INFLUENCE OF LUNG-VOLUME DEPENDENCE OF UPPER AIRWAY-RESISTANCE DURINGCONTINUOUS NEGATIVE AIRWAY PRESSURE

To quantify the contribution of lung volume dependence of upper airway (UA) on continuous negative airway pressure (CNAP)induced increase in upper airway resistance, we compared the changes in supralaryngeal re sistance during an isolated decrease in lung volume and during CNAP in eight normal awake subjects. Inspiratory supralaryngeal resistance wa s measured at isoflow during four trials, during two CNAP trials where the pressure in a nasal mask was progressively decreased in 3- to 5-c mH(2)O steps and during two continuous positive extrathoracic pressure (CPEP) trials where the pressure around the chest (in an iron lung) w as increased in similar steps. The CNAP and CPEP trials were done in r andom order. During the CPEP trial, the neck was covered by a rigid co llar to prevent compression by the cervical seal of the iron lung. In each subject, resistance progressively increased during the experiment s. The increase was linearily correlated with the pressure increase in the iron lung and with the square of the mask pressure during CNAP. T here was a highly significant correlation between the rate of rise in resistance between CNAP and CPEP: the steeper the increase in resistan ce with decreasing lung volume, the steeper the increase in resistance with decreasing airway pressure. Lung volume dependence in UA resista nce can account for 61% of the CNAP-induced increase in resistance. We conclude that in normal awake subjects the changes in supralaryngeal resistance induced by CNAP can partly be explained by the lung volume dependence of this resistance.