How does accelerography work in assessing neuromuscular blockade?

How Accelerography Works in Neuromuscular Blockade Assessment

Accelerography (acceleromyography) measures neuromuscular blockade by detecting the acceleration of muscle movement in response to peripheral nerve stimulation using a piezoelectric transducer attached to the thumb or other muscle, converting mechanical acceleration into an electrical signal that quantifies the degree of neuromuscular block. 1, 2

Technical Mechanism

Core Technology

• A piezoelectric ceramic wafer with electrodes on each side is attached to the thumb (typically over the adductor pollicis muscle) 2
• When the ulnar nerve is electrically stimulated, the resulting muscle contraction causes thumb movement 2
• The acceleration of this movement generates a voltage difference between the two electrodes on the transducer 2
• This voltage is measured and registered in a computing unit that calculates the train-of-four (TOF) ratio 2

Key Advantage Over Other Methods

• Accelerography does not require preload tension on the muscle, only that the muscle can move freely 2
• This makes setup significantly faster than mechanomyography—reliable evaluation can be achieved in less than 2 minutes in clinical practice 2

Clinical Application and Monitoring Protocol

Essential Requirements

• Quantitative neuromuscular monitoring with acceleromyography is essential when administering neuromuscular blocking agents, and should be used throughout all phases of anesthesia 1
• The monitor must be activated after induction of general anesthesia but before neuromuscular blockade 1
• The ulnar nerve with thumb adduction (adductor pollicis) is the most reliable monitoring site 1

Train-of-Four Interpretation

• Four electrical stimuli are delivered in rapid succession 1
• With increasing neuromuscular blockade depth, twitches decrease progressively: T4 is lost first, then T3, T2, and finally T1 3
• The critical "monitoring gap" exists between TOF ratio 0.4 and 0.9—absence of visible fade only indicates recovery to 0.4 or greater, not adequate recovery 1
• A TOF ratio >0.9 must be documented before extubation to prevent residual paralysis complications 1

Important Limitations and Pitfalls

Accuracy Concerns

• Acceleromyography can underestimate block depth during onset (maximal at ~50% twitch depression) and overestimate block during recovery 4
• When one device reads TOF 0.7, acceleromyography readings can vary between 0.4 and 1.0 4
• Acceleromyography is more susceptible to drift than mechanomyography 4
• Different acceleromyography devices are not interchangeable—clinical decisions for deep neuromuscular blockade should be made cautiously due to significant variability between devices 5

Patient and Technical Factors Affecting Accuracy

• If thumb movement is impeded (hand inaccessible during surgery), readings become unreliable with acceleromyography devices 1
• Patient factors that influence results include: monitoring site location, patient temperature, diaphoresis, peripheral edema, and skin resistance 1
• In neonates and small infants, automatic baseline calibration may fail—manual gain adjustment is often required to achieve 100% baseline twitch height 6
• Hypothermia can cause inadequate response despite apparent adequate TOF readings 1

Site-Specific Variations

• Facial nerve monitoring increases the risk of residual paralysis five-fold compared to ulnar nerve monitoring—revert to ulnar nerve at surgery end 1
• Response differs between adductor pollicis, orbicularis oculi, and respiratory muscles (chest wall/diaphragm) due to varying nicotinic receptor density 1
• Accelerographic monitoring over vastus medialis shows faster onset and higher TOF ratios than thumb monitoring 7

Clinical Integration

Comprehensive Assessment Required

• Acceleromyography must be incorporated into a more comprehensive patient assessment that includes clinical evaluation—TOF monitoring alone should not determine adequacy of reversal 1, 8
• Clinical tests (sustained head-lift, hand grip, tongue depressor) have sensitivities of only 10-30% and cannot exclude residual blockade 1
• Continue monitoring for adequacy of reversal for a period ensuring full recovery based on patient condition and pharmacokinetics of both the neuromuscular blocking agent and reversal agent used 8

Alternative Monitoring Options

• If acceleromyography is unreliable (hand inaccessible, excessive edema), consider electromyography devices or compressomyography (TOF-Cuff) 1
• Ultrasound guidance may help locate the ulnar nerve in patients with peripheral edema 1