Helium flow-volume curves in the detection of early small airway disease.

To assess small airway disease in cystic fibrosis (CF) patients with minimal 
pulmonary involvement, maximal expiratory flow volume (MEFV) curves were obtained while 
the patients were breathing first air and then an 80% helium-20% 
oxygen gas mixture. Fifteen CF patients and 24 controls were studied. 
Flow rates at 50% and 25% vital capacity (VC) were calculated 
from the air and helium-mixture MEFV curves and were compared to 
give flow ratios at these lung volumes. At 50% VC, the 
helium/air flow ratios were similar in CF patients and normal subjects. 
At 25% VC, the flow ratio was significantly lower (p < 
0.05) in the CF patients. The air and helium MEFV curves 
were superimposed and the point where the curves crossed (point of 
identical flow, PIF) was determined and expressed in % VC. For 
the CF patients, the mean PIF was 18% and for the 
controls, 5% (p < .001). Nine of the 15 CF patients 
had PIF values greater than 2 standard deviations (SD) from the 
normal mean. None of the following tests were abnormal in more 
than three patients: FEV1/FVC, MMEF, RV/TLC, Vmax 50% TLC (TLC/sec), and 
Vmax 25% VC (TLC/sec). Arterial oxygen tensions were below 86 mm 
Hg in four patients. Closing volumes by the 100% helium bolus 
technique were normal in all patients. Determination of the PIF appears 
to be a simple, noninvasive, and sensitive test for the detection 
of early small airway involvement. 