TENSION-TIME INDEX OF INSPIRATORY MUSCLES IN COPD PATIENTS - ROLE OF AIRWAY-OBSTRUCTION

Inspiratory muscle function has been shown to be related to general mu scle weakness, weight loss, blood gas tensions, airway obstruction and hyperinflation. The aim of this study was to define (1) the factor th at is the main determinant of the tension-time index of the inspirator y muscles (TTmus), and which thus increases the risk of inspiratory mu scle fatigue; and (2) whether a breathing strategy is adopted to avoid inspiratory muscle fatigue. Twenty-seven normal volunteers and 35 sta ble COPD outpatients (FEV1% predicted, range: 21-89%; and FRC/TLC, ran ge: 49-77%) were studied. The TTmus was determined as follows: TTmus = (P) over bar I/PImax . TI/Ttot, where (P) over bar I is the mean insp iratory pressure calculated from the mouth occlusion pressure (P-0.1), PImax is the maximal inspiratory pressure, TI is the inspiratory time , and Ttot is the total time of the breathing cycle. COPD patients sho wed significantly lower PImax and higher P-0.1, (P) over bar I, (P) ov er bar I/PImax, and TTmus than normal subjects. No patient had a TTmus value higher than the inspiratory muscle fatigue threshold of 0.33. T he FEVI was significantly correlated with TTmus and all its components in the patients. The FRC/TLC was also correlated with all components except (P) over bar I. Body weight was only correlated with PImax. In a forward and backward stepwise regression analysis, FEV1 appeared to be the only significant factor explaining the variance of log ((P) ove r bar I/PImax) and log (TTmus), whereas FRC/TLC was the principal dete rminant of PImax, In COPD patients, a non-linear relationship was foun d between TI and P-0.1. A negative linear relationship was found betwe en TI/Ttot and (P) over bar I/PImax. In conclusion, although hyperinfl ation predominantly affected inspiratory muscle strength in a group of stable COPD patients with a wide range of severity, airway obstructio n was the principal factor determining the magnitude of TTmus. In addi tion, in order to remain below the inspiratory muscle fatigue threshol d, as the severity of airway obstruction increased, patients adopted a breathing strategy characterized by decreased TI/Ttot as inspiratory pressure demand increased.