Electrical impedence tomography and heterogeneity of pulmonary perfusion and ventilation in porcine acute lung injury

Background: The heterogeneity of pulmonary ventilation (V), perfusion (Q) and V/Q matching impairs gas exchange in an acute lung injury (ALI). This study investigated the feasibility of electrical impedance tomography (EIT) to assess the V/Q distribution and matching during an endotoxinaemic ALI in pigs. Methods: Mechanically ventilated, anaesthetised pigs (n=11, weight 30-36 kg) were studied during an infusion of endotoxin for 150 min. Impedance changes related to ventilation (Z_V) and perfusion (Z_Q) were monitored globally and bilaterally in four regions of interest (ROIs) of the EIT image. The distribution and ratio of Z_V and Z_Q were assessed. The alveolar-arterial oxygen difference, venous admixture, fractional alveolar dead space and functional residual capacity (FRC) were recorded, together with global and regional lung compliances and haemodynamic parameters. Values are mean±standard deviation (SD) and regression coefficients. Results: Endotoxinaemia increased the heterogeneity of Z_Q but not Z _V. Lung compliance progressively decreased with a ventral redistribution of Z_V. A concomitant dorsal redistribution of Z _Q resulted in mismatch of global (from Z_V/Z_Q 1.1±0.1 to 0.83±0.3) and notably dorsal (from Z_V/Z _Q 0.86±0.4 to 0.51±0.3) V and Q. Changes in global Z_V/Z_Q correlated with changes in the alveolar-arterial oxygen difference (r²=0.65, P<0.05), venous admixture (r ²=0.66, P<0.05) and fractional alveolar dead space (r ²=0.61, P<0.05). Decreased end-expiratory Z_V correlated with decreased FRC (r²=0.74, P<0.05). Conclusions: EIT can be used to assess the heterogeneity of regional pulmonary ventilation and perfusion and V/Q matching during endotoxinaemic ALI, identifying pivotal pathophysiological changes.