Force, torque, and stress relaxation with direct laryngoscopy

Anesth Analg. 1996 Mar;82(3):456-61. doi: 10.1097/00000539-199603000-00004.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Anesthesia, General
  • Elective Surgical Procedures
  • Equipment Design
  • Female
  • Glottis / anatomy & histology
  • Humans
  • Intubation, Intratracheal
  • Laryngoscopes
  • Laryngoscopy / methods*
  • Larynx / anatomy & histology
  • Male
  • Middle Aged
  • Pharynx / anatomy & histology
  • Rotation
  • Stress, Mechanical
  • Time Factors
  • Transducers
  • Viscosity
  • Wrist / physiology