What are the pharmacologic agents, dosages, onset, duration, adjuncts, contraindications, monitoring requirements, and alternative techniques for spinal anesthesia?

Pharmacology of Spinal Anesthesia

Local Anesthetic Agents and Dosing

Bupivacaine remains the most widely used agent for spinal anesthesia, with hyperbaric 0.5% bupivacaine at doses of 10-15 mg providing reliable surgical anesthesia for 2-6 hours depending on the procedure. 1, 2

Primary Local Anesthetics

Bupivacaine (Hyperbaric 0.5%)

• Standard dosing: 10-12.5 mg (2-2.5 mL) for hip/lower extremity surgery 2
• Lower doses (<10 mg) reduce hypotension in elderly patients with hip fractures 1
• Produces 2-6 hours of surgical anesthesia depending on dose and site 3, 4
• Hyperbaric solutions produce more predictable blocks with fewer high blocks compared to isobaric solutions 2

Ropivacaine

• Generally produces less intense motor block of shorter duration compared to bupivacaine 5
• Particularly useful in obstetrics and ambulatory surgery where earlier mobilization is desired 5
• Maximum dose for peripheral blocks: 3 mg/kg when using 0.2% solution 3

Levobupivacaine

• Pure S(-)-enantiomer with lower cardiovascular and CNS toxicity risk than racemic bupivacaine 4
• Similar clinical profile to bupivacaine but improved safety margin 5

Shorter-Acting Agents for Day Surgery

• Hyperbaric prilocaine 2% and 2-chloroprocaine are preferred for day surgery procedures 1
• Low-dose techniques with shorter-acting agents facilitate faster recovery and discharge 1, 2
• Patients should achieve straight-leg raise by 4 hours after last dose; failure warrants further evaluation 2

Procedure-Specific Block Levels and Dosing

Upper Abdominal Surgery

• Target block level: T4-T6 2
• Bupivacaine 0.75%: 15-25 mL (113-188 mg) with 10-20 minute onset 3

Lower Abdominal Surgery

• Target block level: T10 2
• Bupivacaine 0.5%: 15-30 mL (75-150 mg) with 15-30 minute onset 3

Hip Replacement

• Target block level: T10 2
• Bupivacaine 0.5% hyperbaric: 10-12.5 mg 2

Lower Extremity Surgery

• Target block level: L1-L2 2
• Lateral positioning for unilateral procedures (e.g., knee arthroscopy) helps target the block 2

Perineal/Perianal Procedures

• Target block level: S2-S4 2
• Achieved with sitting position during injection 2

Cesarean Section

• Bupivacaine 0.5%: 20-30 mL (100-150 mg) or 0.75%: 15-20 mL (113-150 mg) 3
• Pencil-point spinal needles (25G) mandatory to minimize post-dural puncture headache (<1% incidence) 1

Adjuvant Medications

Clonidine is the most evidence-based non-narcotic adjuvant for neuraxial blocks, providing prolonged analgesia and enhanced block duration. 6

Alpha-2 Agonists

Clonidine (Preservative-Free)

• Dosing: 1-2 micrograms/kg (or 30-75 micrograms in adults) 6, 1
• First-line additive for subarachnoid blocks requiring prolonged analgesia 6
• Mandatory use of preservative-free formulations for neuraxial administration 6
• Enhances block quality and duration in pediatric regional anesthesia 6

Opioid Adjuvants

Fentanyl (Lipophilic)

• Preferred over morphine or diamorphine in elderly patients due to less respiratory and cognitive depression 1
• Labor analgesia: 12.5-15 micrograms combined with 2.5 mg bupivacaine 6
• Continuous infusion in pediatrics: 2-2.5 micrograms/mL with bupivacaine 0.0417-0.1% at 1-3 mL/hr 6

Morphine (Hydrophilic)

• Caudal block adjunct in pediatrics: 30-50 micrograms/kg (requires adequate monitoring) 1
• Provides prolonged postoperative analgesia but associated with greater respiratory depression 1

Critical Safety Requirement: Preservative-free formulations are mandatory for all neuraxial adjuvants to prevent neurotoxicity 6

Onset and Duration

Onset Times by Concentration

• Bupivacaine 1.0%: 10-20 minutes 3
• Bupivacaine 0.75%: 10-20 minutes 3
• Bupivacaine 0.5%: 15-30 minutes 3
• The total dose of local anesthetic is the primary determinant of both therapeutic and unwanted effects 7

Duration by Concentration

• Bupivacaine 1.0%: 4-6 hours 3
• Bupivacaine 0.75%: 3-5 hours 3
• Bupivacaine 0.5%: 2-4 hours 3
• Addition of clonidine significantly extends duration 6

Contraindications

Absolute Contraindications

• Patient refusal
• Infection at injection site
• Severe hypovolemia or shock
• Coagulopathy or therapeutic anticoagulation (risk of neuroaxial hemorrhage) 1
• Increased intracranial pressure
• Severe aortic or mitral stenosis

Relative Contraindications

• Anticipated coagulopathy (e.g., nerve agent intoxication) requires careful risk-benefit assessment 1
• Pre-existing neurological disease
• Severe spinal deformity
• Uncooperative patient

Monitoring Requirements

Minimum monitoring standards include continual presence of anesthesiologist, pulse oximetry, capnography, ECG, and non-invasive blood pressure monitoring. 1

