Comparison of peak cough flows measured by pneumotachograph and a portable peak flow meter

Objective: To compare peak cough flows (PCF) obtained with a pneumotachograph (PCFp) with those measured using a portable peak flow meter (PCFm) in a population of healthy subjects and patients with neuromuscular disease. Design: A total of 30 healthy subjects and 32 medically stable patients with neuromuscular diseases were studied. Using an oronasal mask connected, in a randomized order, to a pneumotachograph and to a portable peak flow meter, PCFp and PCFm were measured as every subject performed maximal cough efforts from total lung capacity. Results: PCFp measurements were 377.70 +/- 179.28 liters/min and PCFm measurements were 377.50 +/- 172.98 liters/min (not significant). The two measurements were correlated (r = 0.95, P < 0.001). The random error obtained was 9.37%. The mean of the differences between the values obtained using both 'instruments +/- limits of agreement (+2 SD) was 0.20 +/- 109.78 liters/min (not significant). When the agreement was tested over three ranges of flows (<270 liters/min, 270-480 liters/min, and >480 liters/min), no statistically significant differences were obtained for the population as a whole in any range; however, in the lower flow range, 14 patients had PCFm significantly greater than PCFp (213.93 +/- 55.92 vs. 180.86 +/- 57.22, P = < 0.01). Conclusion: PCF measurements made with a portable peak flow meter such as the asmaPLAN+ (Vitalograph, Ennis, Ireland) are reproducible and reasonably accurate when flows are >270 liters/min. However, some caution must be exercised in clinical practice at lower PCF because of the tendency of this portable device to overestimate the lower flows.