title: Force, torque, and stress relaxation with direct laryngoscopy.
abstract: 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.
authors.affiliation: Department of Anesthesiology, University of California San Diego, USA.
authors.forename: R H
authors.initials: RH
authors.lastname: Hastings
authors.forename: E D
authors.initials: ED
authors.lastname: Hon
authors.forename: C
authors.initials: C
authors.lastname: Nghiem
authors.forename: E A
authors.initials: EA
authors.lastname: Wahrenbrock
mesh_terms: D000328:Adult; D000368:Aged; D000768:Anesthesia, General; D017558:Elective Surgical Procedures; D004867:Equipment Design; D005260:Female; D005931:Glottis / Q000033:anatomy & histology; D006801:Humans; D007442:Intubation, Intratracheal; D020704:Laryngoscopes; D007828:Laryngoscopy / Q000379:methods*; D007830:Larynx / Q000033:anatomy & histology; D008297:Male; D008875:Middle Aged; D010614:Pharynx / Q000033:anatomy & histology; D012399:Rotation; D013314:Stress, Mechanical; D013997:Time Factors; D014159:Transducers; D014783:Viscosity; D014953:Wrist / Q000502:physiology