Time Course and Degree of Hyperinflation with Metronome-Paced Tachypnea in COPD Patients

In COPD patients, tachypnea should increase (dynamic) hyperinflation by shortening expiratory time. We developed a method to evaluate the time course and degree of dynamic hyperinflation during metronome-paced tachypnea. Fourteen patients with stable COPD (FEV(1) 43 +/- 13% predicted) were studied. Inspiratory capacity (IC) was measured breathing through a flow transducer. Subjects paced their respiratory rate (f(R)) at 20/min, 30/min and 40/min for 60-second periods in response to audible tones generated by a computer. IC measurements were obtained at baseline and after 30 and 60 seconds at each f(R). End-tidal carbon dioxide was monitored and f(R) was allowed to return to baseline between periods of tachypnea. Tachypnea produced reductions in IC of 200 +/- 240 ml, 380 +/- 330 ml and 540 +/- 300 ml after 30 seconds at 20/min, 30/min and 40/min, respectively. IC reduction at 60 seconds was similar to 30 seconds for each f(R). In patients with moderate-to-severe COPD, the dynamic hyperinflation induced by metronome-paced tachypnea was shown to occur rapidly and be complete by 30 seconds for a given f(R). Controlled increments in f(R) produced stepwise increases in dynamic hyperinflation. This standardized method could be a useful and easier method of assessing dynamic hyperinflation in COPD patients before and after therapeutic interventions.