Chest
Volume 122, Issue 3, September 2002, Pages 998-1005
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Laboratory and Animal Investigations
Effects of Positive End-Expiratory Pressure and Body Position on Pulmonary Blood Flow Redistribution in Mechanically Ventilated Normal Pigs

https://doi.org/10.1378/chest.122.3.998Get rights and content

Study objectives

To assess the respective effects of position and positive end-expiratory pressure (PEEP) on the distribution of regional pulmonary blood flow (PBF).

Design

Prospective randomized animal study.

Setting

Animal research facility in a university hospital.

Participants

Normal pigs that were tracheostomized, anesthetized, and mechanically ventilated.

Interventions

PBF was measured in seven pigs in the supine position (SP) and the prone position (PP) at both zero end-expiratory pressure (ZEEP) and 10 cm H2O of PEEP. The regional PBF was assessed by the radioactive microsphere method. The lungs from each pig were sliced into 90 samples. The heterogeneity of PBF was estimated from its coefficient of variation.

Measurements and results

The lung samples had a mean (± SD) weight of 1.60 ± 0.39 g. Changing position from SP to PP at ZEEP redistributed PBF toward the anterior, superior, and peripheral regions and did not significantly reduce the coefficient of variation for regional PBF (reduction, 44.7 ± 7% to 42.2 ± 8%). Changing from the SP to PP position at PEEP induced a similar, but more marked, redistribution of PBF and a significant reduction in the coefficient of variation from 53 ± 13% to 30.4 ± 7% (p < 0.001). In the SP, PEEP redistributed PBF toward the posterior, inferior, and central regions without changing the heterogeneity of PBF. In the PP, PEEP had little effect on the PBF redistribution but significantly reduced the coefficient of variation of PBF from 42.2 ± 8% to 30.4 ± 7% (p < 0.05).

Conclusions

Pigs in the PP had altered gravitational dependence of PBF compared to that observed when pigs were in the SP. This effect was enhanced by using a PEEP of 10 cm H2O.

Section snippets

Animal Preparation and Experimental Protocol

The study was approved by the University of Lyon Animal Research Committee. The investigation was performed in seven healthy pigs (mean [± SD] weight, 26.1 ± 1.1 kg) of both genders, who were premedicated with an IM injection of xylazine (20 mg), ketamine (70 mg), and droperidol (5 mg). Anesthesia was induced with 100 mg IV propofol followed by a continuous infusion of 350 mg/h. Analgesia was performed with repeated injections of fentanyl (100 μg) every 30 min. A continuous IV infusion of

Hemodynamics and Gas Exchange

Mean pulmonary artery pressures and pulmonary wedge pressures were significantly higher in PP-PEEP than in PP-ZEEP (Table 1). Cardiac output was, on average, significantly lower in PP-PEEP than in SP-PEEP. Arterial oxygenation was significantly higher in the PP than in the SP at both ZEEP and PEEP.

PBF Heterogeneity

The complete extraction of microspheres by the pulmonary circulation was achieved in our study since no radioactivity was found in the kidney or the heart. The number of microspheres was large,

Discussion

In this study, we found the following in mechanically ventilated normal pigs: (1) the magnitude of redistribution of PBF toward the anterior and peripheral regions in the PP-ZEEP condition was significantly increased in the PP-PEEP condition; (2) the PEEP-induced shift of PBF to the posterior in the SP was reversed by the PP; and (3) compared with the SP, the PP resulted in a more homogeneous distribution of PBF.

Conclusion

In normal mechanically ventilated pigs, the PP reduced anterior-to-posterior difference through PBF redistribution along the vertical axis. In addition, the PP induced a shift in perfusion toward the superior and peripheral regions. Both effects were more marked at PEEP.

Acknowledgment

The authors thank Véronique Berthier for her help with the preparation of the animals.

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