Direct measurement of subglottic pressure and laryngeal resistance in normal subjects and in spasmodic dysphonia.

This study tested the accuracy of indirect methods of measurement of laryngeal airway resistance in normal subjects and in spasmodic dysphonia (SD). The indirect method assumes that subglottic air pressure remains constant during the voiced segment of a syllable. In this study subglottic air pressure was directly measured via puncture of the cricothyroid membrane in seven normal subjects and seven subjects with SD. The true laryngeal airway resistance was calculated and compared with airway resistance measured using indirect techniques based on intraoral air pressure. In five of the seven normal subjects, subglottic air pressure did not remain constant during the voiced segment. As a result, the error produced using indirect method of calculating average laryngeal resistance for the normal subjects varied from -44% to +50%. For SD subjects the error ranged from -49% to +22%. In general, the indirect technique overestimated laryngeal airway resistance in normal subjects and underestimated the resistance in subjects with SD.