Helium Flow-Volume Curves in the Detection of Early Small Airway Disease

Abstract

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/see), 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.