Intubation biomechanics: laryngoscope force and cervical spine motion during intubation in cadavers—effect of severe distractive-flexion injury on C3–4 motion

Abstract

<jats:sec> <jats:title>OBJECTIVE</jats:title> <jats:p>With application of the forces of intubation, injured (unstable) cervical segments may move more than they normally do, which can result in spinal cord injury. The authors tested whether, during endotracheal intubation, intervertebral motion of an injured C3–4 cervical segment 1) is greater than that in the intact (stable) state and 2) differs when a high- or low-force laryngoscope is used.</jats:p></jats:sec> <jats:sec> <jats:title>METHODS</jats:title> <jats:p>Fourteen cadavers underwent 3 intubations using force-sensing laryngoscopes while simultaneous cervical spine motion was recorded with lateral fluoroscopy. The first intubation was performed with an intact cervical spine and a conventional high-force line-of-sight Macintosh laryngoscope. After creation of a severe C3–4 distractive-flexion injury, 2 additional intubations were performed, one with the Macintosh laryngoscope and the other with a low-force indirect video laryngoscope (Airtraq), used in random order.</jats:p></jats:sec> <jats:sec> <jats:title>RESULTS</jats:title> <jats:p>During Macintosh intubations, between the intact and the injured conditions, C3–4 extension (0.3° ± 3.0° vs 0.4° ± 2.7°, respectively; p = 0.9515) and anterior-posterior subluxation (−0.1 ± 0.4 mm vs −0.3 ± 0.6 mm, respectively; p = 0.2754) did not differ. During Macintosh and Airtraq intubations with an injured C3–4 segment, despite a large difference in applied force between the 2 laryngoscopes, segmental extension (0.4° ± 2.7° vs 0.3° ± 3.3°, respectively; p = 0.8077) and anterior-posterior subluxation (0.3 ± 0.6 mm vs 0.0 ± 0.7 mm, respectively; p = 0.3203) did not differ.</jats:p></jats:sec> <jats:sec> <jats:title>CONCLUSIONS</jats:title> <jats:p>The authors' hypotheses regarding the relationship between laryngoscope force and the motion of an injured cervical segment were not confirmed. Motion-force relationships (biomechanics) of injured cervical intervertebral segments during endotracheal intubation in cadavers are not predicted by the in vitro biomechanical behavior of isolated cervical segments. With the limitations inherent to cadaveric studies, the results of this study suggest that not all forms of cervical spine injury are at risk for pathological motion and cervical cord injury during conventional high-force line-of-sight intubation.</jats:p></jats:sec>

Airway management in patients with suspected or confirmed traumatic spinal cord injury: a narrative review of current evidence

“…Cervical spinal canal diameter is more accurately assessed by examining changes in SAC (Fig. 1) but unfortunately this outcome measure is used 1124 infrequently (Table 2) [10,27,[40][41][42][43][44]. Future research in this area needs to focus on outcomes such as vertebral canal dimension and SAC.…”

“…This is not to say that direct laryngoscopy increases the risk of spinal cord injury, as the studies that have measured SAC in this regard have shown only minor differences between different devices. In cadaveric models of C3/C4 injury and type‐2 odontoid peg fracture, there was no difference in the change in SAC with a Macintosh blade and the Airtraq videolaryngoscope [41–43]. Yet, in a cadaveric model of atlanto‐occipital instability, the King Vision aBlade did produce a smaller change in SAC compared with a Macintosh blade [27].…”

“…It should also be noted that the maximal insult to the spinal cord occurs at the moment of injury; the force required to cause cervical fractures and disruption of ligaments is 645–7429 N (depending on the force vector) [52]. In comparison, the mean (SD) force applied during direct and videolaryngoscopy (Airtraq) is 49 (16) N and 10 (3) N, respectively [53]. Within the cervical spine's normal range of motion, movement requires very little force and is therefore unlikely to transfer significant energy to the spinal cord; this is further attenuated by the absence of the wave effect seen in trauma that occurs focusing the force [54].…”

<p>Prediction of difficult airway management in traumatic cervical spine injury: influence of retropharyngeal space extension</p>

“…Recently, new intubation equipment, including various types of video laryngoscopes, has been developed and introduced for the management of difficult airway. Video laryngoscopes have been reported to be more useful in managing difficult airway than standard direct laryngoscopy; however, several studies have revealed that standard direct laryngoscopy can be efficaciously and safely used with MILS techniques, even in cases of cervical spine injury 15–21. However, most prospective studies have limitations in that they simulated difficult airway scenarios solely by using MILS.…”

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“…MILS may be useful in reducing overall excessive cervical spine movements but has limited effect at mid cervical point of injury [14][15][16]. The movement of spine during laryngoscopy in unstable C spine do not significantly exceed the physiological values of normal intact spine movement [17,18] Patient with cervical spine injuries, the secondary neurological deterioration after laryngoscopy and intubation is very rare, with a reported incidence of less than 0.05% [19][20][21]. Gentle external laryngeal manipulation and use of bougie guided intubation are often extremely helpful in reducing the number of failed attempts [2].…”