Evaluation of the THRESHOLD trainer for inspiratory muscle endurance training: comparison with the weighted plunger method.

Inspiratory muscle training (IMT) has been shown to enhance exercise performance. The weighted plunger (WP) system of inspiratory threshold loading is the most commonly used method of IMT, but is expensive and cumbersome. We have evaluated a commercially available portable spring-loaded IMT device, the THRESHOLD trainer. The WP and THRESHOLD trainer devices were evaluated with their opening pressures set, in random order, at 10, 20, 30 and 40 cmH2O. Using an airpump, pressure at the valve inlet was recorded at the point at which the valve opened, and at airflow rates of 20, 40, 60, 80 and 100 L.min-1. Ten THRESHOLD trainers were then compared using the same opening pressures and airflow rates. Finally, 10 patients with stable chronic heart failure (CHF) inspired, in random order, through the WP and THRESHOLD trainer for 4 min each. The pressure-time product (PTP) was calculated for each 4 min period, to compare the work performed on inspiring through each device. The mean measured opening pressures for the WP set at 10, 20, 30 and 40 cmH2O, were 9.0, 19.3, 27.9 and 39.2 cmH2O, respectively, and there was little change over the range of flow tested. Corresponding values for the THRESHOLD trainer were 7.5, 16.9, 26.2 and 39.1 cmH2O, with the pressure being closer to the set pressure as flow increased to that seen in clinical practice. The 10 different trainers tested performed very similarly to one another. Work performed (as measured by PTP) on inspiring through the WP and THRESHOLD trainer was not significantly different. Although less accurate than the weighted plunger, the THRESHOLD trainer is an inexpensive device of consistent quality. In a clinical setting it would be a satisfactory option for inspiratory muscle training in most patients, but less so in patients with very low inspiratory flow rates.