Effect of PEEP and Tidal Volume on Ventilation Distribution and End-Expiratory Lung Volume: A Prospective Experimental Animal and Pilot Clinical Study

INTRODUCTION: Lung-protective ventilation aims at using low tidal volumes (V(T)) 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 V(T) and PEEP on ventilation distribution, regional respiratory system compliance (C(RS)), 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. METHODS: EIT examinations were performed in 10 anaesthetized pigs with normal lungs ventilated at 5 and 10 ml/kg body weight V(T) and 5 cmH(2)O PEEP. After ALI induction, 10 ml/kg V(T) and 10 cmH(2)O PEEP were applied. Afterwards, PEEP was set according to the pressure-volume curve. Animals were randomized to either low or high V(T) ventilation changed after 30 minutes in a crossover design. Ventilation distribution, regional C(RS) and changes in EELV were analyzed. The same measures were determined in five ARDS patients examined during low and high V(T) ventilation (6 and 10 (8) ml/kg) at three PEEP levels. RESULTS: In healthy animals, high compared to low V(T) increased C(RS) and ventilation in dependent lung regions implying tidal recruitment. ALI reduced C(RS) and EELV in all regions without changing ventilation distribution. Pressure-volume curve-derived PEEP of 21±4 cmH(2)O (mean±SD) resulted in comparable increase in C(RS) in dependent and decrease in non-dependent regions at both V(T). This implied that tidal recruitment was avoided but end-inspiratory overinflation was present irrespective of V(T). In patients, regional C(RS) differences between low and high V(T) revealed high degree of tidal recruitment and low overinflation at 3±1 cmH(2)O PEEP. Tidal recruitment decreased at 10±1 cmH(2)O and was further reduced at 15±2 cmH(2)O PEEP. CONCLUSIONS: Tidal recruitment and end-inspiratory overinflation can be assessed by EIT-based analysis of regional C(RS).