Marked differences between prone and supine sheep in effect of PEEP on perfusion distribution in zone II lung

Abstract

The classic four-zone model of lung blood flow distribution has been questioned. We asked whether the effect of positive end-expiratory pressure (PEEP) is different between the prone and supine position for lung tissue in the same zonal condition. Anesthetized and mechanically ventilated prone (n = 6) and supine (n = 5) sheep were studied at 0, 10, and 20 cm H2O PEEP. Perfusion was measured with intravenous infusion of radiolabeled 15-microm microspheres. The right lung was dried at total lung capacity and diced into pieces (approximately 1.5 cm3), keeping track of the spatial location of each piece. Radioactivity per unit weight was determined and normalized to the mean value for each condition and animal. In the supine posture, perfusion to nondependent lung regions decreased with little relative perfusion in nondependent horizontal lung planes at 10 and 20 cm H2O PEEP. In the prone position, the effect of PEEP was markedly different with substantial perfusion remaining in nondependent lung regions and even increasing in these regions with 20 cm H2O PEEP. Vertical blood flow gradients in zone II lung were large in supine, but surprisingly absent in prone, animals. Isogravitational perfusion heterogeneity was smaller in prone than in supine animals at all PEEP levels. Redistribution of pulmonary perfusion by PEEP ventilation in supine was largely as predicted by the zonal model in marked contrast to the findings in prone. The differences between postures in blood flow distribution within zone II strongly indicate that factors in addition to pulmonary arterial, venous, and alveolar pressure play important roles in determining perfusion distribution in the in situ lung. We suggest that regional variation in lung volume through the effect on vascular resistance is one such factor and that chest wall conformation and thoracic contents determine regional lung volume.

Species-Specific Pulmonary Arterial Asymmetry Determines Species Differences in Regional Pulmonary Perfusion

“…Removing the two most gravitationally dependent slice values (for prone and supine only) gives gradients of −3.77%/cm and −4.63%/cm (see Table 1 for gradient and r 2 values). The larger gradient in the quadruped lung model (after removal of zone 4 flow data) compared with the human arterial model is consistent with previous experimental measurements 7,15,40-42…”

A Comprehensive Review of Prone Position in ARDS

“…In PP at 0 and 10 cm H 2 O PEEP, perfusion to the nondependent (dorsal) lung was unaltered, and it actually increased at a PEEP of 20 cm H 2 O. 76 Less perfusion heterogeneity within horizontal planes also was found in PP at all levels of PEEP. Decreased heterogeneity in pulmonary perfusion distribution with PEEP during PP has been reported by others.…”

Effect of time and body position on ventilation in premature infants

“…Previous studies have also shown ventilation distribution favoring the posterior (dorsal) lung regions (13,14,18,25). It has been reported that the utilization of PEEP redistributes ventilation toward the dependent areas of the lungs (26) although PEEP redistributes pulmonary perfusion to dependent lung regions in patients who are supine but not in prone (27). The normal distribution of ventilation and blood flow change under pathological conditions and during mechanical ventilation due to changes in diaphragmatic and chest wall motion in response to the addition of PEEP (28).…”