The effect of lung deformation on the spatial distribution of pulmonary blood flow.

KEY POINTS: Pulmonary perfusion measurement using magnetic resonance imaging combined with deformable image registration enabled us to quantify the change in the spatial distribution of pulmonary perfusion at different lung volumes. The current study elucidated the effects of tidal volume lung inflation [functional residual capacity (FRC) + 500 ml and FRC + 1 litre] on the change in pulmonary perfusion distribution. Changes in hydrostatic pressure distribution as well as transmural pressure distribution due to the change in lung height with tidal volume inflation are probably bigger contributors to the redistribution of pulmonary perfusion than the changes in pulmonary vasculature resistance caused by lung tissue stretch. ABSTRACT: Tidal volume lung inflation results in structural changes in the pulmonary circulation, potentially affecting pulmonary perfusion. We hypothesized that perfusion is recruited to regions receiving the greatest deformation from a tidal breath, thus ensuring ventilation-perfusion matching. Density-normalized perfusion (DNP) magnetic resonance imaging data were obtained in healthy subjects (n = 7) in the right lung at functional residual capacity (FRC), FRC+500 ml, and FRC+1.0 l. Using deformable image registration, the displacement of a sagittal lung slice acquired at FRC to the larger volumes was calculated. Registered DNP images were normalized by the mean to estimate perfusion redistribution (nDNP). Data were evaluated across gravitational regions (dependent, middle, non-dependent) and by lobes (upper, RUL; middle, RML; lower, RLL). Lung inflation did not alter mean DNP within the slice (P = 0.10). The greatest expansion was seen in the dependent region (P < 0.0001: dependent vs non-dependent, P < 0.0001: dependent vs middle) and RLL (P = 0.0015: RLL vs RUL, P < 0.0001: RLL vs RML). Neither nDNP recruitment to RLL [+500 ml = -0.047(0.145), +1 litre = 0.018(0.096)] nor to dependent lung [+500 ml = -0.058(0.126), +1 litre = -0.023(0.106)] were found. Instead, redistribution was seen in decreased nDNP in the non-dependent [+500 ml = -0.075(0.152), +1 litre = -0.137(0.167)) and increased nDNP in the gravitational middle lung [+500 ml = 0.098(0.058), +1 litre = 0.093(0.081)] (P = 0.01). However, there was no significant lobar redistribution (P < 0.89). Contrary to our hypothesis, based on the comparison between gravitational and lobar perfusion data, perfusion was not redistributed to the regions of the most inflation. This suggests that either changes in hydrostatic pressure or transmural pressure distribution in the gravitational direction are implicated in the redistribution of perfusion away from the non-dependent lung.