BACKGROUND--Ultrasound allows observation of the thickness of the diaphragm in the zone of apposition in vivo during relaxation and maximum inspiratory efforts. METHODS--Changes of diaphragm thickness were studied by B mode (two dimensional) ultrasound in 13 healthy men aged 29-54 years in the seated position. A high resolution 7.5 MHz ultrasound transducer was held perpendicular to the chest wall in the line of a right intercostal space between the anteroaxillary and mid-axillary lines to observe the diaphragm in the zone of apposition 0.5-2 cm below the costophrenic angle. The changes of thickness were observed while breath holding at total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). At FRC the thickness while relaxing against a closed mouthpiece and during a maximum inspiratory mouth pressure (PImax) manoeuvre was recorded. The thickening ratio (TR) was calculated as TR = thickness during PImax manoeuvre/thickness while relaxing. RESULTS--Mean (SD) thickness was 4.5 (0.9) mm at TLC, 1.7 (0.2) mm at FRC, and 1.6 (0.2) mm at RV. During the PImax manoeuvre at FRC mean thickness increased from 1.7 (0.2) mm during relaxation to 4.4 (1.4) mm, while mean PImax and TR were -104 (33) cm H2O and 2.6 (0.7), respectively. There was a high degree of correlation between TR and the pressure achieved during the maximum inspiratory manoeuvre (r = -0.82). CONCLUSIONS--Ultrasound provides a non-invasive assessment of diaphragm thickness with change of lung volume and during the PImax manoeuvre which should prove useful in assessing diaphragm mass and contraction in respiratory and muscle disease.
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