Abstract
IntroductionLung-protective ventilation aims at using low tidal volumes (VT) at optimum positive end-expiratory pressures (PEEP). Optimum PEEP should recruit atelectatic lung regions and avoid tidal recruitment and end-inspiratory overinflation. We examined the effect of VT and PEEP on ventilation distribution, regional respiratory system compliance (CRS), and end-expiratory lung volume (EELV) in an animal model of acute lung injury (ALI) and patients with ARDS by using electrical impedance tomography (EIT) with the aim to assess tidal recruitment and overinflation.MethodsEIT examinations were performed in 10 anaesthetized pigs with normal lungs ventilated at 5 and 10 ml/kg body weight VT and 5 cmH2O PEEP. After ALI induction, 10 ml/kg VT and 10 cmH2O PEEP were applied. Afterwards, PEEP was set according to the pressure-volume curve. Animals were randomized to either low or high VT ventilation changed after 30 minutes in a crossover design. Ventilation distribution, regional CRS and changes in EELV were analyzed. The same measures were determined in five ARDS patients examined during low and high VT ventilation (6 and 10 (8) ml/kg) at three PEEP levels.ResultsIn healthy animals, high compared to low VT increased CRS and ventilation in dependent lung regions implying tidal recruitment. ALI reduced CRS and EELV in all regions without changing ventilation distribution. Pressure-volume curve-derived PEEP of 21±4 cmH2O (mean±SD) resulted in comparable increase in CRS in dependent and decrease in non-dependent regions at both VT. This implied that tidal recruitment was avoided but end-inspiratory overinflation was present irrespective of VT. In patients, regional CRS differences between low and high VT revealed high degree of tidal recruitment and low overinflation at 3±1 cmH2O PEEP. Tidal recruitment decreased at 10±1 cmH2O and was further reduced at 15±2 cmH2O PEEP.ConclusionsTidal recruitment and end-inspiratory overinflation can be assessed by EIT-based analysis of regional CRS.
Highlights
Lung-protective ventilation aims at using low tidal volumes (VT) at optimum positive end-expiratory pressures (PEEP)
pressure of CO2 (Pco2) increased to about 60 torr during ventilation with high VT and inactive ILA whereas ventilation with low VT and active ILA resulted in normocapnia
The simultaneous decrease in regional CRS in the ventral, non-dependent regions reflected regional overinflation. These results indicate that the distribution of regional CRS and regional differences in CRS are crucial for the interpretation of the PEEP and VT effects and that the threshold PEEP, where tidal recruitment begins or ceases, might be the optimal PEEP to achieve best possible recruitment and minimal overinflation
Summary
Lung-protective ventilation aims at using low tidal volumes (VT) at optimum positive end-expiratory pressures (PEEP). We examined the effect of VT and PEEP on ventilation distribution, regional respiratory system compliance (CRS), and endexpiratory lung volume (EELV) in an animal model of acute lung injury (ALI) and patients with ARDS by using electrical impedance tomography (EIT) with the aim to assess tidal recruitment and overinflation. The current recommendations advocate VT of 6 ml/kg predicted body weight and Pplat lower than 30 cm H2O in patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Whether this recommended VT is optimal or whether the risk of ventilator-induced lung injury can be reduced by further reduction of VT is still under debate. VT lower than 6 ml/kg predicted body weight poses the risk of impaired alveolar ventilation and oxygenation [6,7]
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