PaO2 greater than 300 mmHg promotes an inflammatory response during extracorporeal circulation in a rat extracorporeal membrane oxygenation model

Abstract

Background: Extracorporeal membrane oxygenation (ECMO) is being increasingly used for mechanical support of respiratory and cardio-circulatory failure. An excessive systemic inflammatory response is observed during sepsis and after cardiopulmonary bypass (CPB) with similar clinical features. We hypothesized that hyperoxia condition encourages the systemic inflammatory response and organ disorder during ECMO. To prove this hypothesis correct, we investigated the systemic inflammatory responses at normal and high levels of arterial oxygen pressure (PaO₂) in the rat ECMO model.

Methods: Rats were randomly assigned to one of the following groups depending on the value of PaO₂ during ECMO: A group (n=11, PaO₂ 100-199 mmHg), B group (n=10, PaO₂ 200-299 mmHg), C group (n=8, PaO₂ 300-399 mmHg), and D group (n=11, PaO₂ >400 mmHg). Serum cytokine levels [tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-10 (IL-10)] were measured before, 60, and 120 min after the initiation of ECMO. The wet-to-dry weight (W/D) ratio of the left lung was also measured, and dihydroethidium (DHE) staining, reflecting superoxide generation, of lung and liver tissues was performed 120 min after ECMO initiation.

Results: In the C and D groups, the pro-inflammatory cytokines (TNF-α and IL-6) significantly increased during ECMO compared with the other groups. On the other hand, the increase in anti-inflammatory cytokines (IL-10) was more suppressed in the C and D groups than in the other groups. The W/D ratio increased significantly more in the C and D groups than in the other groups. In addition, DHE fluorescence had a tendency to increase as the PaO₂ rose.

Conclusions: These data demonstrate that it is better to avoid administration of too much oxygen during ECMO to attenuate lung injury linked to generation of superoxide and the systemic inflammatory response.

Keywords: Extracorporeal circulation (ECC); cytokine; extracorporeal membrane oxygenation model (ECMO model); hyperoxia; superoxide; systemic inflammation.

Figure 1

The rat extracorporeal membrane oxygenation (ECMO) model.

Figure 2

Serum TNF-α (A), IL-6 (B), IL-10 (C). †, P<0.05 vs. the A group at the same point in time; *, P<0.05 vs. the B group at the same point in time.

Figure 3

The relationship between partial pressure of oxygen during ECMO and cytokine levels (plotted display and approximate curve). The relationships between PaO₂ and TNF-α (A), between PaO₂ and IL-6 (B), and between PaO₂ and IL-10 (C).

Figure 4

Wet-to-dry ratio of the lung at the end of ECMO. †, P<0.05 vs. the A group; *, P<0.05 vs. the B group.

Figure 5

Representative examples of in situ detection of superoxide in each group’s lung and liver. Confocal microscope sections of organ were labeled with the fluorescent oxidative dye dihydroethidium (red fluorescence when oxidized to ethidium bromide by superoxide). Scale bar: 100 µm.

Figure 6

Mean fluorescence intensity after deducting the values of the A group. †, P<0.05 vs. the A group; *, P<0.05 vs. the B group; **, P<0.05 vs. the C group. a.u., arbitrary unit.