A more homogeneous distribution of strain within lung parenchyma attenuates ventilator-induced lung injury in the prone position
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KeywordsSupine Position Tidal Volume Prone Position Lung Volume Homogeneous Distribution
The prone position has been shown to attenuate ventilator-induced lung injury in experimental models [1, 2]. Recently, it has been suggested that the mechanism may be due to a more homogeneous distribution of strain within the lung parenchyma . This study was performed to explore this hypothesis.
A total of 30 animals were ventilated in supine (n = 15) or prone (n = 15) positions until a similar ventilator-induced lung injury (VILI) was reached. To do so, experiment was interrupted when respiratory system elastance (Ers) was 150% of baseline . Thereafter VILI was assessed as the lung wet-to-dry (W/D) ratio and histology (H&H stain). In five more animals CT scans (GE Medical System Light Speed QX/I, 0.6 mm thickness, 100 mA, 100 Kv) were taken at end-expiration and end-inspiration (90% of inspiratory capacity) in both supine and prone positions. Quantitative analysis (Maluna, 2.02 Mannheim, Germany) was performed on the entire lung and dividing the lung into four zones along the vertical axis, from ventral to dorsal. We also considered the area of the lung as seen from a lateral view considering the height at each scan, dividing it into upper and lower regions according to maximal height of the lung at end-expiration (Osiris, Medical imaging software 3.6, Geneva, Switzerland). The tidal volume distribution was then calculated as the ratio between the area at end-inspiration and end-expiration. This was calculated for upper and lower regions in both the supine and prone positions. Data are shown as mean ± SD.
Rats were ventilated with comparable ventilator settings (tidal volume of 88.7 ± 8% of inspiratory capacity vs 88.6 ± 4.9, supine vs prone, P = not significant [NS]). Similar VILI was reached, as assessed by Ers, W/D and histology. The time taken to achieve the target VILI was longer with prone position (73 ± 37 min vs 112 ± 42, supine vs prone, P < 0.05).
When considering CT scan analysis, lung volumes were similar between groups, both at end-expiration (1.84 ± 0.25 ml vs 1.97 ± 0.16, supine vs prone, P = NS) and end-inspiration (10.07 ± 2.79 mL vs 9.52 ± 2.49, supine vs prone, P = NS). However, at end-expiration, lung density along the vertebral axis was similar in the prone position, while it was significantly decreased in the supine position (ANOVA, P < 0.05). The ratio between end-inspiratory and end-expiratory areas was greater in the upper region as opposed to the lower region in the supine position (1.64 ± 0.12 vs 0.74 ± 0.07, upper vs lower region, P < 0.05), while in the prone position it was similar (1.21 ± 0.12 vs 1.13 ± 0.07, upper vs lower, P = NS).
The prone position attenuates VILI, allowing one to buy time on its progression. Both lung volume at end-expiration and tidal volume are more homogeneously distributed in the prone position, suggesting a more homogeneous distribution of strain within lung parenchima. This may explain the protective role of the prone position on the progression of VILI.
- 3.Gattinoni : Eur Respir J 2003, 22(Suppl 47):1s-11s.Google Scholar