Hemorrhage causes interalveolar perfusion maldistribution in the lungs of anesthetized rats

Lung injury often occurs following hemorrhage and we hypothesized that this might be due to the effects of hemorrhage on perfusion distribution among alveoli. To test this, we measured interalveolar perfusion distribution in anesthetized, spontaneously breathing rats subjected to blood losses of 0%, 10%, 20%, or 30% of calculated blood volume.

We measured interalveolar perfusion distribution by analyzing trapping patterns of 4-mum diameter fluorescent latex particles infused into the pulmonary circulation. The particles (2 x 10) were infused 1 hour after each animal had been bled, and the lungs were then removed and air-dried. Using a confocal fluorescence microscope, we collected images of the particles in eight sections of each lung. Each image encompassed 3,360 x 3,360 x 100 microm (approximately 5,000 alveoli), and included 3-4,000 particles. Particle distributions in the images were measured using the method of dispersion index (DI) analysis. A DI value of zero corresponds to a statistically random distribution; the more DI exceeds zero, the more the distribution is clustered or inhomogenous.

The largest DI values for the four groups were: 0%, 0.69 +/- 0.41; 10%, 0.57 +/- 0.58; 20%, 0.72 +/- 0.34; 30%, 1.38 +/- 0.41. The 30% blood loss group had a max DI value approximately twofold greater than those of the other three (p < 0.0001).

Our results suggest that interalveolar perfusion distribution becomes markedly maldistributed at blood losses of 30%. This contributes to ventilation-perfusion mismatching, and may be a precipitating event for lung injury following hemorrhage.