Effects of severe hemorrhage on pulmonary mechanics in ventilated pigs with ARDS

Introduction

The effects of hemorrhagic shock on respiratory system mechanics have rarely been investigated and published data are controversial. Pulmonary compliance depends in part on intrapulmonary blood and interstitial fluid volume. When compliance is severely reduced, small modification of these components may have important effects. The present analysis explored the effect of hemorrhagic shock on respiratory system mechanics and oxygenation parameters in a model of pigs with ARDS.

Methods

We evaluated the dynamic respiratory system compliance (C_rs = VT/(inspiratory airways pressure - PEEP)) of 14 domestic pigs. Animals were mechanically ventilated: tidal volume (VT) set at 10 ml/kg; respiratory rate at 15 bpm; PEEP at 0 cmH₂O. Animals were separated into a control group (n = 9) and an ARDS group (n = 5). ARDS was induced by lung lavage with NaCl 0.9%. During hemorrhage 40% of the total blood volume was removed. The blood was then infused during the re-transfusion phase.

Results

In the control group, C_rs (ml/cmH₂O) did not change during hemorrhage or re-transfusion (Figure 1). In the ARDS group, C_rs decreased with lung lavage (31.2 ± 5.7 (baseline) to 16.4 ± 3.0; *P < 0.01). After hemorrhage C_rs increased (21.5 ± 2.9; **P < 0.001 compared with lavage) and then decreased again after re-transfusion (18.7 ± 2.7; ***P < 0.05). In the same group PaO₂/FiO₂ (mmHg) decreased after ARDS (469 ± 50 (baseline) to 105 ± 38; P < 0.001), increased during hemorrhage (218 ± 105; P < 0.05) and did not change after re-transfusion (207 ± 125; P = 0.82). The shunt fraction (%) decreased during hemorrhage in the ARDS group (26.2 ± 14.9 (lavage) to 6.4 ± 6.6; P < 0.05) but did not change significantly after re-transfusion (13.9 ± 17.0; P = 0.3).

Figure 1

Respiratory system compliance. Tidal volume, 10 ml/kg.

Conclusions

Acute reduction of blood volume is associated with an increase of respiratory system compliance and oxygenation parameters. Reduction of intrapulmonary blood and interstitial fluid volume or thoracic cage compliance could be responsible for this effect.