Therapeutic Hypercapnia Is Not Protective in the in vivo Surfactant-Depleted Rabbit Lung

Abstract

Permissive hypercapnia because of reduced tidal volume is associated with improved survival in lung injury, whereas therapeutic hypercapnia-deliberate elevation of arterial PCO 2 -protects against in vivo reperfusion injury and injury produced by severe lung stretch. No published studies to date have examined the effects of CO 2 on in vivo models of neonatal lung injury. We used an established in vivo rabbit model of surfactant depletion to investigate whether therapeutic hypercapnia would improve oxygenation and protect against ventilator-induced lung injury. Animals were randomized to injurious (tidal volume, 12 mL/kg; positive end-expiratory pressure, 0 cm H 2 O) or protective ventilatory strategy (tidal volume, 5 mL/kg; positive end-expiratory pressure, 12.5 cm H 2 O), and to receive either control conditions or therapeutic hypercapnia (fraction of inspired CO 2 , 0.12). Oxygenation (alveolar-arterial O 2 difference, arterial PO 2 ), lung injury (alveolar-capillary protein leak, impairment of static compliance), and selected bronchoalveolar lavage and plasma cytokines (IL-8, growth-related oncogene, monocyte chemoattractant protein-1, and tumor necrosis factor-␣) were measured. Injurious ventilation resulted in a large alveolar-arterial O 2 gradient, elevated peak airway pressure, increased protein leak, and impaired lung compliance. Therapeutic hypercapnia did not affect any of these outcomes. Tumor necrosis factor-␣ was not increased by mechanical stretch in any of the groups. Therapeutic hypercapnia abolished the stretch-induced increase in bronchoalveolar lavage monocyte chemoattractant protein-1, but did not affect any of the other mediators studied. Therapeutic hypercapnia may attenuate the impairment in oxygenation and inhibit certain cytokines. Because hypercapnia inhibits certain cytokines but does not alter lung injury, the pathogenic role of these cytokines in lung injury is questionable. Mechanical ventilation is central to neonatal and pediatric critical care. Limiting V t during mechanical ventilation in the setting of acute lung injury results in several important effects, including elevation of PaCO 2 (1, 2), alteration of lung inflammatory events (3), decreased duration of mechanical ventilation (4), and improved survival (5). Although the associations among these effects are clear, the mechanistic relationships are uncertain. Lung stretch is associated with a complex series of pulmonary and systemic events, including neutrophil recruitment (6) and release of inflammatory prostanoids (7,8) and cytokines (9 -11) as well as morphologic injury (6,12).Alterations in CO 2 tension may be important in this paradigm for several reasons (13

Bench-to-bedside review: Hypercapnic acidosis in lung injury - from 'permissive' to 'therapeutic'

“…HCA may have protective effects against lung injury as has been shown in multiple in vivo models of lung injury, including ALI induced by bacterial [12,73] or endotoxin instillation [10,74], systemic [9] and pulmonary ischemia-reperfusion [70] and stretch-induced lung injury [8,11,75-77], as outlined below.…”

Effect of elevated carbon dioxide on bronchial epithelial innate immune receptor response to organic dust from swine confinement barns

“…Some link these effects not to reduced mechanical stretch of the lungs, but to elevated CO 2 (Curley et al , 2010). In contrast, others show increased inflammation due to hypercapnia (Nichol et al , 2009), and animal studies have yielded conflicting results, with some showing reductions in cytokines, while others show increased inflammation in other cell systems (Rai et al , 2004,Abolhassani et al , 2009,Sinclair et al , 2002). …”

Hypercapnic acidosis attenuates ventilation-induced lung injury by a nuclear factor-κB–dependent mechanism

“…HCA exerts protective effects in severe ventilation-induced lung injury (VILI) (19)(20)(21). However, the effects of HCA in less severe VILI are less clear (22,23).…”