Intubation biomechanics: laryngoscope force and cervical spine motion during intubation with Macintosh and Airtraq laryngoscopes

Abstract

Introduction: 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.

Discussion: 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.

Fig. 1

Mounted sensor arrays and corresponding schematics showing individual sensing elements for (A) Macintosh and (B) Airtraq laryngoscopes.

Fig. 2

Laryngoscope force and overall (Oc-C5) cervical spine extension for Macintosh (blue square and lines) and Airtraq (red circle and lines) laryngoscopes during four stages of intubation. Stage 1—Pre-intubation baseline, defined as zero force and zero extension. Stage 2—Laryngoscope Introduction. Stage 3—Laryngoscope Placement (“Best View”). Stage 4—Intubation. *Macintosh Oc-C5 extension (10.3±12.7 degrees) at 10.4±2.8 N of force (see text). †Macintosh Oc-C5 extension (16.2±12.6 degrees) at 20.0±0 N of force (see text). Values are expressed as mean ± SD.

Fig. 3

(A) Schematic illustration of baseline (Stage 1) position of the cervical spine (occiput to C5) prior to intubation. At this stage, the weight of the head (W_HEAD) and the force of gravity ( keep the occiput resting on the pad. (C) During laryngoscopy (Stage 3, “Best View”), the force of intubation (F_INT) is comprised of both inferiorly- (F_INT-INFERIOR) and anteriorly- directed forces (F_INT-ANTERIOR). Because F_INT-ANTERIOR results in both airway tissue displacement/deformation and cervical spine motion, the amount of F_INT-ANTERIOR transmitted to cervical spine is variably less than () total F_INT-ANTERIOR. During laryngoscopy, two predominant forces (F_INT-ANTERIOR and W_HEAD) act across two distances from the C5 body (l/2 and l, respectively) to create two moments (moment = force * distance) in opposing directions that induce intervertebral rotation (extension). Increases in intervertebral rotation between stages -1 and -3 are qualitatively illustrated with dashed lines passing through each vertebral body and the occiput. The force of friction (F_FRICTION) between the pad and table acts at a vertical distance d from C5, creating an opposing moment (F_FRICTION * d) that acts to cause cervical spine flexion. (B) Lateral radiograph of a patient at intubation stage 3 with the Airtraq, with simultaneous laryngoscope force measurement (inset). The force vector (bold arrow, force (F)=12.7 N) acts in a direction normal (90 degrees) to the laryngoscope surface through the center of force (COF, small arrow) located at 55 mm from the distal tip of the laryngoscope. (D) Lateral radiograph and force measurement at intubation stage 3 with the Macintosh in the same patient. The force vector (bold arrow, force (F)=48.3 N) acts in a direction normal (90 degrees) to the laryngoscope surface through the center or force (COF, small arrow) located at 47 mm from the distal tip of the laryngoscope.