Intraoperative Neuromuscular Monitoring

1. Clinical/Bedside Assessment (Least Reliable - Alone Insufficient)

"Neither time from last neuromuscular blocking drug administration nor physical assessments should be used to assess recovery from neuromuscular blockade." - Barash Clinical Anesthesia, 9e

2. Peripheral Nerve Stimulator (PNS) - Qualitative Monitoring

Stimulation Patterns

A. Single Twitch (ST)

• Frequency: 0.1-10 Hz
• Use: Onset of blockade only - loss of twitch indicates deep block
• Limitation: Requires a baseline measurement; does not detect partial paralysis

B. Train-of-Four (TOF) - Most Common

• 4 stimuli at 2 Hz, not repeated more than every 15 seconds
• Results in 4 twitches (T1-T4); compare T4/T1 = TOF ratio
• With nondepolarizing block, progressive fade occurs (T4 disappears first, then T3, T2, T1)
• TOF count (number of visible twitches) is used during deep block:
  • 0 twitches = profound/deep block
  • 1-2 twitches = moderate-deep block
  • 3-4 twitches = moderate block; fade analysis needed
• Key thresholds:
  • TOF ratio ≥ 0.90 = adequate neuromuscular recovery
  • TOF ratio < 0.90 = residual paralysis (adverse respiratory events, aspiration risk, pharyngeal dysfunction)
• Critical limitation: Clinicians cannot subjectively detect fade when TOF ratio exceeds 0.30-0.40 - meaning significant residual block is missed by feel/sight

C. Tetanic Stimulation

• High-frequency stimulation at 50 Hz (5 sec) or 100 Hz
• With nondepolarizing block: sustained tetanus = no significant block; fade during tetanus = residual block
• Painful in awake patients; not repeated within 6 minutes (alters subsequent responses)
• More sensitive than TOF for detecting partial block, but still subjective

D. Post-Tetanic Count (PTC)

• Used during profound/deep block (when TOF count = 0)
• A 50 Hz tetanic stimulus is followed after 3 seconds by single twitches at 1 Hz
• Count the number of post-tetanic twitches:
  • PTC 0-1 = very deep block
  • PTC 6+ = TOF responses will reappear soon
• Predicts time to spontaneous recovery and guides decisions on reversal timing

E. Double-Burst Stimulation (DBS)

• Two short tetanic bursts (3 impulses at 50 Hz each, separated by 750 ms)
• More sensitive than TOF for detecting fade visually/tactilely
• Can detect fade at TOF ratios up to 0.60 (vs. ~0.30-0.40 for standard TOF)
• Variants: DBS 3,3 (most common) and DBS 3,2
• Still qualitative - cannot confirm TOF ≥ 0.90

3. Quantitative Monitoring - Gold Standard

A. Mechanomyography (MMG) - Reference Standard

• Measures actual force of muscle contraction (isometric)
• Requires precise positioning and immobilization
• Reference standard for research; impractical at the bedside
• All other technologies are compared against MMG

B. Electromyography (EMG)

• Measures the compound muscle action potential (electrical signal) of the stimulated muscle
• Electrode placement over the target muscle (e.g., thenar muscles, first dorsal interosseous)
• Closest to MMG in accuracy; values slightly higher than MMG in some studies
• Commercially available devices: TetraGraph, NMT module on GE monitors
• Does not require free movement of the thumb - usable in more clinical situations

C. Acceleromyography (AMG)

• Measures acceleration of the thumb using a piezoelectric sensor (Newton's second law: F = ma)
• Most widely used quantitative device commercially
• Bias: Tends to overestimate TOF ratio by ~5-10% compared to MMG
  • Suggested goal: TOF ratio ≥ 0.95-1.0 to confirm actual recovery ≥ 0.90
• Requires free movement of the thumb (thumb must not be restricted)
• Devices: TOF-Watch, TOFscan

D. Kinemography (KMG)

• Measures thumb displacement using a bending sensor
• Similar accuracy to AMG; slight overestimation vs. MMG
• Less common commercially

E. Phonomyography

• Measures low-frequency sounds produced by muscle contraction using a microphone
• Still largely investigational

Quantitative Monitor Comparison Table

4. Nerve-Muscle Recording Sites

Laryngeal and diaphragmatic muscles are more resistant to neuromuscular block than the adductor pollicis - a full block at the wrist often underestimates how much block remains at the airway.

