What is the recommended target-controlled infusion (TCI) regimen for propofol and fentanyl for a 62-year-old patient weighing 55 kg with hypertension on losartan (angiotensin II receptor antagonist) maintenance, undergoing spine decompression T6-L1 with intraoperative neuromonitoring (IONM)?

TCI Regimen for Propofol and Fentanyl in Spine Decompression with IONM

For this 62-year-old, 55 kg patient on losartan undergoing T6-L1 spine decompression with IONM, use propofol TCI targeting 2.0-2.5 μg/mL effect-site concentration for induction (reduced from standard 3.0 μg/mL due to age and losartan) and 1.5-2.0 μg/mL for maintenance, combined with fentanyl 25-50 μg boluses (0.5-1.0 μg/kg) with infusion at 0.5-1.0 μg/kg/h, titrating to maintain motor evoked potentials (MEPs) and somatosensory evoked potentials (SSEPs) while avoiding excessive anesthetic depth that impairs IONM signals.

Critical Considerations for IONM Compatibility

• IONM requires careful anesthetic management because propofol and opioids can suppress evoked potentials, particularly MEPs which are more sensitive than SSEPs to anesthetic depth 1, 2.
• Maintain propofol effect-site concentration at the lower end of the therapeutic range (1.5-2.0 μg/mL during maintenance) to preserve adequate MEP amplitudes while providing sufficient anesthesia 1, 3.
• Avoid neuromuscular blocking agents after intubation or use minimal doses with train-of-four monitoring showing at least 2-3 twitches, as complete neuromuscular blockade abolishes MEPs 4.
• Fentanyl is preferred over remifentanil for spine surgery with IONM because its longer duration provides more stable analgesia without the rapid offset that can cause sudden increases in anesthetic requirements 5.

Propofol TCI Dosing Algorithm

Induction Phase

• Set initial propofol TCI target at 2.0-2.5 μg/mL effect-site concentration (not the standard 3.0 μg/mL) because this patient is 62 years old and on losartan, which increases risk of hypotension 6, 3.
• The FDA label recommends 1.0-1.5 mg/kg for elderly patients (approximately 20 mg every 10 seconds), which translates to roughly 55-82 mg total for this 55 kg patient 6.
• Indian population data shows mean induction dose of 1.17 mg/kg (approximately 64 mg for this patient) with mean effect-site concentration of 2.34 μg/mL at loss of consciousness 3.
• Titrate slowly over 2-3 minutes rather than rapid bolus to minimize hypotension, especially critical given losartan's vasodilatory effects 6, 7.

Maintenance Phase

• Target propofol effect-site concentration of 1.5-2.0 μg/mL during surgical maintenance to balance adequate anesthesia with IONM signal preservation 1, 3.
• Expected infusion rate will be 50-80 μg/kg/min (approximately 2.75-4.4 mg/kg/h for this 55 kg patient), which is lower than standard dosing due to age and opioid synergy 2, 3.
• Indian population data shows mean maintenance dose of 81 μg/kg/min, supporting this lower range 3.
• Monitor state entropy targeting 40-50 to ensure adequate anesthetic depth without excessive suppression that impairs IONM 3.

Recovery Phase

• Stop propofol infusion when effect-site concentration reaches approximately 1.0-1.2 μg/mL (approximately 50% of induction concentration) to allow smooth emergence 3.
• Recovery occurs at mean propofol Ce of 1.02 μg/mL with state entropy of 87, providing predictable emergence timing 3.

Fentanyl Dosing Strategy

Initial Dosing

• Administer fentanyl 25-50 μg (0.5-1.0 μg/kg) bolus 2-3 minutes before propofol induction to provide baseline analgesia and reduce propofol requirements 5, 6.
• This pretreatment reduces propofol maintenance infusion rates by providing synergistic analgesia 6.

Maintenance Infusion

• Start fentanyl infusion at 25-50 μg/h (0.5-1.0 μg/kg/h) for continuous analgesia during the multi-level spine decompression 5.
• Administer additional 25-50 μg boluses every 30-60 minutes as needed for surgical stimulation, particularly during bone work and instrumentation 5, 6.
• Fentanyl has 1-4 hour duration, making it suitable for this lengthy procedure without excessive accumulation 5.

IONM-Specific Fentanyl Considerations

• Fentanyl provides moderate anti-shivering effects which may be beneficial if hypothermia develops during prolonged surgery 5.
• Opioids have less impact on MEPs than propofol, allowing higher relative opioid dosing to reduce propofol requirements 5, 2.

Losartan-Specific Hemodynamic Management

• Losartan (angiotensin II receptor blocker) increases risk of induction hypotension because it prevents compensatory angiotensin-mediated vasoconstriction when propofol causes vasodilation 5, 6.
• Propofol causes dose-dependent decreases in cardiac output, systemic vascular resistance, and blood pressure, which is exacerbated by losartan 2, 6.
• Have vasopressors immediately available (phenylephrine 50-100 μg boluses or norepinephrine infusion) before induction 5.
• Consider reducing initial propofol target by 20-30% compared to patients not on antihypertensives 6, 7.
• Avoid rapid bolus administration which dramatically increases hypotension risk in this population 6, 7.

TCI System Selection and Setup

• Use modified Marsh pharmacokinetic model for propofol TCI, which is validated and commonly used 1, 3.
• Program patient parameters: 62 years old, 55 kg, male or female as appropriate 1.
• The TCI computer calculates infusion rates to achieve target effect-site concentrations based on three-compartment pharmacokinetic modeling 1, 2.
• Allow 3-5 minutes between target adjustments to assess clinical effects and avoid overshooting 6.

Monitoring Requirements

• Continuous pulse oximetry, ECG, and invasive arterial blood pressure monitoring are essential given the hemodynamic risks with losartan 1, 2.
• Processed EEG monitoring (BIS or entropy) targeting 40-50 helps titrate propofol to adequate depth without excessive suppression 8, 3.
• IONM baseline signals must be established before surgical incision, with continuous monitoring of MEPs and SSEPs throughout the procedure 1.
• Communicate with neurophysiologist if MEP amplitudes decrease >50% or SSEPs decrease >50% amplitude or increase >10% latency, requiring immediate propofol reduction 1.

Common Pitfalls and How to Avoid Them

• Pitfall: Using standard propofol doses in elderly patients on antihypertensives causes severe hypotension - Avoid by reducing initial target to 2.0-2.5 μg/mL and titrating slowly 6, 7.
• Pitfall: Excessive propofol depth abolishes MEPs - Avoid by maintaining effect-site concentration ≤2.0 μg/mL during critical monitoring periods 1, 3.
• Pitfall: Inadequate analgesia leads to movement or hypertension during bone work - Avoid by ensuring adequate fentanyl loading (50 μg) and maintenance infusion 5, 6.
• Pitfall: Propofol has no analgesic properties - Must combine with opioids for this painful procedure 2, 6.
• Pitfall: Rapid propofol boluses cause hypertension with tachycardia paradoxically at low doses - Avoid by slow titration over 2-3 minutes 7.

Alternative Considerations

• If MEPs become unacceptably suppressed, reduce propofol target to 1.0-1.5 μg/mL and increase fentanyl to 1.5-2.0 μg/kg/h to maintain anesthesia 5, 1.
• If hypotension persists despite vasopressors, consider adding low-dose ketamine (0.25-0.5 mg/kg bolus, then 0.5-1.0 mg/kg/h infusion) which has sympathomimetic effects and less MEP suppression 5.
• Avoid benzodiazepines as they provide no benefit for this case and increase delirium risk in elderly patients 5, 9.