Background: We sought to determine the distances of the three segments of the airway from upper incisors to carina in intubated patients in three different neck positions.
Methods: Twenty patients undergoing elective surgery were studied. The airway was divided into three segments: upper incisor to vocal cords (UI-VC), vocal cords to sternal notch (VC-SN), and sternal notch to the carina (SN-CA). After general anesthesia and tracheal tube placement, the circuit was connected and the lungs ventilated. A bronchoscope was inserted through a ported elbow adapter until the tip just contacted carina. A marker tape was placed on the bronchoscope immediately above the adapter port. As the bronchoscope was withdrawn to the sternal notch (by transillumination), vocal cords and upper incisor (endoscopic visualization), three corresponding markers were placed along the bronchoscope. The three segments of the airway were obtained by measuring the distances between the four markers. Measurements were taken with the patient's neck in flexion (10 cm pillow), neutral (5 cm pillow), and extension (no pillow) positions. Repeated measure analysis of variance and paired t-tests were used for analysis of the data.
Results: The UI-VC, VC-SN, and SN-CA distances were 12.01 +/- 1.49, 5.37 +/- 0.95, 8.24 +/- 1.16 cm. From neck flexion to extension, UI-VC and VC-SN increased by 0.36 +/- 0.68 cm (P = 0.027) and 1.74 +/- 0.48 cm (P < 0.001) respectively; SN-CA decreased by 0.12 +/- 0.70 cm (NS). Overall, UI-CA increased by 1.99 +/- 0.70 cm (P < 0.001). SN-CA represented 64%, 61%, 56% of the VC-CA distance with the neck in flexion, neutral, and extension respectively. SN-CA did not change significantly among the head positions (NS).
Conclusions: From neck flexion to extension, the UI-CA distance increased by 1.99 cm. The major contribution to this lengthening was an increase of the VC-SN distance by 1.74 cm; UI-VC increased by 0.36 cm whereas SN-CA did not change significantly. Averaging the three neck positions, SN-CA represented 60% of the VC-CA distance. Our findings may explain why tracheal tubes fixed at the mouth ascend in the trachea with neck extension.