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Background: Although breathlessness is common in chronic heart failure (CHF), the role of inspiratory muscle dysfunction remains unclear. We hypothesised that inspiratory muscle endurance, expressed as a function of endurance time (Tlim) adjusted for inspiratory muscle load and inspiratory muscle capacity, would be reduced in CHF. Methods: Endurance was measured in 10 healthy controls and 10 patients with CHF using threshold loading at 40 % maximal oesophageal pressure (Poesmax). Oesophageal pressure-time product (PTPoes per cycle) and Poesmax were used as indices of inspiratory muscle load and capacity, respectively. Results: Although Poesmax was slightly less in the CHF group (2117.7 (23.6) v 2100.0 (18.3) cm H2O; 95 % CI 237.5 to 2.2 cm H2O, p = 0.1), Tlim was greatly reduced (1800 v 306 (190) s; 95 % CI 1368 to 1620 s, p,0.0001) and the observed PTPoes per cycle/Poesmax was increased (0.13 (0.05) v 0.21 (0.04); 95 % CI 20.11 to 20.03, p = 0.001). Most of this increased inspiratory muscle load was due to a maladaptive breathing pattern, with a reduction in expiratory time (3.0 (5.8) v 1.1 (0.3) s; 95 % CI 0.3 to 3.5 s, p = 0.03) accompanied by an increased inspiratory time relative to total respiratory cycle (Ti/Ttot) (0.43 (0.14) v 0.62 (0.07); 95 % CI 20.3 to 20.1, p = 0.001). However, log Tlim, which incorporates the higher inspiratory muscle load to capacity ratio caused by this altered breathing pattern, was>85