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Synopsis: In a methods comparison study involving 27 critically ill pediatric patients, poor agreement was found between observed muscle paralysis and corresponding train-of-four scores obtained from peripheral nerve stimulation during and after neuromuscular blocking agent infusion.
Source: Pena O, et al. Heart Lung 2000;29:309-318.
This study was conducted to determine the degree of agreement between observed muscle movement and train-of-four (TOF) scoring obtained from a peripheral nerve stimulator during a period of therapeutic paralysis. Data were collected from 27 critically ill pediatric patients (mean age 3.5 ± 5.6 years, range, 0.01-17.88 years) at two observation points: 1) during an infusion of a neuromuscular blocking agent (NMBA), and 2) one hour after the infusion was discontinued. At both observation points, patients were observed for any muscle movement in response to eye care or oral care, or to gentle abdominal stimulation. After this observation point, data collectors obtained a TOF stimulation score from the ulnar nerve. If the patient had no response to ulnar nerve TOF stimulation, or if edema was observed, the facial nerve site was used to obtain a TOF score.
Agreement between the two measurement techniques (observed muscle movement and TOF scores) before and after NMBA infusion was evaluated with a Cohen (kappa) statistic, an appropriate statistic to assess an index of agreement. Agreement was defined as a TOF of 0-2 twitches out of four and no muscle movement observed and 3-4 twitches with muscle movement observed. Level of agreement was defined according to the Table below.
|Kappa||Level of Agreement|
Thirty observations were taken from 27 patients (3 patients were observed twice). Twenty-two observations were taken from patients receiving vecuronium bromide and eight were taken from patients receiving cisatricurium besylate infusions. As expected, Pena and colleagues found little observable movement in patients during the NMBA infusion. However, TOF scores did not agree with this observation. Fair agreement was found between observed muscle movement and corresponding TOF scores during the infusion (kappa ranges, < 0.30). During this time, there were patients with low TOF scores (0-2 of 4 twitches) who were observed to have muscle movement and there were patients with high TOF scores (3-4 twitches), with no muscle movement observed in response to tactile stimuli. During the second observation (50-60 minutes after NMBA infusion ceased), patients exhibited more muscle response to tactile stimulation and there was fair-to-moderate agreement (kappa ranges, 0.39-0.70) between observed muscle movement and corresponding TOF scores. The indices of agreement were considerably higher when using the facial nerve site (kappa ranges, 0.34-0.70) to obtain TOF scores than the ulnar nerve site (kappa ranges, 0.06-0.39) in all cases after the drug was discontinued. Pena et al concluded that the disagreement between assessment techniques during the NMBA infusion corroborated the initial observations of their critical care unit’s nurses that TOF monitoring was not a reliable indicator of the degree of neuromuscular blockade.
Reports of prolonged paralysis following cessation of neuromuscular blockade infusions emphasized the importance of giving the lowest possible dose of NMBA to achieve desired effects. To more effectively estimate the level of blockade, peripheral nerve stimulation and TOF monitoring protocols have been advocated to prevent prolonged paralysis in critically ill patients of all ages. However, Pena et al found poor agreement between muscular movement and corresponding TOF during NMBA infusion, and substantial agreement between the two techniques after the infusion was discontinued. Pena et al conclude that TOF monitoring may not be a reliable indicator of the level of neuromuscular blockade. Do we now add TOF monitoring to the list of monitoring devices used in critical care units (e.g., pulse oximetry, hemodynamic monitoring), that clinicians find data inaccurate and unreliable?
These results and conclusions raise issues analogous to those that have been raised with respect to the use of the pulmonary artery catheter. In order to fairly evaluate technology, the proper use of the technology must first be controlled. It was Homer who said, "A fool with a tool is still a fool." Optimal use of any technology is dependent upon the clinicians obtaining the data. Pena et al state that "educational training of the investigators and nursing staff in the use of TOF monitoring was provided." However, content and length of the training, experience of the nursing staff, and assessment of competency were not addressed.
Before we conclude that TOF monitoring is not reliable, we must further investigate the following: 1) technical aspects of TOF monitoring (electrode placement, site selection); 2) variables affecting accuracy (edema, site selection); 3) interpretation (visual vs tactile assessment) of the muscle’s mechanical response, and 4) drug therapy (when and how much should NMBA use be titrated). Just like with the controversy over the value of the pulmonary artery catheter in terms of decision making in the care of critically ill patients, the value of TOF monitoring cannot be addressed without first improving the knowledge and practice of the clinicians who use the technology.