ASSESSMENT OF THE RESPIRATORY COMPLIANCE IN AWAKE SUBJECTS USING PRESSURE SUPPORT

Pressure support (PS), a new mode of ventilatory assistance, is known to induce respiratory muscle relaxation. It was used to obtain reliabl e measurements of the compliance of the respiratory system (Crs) in aw ake subjects. PS was applied, through a mouthpiece, at four successive levels (0, 0.75, 1 and 1.25 kPa) to 30 healthy subjects. At the highe st PS level, the subject's relaxation was obtained as assessed by a de crease in the occlusion pressure from 0.10+/-0.06 to 0.05+/-0.04 kPa, whereas the minute ventilation increased (from 7.5+/-1.5 to 13.8+/-3.3 l.min-1), and the end-tidal carbon dioxide tension (Pco2) decreased ( from 5.0+/-0.4 to 3.2+/-0.5 kPa) below its apnoea threshold. In three subjects, respiratory muscle relaxation was confirmed by a fall in dia phragmatic electromyographic activity. Crs was calculated as the ratio of the tidal volume to the corresponding end-inspiratory airway press ure (i.e. PS level) since, at end inspiration, a zero-flow period was obtained. Crs was highly correlated (r=0.77) to the height (Ht) of the subJects: Crs (l.kPa-1) = 3.56 x Ht (m) -4.86 (+/-0.23), allowing nor mal values to be determined. In order to evaluate the applicability of the method to patients, Crs was measured in four patients with scolio sis, and was found to range from 4.5-82% of the predicted values. It i s suggested that this simple method of Crs determination may be used t o characterize various chest wall or pulmonary diseases.