Therapeutic Hypercapnia and Ventilation-Perfusion Matching in Acute Lung Injury

Study objectives Hypercapnic acidosis has antiinflammatory effects in animal models of acute lung injury (ALI) and improves ventilation-perfusion (V/Q) matching in normal lungs. The effect of hypercapnia on V/Q matching in ALI is conflicting. Hypercapnic acidosis produced by reduced tidal volumes (V t s) was associated with an increased shunt fraction (QS/QT) in patients with ALI compared with control subjects. V t differences between groups make the assessment of hypercapnic acidosis on V/Q matching difficult. Adding CO 2 to the inhaled gas allows the comparison of gas exchange under identical V t conditions. We hypothesized the presence of hypercapnic acidosis from inspired carbon dioxide (ICD) would improve gas exchange in ALI and would be superior to that of low minute ventilation (LV e ) produced by reduced respiratory rate, rather than V t . Design University laboratory study of anesthetized New Zealand White rabbits. Interventions Assessment of V/Q relationships using the multiple inert gas elimination technique was performed in 10 saline solution-lavaged animals, which were ventilated with 6 mL/kg V t s and a positive end-expiratory pressure of 8 cm H 2 O. Each rabbit was studied while it was in eucapnia, followed by hypercapnia (Pa co 2 , 95 to 100 mm Hg) induced by LV e from decreased respiratory rate and by 10% ICD, in random order. Measurements and results The Pa o 2 was greater in ICD and LV e compared to eucapnia, but no significant differences in alveolar-arterial oxygen pressure difference or Pao 2 /fraction of inspired oxygen ratio occurred. LV e statistically reduced the mean V/Q distributions compared with ICD and eucapnia. Log SDs of ventilation and combined retention and excretion curves of the dispersion index were both increased during LV e , indicating the presence of unfavorable changes in ventilation distribution. Neither LV e nor ICD altered the QS/QT. Conclusions LV e slightly impairs overall gas exchange and ventilation distribution, but does not increase QS/QT compared with eucapnia and ICD. While ICD does not significantly improve gas exchange, it may be superior to LV e in achieving the antiinflammatory effects of “therapeutic” hypercapnia, since it does not adversely alter gas exchange and has the potential to make the lung more uniformly acidotic.