Cinefluorographic techniques were used in order to locate the sites of obstruction in the upper airways when artificial respiration was given by mouth-to-mouth and mouth-to-nose methods. In 15 adult volunteer subjects anesthetized to apnea 3 sites of obstruction were observed with the subject's head in the neutral position. Two were areas of contact of the pharyngeal wall with (1) the soft palate and (2) the base of the tongue; the third was at the lips. Obstruction at (1) and (2) was relieved when the head was moved back into the extended position by the two-handed hyperextension maneuver. This head-tilt maneuver is a reliable, simple, versatile method of clearing the upper airways for mouth-to-mouth and mouth-to-nose artificial respiration.
Greene DG, Elam JO, Dobkin AB, Studley CL. Cinefluorographic Study of Hyperextension of the Neck and Upper Airway Patency. JAMA. 1961;176(7):570-573. doi:10.1001/jama.1961.03040200006002