Intraoperative Monitoring

• Continuous pulse oximetry 1
• Capnography 1
• ECG 1
• Non-invasive blood pressure monitoring 1
• Core temperature monitoring routinely 1
• Point-of-care hemoglobin analyzers at end of surgery to guide transfusion 1

Post-Spinal Monitoring

• Frequent non-invasive blood pressure and fetal heart rate monitoring for 30 minutes after initiation 1
• During maintenance, monitoring identical to epidural catheter protocols 1
• Supplemental oxygen should always be provided during spinal anesthesia 1

Recovery Criteria for Mobilization

• Return of sensation to peri-anal area (S4-5) 1
• Plantar flexion of foot at pre-operative strength levels 1
• Return of proprioception in big toe 1
• Straight-leg raise by 4 hours after last dose 2

Management of Complications

Hypotension

Lower doses of intrathecal bupivacaine (<10 mg) significantly reduce associated hypotension, particularly in elderly patients. 1

• Attempted lateralization using hyperbaric bupivacaine with fractured hip inferior may ameliorate hypotension 1
• Volume expansion and vasoactive drugs successfully treat cardiovascular effects 4, 7
• High blocks (above T4) pose risk of severe hypotension and bradycardia 2

Total Spinal Anesthesia

• Can occur with inadvertent high spread 2
• Requires immediate airway management and cardiovascular support 2
• Equipment and personnel for managing failed intubation, respiratory depression, and local anesthetic systemic toxicity must be immediately available 1

Post-Dural Puncture Headache

• Use pencil-point spinal needles (25G) instead of cutting-bevel needles to minimize risk (<1%) 1
• Information on post-dural puncture headache and management should be included in discharge instructions 1
• Consider leaving intrathecal catheter in place for 24 hours if accidental dural puncture occurs 1
• Sterile saline injection into intrathecal catheter may help, though evidence is limited 1

Inadequate Block

• May require supplemental analgesia or conversion to general anesthesia 2
• Test dose (3-5 mL short-acting local anesthetic with epinephrine) should be used prior to complete block 3
• Repeat test dose if patient repositioned in manner that could displace catheter 3

Special Techniques and Considerations

Combined Spinal-Epidural (CSE)

• CSE produces greater sensorimotor anesthesia and prolonged recovery compared to single-shot spinal 8
• More frequent hypotension and vasoconstrictor use despite identical intrathecal doses 8
• May be used for labor with anticipated duration longer than spinal drug effects 1
• Provides rapid onset with ability to extend duration via epidural catheter 1

Continuous Spinal Anesthesia

• For cesarean section: gradual titration to T4 level with full monitoring in facility equipped to manage high/total spinal 1
• Bupivacaine/ropivacaine combined with lipophilic opioids and morphine or diamorphine are suggested 1
• Clear labeling mandatory; precautions to avoid cerebrospinal fluid leakage 1
• Structured handover notification to all staff members essential 1

Labor Analgesia

• Neuraxial analgesia should be offered on individualized basis regardless of cervical dilation 1
• Does not increase incidence of cesarean delivery 1
• Initial dose: Ropivacaine 0.2% 10-20 mL (20-40 mg) with 10-15 minute onset 3
• Continuous infusion: 6-14 mL/hr (12-28 mg/hr) 3
• Restrict IV fluids to ≤500 mL to reduce urinary retention incidence 1

Day Surgery Applications

• Low-dose spinal techniques with hyperbaric prilocaine 2% or 2-chloroprocaine are accepted for day surgery 1
• Appropriate dosing targeted to surgical site minimizes side effects 1
• Patients may be discharged with residual blockade if limb protected and appropriate home support available 1
• Written instructions regarding conduct until normal sensation returns mandatory 1

Pediatric Considerations

• Caudal block: Bupivacaine 0.25% 1.0 mL/kg with clonidine 1-2 micrograms/kg 1
• Lumbar epidural: Bupivacaine 0.25% 0.5 mL/kg (max 15 mL) initially 1
• Morphine 30-50 micrograms/kg for caudal blocks requires adequate monitoring 1

High-Risk Populations

Elderly Patients with Hip Fractures

• Lower bupivacaine doses (<10 mg) reduce hypotension 1
• Peripheral nerve blockade should always be considered as adjunct 1
• Opioid analgesics as sole adjunct not supported due to respiratory depression and confusion risk 1
• Sedation should be used cautiously; midazolam and propofol commonly used 1
• Ketamine may counteract hypotension but associated with postoperative confusion 1

Obstetric Patients

• Early neuraxial catheter insertion for complicated parturients (twin gestation, preeclampsia, anticipated difficult airway, obesity) reduces need for general anesthesia 1
• Neuraxial techniques preferred over general anesthesia for most cesarean deliveries 1
• Indwelling epidural catheter may be used for urgent cesarean delivery as alternative to spinal or general anesthesia 1

Common Pitfalls and How to Avoid Them

The most critical pitfall is using excessive doses of local anesthetic, which increases both therapeutic effects and complications. 7

• Avoid rapid injection of large volumes; always use fractional (incremental) doses 3
• Do not use disinfecting agents containing heavy metals (mercury, zinc, copper) as they cause swelling and edema 3
• Avoid mixing ropivacaine with alkaline solutions (pH >6) due to precipitation risk 3
• Do not leave flexible container bags in place for >24 hours 3
• Any solution remaining from opened container should be discarded promptly 3
• Amide local anesthetics preferred over ester types in patients with cholinesterase inhibition 1
• Ensure adequate test dose and allow sufficient time for onset before proceeding 3