What are the commonly used intrathecal additives for spinal anaesthesia and their recommended adult doses?

Additive Drugs Used in Spinal Anaesthesia and Their Doses

Opioid Adjuvants

The most commonly used intrathecal additives are opioids, with morphine and fentanyl being the gold standard agents that significantly enhance analgesia while allowing dose reduction of local anaesthetics. 1, 2

Morphine (Hydrophilic Opioid)

• Standard dose: 100 µg (0.1 mg) preservative-free morphine for postoperative analgesia in most surgical procedures 3, 1
• Alternative dose: 300 µg diamorphine (UK equivalent to morphine) when morphine is unavailable 3, 4
• Dose range studied: 0.05-2 mg, though higher doses increase side effects without improving analgesia 2, 5
• Provides prolonged postoperative analgesia averaging 503 minutes (8.4 hours) beyond local anaesthetic alone 2
• Do not exceed 100 µg morphine as higher doses markedly increase respiratory depression risk without additional analgesic benefit 4

Side effect profile: Morphine increases risk of nausea (NNH 9.9), vomiting (NNH 10), urinary retention (NNH 6.5), pruritus (NNH 4.4), and respiratory depression (NNH 38-59 with 0.05-0.5 mg doses) 2

Fentanyl (Lipophilic Opioid)

• Standard dose: 10-25 µg fentanyl for intraoperative and early postoperative analgesia 1, 6, 2
• Most commonly used dose: 12.5-25 µg in combination with bupivacaine 3, 1
• Lower dose range: 5-20 µg for labour analgesia 1, 6
• Provides moderate prolongation of analgesia averaging 114 minutes beyond local anaesthetic alone 2
• Onset is faster than morphine but duration is shorter, making it ideal for intraoperative enhancement 7, 5

Side effect profile: Fentanyl increases risk of pruritus (NNH 3.3) but does not significantly increase respiratory depression at doses of 10-40 µg 2

Sufentanil (Lipophilic Opioid)

• Standard dose: 2.5-7.5 µg sufentanil as an alternative to fentanyl 3, 1
• Most commonly used: 2-5 µg in combination with bupivacaine 6, 7
• Provides potentially faster onset and more profound analgesia than fentanyl at equipotent doses 1, 7
• Used in continuous infusions at 0.75-1 µg/mL concentration for labour analgesia 3

Alpha-2 Adrenergic Agonists

Dexmedetomidine

• Dose: 2.5-5 µg dexmedetomidine added to local anaesthetic 8, 7
• Produces prolonged motor and sensory blockade without opioid-related side effects (no pruritus, nausea, or respiratory depression) 8
• Accelerates onset and prolongs duration of block and analgesia through alpha-2 receptor agonism 7

Critical caveat: Intraoperative hypotension is more frequent with dexmedetomidine compared to morphine, requiring vigilant blood pressure monitoring and vasopressor availability 8

Clonidine

• Dose range: 15-150 µg clonidine (though specific optimal dose not well-established in guidelines) 7
• Accelerates onset and prolongs duration of sensory and motor block 7
• Side effects include hypotension, sedation, and potential respiratory depression, limiting its routine use 7

Other Adjuvants (Less Commonly Used)

Magnesium Sulphate

• Primarily potentiates analgesic action of intrathecal opioids rather than providing independent analgesia 7
• Does not produce significant side effects when used as adjuvant 7
• Specific intrathecal dosing not well-established in current guidelines 7

Adrenaline (Epinephrine)

• May prolong block duration through local vasoconstriction 7
• Increases intensity of motor block, which may be undesirable in ambulatory settings 3
• Used in some labour analgesia protocols but not routinely recommended 3

Clinical Decision Algorithm

For surgical anaesthesia requiring postoperative analgesia:

• Add 100 µg preservative-free morphine to local anaesthetic for procedures requiring prolonged postoperative pain control 3, 4
• Add 15-25 µg fentanyl for enhanced intraoperative anaesthesia with moderate postoperative benefit 1, 2

For labour analgesia (intrathecal catheter after accidental dural puncture):

• Initial bolus: Bupivacaine 1.5-2.5 mg + fentanyl 5-20 µg 1, 6
• Maintenance infusion: Bupivacaine 0.0417-0.1% + fentanyl 2-2.5 µg/mL at 1-3 mL/hour 3, 1

For patients at high risk of opioid side effects:

• Consider dexmedetomidine 2.5-5 µg instead of opioids, accepting increased hypotension risk 8
• Use fentanyl 10-20 µg rather than morphine to minimize respiratory depression and delayed side effects 2, 5

For elderly or frail patients:

• Fentanyl is strongly preferred over morphine due to lower risk of respiratory and cognitive depression 4
• Use lower end of dosing ranges for all adjuvants 4

Critical Safety Considerations

Monitoring requirements when intrathecal morphine is used:

• Assess respiratory rate and sedation hourly for 12 hours, then every 2 hours for the next 12 hours 1
• This extended monitoring is mandatory due to morphine's hydrophilic nature causing delayed rostral spread 5

Common pitfall to avoid:

• Do not assume dose-responsiveness exists for opioid adjuvants - studies show no clear dose-response relationship for either beneficial or harmful effects, making minimal effective doses the safest choice 2
• Never use epidural dose volumes through spinal needles when adding adjuvants, as this is a common cause of catastrophic overdosing 4

Drug error prevention:

• Adding multiple adjuvants increases risk of drug errors and delays administration 3
• The potential benefit of additional medications must be balanced against increased error risk 3