title: Intubation biomechanics: laryngoscope force and cervical spine motion during intubation with Macintosh and Airtraq laryngoscopes.
abstract: BACKGROUND
  Laryngoscopy and endotracheal intubation in the presence of cervical spine instability may put patients at risk of cervical cord injury. Nevertheless, the biomechanics of intubation (cervical spine motion as a function of applied force) have not been characterized. This study characterized and compared the relationship between laryngoscope force and cervical spine motion using two laryngoscopes hypothesized to differ in force.
  
  
  METHODS
  Fourteen adults undergoing elective surgery were intubated twice (Macintosh, Airtraq). During each intubation, laryngoscope force, cervical spine motion, and glottic view were recorded. Force and motion were referenced to a preintubation baseline (stage 1) and were characterized at three stages: stage 2 (laryngoscope introduction); stage 3 (best glottic view); and stage 4 (endotracheal tube in trachea).
  
  
  RESULTS
  Maximal force and motion occurred at stage 3 and differed between the Macintosh and Airtraq: (1) force: 48.8 ± 15.8 versus 10.4 ± 2.8 N, respectively, P = 0.0001; (2) occiput-C5 extension: 29.5 ± 8.5 versus 19.1 ± 8.7 degrees, respectively, P = 0.0023. Between stages 2 and 3, the motion/force ratio differed between Macintosh and Airtraq: 0.5 ± 0.2 versus 2.0 ± 1.4 degrees/N, respectively; P = 0.0006.
  
  
  CONCLUSIONS
  The relationship between laryngoscope force and cervical spine motion is: (1) nonlinear and (2) differs between laryngoscopes. Differences between laryngoscopes in motion/force relationships are likely due to: (1) laryngoscope-specific cervical extension needed for intubation, (2) laryngoscope-specific airway displacement/deformation needed for intubation, and (3) cervical spine and airway tissue viscoelastic properties. Cervical spine motion during endotracheal intubation is not directly proportional to force. Low-force laryngoscopes cannot be assumed to result in proportionally low cervical spine motion.
authors.affiliation: From the Department of Anesthesia, University of Iowa Roy J. and Lucille A. Carver College of Medicine, Iowa City, Iowa (B.J.H., R.P.F., M.M.T.); Foundation for Orthopaedic Research and Education, Tampa, Florida (B.G.S.); and Department of Mechanical Engineering, School of Biomedical Engineering, Orthopaedic Bioengineering Research Laboratory, Colorado State University, Fort Collins, Colorado (C.M.P.).
authors.forename: Bradley J
authors.initials: BJ
authors.lastname: Hindman
authors.forename: Brandon G
authors.initials: BG
authors.lastname: Santoni
authors.forename: Christian M
authors.initials: CM
authors.lastname: Puttlitz
authors.forename: Robert P
authors.initials: RP
authors.lastname: From
authors.forename: Michael M
authors.initials: MM
authors.lastname: Todd
mesh_terms: D000328:Adult; D000368:Aged; D000768:Anesthesia, General; D001696:Biomechanical Phenomena; D002574:Cervical Vertebrae / Q000033:anatomy & histology / Q000502:physiology; D003680:Deglutition Disorders / Q000453:epidemiology / Q000209:etiology; D005260:Female; D005931:Glottis / Q000033:anatomy & histology; D006257:Head / Q000033:anatomy & histology; D006801:Humans; D007442:Intubation, Intratracheal / Q000009:adverse effects / Q000295:instrumentation* / Q000379:methods; D020704:Laryngoscopes; D007828:Laryngoscopy / Q000009:adverse effects / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009038:Motion; D009333:Neck / Q000033:anatomy & histology; D010147:Pain Measurement; D010149:Postoperative Pain / Q000453:epidemiology; D011183:Postoperative Complications / Q000453:epidemiology; D013131:Spine / Q000033:anatomy & histology / Q000502:physiology*; D016683:Supine Position; D018677:Tooth Injuries / Q000453:epidemiology / Q000209:etiology; D014832:Voice Disorders / Q000453:epidemiology / Q000209:etiology
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