Influence of different PEEP levels and tidal volumes on the regional nonaerated tissue: experimental study

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Introduction

In the Acute Respiratory Distress Syndrome (ARDS), although the edema is homogeneously distributed, a hydrostatic pressure gradient squeezes the gas out from the alveoli, causing a lung density increase along a gravity gradient. We studied by CT scan, in an oleic acid induced ARDS experimental model, the influence of different V_T and PEEP levels on the regional nonaerated tissue during volume-controlled ventilation.

Methods

ARDS was induced in six sedated, paralyzed and mechanically-ventilated dogs by an oleic acid dose (0.075 mg/kg) injected into the pulmonary circulation. After ARDS induction, the volume-controlled ventilation was maintained, changing three tidal volumes (V_T) (12, 24 and 36 ml/kg) and two PEEP levels (5 and 15 cmH₂O).During each of the six randomized steps a CT scan section has been taken, at end-expiration and end-inspiration, 5 cm above the diaphragm. For data analysis, we divided the CT scan section into an upper and a lower part. The nonaerated tissue mass of the upper or lower region was calculated from the densities (ρ) (-200 H <CT <+100 H) and the volume (V), according to the formula m=ρ×V [1]; then it was standardized for the area of each lung region. To evaluate the effect of VT, PEEP level and lung region (upper/lower) on the amount of nonaerated tissue, we performed analysis of variance, in condition of end-expiration and end-inspiration, respectively.

Results

The behavior of nonaerated tissue (g/cm²) is shown in the table (data expressed as mean ± SD).

The amount of nonaerated tissue is greater in the lower region than in the upper one, in both end-expiration and end-inspiration (P<0.01). In both regions PEEP and V_T independently affect the amount of nonaerated tissue (P<0.01). There is interaction between PEEP and V_T only at end-inspiration (end-expiration: P=n.s.; end-inspiration: P<0.01).

Conclusion

The effects of PEEP level and V_T on lung recruitment are greater in dependent lung regions and at end-inspiration.

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