Diaphragm ultrasonography as an alternative to whole-body plethysmography in pulmonary function testing

Objective. Whole-body plethysmography is a common method of measuring pulmonary function. Although this technique provides a sensitive measure of pulmonary function, it can be problematic and unsuitable in some patients. The development of more accessible techniques would be beneficial. Methods. A prospective study was performed to validate diaphragm ultrasonography as an alternative to whole-body plethysmography in patients referred for pulmonary function testing. Diaphragm movement and position were assessed by ultrasonography after standard pulmonary function testing using whole-body plethysmography. Results. A wide range of lung function was observed. Standard lung volumes were as follows: total lung capacity, 5.57 +/- 1.31 L, residual volume, 2.27 +/- 0.56 L; and vital capacity, 3.30 +/- 0.98 L (mean SD). The ratio of forced expiratory volume in 1 second to forced vital capacity was calculated as 0.69 +/- 0.08. Ultrasonography showed that mean diaphragm excursion values were 11.1 +/- 3.8 mm (2-dimensional), 14.7 +/- 4.1 mm during quiet breathing (M-mode), and 14.8 +/- 3.9 mm during a maximal sniff (M-mode). The velocity of diaphragm movement rose sharply during the sniff maneuver from 15.2 +/- 5.8 mm/s during quiet breathing to 104.0 +/- 33.4 mm/s. Static 2-dimensional measures of diaphragm position at the end of quiet inspiration or expiration correlated with standard measures of lung volume on plethysmography (eg, a correlation coefficient of 0.83 was obtained with end inspiration and vital capacity). All measures of diaphragm movement (whether by 2-dimensional or M-mode techniques) were poorly correlated with any lung volumes measured. Conclusions. These data suggest that dynamic measurements using diaphragm ultrasonography provide a relatively poor measure of pulmonary function in relation to whole-body plethysmography.