Anesthetic Management of a 6-Month-Old Child for MRI

1. Pre-Anesthetic Assessment

Pediatric-Specific Concerns at 6 Months

• Airway: Proportionally larger occiput causes neck flexion when supine - place a shoulder roll; obligate nasal breather; narrow subglottic area (most narrow at cricoid ring); short trachea with risk of endobronchial intubation
• Fasting (NPO) guidelines: Breast milk 4 hrs, formula/non-human milk 6 hrs, solids 6-8 hrs; clear fluids up to 2 hrs before
• Cardiovascular: Cardiac output is heart-rate dependent; avoid bradycardia
• Pharmacology: Immature hepatic metabolism; lower plasma protein binding; larger volume of distribution for water-soluble drugs; reduced MAC for inhalational agents
• Temperature regulation: High surface area to body weight ratio; prone to hypothermia in cold MRI suite

History & Examination

• Premature birth (corrected age matters - ex-premies have apnea risk up to 60 weeks post-conceptional age)
• Upper respiratory infection (URI) within 2-6 weeks - increases laryngospasm and airway complications
• Known cardiac or neurological disease (common reason for the MRI)
• Weight (for drug dosing)
• Parental anxiety and consent

2. MRI-Specific Challenges

3. Monitoring (MRI-Compatible)

• Pulse oximetry (fiber-optic probe - place away from magnetic isocenter to avoid burns)
• ECG (MRI-safe leads; ECG trace distorted by magnetohydrodynamic effect - unreliable for ST analysis)
• Non-invasive blood pressure (NIBP)
• Capnography via side-stream sampling adapter - essential for spontaneously breathing or intubated infants
• Temperature probe
• Precordial stethoscope via long tubing

4. Sedation vs. General Anesthesia Decision

Sedation Approach

• Advantages: No airway instrumentation, faster turnover, lower cost
• Disadvantages: Higher failure rate, risk of oversedation/airway obstruction, no analgesia
• Suitable only for very brief, simple scans in a stable infant

General Anesthesia Approach

• Preferred for a 6-month-old (cannot cooperate, unpredictable sedation depth, high aspiration risk without controlled airway)
• More reliable image quality
• Same standards of care as in the OR apply in NORA settings

"The standards of care for pediatric patients undergoing sedation and/or general anesthesia for MRI and CT imaging are the same as those in the OR." - Barash Clinical Anesthesia, 9e

5. Airway Management Options

Option A: Tracheal Intubation

• Preferred for longer scans, airway compromise, or significant comorbidities
• Use uncuffed ETT (Cole or straight ETT): size = (age/4) + 4; for 6 months ~3.5 mm uncuffed (or 3.0 cuffed at low inflation pressure)
• Secure tube very well before entering bore - impossible to reintubate inside magnet
• Use MRI-compatible ventilator

Option B: Laryngeal Mask Airway (LMA)

• Acceptable for stable infants with clear airways and no aspiration risk
• Size 1 or 1.5 for a 6-month-old (~6-7 kg)
• Allows spontaneous ventilation; must verify adequate ventilation before entering bore
• Less secure than ETT

Option C: Sedation with Natural Airway

• Reserve for brief, cooperative-adjacent scenarios or with experienced pediatric sedation team
• Requires meticulous monitoring of breathing via capnography

6. Drug Choices

Induction

• Sevoflurane inhalation induction (preferred when IV access not established): smooth, rapid
• Propofol IV 2-3 mg/kg once IV established

Maintenance

Propofol Infusion (TIVA)

• Loading: 1-2 mg/kg IV
• Infusion: 100-250 mcg/kg/min (6-15 mg/kg/hr) titrated to effect
• Advantages: Rapid onset, rapid recovery, reduces nausea, clear field of view from magnet bore
• Cautions: Respiratory depression, hypotension, apnea; propofol infusion syndrome risk with prolonged high-dose infusions (>4.5 mg/kg/hr for >24 hrs) - though not a concern for brief MRI scans
• No analgesic properties

Dexmedetomidine (DEX)

• Alpha-2 agonist; provides sedation with less respiratory depression than propofol
• Loading dose: 1-2 mcg/kg over 10 minutes
• Maintenance: 0.5-1 mcg/kg/hr
• Advantages: Preserves airway reflexes, blunts sympathetic response, less respiratory depression
• Disadvantages: Slower onset, longer recovery, risk of bradycardia and hypotension; hemodynamic monitoring essential
• Particularly useful when airway protection is paramount

One study comparing DEX vs propofol for pediatric MRI found: onset and recovery were faster with propofol, but hypotension, respiratory depression, and desaturation were more common with propofol vs. dexmedetomidine. - Miller's Anesthesia, 10e

Sevoflurane (Inhalational)

• Via LMA or ETT with long circuit (3-5 m) extending out of bore
• MAC reduced in infants - use ~1.5-2% for maintenance
• MRI-compatible vaporizers required (Penlon or equivalent)
• Advantages: Easily titratable, rapid recovery

Ketamine

• Not commonly used alone for MRI (increases secretions, causes dysphoria)
• Can be combined with midazolam or propofol in some protocols

Adjuncts

• Atropine 0.02 mg/kg IV - have ready; essential for treating bradycardia (especially if using DEX)
• Midazolam 0.05-0.1 mg/kg intranasal - premedication to ease parental separation
• Avoid opioids for routine non-painful MRI

7. Practical Protocol for a 6-Month-Old

1. Premedication: Intranasal midazolam 0.2 mg/kg (or oral) 20-30 min before; parental presence at induction if feasible
2. Induction: Sevoflurane in O2 via face mask until IV placed; then propofol 2 mg/kg IV
3. Airway: LMA size 1.5 OR intubate with 3.0-3.5 uncuffed ETT based on clinical assessment
4. Maintenance: Propofol infusion 100-200 mcg/kg/min OR sevoflurane 1.5-2% via long circuit OR DEX 0.5-1 mcg/kg/hr (if spontaneous breathing without intubation preferred)
5. Position: Supine with shoulder roll; confirm good ventilation before bore entry
6. Monitoring: Capnography (essential), SpO2 with fiber-optic probe, NIBP every 2-3 min, temperature
7. During scan: All personnel outside; closed-circuit camera surveillance; communicate via intercom; have emergency stop/eject button accessible
8. Recovery: Full PACU care; discharge criteria same as OR recovery; monitor for apnea (especially ex-premies)

8. Emergency Preparedness

• Laryngospasm: Propofol bolus or succinylcholine 2-4 mg/kg IM/IV; have ready
• Bronchospasm: Sevoflurane, salbutamol
• Bradycardia: Atropine 0.02 mg/kg
• Cardiac arrest: Evacuate immediately from bore before CPR (metal defibrillator paddles cannot enter MRI room)
• Full crash cart outside MRI suite (not inside - non-MRI-compatible); fiber-optic laryngoscope if available

9. Neurotoxicity Consideration

Key Points Summary

• Barash Clinical Anesthesia, 9e, p. 2679 (Pediatric Sedation and Anesthesia for MRI and CT Scans)
• Miller's Anesthesia, 10e, Dexmedetomidine section (p. 11394-11395)
• Nonpharmacological Interventions to Reduce Sedation in Pediatric MRI - Meta-analysis, 2023 PMID: 37727937