Effects of respiratory timing and cheek support on resistance measurements, before and after bronchodilation in asthmatic children using the interrupter technique.

The interrupter technique (R-int) is a noninvasive method for assessing respiratory resistance. The aims of this study were to assess whether upper airway support affects the measurement of R-int, if inspiratory or expiratory R-int were most reproducible, and which method of assessing R-int correlated best with spirometry results and was the most sensitive for identifying bronchodilator response. Twenty-four asthmatic children with a mean age of 10.3 years (range, 7-16 years) were included in the study R-int measurements were obtained in inspiration and expiration with cheeks supported and unsupported. Spirometry was then performed. R-int and spirometry measurements were repeated after the inhalation of 600 mcg of salbutamol. The mean R-int supported inspiratory (0.708 KPa/l/sec) and expiratory (0.729 KPaA/sec) values were significantly higher than the unsupported values (inspiratory, 0.622 KPa/l/sec; expiratory, 0.584 KPa/l/sec), P < 0.05 and P < 0.001, respectively The reproducibility of R-int was not different whether cheeks were supported or not, or whether the measurements were carried out during inspiration or expiration. Cheek support improved the correlation with all the lung function results, both in inspiratory and expiratory measurements. The best correlations, however, were found for the inspiratory supported R-int results. The most sensitive method to ascertain bronchodilator response (BD) was the inspiratory supported R-int measurement, as 83.3% of children were identified as having a positive response to bronchodilator therapy as defined by a reduction of twice the coefficient of variation of the measurement. In conclusion, cheek support increases R-int but does not impact on reproducibility, though it improves the correlation with spirometric indices. R-int with cheek support on inspiration correlates best with spirometric indices and appears to be the most sensitive measure of response to bronchodilators.