PATHOPHYSIOLOGIC BASIS OF ACUTE RESPIRATORY-DISTRESS IN PATIENTS WHO FAIL A TRIAL OF WEANING FROM MECHANICAL VENTILATION

To determine the mechanisms of acute respiratory distress and failure in patients with chronic obstructive pulmonary disease (COPD), we stud ied 17 ventilator-supported patients who failed a trial of spontaneous breathing and 14 patients who tolerated such a trial and were success fully extubated. Immediately before the weaning trials, maximal inspir atory pressure was not statistically different between the two groups (p = 0.48). On discontinuation of the ventilator, the failure group im mediately developed rapid shallow breathing, and higher values of dyna mic lung elastance (Edyn(L)) (p < 0.01) and intrinsic positive end-exp iratory pressure (PEEP(i), p < 0.03) than did the success group. Betwe en the onset and end of the trial, the failure group developed further increases in Edyn(L) (p < 0.0001) and PEEP(i) (p < 0.0001), and incre ases in inspiratory resistance (p < 0.009) and inspiratory pressure-ti me product (PTP) (p < 0.0001). Partitioning of PTP at the end of the t rial revealed a 111% increase in the PEEP(i) component, a 33% increase in the non-PEEP(i) elastic component, and a 42% increase in the resis tive component (all p < 0.0001). Despite the increase in PTP, 13 of th e failure patients developed an increase in Pa-CO2. The product of PTP and Pa-CO2, an index of inefficient CO2 clearance, was more than twic e as high in the failure group than in the success group at the end of the trial (p < 0.0005). Thus, development of acute respiratory distre ss during a failed weaning attempt was due to worsening of pulmonary m echanics, which in conjunction with rapid shallow breathing led to ine fficient clearance of CO2.