Critique on application of diaphragmatic time-tension index to spontaneously breathing humans

Bellemare and Grassino (J. Appl. Physiol. 53: 1196-1206, 1982) have reported that the diphragmatic time-tension index (TTdi) (i.e., the product of mean transdiaphragmatic pressure/maximum transdiaphragmatic pressure and the inspiratory duty cycle) can be used as a predictor of diaphragmatic fatigue in humans. However, the publications of these authors do not directly address the question of whether inspiratory flow or transdiaphragmatic pressure should be used to calculate the inspiratory duty cycle. To gather data on this point, we computed TTdi by both methods in spontaneously breathing normal adult males (AMN) and age-matched males with chronic obstructive pulmonary disease (COPD) at rest and during treadmill exercise. During rest and exercise in both AMN and COPD, the fraction of the breathing cycle over which diaphragmatic tension was maintained (Tdi/TT) exceeded the fraction of the breathing cycle over which inspiratory airflow was maintained (TI/TT). Therefore, TTdi calculations using Tdi/TT were greater (P less than 0.05) than TTdi computations using TI/TT. However, this difference in TTdi values was relatively small (approximately 15%).