In Reply:-As inventors and developers of the ParaGraph muscle relaxation monitor, we support the findings of Roberts and Dorsch regarding the placement of the piezo-electric motion sensor of the ParaGraph monitor.

Historically, the piezo-electric motion sensor was placed on the thumb to record movement of the adductor pollicis muscle to facilitate clinical comparison with an isometric mechanomyogram. The mechanomyogram is the clinical standard for measurement of muscle relaxation. It requires the hand be fixed to an arm board and the thumb to be placed in a ring so that force generated by the thumb in response to stimulation can be quantified. These research studies required placing the ParaGraph motion sensor over the thumb, to compare its response with the mechanomyogram. However, during routine clinical use, the motion sensor should be placed over the muscle group that is most responsive to the stimulation, as suggested by Roberts and Dorsch.

We endorse the motion sensor placement procedure described by Roberts and Dorsch as:

1) Place the stimulating pad over the ulnar nerve before administration of any muscle relaxant.

2) Electrically stimulate the nerve using a train-of-four or single twitch stimulation.

3) Observe visually where the maximum motion on the hand occurs in response to electrical stimulation.

4) Place the piezo-electric motion sensor over the joint of the hand where maximum motion was observed.

We realize that it is not always possible to observe muscle movement and position the motion sensor before muscle relaxants are given because there may not be adequate analgesia to use the stimulator. In these situations, placement over the intersection of the hypothenar eminence and the fifth digit, as described by Roberts and Dorsch, is appropriate.

Joseph A. Orr, Ph.D., Research Instructor; Dwayne R. Westenskow, Ph.D., Professor, University of Utah, Department of Anesthesiology, University of Utah Medical Center, 50 North Medical Drive, Salt Lake City, Utah 84132.

(Accepted for publication August 21, 1996.)

Kern SE, Johnson JO, Westenskow DR, Orr JA: An effectiveness study of a new piezoelectric sensor for train-of-four measurement. Anesth Analg 1994; 78:978-82.