What are the typical doses for subarachnoid administration of morphine, fentanyl, and bupivacaine?

Subarachnoid (Intrathecal) Drug Dosing

For subarachnoid administration, use bupivacaine 10-15 mg, morphine 0.1-0.2 mg (100-200 mcg), and fentanyl 10-25 mcg as typical starting doses, with all medications being preservative-free and administered through a filter.

Bupivacaine Dosing

Intrathecal bupivacaine doses range from 10-15 mg for most surgical procedures, with lower doses (<10 mg) recommended in elderly patients to reduce hypotension 1. Specific dosing includes:

• 12.5 mg of 0.5% hyperbaric bupivacaine is commonly used for cesarean sections and lower limb orthopedic procedures 2, 3, 4
• Top-up doses of 2.5 mg bupivacaine (or equivalent) may be required through intrathecal catheters for labor analgesia if initial dosing is only partially effective 1
• Maximum safe dose is 3.0 mg/kg with epinephrine or 2.5 mg/kg without epinephrine for all routes of local anesthetic administration 1

The dead space of intrathecal catheters and filters (0.5-1 mL) must be accounted for when calculating effective doses delivered to the subarachnoid space 1.

Morphine Dosing

Intrathecal morphine 0.1-0.2 mg (100-200 mcg) provides prolonged postoperative analgesia lasting up to 24 hours 2, 4, 5. Specific considerations:

• 50 mcg morphine combined with fentanyl and bupivacaine is effective for cesarean section 3
• 100 mcg (0.1 mg) morphine is comparable to 250 mcg diamorphine for post-cesarean analgesia 4
• 2.85 mcg/kg morphine (approximately 200 mcg for a 70 kg patient) provides prolonged two-segment regression time and effective analgesia 2

Critical safety warning: Accidental overdosing with 4 mg intrathecal morphine (40 times the typical dose) has been reported, causing severe complications including atrial fibrillation after naloxone administration 6. This underscores the importance of careful drug verification before administration.

Fentanyl Dosing

Intrathecal fentanyl 10-25 mcg provides shorter-duration analgesia compared to morphine but with faster onset 1, 2. Recommended doses:

• 12.5 mcg fentanyl combined with bupivacaine and morphine for cesarean section 4
• 20 mcg (0.02 mg) fentanyl added to bupivacaine and morphine for lower limb orthopedic procedures 5
• 25 mcg fentanyl combined with bupivacaine and morphine for cesarean delivery 3
• 0.35 mcg/kg fentanyl (approximately 25 mcg for a 70 kg patient) provides effective but shorter-duration analgesia than morphine 2

Fentanyl is preferred over morphine when avoiding delayed respiratory depression and cognitive effects is prioritized, particularly in elderly patients 1.

Critical Safety Considerations

All intrathecal medications must be preservative-free and administered through a filter connected to the catheter 1. Key safety measures include:

• Only anesthetists should administer bolus medications through intrathecal catheters 1
• Clearly label intrathecal catheters on both infusion pumps and tubing to prevent accidental epidural dose administration, which would cause high or total spinal anesthesia 1
• Do not flush intrathecal catheters with saline after top-ups, as this creates unquantifiable effects on baricity and drug delivery 1
• Account for catheter dead space (0.5-1 mL) when calculating doses 1

Combination Therapy

Combining opioids with local anesthetics enhances block quality while reducing total drug requirements 2. The most common combinations are:

• Bupivacaine 12.5 mg + fentanyl 12.5-25 mcg + morphine 50-100 mcg for cesarean section provides both intraoperative and prolonged postoperative analgesia 3, 4
• Morphine provides superior prolonged analgesia (two-segment regression time, lower pain scores, fewer epidural boluses required) compared to fentanyl when added to bupivacaine 2

The addition of fentanyl 20 mcg to isobaric bupivacaine with morphine does not alter the maximal block height or time to maximal block 5.