All medical records coded with the diagnosis of ARDS during a period from 1983 to 1987 were reviewed for inclusion in the study. Patients were considered to have ARDS when all the following criteria were fulfilled: occurrence of an event or condition recognized as a risk factor for ARDS, acute onset of respiratory failure requiring intubation and mechanical ventilation, rapid development of bilateral diffuse interstitial or alveolar infiltrates, initial Pw < 18 mm Hg, total static Clbnt <50 ml/cm H2() immediately after initiation of mechanical ventilation, and PaO/FIo2 <150 mm Hg (<200 mm Hg for patients receiving some degree of PEEP).
Patients fulfilling the criteria for ARDS were included in the study if they continued to require intubation for at least seven days after the onset of ARDS, and if they survived an additional seven days to day 14 of respiratory failure. Survival to the 14th day of ARDS was a necessary entrance criterion so as to allow determination of the presence or absence of prolonged intubation, which was defined as >14 days.
The study patients were divided into two groups on the basis of their duration of intubation and mechanical ventilation. Croup 1 patients required endotracheal intubation for >7 days but <=14 days, and group 2 patients required endotracheal intubation for >14 days.
Daily clinical information was obtained from the medical records beginning with day 0, which was defined as the first day that patients fulfilled diagnostic criteria for ARDS. Day 0 data were obtained from medical record information charted either immediately after nonintubated patients underwent intubation and initiation of mechanical ventilation for respiratory failure, or after patients already intubated for other indications fulfilled diagnostic criteria for ARDS. Clinical information on subsequent hospital days was extracted from data charted daily between 0600 and 0800 in nursing notes, 1С U charts, laboratory reports and respiratory therapy records.
Total static thoracic compliance was calculated by dividing the actual exhaled tidal volume (Vt) by the Ps: Clbor = Vt/(Ps-PEEP). The Vt was corrected by subtracting tube compression volume from the measured Vt.
The progression of hypoxemia was determined by calculating the ratio of PaOs to Pa02 utilizing the alveolar gas equation: Ра(уРл02 = Pa(V(PB-PM2<))FIo2 – PaCXy0.8 where P„ = sea-level PB (760 mm Hg) and Р„20 = water vapor pressure (47 mm Hg).
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Chest radiographs were available from six of the ten group 1 patients and nine of the 14 group 2 patients. Day 0 radiographs were evaluated as a part of the diagnostic criteria for ARDS. Day 7 radiographs were graded in a blinded fashion by both investigators independently using two scoring systems. The first system determined whether the severity of pulmonary infiltrates was improved, unchanged or worse on day 7 compared with day 0. The second system determined whether the day 7 chest radiograph was normal or demonstrated diffuse interstitial infiltrates, alveolar infiltrates involving <50 percent of the lung fields, or alveolar infiltrates involving <50 percent of the lung fields. A numerical severity score of 1 through 4 was applied to the increasing severity of the radiographic findings to allow statistical comparisons between groups. Results of the readers’ scores were averaged.
All data are expressed as mean ± SEM except for range of values where noted. Statistical significance between groups was determined using unpaired Students t test. Differences between groups in PEEP and РаО2/РлO2 values over time were compared using two-way ANOVA. A p value <0.05 was considered significant.