Metabolic acidosis may be as protective as hypercapnic acidosis in an ex-vivo model of severe ventilator-induced lung injury: a pilot study

Sixty New Zealand white rabbit ventilated and perfused heart-lung preparations were used. Six study groups were evaluated. Respiratory acidosis (RA), metabolic acidosis (MA) and normocapnic-normoxic (Control - C) groups were randomized into high and low peak inspiratory pressures, respectively. Each preparation was ventilated for 1 hour according to a standardized ventilation protocol. Lung injury was evaluated by means of pulmonary edema formation (weight gain), changes in ultrafiltration coefficient, mean pulmonary artery pressure changes as well as histological alterations.HPC group gained significantly greater weight than HPMA, HPRA and all three LP groups (P = 0.024), while no difference was observed between HPMA and HPRA groups regarding weight gain. Neither group differ on ultrafiltration coefficient. HPMA group experienced greater increase in the mean pulmonary artery pressure at 20 min (P = 0.0276) and 40 min (P = 0.0012) compared with all other groups. Histology scores were significantly greater in HP vs. LP groups (p < 0.001).In our experimental VILI model both metabolic acidosis and hypercapnic acidosis attenuated VILI-induced pulmonary edema implying a mechanism other than possible synergistic effects of acidosis with CO2 for VILI attenuation.The ARDS Network study [1] documented that a low tidal volume (VT) of 6 ml/kg has beneficial effects on outcomes compared to the traditionally used volume of 12 ml/kg. However, these "protective" ventilation strategies may lead to the development of hypercapnic acidosis (HA), a fact initially viewed as an undesirable side effect to be corrected. Since low VT ventilation is now increasingly used in order to minimize ventilator-induced lung injury (VILI), hypercapnic acidosis, also termed "permissive hypercapnia", is frequently confronted by clinicians. Either a side-effect to be tolerated or a desirable beneficial event, hypercapnic acidosis is still an intriguing issue having received much debate.The mounting exper