The use of the train-of-four (TOF) stimulation for controlling clinical paralysis has become extremely popular since its introduction by Ali et al. (1). Nevertheless, the various paralysis levels monitored by this stimulation pattern are quite often not profound enough to satisfy some special clinical situations—such as a “no cough maneuver” during tracheal suctioning or an immediate “full hiccup control” during delicate intraabdominal surgery—very often requiring additional muscle relaxant administration (2). Viby-Mogensen et al. (3) proposed a very elegant practical solution—the posttetanic count (PTC) methodology—based on the posttetanic facilitation phenomenon for managing deeper paralysis levels than those monitored with the sole TOF stimulation.
The present study was thus designed to make this quantitative distinction using a previously described method for stabilizing precise targeted paralysis levels by a constant or adjusted muscle relaxant infusion rate (4–8).
To optimize the quality of the scheduled observations, the following approaches were preselected: first, the use of a noncumulative muscle relaxant with short duration of action—mivacurium (MIV) (5–8); second, an anesthetic drug regimen providing time-independent observation conditions (7); and third, the choice of precise targeted paralysis levels, TOF and PTC counts of 2, monitored on an adductor pollicis maintained in normal conditions.
After local ethical committee approval of the study design and conditions, 10 ASA physical status I–II adult patients scheduled for elective abdominal surgery were enrolled. Written informed consent was obtained from all subjects. Patients exceeding 130% of ideal body weight, those with clinical biochemical evidence of hepatic, renal, electrolyte, and neuromuscular disorders or those taking medications known to interfere with neuromuscular transmission were excluded. Moreover, total pseudocholinesterases together with dibucaine and fluoride numbers were screened to eliminate the patients with grossly abnormal plasma cholinesterase patterns (9,10).
The patients received 7.5 mg of midazolam orally, 1 h before their arrival in the anesthesia room where they were equipped for monitoring with a 3-lead electrocardiogram, upper extremity noninvasive arterial blood pressure, and pulse oximetry. An infusion line, with a 3-way stopcock, loaded with Ringer’s lactate solution, was placed in a large forearm vein. After 3 min of 100% oxygen administration, anesthesia was induced with sufentanil 0.04 μg/kg IV followed by propofol administered to provide effect compartment target concentrations between 4–5 μg/mL (Diprifusor™; Vial Medical, Brezins, France). After loss of consciousness, assessed verbally, ventilation was controlled manually. Tracheal intubation was performed after the administration of MIV 200 μg/kg. Thereafter, ventilation was controlled mechanically (semiclosed circuit, 35%–45% oxygen in air) and adjusted to produce an end-tidal carbon dioxide concentration in the range of 4.1–4.7 kPa. According to the patient’s reactions to surgical stimulations (modification of arterial blood pressure and/or heart rate of >15%), the targeted effect site compartment propofol concentration was changed and an additional bolus of sufentanil 0.01 μg/kg was injected when requested.
The skin thenar temperature was measured with a surface electrothermometer and the oropharyngeal temperature was obtained with a bulb electrothermometer. Heat loss from the body surface and the arms was controlled by using a hot-air warming mattress placed on the upper trunk, face, and arms of the patient. Occasionally, the IV infusion line was warmed. Oropharyngeal temperature was maintained between 36° and 36.5°C and thenar skin temperatures above 32.5°C during the entire study period.
For eliciting adductor pollicis movements, the ulnar nerve was stimulated by surface electrodes placed at the wrist. The paralysis levels targeted were assessed by gentle contact between the patient’s freely moving thumb and the index finger of the investigator applied during either a TOF or a PTC stimulation (11). TOF and PTC count stimulation patterns (50 mA, 0.2-ms duration of constant rectangular current) were provided by a peripheral nerve stimulator (TOF-Watch™; Organon Ireland Ltd., Dublin, Ireland). After recovery from the initial MIV dose with 2 responses to TOF stimulation, a 50-mL electrically driven syringe of 2 mg/mL native MIV solution was started. Manual flow adjustments of the syringe were allowed every 5 min to produce 2 predetermined paralysis levels: first, stable TOF counts of 2 and second, a constant PTC of 2. An interval of about 5 min was considered sufficient between repeated PTC stimulations to minimize the interference of the posttetanic facilitation phenomenon (11) on the thumb movements observed. The sequence TOF/PTC stimulations was always performed in that order. These paralysis levels noted every 5 min were considered as stable if they remained constant during 4 consecutive determinations, thus after at least a 15-min period; similarly, MIV infusion rates, calculated in μg · kg−1 · min−1, were considered stable if they remained constant during 3 consecutive periods of 5 min (12).
