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Department of Anesthesiology, University of Montréal, Montréal, Québec, Canada, francois.donati{at}umontreal.ca
In Response:
Dr. Eikermann (1) suggests a possible ideal nerve stimulation frequency between 50 and 100 Hz to detect residual blockade clinically. His recommendation deserves to be tested experimentally, keeping two points in mind: one can measure fade with tetanic stimulation in the absence of neuromuscular blocking drugs, and fade measurable by mechanomyography might remain undetectable by visual or tactile means.
Mechanomyography may measure fade in the presence of volatile anesthetics alone (without nondepolarizing neuromuscular blocking drugs), the magnitude of fade being dependent on end-tidal concentration and stimulation frequency. For example, at 6% end-tidal desflurane, the tetanic ratio (force at the end divided by force at the beginning of the tetanic response) is 84% at 50 Hz, 64% at 100 Hz, and 29% at 200 Hz, while the train-of-four (TOF) ratio is preserved (97%) (2). Fade after high-frequency (100 Hz), but not low-frequency (50 Hz or TOF) tetanic stimulation, is also accentuated by residual concentrations of neuromuscular blocking drugs. For example, after vecuronium blockade and neostigmine reversal, a tetanic ratio >90% is observed with 100 Hz, 50 Hz and TOF in the absence of inhaled anesthetic. However, with isoflurane added, fade is virtually unchanged after 50 Hz and TOF stimulation, whereas 100 Hz tetanic ratio decreases markedly, to 30%–40% (3).
However, the presence of measurable fade does not mean that it can be detected by visual or tactile means. This applies to TOF, double-burst, and tetanic stimulation patterns. In fact, if human senses could detect TOF fade up to a TOF ratio of 0.9, all other stimulation modalities would be superfluous. However, TOF fade is seen or felt only when its actual value is approximately 0.4 or less (4,5). The ideal stimulation frequency should be associated with fade detection when TOF ratio is 
0.9, and no fade detection if TOF ratio is >0.9, both in the presence and absence of volatile anesthetics. The mechanomyography-measured tetanic ratio when fade becomes detectable by visual or tactile methods has not been measured, but analogy with the TOF pattern and results with 50-Hz stimulation suggest that this value is likely low (0.4 or perhaps less). The idea then would be to find a stimulation frequency that reaches that detection threshold when the TOF ratio is approximately 0.9. Our results (5) suggest, but do not prove, that this optimal stimulation frequency might be an elusive target, because tetanic fade at frequencies >50 Hz are affected by factors other than neuromuscular blocking drugs, such as volatile anesthetics. In all likelihood, these other factors increase the variability of the 100-Hz test and make it unreliable.
REFERENCES
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