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Assessment of neuromuscular blockade at the vastus medialis

Saskatoon, Saskatchewan

To the Editor:

I would like to comment on the interesting experiment on neuromuscular blockade (NMB) monitoring by Saitoh et al.¹ Their article may be more interesting than they realize. The article alludes to the phenomenon of direct muscle stimulation, but fails to observe that the innervation of the vastus medialis is via a motor nerve entering the muscle from its deep surface.² There may be an additional small branch entering anteriorly. Nonetheless, all of the motor innervation enters the muscle near its midpoint,³ well above the placement of the stimulating electrodes.

This makes specific motor nerve stimulation impossible with the set-up illustrated, and suggests that the response is due to muscle stimulation. Thus, the changes in acceleration shown, which closely mirror the change in NMB, must be due to stimulation of small im branches of the vastus medialis nerve.

The implications are important. Traditional NMB monitoring uses the adductor pollicis because the muscle is remote from the site of stimulation and avoids direct muscle stimulation. The authors may have shown the way to using the accelerometer-based instrument on any muscle, anywhere, without regard to the anatomy of innervation. This hypothesis, of course, requires scientific study.

References

1 Saitoh Y, Nakajima H, Hattori H, Aoki K, Katayama T, Murakawa M. Neuromuscular blockade can be assessed accelerographically over the vastus medialis muscle in patients positioned prone. Can J Anesth 2003; 50: 342–7.[Abstract/Free Full Text]

2 Warwick R, Williams PL. Gray’s Anatomy, 35th ed. Edinburgh: Longman; 1973: 565.

3 Basmajian JV. Grant’s Method of Anatomy, 10th ed. Baltimore: Williams & Wilkins; 1980: 254.

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REPLY

Yuhji Saitoh
CJA 2003 50: 865. [Full Text]  

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