CHANGES IN BREATHING PATTERN AND RESPIRATORY MUSCLE PERFORMANCE PARAMETERS DURING DIFFICULT WEANING

Objective: This study examined, using noninvasive means, the changes i n breathing pattern and inspiratory muscle pressure-time indices durin g difficult progressive withdrawal of pressure support ventilation. De sign: A prospective analysis of the temporal evolution of several resp iratory variables in difficult-to-wean patients. Setting: A university hospital intensive care unit. Patients: A heterogeneous group of 17 p atients receiving prolonged mechanical ventilation. Interventions: Dai ly measurements of breathing pattern and respiratory muscle performanc e parameters in difficult-to-wean patients. Measurements and Main Resu lts: We examined breathing pattern variables, rapid shallow breathing (respiratory rate/tidal volume), tracheal occlusion pressure, maximal inspiratory pressure (P(I)max), and the tension-time index of the insp iratory muscles (TTmus = P-I/P(I)max x Ti/Ttot) (where Ti/Ttot is insp iratory fraction of the cycle). All measurements were repeated at 24-h r intervals throughout the difficult weaning period. The patients were extubated on satisfying ten of 12 classical weaning criteria. Eleven patients were successfully weaned from mechanical ventilation while si x patients were not. Weaning failure was associated with the following : a) longer periods of mechanical ventilation before weaning; b) high Values of tracheal occlusion pressure, respiratory rate, minute ventil ation, and effective impedance maintained throughout the difficult wea ning period; and c) persistent high Paco(2) and intrinsic positive end -expiratory pressure values. As the weaning failure patients' inspirat ory muscles confronted an increasing inspiratory load, Values of the t ension-time index of the inspiratory muscles entered or remained in th e fatigue zone. In contrast, weaning success patients normalized their breathing pattern and decreased their tracheal occlusion pressure, ef fective impedance, and tension-time index values. Conclusions: Breathi ng pattern alterations and respiratory muscle performance impairments lead to ventilator dependency after prolonged mechanical ventilation. The measurement of variables such as the noninvasive tracheal occlusio n pressure, inspiratory power of breathing, and tension-time index of the inspiratory muscles facilitate the management of difficult-to-wean patients.