Comments on 'Flow dependence and repeatability of interrupter resistance in lower airways and nose'.

The authors (Eiser et al 2005 Physiol. Meas. 26 143-56) have undertaken a much needed investigation into the extent of the flow dependence of resistance, as measured by the interrupter technique (Rint). The clear explanations of the general principles should prove invaluable to the readers. However, I should like to highlight some small technical errors as well as raising some points of concern regarding the accuracy of the findings due to the relatively slow sampling rate for flow and subjects being asked to breathe 'deeper and faster', if the device was not triggering at higher flows.

Pressure oscillation amplitude after interruption of tidal breathing as an index of change in airway mechanics in preschool children.

Bronchodilator reversibility testing using change in airway resistance during interruption (Rint) is feasible in preschool children. Analysis of postocclusion oscillations of the mouth pressure-time transient (Pmo(t)), recorded during airflow interruption, may offer an alternative index of change in airway mechanics. We analyzed Pmo(t) oscillation amplitude in three different ways: 1) difference between the first relative maximum and minimum (AMxMn); 2) detection of the dominant frequency using Fourier analysis (AFS); and 3) curve-fitting based on a mathematical model (ACurv). In 25 asymptomatic asthmatic children, aged 2.5-5.6 years, who had undertaken reversibility testing, the correlation coefficients between baseline Rint and amplitude were: AMxMn r = -0.84, AFS r = -0.82, ACurv r = -0.84. The coefficient of variation (CoV) of readings contributing to baseline Rint measurement, as median (range), was 12% (5-24%), which was not significantly different from AFS or ACurv (P > 0.05). All parameters were significantly different postbronchodilator (P < 0.001). Using the sensitivity index, i.e., the change after intervention divided by the baseline standard deviation, ACurv was the most sensitive and Rint the least sensitive, with median (range) at 2.72 (-0.84 to 12.10) and 1.91 (-1.17 to 9.50), respectively (P = 0.005). Our results suggest that oscillation amplitude analysis may provide a sensitive index of change in airway mechanics in preschool children undertaking bronchodilator reversibility testing.

Handgrip-induced airway dilation in asthmatic patients with bronchoconstriction induced by MCh inhalation.

We investigated the effects of static and rhythmic handgrip on the time course of recovery of airway resistance measured with the interrupter technique (Rint) following bronchoconstriction induced by methacholine (MCh) inhalation in 17 asthmatic patients. On three separate occasions, a 100 +/- 5% increase in baseline Rint was induced by MCh inhalation. Subsequently, patients either rested [control trials (CTs)] or performed 3-min bouts of static or rhythmic handgrip. Respiratory and cardiovascular variables were continuously monitored. Rint changes were assessed at 1-min intervals for 30 min after rest and both types of handgrip. Plasma catecholamine concentrations were also determined at scheduled intervals. Bronchoconstriction increased ventilation (P < 0.01) but did not affect cardiovascular variables and plasma catecholamine concentrations. Handgrip provoked an increase in cardiovascular variables (P < 0.01) and plasma norepinephrine concentrations (P < 0.05) but caused no additional changes in ventilation. Rint only partially recovered within 30 min after CTs, whereas it consistently decreased 1 min after both handgrip paradigms and remained lower than after CTs (P always <0.01) for the whole 30-min observation period. Sympathetic activation and withdrawal of cholinergic input to the airway smooth muscle reflexly induced by activation of skeletal muscle and carotid sinus receptors may be the primary events accounting for the bronchodilator response induced by handgrip. Mediators co-released in response to sympathetic activation may also have contributed.