Relationship between normalized adductor pollicis train-of-four ratio and manifestations of residual neuromuscular block: a study using acceleromyography during near steady-state concentrations of mivacurium.
BACKGROUND
Baseline acceleromyographic adductor pollicis train-of-four (TOF) ratio varies significantly between individuals and is often greater than unity. Thus, normalization of acceleromyography data is necessary. The relationship between normalized acceleromyographic TOF ratio, lung volumes, and clinical signs of residual neuromuscular block was studied.


METHODS
In 12 healthy volunteers, three steady-state levels of neuromuscular block were achieved with mivacurium infusions. TOF ratio was measured acceleromyographically at the adductor pollicis using a preload. Lung volume measurements and a series of clinical tests were made at each stable block and reconciled to the normalized TOF measures.


RESULTS
None experienced airway obstruction or arterial oxygen desaturation, even at normalized TOF ratio less than 0.4. Functional residual capacity remained unchanged whereas vital capacity decreased linearly with decreasing TOF ratio. The ability to protrude the tongue was preserved at all times. The ability to clench the teeth was lost in one volunteer at normalized TOF ratio of 0.84 but retained in four at normalized TOF ratio less than 0.4. Four volunteers lost the ability both to raise the head more than 5 s and to swallow, with the most sensitive individual demonstrating these effects at normalized TOF ratio of 0.60. At mean normalized TOF ratio of 0.42, the mean handgrip strength was approximately 20% of baseline value.


CONCLUSIONS
Lung vital capacity decreased linearly with decreasing TOF ratio. Responses to clinical tests of muscle function varied to a large extent among individuals at comparable TOF ratios. None of the volunteers had significant clinical effects of neuromuscular block at normalized acceleromyographic TOF ratio greater than 0.90.
Heier
Tom
T
Department of Anesthesia, University of California, San Francisco, California, USA. tom.heier@medisin.uio.no
Caldwell
James E
JE
Feiner
John R
JR
Liu
Luke
L
Ward
Theresa
T
Wright
Peter M C
PM
D000293:Adolescent; D000328:Adult; D000762:Anesthesia Recovery Period; D003679:Deglutition / Q000187:drug effects / Q000502:physiology; D004576:Electromyography; D005260:Female; D005652:Functional Residual Capacity; D018737:Hand Strength / Q000502:physiology; D006801:Humans; D007546:Isoquinolines; D007568:Jaw / Q000502:physiology; D008297:Male; D000077590:Mivacurium; D009068:Movement / Q000187:drug effects; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D019148:Neuromuscular Blockade; D003473:Neuromuscular Nondepolarizing Agents; D012115:Residual Volume; D012129:Respiratory Function Tests; D013060:Speech / Q000187:drug effects; D014059:Tongue / Q000502:physiology; D014785:Vision, Ocular / Q000187:drug effects; D055815:Young Adult
D007546:Isoquinolines; D003473:Neuromuscular Nondepolarizing Agents; D000077590:Mivacurium