What is the recommended timing and dose of ketamine and Morphine for a 50-year-old obese patient, 5ft 10in, 106kg, with a history of thyroidectomy, on Sevoflurane plus Remifentanil and Rocuronium balance anesthesia to prevent breakthrough pain or hyperalgesia?

Ketamine and Morphine Dosing for Prevention of Opioid-Induced Hyperalgesia

For this 106kg patient undergoing thyroidectomy on remifentanil-based anesthesia, administer ketamine 0.5 mg/kg (53mg) IV bolus after anesthesia induction, followed by continuous infusion at 0.125-0.25 mg/kg/h (13-26.5 mg/h), stopping 30 minutes before surgery ends; morphine should be dosed at 0.05-0.1 mg/kg (5-10mg) IV titrated to effect in the PACU for breakthrough pain. 1

Ketamine Administration Protocol

Timing and Initial Dosing

• Administer ketamine AFTER anesthesia induction to prevent psychodysleptic side effects (hallucinations), not before 1
• Bolus dose: 0.5 mg/kg (maximum), which equals approximately 53mg for this 106kg patient 1
• For thyroidectomy specifically, esketamine 0.4 mg/kg has been shown optimal for reducing remifentanil-induced hyperalgesia without increasing adverse effects 2

Maintenance Infusion

• Continue infusion at 0.125-0.25 mg/kg/h (13-26.5 mg/h for this patient) throughout the procedure 1
• Stop infusion 30 minutes before end of surgery to minimize postoperative hallucination risk while maintaining analgesic benefit 1
• The guideline explicitly states that continuing ketamine into the postoperative period increases hallucination risk without significantly improving analgesia 1

Evidence-Based Rationale

• Ketamine is the first-line anti-hyperalgesic agent for patients on remifentanil-based anesthesia, which creates high vulnerability to opioid-induced hyperalgesia 1
• This regimen reduces 24-hour morphine consumption by approximately 15mg (mean reduction) and decreases acute pain intensity 1, 3
• Reduces chronic postoperative pain incidence by 30% at 3 months, though evidence quality is moderate 1

Morphine Administration Protocol

Postoperative Dosing Strategy

• Do NOT administer morphine intraoperatively as part of the balanced anesthesia—the remifentanil infusion provides intraoperative analgesia 1
• Initial PACU dose: 0.025-0.1 mg/kg IV (2.5-10mg for this patient), titrated to effect based on pain scores 1
• Morphine remains the reference opioid for postoperative pain management 1

Titration Guidelines

• Administer in small increments of 2-3mg IV every 5-10 minutes until pain is controlled (VAS <4/10) 1
• For this obese patient with thyroidectomy history, use multimodal opioid-sparing approach to minimize respiratory depression risk 1
• Target total 24-hour morphine consumption should be reduced by the ketamine pretreatment effect 3

Critical Obesity Considerations

• Obese patients show increased sensitivity to opioid respiratory depression, requiring careful titration and extended monitoring 1
• Use short-acting opioids and minimal doses as part of enhanced recovery protocols 1
• Maintain head-up position throughout recovery and monitor oxygen saturation until mobile 1
• Consider level-2 care if long-acting opioids required and patient not on preoperative CPAP 1

Multimodal Adjuncts to Reduce Opioid Requirements

Additional Intraoperative Agents

• Dexamethasone 8mg IV at induction reduces postoperative pain and PONV 1
• IV lidocaine: bolus 1-2 mg/kg (106-212mg) followed by infusion 1-2 mg/kg/h for major surgery if no regional analgesia used 1
• These adjuncts further reduce morphine requirements and improve recovery 1

Postoperative Multimodal Analgesia

• Paracetamol 1g IV provides similar efficacy to ketorolac 30mg for thyroidectomy pain 4
• NSAIDs (if not contraindicated) reduce opioid consumption significantly 1
• Local wound infiltration with bupivacaine 0.5% reduces opioid requirements 1

Critical Safety Monitoring

Respiratory Monitoring Requirements

• Continuous pulse oximetry until patient is mobile postoperatively 1
• Observe for apnea/hypopnea episodes while unstimulated before PACU discharge 1
• Respiratory rate must be normal with no hypopnea for at least 1 hour before ward transfer 1
• Approximately 10% of patients receiving higher opioid doses develop persistent postoperative respiratory depression 5, 6

Ketamine-Specific Monitoring

• CNS adverse events occur in only 5% of patients receiving perioperative ketamine, similar to placebo (4%) 3
• No hallucinations reported when ketamine stopped 30 minutes before surgery end 1, 7
• Cardiovascular stability maintained with sub-anesthetic ketamine doses 7

Common Pitfalls to Avoid

• Do NOT give ketamine before induction—this increases psychodysleptic effects 1
• Do NOT continue ketamine infusion into PACU—this increases hallucination risk without analgesic benefit 1
• Do NOT use intramuscular morphine in obese patients—IV titration is essential for safety 1
• Do NOT discharge from PACU until respiratory parameters return to baseline without supplemental oxygen (or with home CPAP levels) 1
• Avoid excessive morphine boluses—use 2-3mg increments rather than standard 5-10mg doses given obesity-related respiratory sensitivity 1