What local anaesthetic agents and adjuvant drugs are used for spinal anaesthesia, including typical doses and patient‑specific considerations?

Drugs Used in Spinal Anaesthesia

Local Anaesthetic Agents

Bupivacaine (Most Common)

Bupivacaine remains the gold standard local anaesthetic for spinal anaesthesia, available in both hyperbaric and isobaric formulations. 1

Hyperbaric bupivacaine 0.5% produces more predictable cephalad spread with fewer high spinal blocks compared to isobaric solutions. 2

Standard Dosing by Procedure:

• Hip/lower extremity surgery: 10-12.5 mg (2-2.5 ml) of 0.5% hyperbaric bupivacaine 2
• Lower abdominal surgery: Requires T10 block level 2
• Upper abdominal surgery: Requires T4-T6 block level 2
• Perineal/perianal procedures: Target S2-S4 level using sitting position during injection 2

Special Techniques:

• Ultra-low-dose technique for high-risk patients with difficult airway or respiratory disease: 3.6 mg bupivacaine (3.6 ml of 0.1%) combined with 10 μg fentanyl, though supplemental local infiltration may be required 2
• Incremental dosing: 1.25-2.5 mg every 3 minutes until adequate surgical level achieved, with total doses typically 7.5-15 mg 3

Levobupivacaine

Levobupivacaine is an isomerically pure S-enantiomer with a favorable safety profile compared to racemic bupivacaine. 1 Studies show no significant difference between levobupivacaine, bupivacaine, and ropivacaine for postoperative pain scores or supplementary analgesia requirements. 4

Ropivacaine

Ropivacaine produces less intense motor block of shorter duration compared to levobupivacaine, making it particularly useful for ambulatory surgery and obstetrics where earlier mobilization is desired. 1 This characteristic facilitates faster discharge from hospital. 1

Shorter-Acting Agents for Day Surgery

For day surgery procedures, hyperbaric prilocaine 2% or 2-chloroprocaine are recommended to facilitate faster recovery and discharge. 4, 2

2-Chloroprocaine Dosing (FDA-Approved):

• Without epinephrine: Maximum 11 mg/kg, not exceeding 800 mg total 5
• With epinephrine (1:200,000): Maximum 14 mg/kg, not exceeding 1000 mg total 5
• Dosage should be reduced for elderly, debilitated patients, and those with cardiac or liver disease 5

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Opioid Adjuvants

Morphine (First-Line for Prolonged Analgesia)

For procedures requiring prolonged postoperative analgesia, intrathecal morphine 50-100 μg is the first choice, providing effective analgesia with optimal side effect profile. 6

Dosing Strategy:

• Standard dose: 50-100 μg for most procedures 6
• High-intensity pain procedures: 150-300 μg 6
• Pediatric dosing: 30-50 μg/kg with appropriate monitoring 6

Critical Safety Requirement: Respiratory monitoring for 24 hours is mandatory due to delayed respiratory depression risk. 6

Side Effects:

Morphine is associated with pruritus, nausea, and vomiting, with increased risk at higher doses. 6 The combination of bupivacaine plus morphine is superior to bupivacaine alone for decreasing pain scores, supplementary analgesic consumption, and time to first analgesic request. 4

Fentanyl (Rapid Onset, Shorter Duration)

For shorter procedures or when rapid onset is needed, intrathecal fentanyl 25 μg is the first choice. 6

• Provides rapid onset sensory enhancement with moderate duration 6
• Caution: May lead to acute opioid tolerance when combined with morphine 6
• For labor analgesia: 2.5 mg bupivacaine with up to 15 μg fentanyl 3

Comparative Evidence: A 2023 study found that nalbuphine 1 mg provided longer duration of effective analgesia (388 minutes) compared to fentanyl 25 μg (305 minutes) when added to 15 mg hyperbaric bupivacaine. 7

Sufentanil

Alternative to fentanyl at 10 μg intrathecally, though less commonly used. 6 For labor analgesia via intrathecal catheter: 2-7.5 μg combined with bupivacaine. 3

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Alpha-2 Adrenergic Agonist Adjuvants

Clonidine

Clonidine 1-2 μg/kg prolongs block duration and enhances analgesia but carries significant hemodynamic risks. 6

Evidence for Efficacy:

