The effectiveness of the larynx as a valve under conditions of complete closure is impressive. Intrathoracic pressures as high as 185 mmHg can be supported during voluntary Valsalva maneuvers (46), and transient values of 250-300 mmHg have been recorded during protracted coughing (196) and during weightlifting (46) in which breath holding helps to stabilize the thorax. Much of the ability of the closed glottis to withstand outwardly directed pressures seems to depend on the passive mechanical properties of the ventricular folds or false vocal cords (40, 187). The false cords are said to be prominent in arboreal mammals, which presumably use Valsalva maneuvers for postural stabilization of the thorax while climbing (40). Inwardly directed pressures of up to 140 mmHg can be supported by the true vocal cords (40).
-Bartlett D, Jr. Respiratory functions of the larynx. Physiological reviews 1989;69:33-57.
196 Sharpey-Schafer EP. The mechanism of syncope after coughing. British medical journal 1953;2:860-863.