5. Depth of Block Definitions

Key Clinical Points

• Qualitative PNS alone is insufficient - subjective fade detection fails above TOF ratio 0.30-0.40
• Quantitative monitoring is the only way to confirm a TOF ratio ≥ 0.90 and exclude residual block
• DBS is better than TOF for subjective fade detection but still qualitative
• AMG overestimates recovery - aim for TOF ≥ 0.95-1.0 with AMG devices
• EMG most closely mirrors MMG and is the preferred quantitative method per recent guidelines
• Residual block with TOF < 0.90 causes pharyngeal dysfunction, aspiration risk, and upper airway obstruction even in patients who appear clinically awake

Lung Isolation Techniques in a 2-Year-Old Child

Why This Age Group Is Challenging

• Trachea is narrow and short (~5-6 cm from cords to carina)
• Double-lumen tubes (DLTs) cannot be used - the smallest DLT available is 26 Fr (external diameter 8.7-9.3 mm, equivalent to a 6.5 ETT), which should not be used in children under 8 years of age
• Small ETT sizes (typically 4.0-4.5 mm ID uncuffed or 3.5-4.0 mm cuffed in a 2-year-old) limit device passage through the lumen
• Reduced respiratory reserve with greater risk of hypoxemia in the lateral decubitus position (HPV response is less effective; FRC is reduced)
• Extraluminal placement of a bronchial blocker is the only viable option for children under 2 years of age due to ETT size constraints

Available Techniques (Applicable at Age 2)

1. Intentional Mainstem (Endobronchial) Intubation

• A standard single-lumen ETT (same size used for tracheal intubation) is deliberately advanced into the mainstem bronchus of the dependent (operative field - opposite) lung - i.e., if right thoracotomy, advance the ETT into the left mainstem bronchus to ventilate the left lung while the right collapses
• Position confirmed by auscultation ± fiberoptic bronchoscopy
• For left-sided surgery, advance into the right mainstem bronchus (easier due to the more vertical angle of the right bronchus)

• No special equipment
• Works in any size patient including neonates
• Immediate - fastest to establish isolation

• Cannot suction or provide CPAP/O2 to the isolated lung
• Risk of right upper lobe obstruction when right mainstem intubation is performed (right upper lobe orifice is only ~2 cm from the carina)
• Difficult to achieve selective lobar isolation
• Tube can migrate back to trachea with repositioning

2. Bronchial Blockers (BBs) - Preferred Technique at This Age

Placement Options

• Intraluminal (through the ETT): Requires the ETT to be wide enough to accommodate both the bronchoscope and the blocker simultaneously. The smallest ETT this is feasible through is 5.0 mm ID (using the 5 Fr Arndt blocker with a ~2.2 mm scope).
  • A standard 2-year-old will typically have a 4.0-4.5 mm ETT - this may be too small for intraluminal placement.
• Extraluminal (alongside the ETT): The blocker is placed external to the ETT, through the glottis separately. This is the only viable option for children under 2 years of age and is appropriate at this age.

Types of Bronchial Blockers

1. Intubate trachea with appropriately sized ETT; use multiport adapter
2. Pass the 5 Fr Arndt blocker alongside (external to) the ETT through the vocal cords
3. Advance fiberoptic bronchoscope through the ETT lumen to the carina
4. Use the blocker's wire loop snared on the scope to guide blocker into the target mainstem bronchus under direct vision
5. Inflate cuff under fiberoptic visualization
6. Confirm position with scope; recheck after lateral positioning

• Allows selective bronchial or even lobar isolation
• The inner lumen permits lung deflation via suction, and administration of O2 or CPAP to the isolated lung (critical in infants prone to hypoxemia)
• ETT remains in trachea - easier to switch back to two-lung ventilation

• Requires fiberoptic bronchoscopy
• More prone to dislodgement than DLT (especially with surgeon manipulation)
• Lung collapses slowly through the small inner lumen
• Peak airway pressure must be kept below 30 cmH2O to prevent cuff bypass

3. Univent Tube

• The blocker is built into the tube wall (not passed through the lumen)
• Pediatric sizes available down to approximately 3.5 mm ID (smallest Univent tube)
• The integrated blocker is advanced into the mainstem bronchus under fiberoptic guidance
• Has a small inner lumen for deflation/CPAP

• Blocker integrated - no second device to position
• Blocker less likely to migrate intraoperatively

• Larger outer diameter than a plain ETT of same ID
• Limited availability of pediatric sizes

4. Double-Lumen Tube (DLT) - NOT Appropriate at Age 2

Summary: What to Use in a 2-Year-Old

Key principle: In children under 2 years, extraluminal bronchial blocker placement is the only viable bronchial blocker option due to the small ETT size. Fiberoptic bronchoscopy is essential for correct placement of any blocker device.