• Epidural clonidine plus local anaesthetic is superior to either agent alone for postoperative analgesia 4
• Clonidine plus morphine is superior to morphine alone for decreasing pain scores and supplementary analgesic use 4

Critical Limitations:

The American Society of Anesthesiologists recommends against routine use of clonidine in total knee arthroplasty due to limited and inconsistent evidence. 6 Significant hemodynamic risks include hypotension and bradycardia requiring frequent blood pressure monitoring. 6

Pediatric Use:

For epidural/caudal blocks in children: 1-2 μg/kg preservative-free clonidine 4

Dexmedetomidine

Intravenous dexmedetomidine 0.5 μg/kg provides prolonged sensory block with better postoperative analgesia than fentanyl. 6

• Administration: 1 μg/kg IV over 10 minutes, followed by 0.5 μg/kg/hour infusion 8
• Intrathecal use: 5 μg dexmedetomidine added to hyperbaric bupivacaine prolongs duration of analgesia (326 minutes vs 217 minutes) and sensory block 9

Critical Safety Requirement: Cardiac monitoring is mandatory due to risk of bradycardia and arrhythmias. 6 A 2022 study demonstrated that under IV dexmedetomidine sedation, adding intrathecal fentanyl to ropivacaine provided no additional benefit. 8

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Other Adjuvants

Magnesium Sulphate

Magnesium sulphate potentiates intrathecal opioid analgesia without significant independent side effects and enhances effect when combined with dexmedetomidine or morphine. 6

Not Recommended Adjuvants

Neostigmine: Not recommended despite analgesic efficacy due to significant nausea and vomiting. 6

Ketamine: Not routinely recommended due to neurotoxicity concerns, despite prolonging analgesia compared to fentanyl. 6

Midazolam: Not recommended due to inferior efficacy compared to other adjuvants and neurotoxicity concerns. 6

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Clinical Decision Algorithm for Adjuvant Selection

For Prolonged Postoperative Analgesia:

1. First choice: Intrathecal morphine 50-100 μg 6
2. Monitor for respiratory depression for 24 hours 6
3. Manage side effects (pruritus, nausea, vomiting) as needed 6

For Shorter Procedures or Rapid Onset:

1. First choice: Intrathecal fentanyl 25 μg 6
2. Alternative: Intrathecal sufentanil 10 μg 6
3. Alternative: IV dexmedetomidine 0.5 μg/kg 6

For Enhanced Block Duration Without Opioids:

1. Consider clonidine 1-2 μg/kg if hemodynamic stability can be maintained 6
2. Ensure frequent blood pressure monitoring and treatment protocols for hypotension/bradycardia 6

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Special Considerations for Day Surgery

Low-dose spinal techniques with shorter-acting agents are essential for day surgery. 4, 2

Recommended Approach:

• Use hyperbaric prilocaine 2% or 2-chloroprocaine 4
• Target spinal dosing to surgical site (lateral positioning for unilateral knee arthroscopy, sitting for perianal procedures) 4
• Restrict IV fluids to ≤500 ml to reduce urinary retention 4
• Use 25G pencil-point needles to reduce post-dural puncture headache incidence to <1% 4

Discharge Criteria After Spinal Anaesthesia:

• Return of sensation to perianal area (S4-5) 4
• Plantar flexion of foot at pre-operative strength 4
• Return of proprioception in big toe 4
• Patients should achieve straight-leg raise by 4 hours after last dose; if not, further evaluation is warranted 2

Patients may be safely discharged with residual sensory blockade provided the limb is protected and appropriate home support is available, with written instructions regarding expected duration and conduct until normal sensation returns. 4

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Common Pitfalls and How to Avoid Them

High Spinal Block Prevention:

• Use hyperbaric solutions for more predictable spread 2, 10
• Employ incremental dosing technique (1.25-2.5 mg bupivacaine every 3 minutes) 3
• Limit increments to 2.5 mg maximum to minimize high block risk 3

Inadequate Analgesia After Block Resolution:

Develop an analgesic plan before spinal/regional anaesthesia, including premedication with oral analgesics and postoperative oral analgesics with written instructions. 4 Otherwise, patients may experience significant pain when the block wears off. 4

Hemodynamic Complications:

• Appropriate spinal dosing targeted to surgical site minimizes hypotension 4
• Restrict IV fluids to 500 ml to reduce urinary retention 4
• Have treatment protocols ready for alpha-2 agonist-induced hypotension and bradycardia 6