Bedside assessment of the effects of positive end-expiratory pressure on lung inflation and recruitment by the helium dilution technique and electrical impedance tomography

Purpose: Higher positive end-expiratory pressure might induce lung inflation and recruitment, yielding enhanced regional lung protection. We measured positive end-expiratory pressure-related lung volume changes by electrical impedance tomography and by the helium dilution technique. We also used electrical impedance tomography to assess the effects of positive end-expiratory pressure on regional determinants of ventilator-induced lung injury. Methods: A prospective randomized crossover study was performed on 20 intubated adult patients: 12 with acute hypoxemic respiratory failure and 8 with acute respiratory distress syndrome. Each patient underwent protective controlled ventilation at lower (7 [7, 8] cmH2O) and higher (12 [12, 13] cmH2O) positive end-expiratory pressures. At the end of each phase, we collected ventilation, helium dilution, and electrical impedance tomography data. Results: Positive end-expiratory pressure-induced changes in lung inflation and recruitment measured by electrical impedance tomography and helium dilution showed close correlations (R2 = 0.78, p < 0.001 and R2 = 0.68, p < 0.001, respectively) but with relatively variable limits of agreement. At higher positive end-expiratory pressure, recruitment was evident in all lung regions (p < 0.01) and heterogeneity of tidal ventilation distribution was reduced by increased tidal volume distending the dependent lung (p < 0.001); in the non-dependent lung, on the other hand, compliance decreased (p < 0.001) and tidal hyperinflation significantly increased (p < 0.001). In the subgroup of ARDS patients (but not in the whole study population) tidal hyperinflation in the dependent lung regions decreased at higher positive end-expiratory pressure (p = 0.05), probably indicating higher potential for recruitment. Conclusions: Close correlations exist between bedside assessment of positive end-expiratory pressure-induced changes in lung inflation and recruitment by the helium dilution and electrical impedance tomography techniques. Higher positive end-expiratory pressure exerts mixed effects on the regional determinants of ventilator-induced lung injury; these merit close monitoring.