Effects of general anesthesia and paralysis on upper airway changes due to head position in humans.
BACKGROUND
In supine patients with their heads in flexion, general anesthesia causes posterior displacement of upper airway structures that is associated with airway obstruction, and extension of the head helps restore patency. However, the independent effects of head position, general anesthesia, and muscle paralysis on upper airway structures are not known.


METHODS
Lateral radiographs of the neck were taken in supine patients with the head in flexion and extension, during consciousness, and after induction of general anesthesia and muscle paralysis. The following measurements were made distances from the horizontal plane to the epiglottis, the hyold, and the thyroid cartilage to detect anteroposterior displacements; distances from the transverse plane to the hyold and the thyroid cartilage to detect cephalocaudad displacements; and widths of the oropharynx, the laryngeal vestibule, and the laryngeal sinus.


RESULTS
With the head in flexion, anesthesia and paralysis compared with the conscious state caused posterior displacement of the epiglottis, narrowing of the oropharynx, and widening of the laryngeal vestibule. With the head in extension, anesthesia and paralysis compared with the conscious state caused anterior displacements of the epiglottis, the hyold, and the thyroid cartilage, narrowing of the oropharynx, and widening of the laryngeal vestibule and the laryngeal sinus.


CONCLUSIONS
Loss of tonic muscular activity due to anesthesia and paralysis results in anteroposterior displacements of the upper airway structures with flexion and extension of the head that are in the same direction as that of the mandible. Anesthesia and paralysis also widen the dimensions of the larynx. These changes might have implications for instrumentation and protection of the airway during general anesthesia or unconsciousness.
Sivarajan
M
M
Seattle Veterans Affairs Medical Center, University of Washington, Washington, USA. murali@u.washington.edu
Joy
J V
JV
D000368:Aged; D000402:Airway Obstruction / Q000209:etiology / Q000517:prevention & control; D000768:Anesthesia, General / Q000009:adverse effects; D006257:Head; D006801:Humans; D008297:Male; D008875:Middle Aged; D009126:Muscle Relaxation / Q000187:drug effects / Q000502:physiology; D011859:Radiography; D012143:Respiratory Physiological Phenomena; D012137:Respiratory System / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D016683:Supine Position / Q000502:physiology