A threshold loading device for testing of inspiratory muscle performance

Threshold loading is a method of incrementally loading the inspiratory muscles, which requires development of an adjustable threshold pressure (Pth) before air will flow. In the ideal device, flow is independent of pressure once Pth is exceeded. A "weighted plunger" apparatus is commonly used, but devices described previously perform unreliably at high inspiratory load. We have modified this apparatus using the diaphragm and valve seating from a threshold valve used to deliver positive airway pressure (AMBU 10, AMBU International, Denmark) to yield improved pressure-flow characteristics. With this device, there was a linear relationship between Pth and weight on the plunger (r = 0.998), and flow was largely independent of pressure once Pth was exceeded: pressure developed at an inspiratory flow of 1.3 +/- 0.1 l.s-1 was 13.6 +/- 7.0% below Pth. Our modified weighted plunger apparatus allows consistent and reliable threshold loading of inspiratory muscles, even at high inspiratory loads.