 mg IV (up to 3 mg total)
   • Glycopyrrolate 0.2-0.4 mg IV
   • Adrenaline 0.1-1 mg IV if severe bradycardia or cardiac arrest threatened
5. Cardiac arrest:
   • Immediately commence CPR (2-minute cycles)
   • Follow ALS/ACLS protocol
   • Adrenaline 1 mg IV every 3-5 minutes
   • Defibrillation for shockable rhythms
   • In obstetric patients: manual left uterine displacement throughout; perimortem caesarean section at 4 minutes if no ROSC (the "4-minute rule")

D - Disability (Neurological)

• Keep patient informed and reassured if conscious - explain what is happening
• Sedation: if intubated, use IV propofol or midazolam infusion (patient may be awake but paralysed - an extremely distressing experience)
• Monitor level of consciousness regularly
• Check pupils

E - Exposure / Monitoring

• Full monitoring: ECG, NIBP/IBP, SpO2, capnography (if intubated), urine output (IDC)
• Position: supine with left lateral tilt (obstetric patients)
• Tilt the table slightly head-up only if haemodynamically stable, to try to limit further cephalad spread; however, do NOT do this if hypotensive - lying flat is preferred for cardiovascular support
• Keep patient warm (sympathetic block causes poikilothermia)

Specific Scenario - Total Spinal After Failed Epidural

• The dose used for epidural is 5-10x higher than a spinal dose, so total spinal from inadvertent intrathecal epidural injection is particularly severe and rapid
• Immediate RSI and cardiovascular support are paramount
• Do not attempt to reposition or repeat the block

When to Extubate

• Patient must demonstrate:
  • Sustained head lift for 5 seconds
  • Hand grip strength returning
  • Adequate tidal volumes (>5 mL/kg)
  • SpO2 >95% on FiO2 0.4
  • Haemodynamic stability without vasopressors
• Typical duration of block: 1-2 hours for lidocaine, up to 3-4 hours for heavy bupivacaine

Summary Algorithm

NEURAXIAL BLOCK → Rapidly ascending block + hypotension/bradycardia
         ↓
CALL FOR HELP + 100% O2
         ↓
Airway at risk? → YES → RSI + Mechanical Ventilation
         ↓
Hypotension → IV fluids + Phenylephrine/Ephedrine
         ↓
Bradycardia → Atropine ± Adrenaline
         ↓
Cardiac Arrest → CPR + ACLS
         ↓
REASSURE CONSCIOUS PATIENT THROUGHOUT
         ↓
Continue supportive care until block resolves (1-4 hours)

Key Exam Points

1. Distinguishing total spinal from high spinal: intracranial spread (LOC, fixed pupils) = total spinal
2. Mechanism after epidural: inadvertent intrathecal injection of a full epidural dose = 5-10x overdose in the CSF
3. Triad of total spinal: hypotension + bradycardia + apnoea (± loss of consciousness)
4. "Every dose is a test dose" - the single most important preventive mantra
5. Treatment is supportive - no reversal agent exists; maintain life until the block wears off
6. Psychological support matters - a paralysed, awake patient is terrified; communication is treatment
7. In obstetrics: left lateral tilt, vasopressors for uteroplacental flow, perimortem CS if cardiac arrest does not resolve within 4 minutes