Change of extravascular lung water in sheep with early acute respiratory distress syndrome

Objective

To determine the change of extravascular lung water (EVLW) and the effect of positive end-expiratory pressure (PEEP) and cardiac output (CO) on EVLW in sheep with early acute respiratory distress syndrome (ARDS).

Methods

The sheep ARDS model was induced by infusion of endotoxin intravenously, then divided into the ARDS model group (n = 12), PEEP group (n = 21) and dobutamine group (n = 13). The PEEP group was divided into three subgroups: PEEP 5 cmH₂O group (n = 7), PEEP 10 cmH₂O group (n = 8) and PEEP 15 cmH₂O group (n = 6). EVLW was measured by the single indicator thermodilution technique. The EVLW, hemodynamics and lung mechanics parameters were observed at 6 hours after the sheep ARDS model was induced, 2 hours after they received mechanical ventilation with PEEP (5, 10, 15 cmH₂O), respectively, and 2 hours after CO increase > 50% of the base value (induced by dobutamine infusion).

Results

(1) When ARDS was induced in sheep, EVLW increased from 12.8 ± 4.7 ml/kg to 18.1 ± 7.1 ml/kg (P < 0.01), and EVLW had no significant change (P > 0.05) during the early phase of ARDS (6 hours). The PaO₂/FiO₂ was 136.8 ± 34.9 mmHg at ARDS 0 hours, which was higher than that before infusion of endotoxin (444.3 ± 127.7 mmHg, P < 0.01). EVLW had no relationship with PaO₂/FiO₂, but EVLW after the ARDS model was induced had good correlation with PaO₂/FiO₂ (r = -0.501, P < 0.001). When ARDS was induced, static lung compliance decreased from 26.9 ± 8.4 to 16.4 ± 5.0 (P < 0.01). (2) After mechanical ventilation with PEEP for 2 hours, EVLW values in the PEEP 10 cmH₂O group and PEEP 15 cmH₂O group were 14.7 ± 4.5 ml/kg and 15.3 ± 3.7 ml/kg, respectively, which was significantly less than that before applied PEEP (16.5 ± 4.7 ml/kg and 18.4 ± 6.0 ml/kg, respectively; all P < 0.05). But EVLW in the 5 cmH₂O group was no different during 2 hours PEEP application. (3) After CO increase > 50% of the base value at 1 hour and 2 hours, EVLW values were 16.3 ± 4.9 ml/kg and 16.9 ± 6.9 ml/kg, respectively, which did not differ from that of the base value (15.1 ± 4.6 ml/kg; all P > 0.05).

Conclusions

EVLW in ARDS sheep increased significantly and remained at the same level during the early phase of ARDS. PEEP had a marked effect on reducing EVLW and the increase of CO induced by dobutamine did not significantly raise the EVLW.