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Anesth Analg. 1996 Mar;82(3):456-61.

Force, torque, and stress relaxation with direct laryngoscopy.

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Department of Anesthesiology, University of California San Diego, USA.


The anesthetist exerts axial force on the laryngoscope handle to expose the glottis. The anesthetist must also apply a perpendicular force to balance the torque on the laryngoscope. Several studies have measured axial force during direct laryngoscopy, but none has measured torque. This study used a newly designed laryngoscope handle to measure force and torque simultaneously during direct laryngoscopy of ASA grade I and II patients requiring general anesthesia and endotracheal intubation for elective surgery. In 58 patients, peak force averaged 38 +/- 2 newtons. Peak torque averaged 4 +/- 0.2 newton-meters, and the perpendicular force was estimated as 40 +/- 2 newtons. The peak torque that can be balanced by the wrist is approximately 6 newton-meters, suggesting that torque may be a limiting factor for laryngoscopy in some situations. Peak force and torque demonstrated stress relaxation, a viscous property of biologic tissues. Force and torque decreased monoexponentially to approximately 70% of peak values with a half-time of 4 +/- 0.3 s. The phenomenon occurred in spite of administration of muscle relaxants, and was probably due to stress relaxation of pharyngeal tissues that are passively stretched during laryngoscopy.

[Indexed for MEDLINE]

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