THE TENSION-TIME INDEX AND THE FREQUENCY TIDAL VOLUME RATIO ARE THE MAJOR PATHOPHYSIOLOGIC DETERMINANTS OF WEANING FAILURE AND SUCCESS/

zWe have previously shown (Am. J. Respir. Grit. Care Med. 1995;152:124 8-1255) that in patients needing mechanical ventilation, the load impo sed on the inspiratory muscles is excessive relative to their neuromus cular capacity. We have therefore hypothesized that weaning failure ma y occur because at the time of the trial of spontaneous breathing ther e is insufficient reduction of the inspiratory load. We therefore pros pectively studied patients who initially had failed to wean from mecha nical ventilation (F) but had successful weaning (S) on a later occasi on. Compared with S, during F patients had greater intrinsic positive end-expiratory pressure (6.10 +/- 2.45 versus 3.83 +/- 2.69 cm H2O), d ynamic hyperinflation (327 +/- 180 versus 213 +/- 175 ml), total resis tance (R-max, 14.14 +/- 4.95 versus 11.19 +/- 4.01 cm H2O/L/s), ratio of mean to maximum inspiratory pressure (0.46 +/- 0.1 versus 0.31 +/- 0.08), tension time index (TTI, 0.162 +/- 0.032 versus 0.102 +/- 0.023 ) and power (315 +/- 153 versus 215 +/- 75 cm H2O x L/min), less maxim um inspiratory pressure (42.3 +/- 12.7 versus 53.8 +/- 15.1 cm H2O), a nd a breathing pattern that was more rapid and shallow (ratio of frequ ency to tidal volume, f/V-T 98 +/- 38 versus 62 +/- 21 breaths/min/L). To clarify on pathophysiologic grounds what determines inability to w ean from mechanical ventilation, we performed multiple logistic regres sion analysis with the weaning outcome as the dependent variable. The TTI and the f/VT ratio were the only significant variables in the mode l. We conclude that the TTI and the f/VT are the major pathophysiologi c determinants underlying the transition from weaning failure to weani ng success.