Aspiration of dead space allows isocapnic low tidal volume ventilation in acute lung injury. Relationships to gas exchange and mechanics

Objective: In acute lung injury (ALI) mechanical ventilation damages lungs. We hypothesised that aspiration and replacement of dead space during expir ation (ASPIDS) allows normocapnic ventilation at higher end-expiratory pres sure (PEEP) and reduced tidal volume (VT), peak and plateau pressures (Paw( peak) Paw(plat)), thus avoiding lung damage. Setting: University Hospital. Patients: Seven consecutive sedated and paralysed ALI patients were studied . Interventions and measurements: Single breath test for CO2 and multiple ela stic pressure volume (Pel/V) curves recorded from different end-expiratory pressures guided ventilatory setting at ASPIDS. ASPIDS was studied at respi ratory rate (RR) of 14 min(-1) and then 20 min(-1) with minute ventilation maintaining stable CO2 elimination. Results: Alveolar and airway dead spaces were 24.3 % and 31.3 % of V-T, res pectively. Multiple Pel/V curves showed a shift towards lower volume at dec reasing PEEP, thus indicating that patients required a higher PEEP. At ASPI DS, PEEP was increased from 8.9 cmH(2)O to 12.6 cmH(2)O and V-T reduced fro m 11 ml/kg to 8.9 ml/kg at RR 14 min(-1) and to 6.9 ml/kg at RR 20 min(-1). A significant decrease in Paw(peak) (36.7 vs 32 at RR 14 min(-1) and 20 at RR 20 min(-1)) and Paw(plat) (29.9 vs 27.3 at RR 14 min(-1) and 24.1 at RR 20 min(-1)) were observed. PaCO2 remained stable. No intrinsic PEEP develo ped. No side effects were noticed. Conclusions: ASPIDS allowed the use of higher PEEP at lower V-T and inflati on pressure and constant PaCO2. Multiple Pel/V curves gave insight into the tendency of lungs to collapse.