Observations with the TOF-Watch/Guard® on train-of-four fade during onset of relaxation

EDITOR: Non-depolarizing muscle relaxants produce blockade that is associated with fade in response to train-of-four stimulation (TOF). Fade is reported to be the prejunctionnal component of this blockade, while twitch depression is believed to be the resultant of postsynaptic acetylcholine receptors blockade [1,2]. In many procedures, muscle relaxants are used at the induction of anaesthesia to facilitate tracheal intubation. We compared the pattern of fade during induction of anaesthesia with an intubating dose of relaxants using a TOF-Guard® accelerometer (Biometer Odense, Denmark). Accelerography frequently generates a TOF ratio >1 before the administration of neuromuscular blocking drugs [3,4] but, subsequent fade detection with this method has not been reported. In addition, some new devices, e.g. the ‘TOF-Watch®’ accelerometers, assess only the TOF ratio but not the twitch height. Therefore during onset, the TOF ratio is the only parameter to quantify neuromuscular blockade. Fifty patients aged 65-83 years scheduled for elective surgery and requiring general anaesthesia with tracheal intubation were evaluated after giving informed consent. When the patient arrived in the operating room, surface electrodes connected to a TOF-Guard® accelerometer were applied overthe ulnar nerve at the wrist with the piezo-electric device placed on the corresponding thumb. A constant supramaximal (60) mA train-of-four stimulation was applied every 15s. Groups were: mivacurium (M), n = 10; rocuronium (R), n = 10; atracurium (A), n = 10; vecuronium (V), n = 10; and pancuronium (P), n = 10. General anaesthesia was induced with fentanyl, 1.5-3 μg kg−1 and propofol, 1-3 mg kg−1. After loss of consciousness, the accelerometer was self-calibrated using its automatic start-up procedure. If the T4/T1 ratio on the display was above 110%, the corresponding thumb was then manually adjusted to obtain a ratio of less than 110% for at least two consecutive stimulation (stabilization time). Therefore, an intubating dose of each relaxant was administered: M, 0.25 mg kg−1; R, 0.6 mg kg−1; A, 0.5 mg kg−1; V, 0.1 mg kg−1; P, 0.1 mg kg−1. General anaesthesia was maintained with a mixture of isoflurane at 1-1.2 MAC and 50% N2O/O2. The following variables were recorded: (a) maximum blockade (%), (b) onset AP: (time to achieve maximum blockade at the adductor pollicis) and (c) TOF ratios at: baseline, T1: 75%, 50%, 25% and duration to 25% T1 recovery. The trachea was intubated when maximum blockade was detected. No significant differences were found between groups for patients' characteristics, onset, stabilization period and baseline TOF ratios. All patients had 100% blockade at the AP (Table 1). At baseline registration, TOF ratios >1 were often encountered (40%) and manually adjusted for 10 patients. Despite this phenomenon, the accelerometer could detect subsequent fade. Within each group, TOF ratio at T1: 25% was significantly lower compared with the TOF ratio at T1: 100% (P < 0.005) for all relaxants. At T1 75%, rocuronium and pancuronium were associated with a lower TOF ratio than mivacurium. At T1 50%, the TOF ratio of pancuronium was significantly lower than all the other groups. At T1: 25%, the TOF ratio for both rocuronium and pancuronium was significantly lower than for the other groups (Table 2). These results suggest that an intubating dose of rocuronium and pancuronium yields more fade than most of the other currently used muscle relaxants. We assessed fade during onset of relaxation, although because fade during recovery is more important, as it bears a fixed relationship to T1 [5]. It is admitted that detection of fade is inversely dose related: although the doses of relaxants used in the present report are not equipotent, they are recommended for tracheal intubation in routine practice. Studies concerning TOF fade are controversial [6,7,8]. When compared with our data, the discrepancy might be associated with factors related to our design which are: difference in equipotency, short stabilization time or assessment of blockade by accelerography. The latter is a convenient method of monitoring [9], is easy to set up and does not necessitate preload and a long stabilization period [10,11]. Although we could detect a statistically significant difference in the accelerographic pattern of fade, we were unable to find major clinical advantage of fade detection during onset. The existence of differential TOF ratio during onset implies that no relationship can be reliably predicted with the T1 value. Unexpectedly, we found the onset time after injection of rocuronium to be longer than values usually encountered. This is mostly explained by the fact that neuromuscular characteristics were assessed during induction of anaesthesia with a very short stabilization period. Additionally, the onset times were recorded at 100% blockade; the auto-calibrating start-up procedure of the accelerometer which currently generates a 60 mA current provides more than a supramaximal stimulus in most patients and might lead to movement artefacts. In summary, an intubating dose of rocuronium or pancuronium generated more accelerographic fade than other relaxants. This difference implies that as with other monitoring techniques, accelerographic TOF ratios might not reliably reflect the exact degree of muscle relaxation during the onset of an intubating dose of non-depolarizing muscle relaxant.Table 1: Patients characteristicsTable 2: Neuromuscular characteristicsC. MOTAMED K. KIROV P. 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