Gas exchange, specific lung elastance and mechanical power in the early and persistent ARDS

Purpose: Aim of this study was to evaluate the effect of acute respiratory distress syndrome (ARDS) duration on gas-exchange, respiratory mechanics, specific lung elastance and mechanical power. Materials and methods: In a single center prospective study 28 ARDS patients (66.4 ± 10.0 years, BMI 23.6[21.3–28.8] kg/m2, PaO2/FiO2 148.9[99.6–173.5]) who still presented ARDS criteria after 7-days of mechanical ventilation were studied in early and persistent phase of the disease (day-1 and after 7-days). Each patient underwent PEEP trial at 5–15 cmH2O in both phases. Results: At both PEEP levels the PaO2 was similar in both phases (early: 70.7[65.1–84.4] vs 102.0[85.5–131.8] mmHg; persistent 70.7[63.0–76.2] vs 97.4[86.5–117.1] mmHg, 5–15 cmH2O respectively), the PaCO2 was significantly higher in the persistent phase at both PEEP levels (early 50.6 ± 10.2 vs 52.1 ± 10.5 mmHg; persistent 57.7 ± 13.4 vs 56.9 ± 12.8 mmHg). Specific lung elastance was not different in the early compared to the persistent phase 12.5 ± 3.1 vs 12.2 ± 3.8 cmH2O. The mechanical power normalized for the functional residual capacity increased with PEEP and was similar in both phases (early 23.4[12.8–32.8] vs 34.3[25.3–47.9], persistent 16.3[10.9–24.1] vs 26.7[19.9–46.0] J/min/L, 5–15 cmH2O respectively). Conclusions: The persistent phase of ARDS for 7-days did not affect the respiratory mechanics while significantly impaired the PaCO2 exchange.