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Relationship between superimposed pressure and pleural pressure gradient in an experimental model of ARDS

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Introduction

During Acute Respiratory Distress Syndrome (ARDS), the lung density increases along a gravity gradient, causing a hydrostatic pressure gradient, even if the edema is homogeneously distributed. In an oleic acid induced ARDS experimental model, we studied by CT scan the influence of lung hydrostatic pressure on regional pleural pressure.

Methods

ARDS was induced in eight sedated, paralyzed and mechanically-ventilated dogs, by an oleic acid dose (0.075 mg/kg) injected in the pulmonary circulation. Before and after ARDS induction, a CT scan basal section was taken, and pleural pressure was directly measured throughout two flat pressure sensors (wafers) positioned into the pleural space, in the most upper part and in the most dependent part of the lung. We calculated the superimposed pressure (SP) by mean density (ρ) and height (h) of the CT scan section, according to the following formula: SP = ρ× h [1]. SP represents an estimation of hydrostatic pressure that weighs at the lowest level of the CT section.

Results

After oleic acid, SP is significantly increased (from 5.0± 0.3 to 10.5± 1.9 cmH2O, P<0.01), and pleuralpressure gradient (calculated as the difference between the pressure measured by the lower and the upper wafer) has a similar trend (from 6.5± 2.0 to 10.0± 4.2 cmH2O, P<0.01). The SP sterno-vertebral gradient (Δ SP) is significantly correlated with the pleural pressure gradient (Δ Ppl) (P=0.01; r=0.76), according to the following equation: Δ Ppl [cmH2O]=(1.33± 1.21) + (0.74± 0.23)* Δ SP   [cmH2O].

Conclusion

Pleural pressure seems to change as a function of superimposed pressure, and both these pressures change as a function of the sterno-vertebral level. However, the slope between Δ Ppl and Δ SP is lower than 1 because also other variables (thorax shape, regional compliance, etc.) probably influence the SP effect on the pleural pressure changes.

References

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Bottino, N., Eccher, G., Pelosi, P. et al. Relationship between superimposed pressure and pleural pressure gradient in an experimental model of ARDS. Crit Care 4, P115 (2000). https://doi.org/10.1186/cc835

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Keywords

  • Oleic Acid
  • Hydrostatic Pressure
  • Acute Respiratory Distress Syndrome
  • Pulmonary Circulation
  • Pleural Space