Spirometry

Measurements
Routine spirometry, obtained with the patients seated in a comfortable armchair, was measured as previously described.
Functional residual capacity (FRC) was measured by the helium dilution technique. The normal values for lung volumes were those of the European Community for Coal and Steel. Arterial blood gas tensions were measured (IL-1304; Instrumentation Laboratory; Milan, Italy).  website 
For mechanical studies, an esophageal latex balloon (length, 10 cm; air volume, 0.5 mL) was introduced via the nose. A marker was placed on the polyethylene tubing 40 cm from the balloon tip. The catheter was connected to a differential pressure transducer (Validyne; Northridge, CA). Total lung resistance (RL) and dynamic lung elastance (Eldyn) were measured during resting breathing. RL was obtained using the isovolume method; predicted values are those proposed by the European Community for Coal and Steel. Eldyn was determined by dividing the difference in esophageal pressure (Pes) between points of zero flow by Vt.
The highest (most negative in sign) Pes during a sniff maneuver (Pessn) was evaluated at FRC and was repeated until three measurements with < 5% variability were recorded. The highest value of Pessn was used for subsequent analysis.