Statistical analysis of the data was performed with Wilcoxon’s test for paired comparisons. Data were noted as: median (min–max).
Patient characteristics including pseudocholinesterases, dibucaine number, fluoride number, durations of MIV exposure, end-tidal carbon dioxide, and central and thenar temperatures are summarized in Table 1.
MIV infusion rates observed varied from 6 (2–11) for maintaining a TOF count of 2 to 17 (3–18) μg · kg−1 · min−1 for obtaining a PTC of 2 (P < 0.001; Wilcoxon’s paired comparison test). Evolutions of individual MIV infusion rates noted for the TOF count of 2 and the PTC of 2 are illustrated in Figure 1.
When a stable paralysis/stable infusion rate method is used, there is a clear quantitative distinction between a TOF count of 2 and a PTC of 2. This implies that a marked MIV infusion rate increase (>200%) is necessary to progress from a TOF count of 2 paralysis level to the PTC of 2.
TOF and PTC stimulations applied to the ulnar nerve to elicit adductor pollicis reactions are popular approaches for monitoring various clinical paralysis levels (11,13–17). Both stimulation patterns could be complementary for correct management of the various paralysis levels needed by the serial phases of a surgical procedure. Whereas PTC-based paralysis levels were termed “intense” by the first authors describing this particular pattern (3), TOF counts of 1 or 2 are often referred to as the “standard” paralysis level. This objective, but purely qualitative, distinction has remained largely accepted. The goal of this study was to provide more practical quantitative information under very basic conditions, including tactile detection of the paralysis levels observed and manual adjustments of an electrical syringe containing a noncumulative short-acting muscle relaxant. This design provides immediately useful data for those clinicians working in cost-contained professional environments.
Numerous authors have designed clinical studies relying partly or totally on tactile or visual detection of muscle movements after standardized stimulation patterns (11,13–17). More recently, the sole use of a tactile TOF count monitoring was advocated, by Pedersen et al. (17), for the management of atracurium and MIV infusions in daily practice. This study confirms their previous conclusions about the feasibility of such a “naked hand” clinical monitoring approach for maintaining stable paralysis levels. The administration of an initial dose followed by an adjustable infusion, once partial recovery is identified, seems to be a very practical method for efficient control of muscle relaxant management because it avoids the occurrence of any major overdosing and limits paralysis level variations. Among the available clinical muscle relaxants, some (i.e., atracurium, cisatracurium, MIV) clearly exhibit noncumulative effects even during long lasting infusions (4–8,12). MIV was selected in this study because its short duration of action offered a rapid resolution of an “intense” PTC of 2 once the study was completed.
Closed level-loop MIV infusions have been described during anesthesia using various drug regimens. In many situations, the servo-controlled infusion rate was influenced either by the magnitude of volatile drug concentrations (18,19) or, in the presence of a constant inhaled concentration, by the duration of volatile drug exposure (7). In the present study, inhaled drugs were not used and anesthesia was maintained with coadministration of propofol and sufentanil providing stable conditions for the observed variables: adductor pollicis activity and MIV infusion rates. It is interesting that the median MIV infusion rate required to maintain stable a clinically assessed TOF count of 2 level, representing approximately an 80% reduction of initial twitch height (15), is not very different than that observed previously with the use of a more sophisticated monitoring method such as close-loop control of MIV infusion on a reduced fraction (circa 5%) of the initial or reference twitch (7,8,18–25) (Table 2).
In anesthetized patients receiving MIV infusion, the use of the stable paralysis/stable infusion rate method established that, for obtaining and maintaining a PTC of 2, MIV infusion rates must be far in excess of the recommendations suggested by the existing literature.
The authors thank Pr. F. Clergue and Dr. A. Foster for their support of this clinical study and Prs. R. Smiley and M. Baurain for their very kind help in editing the manuscript. The expert assistance, for patient testing, provided by the staff members (Mr. N. Mensi and Dr. O. Gollaz) of the institutional clinical chemistry laboratory (Pr. D. Hochstrasser) was also particularly appreciated.
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