Hypercapnic acidosis reduces oxidative reactions in endotoxin-induced lung injury.

BACKGROUND: Hypercapnic acidosis frequently occurs when patients with acute lung injury are initially ventilated with low tidal volume "protective" strategies. Hypercapnic acidosis per se, in the absence of any change in tidal volume or airway pressure, is protective when instituted before the onset of injury. However, the mechanisms by which hypercapnic acidosis confers this protection are incompletely understood, in particular, the effects on pulmonary oxidative reactions, which are potent mediators of tissue damage, have not been previously examined in vivo. METHODS: After anesthesia, tracheostomy, and the intratracheal instillation of endotoxin to establish lung injury, rats were mechanically ventilated for 6 h in normocapnia (21% O2, 0% CO2). Rats were then randomized to either normocapnic (21% O2, 0% CO2) or hypercapnic (21% O2, 5% CO2) ventilation and a nonspecific nitric oxide synthase inhibitor (N-monomethyl-L-arginine) or vehicle. Dihydrorhodamine was administered intravenously, and the lungs were removed for determination of the oxidative formation of rhodamine by spectrofluorimetry after 20 min. Thus, rats were randomly assigned to either: normocapnia-endotoxin (n = 12), normocapnia-endotoxin-N-monomethyl-L-arginine (n = 9), hypercapnia-endotoxin (n = 11), or hypercapnia-endotoxin-N-monomethyl-L-arginine (n = 10). RESULTS: Hypercapnic acidosis significantly reduced the pulmonary oxidative reactions in the inflamed lung compared with normocapnia. Nitric oxide synthase blockade did not alter endotoxin-induced oxidative reactions. CONCLUSIONS: Hypercapnic acidosis reduced oxidative reactions in the acutely injured lung in vivo, within minutes of onset and was not reliant on nitric oxide-dependent peroxynitrite production. This rapid onset antioxidant action is a previously undescribed mechanism by which hypercapnic acidosis could act, even when acute lung injury is well established.

Infection-induced lung injury is worsened after renal buffering of hypercapnic acidosis.

OBJECTIVE: Prolonged hypercapnia is commonly encountered during the treatment of acute respiratory distress syndrome and acute respiratory failure attributable to other causes with protective ventilation strategies. In these circumstances, compensatory renal buffering returns pH to normal establishing a condition of buffered hypercapnia. It is also common intensive care practice to correct the pH more rapidly using bicarbonate infusions. Although it is well-established that hypercapnic acidosis has potent anti-inflammatory and protective effects, the effect of buffered hypercapnia on acute lung injury and acute respiratory distress syndrome is unknown. We therefore wished to determine the effects of buffered hypercapnia on acute lung injury induced by endotoxin or Escherichia coli infection in vivo. DESIGN: Prospective, randomized animal study. SETTING: University research laboratory. SUBJECTS: Adult male Sprague-Dawley rats. INTERVENTIONS: We established buffered hypercapnia by exposing rats to a hypercapnic environment for 3 days before the induction of lung injury. Buffered hypercapnia rats (initial pH >7.35, FiCO2 = 0.05) and normocapnic controls (initial pH >7.35, FiCO2 = 0.00) were then anesthetized, mechanically ventilated, and lung injury induced by intra-tracheal inoculation of endotoxin (series I) or Escherichia coli (series II). MEASUREMENTS AND MAIN RESULTS: Buffered hypercapnia significantly increased both endotoxin and Escherichia coli-induced lung injury when compared to normocapnic controls, as assessed by arterial oxygenation, lung compliance, pro-inflammatory pulmonary cytokine concentrations, and measurements of structural lung damage. In additional in vitro experiments buffered hypercapnia did not alter neutrophil phagocytosis ability but did impaired epithelial wound healing. CONCLUSIONS: Our results demonstrate that infection-induced injury in vivo is worsened after renal buffering of hypercapnic acidosis independently of any changes in tidal volume. These findings have important implications for our understanding of the pathogenesis of infection-induced lung injury during the use protective ventilation strategies that permits buffered hypercapnia and during infective exacerbations of chronic lung diseases associated with sustained hypercapnia.