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 expiration (ASPIDS) allows normocapnic ventilation at higher end-expiratory pressure (PEEP) and reduced tidal volume (V(T)), 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 CO(2) and multiple elastic pressure volume (Pel/V) curves recorded from different end-expiratory pressures guided ventilatory setting at ASPIDS. ASPIDS was studied at respiratory rate (RR) of 14 min(-1) and then 20 min(-1) with minute ventilation maintaining stable CO(2) elimination. RESULTS: Alveolar and airway dead spaces were 24.3% and 31.3% of V(T), respectively. Multiple Pel/V curves showed a shift towards lower volume at decreasing PEEP, thus indicating that patients required a higher PEEP. At ASPIDS, PEEP was increased from 8.9 cmH(2)O to 12.6 cmH(2)O and VT reduced from 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 28.7 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. PaCO(2) remained stable. No intrinsic PEEP developed. No side effects were noticed. CONCLUSIONS: ASPIDS allowed the use of higher PEEP at lower V(T) and inflation pressure and constant PaCO(2). Multiple Pel/V curves gave insight into the tendency of lungs to collapse.