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

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.

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.

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).

The persistent phase of ARDS for 7-days did not affect the respiratory mechanics while significantly impaired the PaCO₂ exchange.