Technical and interpretive problems of peripheral nerve stimulation in monitoring neuromuscular blockade in the intensive care unit.

OBJECTIVE: To review the literature and provide an overview of the technical and interpretive problems associated with peripheral nerve stimulation in monitoring neuromuscular blockade in the intensive care unit. DATA SOURCES: A computerized search on MEDLINE from 1985 through 1994 was performed to identify English-language comparative studies, abstracts, and review articles pertaining to peripheral nerve stimulation, train-of-four monitoring, and neuromuscular blockade in the critical care setting. STUDY SELECTION AND DATA EXTRACTION: Relevant studies in humans were selected and information was extracted on the use of peripheral nerve monitoring in the critically ill. DATA SYNTHESIS: Use of peripheral nerve stimulation is complicated in the intensive care unit. Problems may occur with the patient, the device, as well as operator technique, all of which may lead to errors in interpretation of the depth of paralysis. The critically ill patient has changing comorbid disease states and total body water composition, which may impair the accuracy or reproducibility of measurements. Technical problems relate to the operation of the device, electrode placement, and suboptimal delivery of the desired current. Difficulties in performing peripheral nerve stimulation and interassessor variability contribute to errors of interpretation. CONCLUSIONS: These difficulties compromise the precision, accuracy, and reliability of the peripheral nerve stimulator as a tool for monitoring neuromuscular blockade in the critically ill. Peripheral nerve stimulation should be used in conjunction with clinical parameters to make decisions regarding dose adjustments. Doses should be reduced as much as possible to provide the minimum depth of paralysis that is clinically appropriate. Technical directions and training programs for peripheral nerve stimulation should be developed, and designated individuals should be trained in its application. Large, prospective, controlled studies are necessary to evaluate the incidence of prolonged paralysis or motor neuropathy with administration of neuromuscular blocking agents in patients whose dose is adjusted on the basis of peripheral nerve stimulation.

Comparison of methods of measurement of compliance of the respiratory system in children.

Compliance of the respiratory system (Crs) was compared using 2 previously described methods, namely, the passive expiratory flow and multiple occlusion techniques. These 2 methods have the advantage of being totally noninvasive, but both have limitations precluding their use in some circumstances. The results of this study of 10 intubated patients, who varied widely in age (premature newborn to adolescence) and diseases, showed no significant difference between the Crs obtained by the 2 methods. The Crs measurements can therefore be used interchangeably, the choice of technique being dependent on the subjects' clinical state.

Diaphragmatic work of breathing in premature human infants.

Present methods of assessing the work of breathing in human infants do not account for the added load when intercostal muscle activity is lost and rib cage distortion occurs. We have developed a technique for assessing diaphragmatic work in this circumstance utilizing measurements of transdiaphragmatic pressure and abdominal volume displacement. Eleven preterm infants without evidence of lung disease were studied. During periods of minimal rib cage distortion, inspiratory diaphragmatic work averaged 5.9 g X cm X ml-1, increasing to an average of 12.4 g X cm X ml-1 with periods of paradoxical rib cage motion (P less than 0.01). Inspiratory work was strongly correlated with the electrical activity of the diaphragm as measured from its moving time average (P less than 0.05). Assuming a mechanical efficiency of 4% in these infants, the caloric cost of diaphragmatic work may reach 10% of their basal metabolic rate in periods with rib cage distortion. When lung disease is superimposed, the increased metabolic demands of the diaphragm may predispose preterm infants to fatigue and may contribute to a failure to grow.

Investigation of the heterogeneity of the MM isozyme of creatine kinase.

MM creatine kinase (MM CK) from human skeletal muscle was resolved into 21 subspecies by isoelectric focusing. Rabbit and bovine MM CK's were also shown to have patterns of 21 bands. These results are compatible with the presence of six different M subunit forms in skeletal muscle, which by random association into dimers, could produce 21 different subspecies of creatine kinase. Two-dimensional isoelectric focusing of human MM CK yielded a diagonal pattern, indicating that the various enzyme subspecies were not induced by ampholyte-protein interaction. Heat inactivated serum appeared to stabilize various CK subspecies and also produced one further anodally migrating MM CK form. Fresh serum had combined inhibitory and stabilizing effects on the MM CK subforms, and allowed for the identification of four anodally migrating CK subspecies not present in tissue extract. These findings are of critical importance because of the widespread use of CK isoenzymes in serum for diagnostic purposes.