Validation of a noninvasive tension-time index of inspiratory muscles

The aim of this study was to validate a noninvasive tension-time index (TT) for all the inspiratory muscles estimated from the measurement of mouth occlusion pressure (P0.1), i.e., TT of inspiratory muscles (TTmus = PI/PImax x TI/TT, where PI is mean inspiratory pressure, PImax is maximal PI, TI is time of muscle contraction, and TT is total time of respiratory cycle) compared with TT of the diaphragm (TTdi = Pdi/Pdimax x TI/TT, where Pdi is mean transdiaphragmatic pressure and Pdimax is maximal Pdi). PI was estimated as PI = 5 P0.1 x TI. Eleven patients with chronic obstructive pulmonary disease and seven normal subjects were studied at rest in the sitting position. After 5 min of steady state, we measured breathing pattern, gastric and esophageal pressures, Pdi, mean inspiratory transpulmonary pressure swing, PImax, and Pdimax. By linear regression analysis, significant positive correlations were found between PI and mean inspiratory transpulmonary pressure swing, PI and Pdi, PImax and Pdimax, and PI/PImax and Pdi/Pdimax, with P < 0.001 for all subjects combined. These led to the highly significant correlation between TTmus and TTdi for all subjects combined (TTmus = 2.1 TTdi + 0.012; r = 0.97; P < 0.001) and for patients only (TTmus = 2.0 TTdi + 0.024; r = 0.97; P < 0.001). Therefore, patterns of breathing that lie near fatigue thresholds can be identified with TTmus or TTdi. In conclusion, noninvasive and clinically easily determined TTmus seems valid for situating patients of chronic obstructive pulmonary disease in reference to the inspiratory muscle fatigue.