Acute graded hypercapnia increases collateral coronary blood flow in a swine model of chronic coronary artery obstruction

Objective: To evaluate the effect of acute hypercapnia on regional myocardi al blood flow in a swine model of chronic, single-vessel coronary artery ob struction. Permissive hypercapnia is being used frequently in critical care settings. One possible detrimental effect of hypercapnia is the initiation of coronary "steal" in patients with coronary artery disease. The effects of hypercapnia on collateral coronary blood flow in the setting of coronary obstruction have not been defined. Design: Prospective controlled experimental study. Setting: Institutional animal research facility. Subjects: Eight juvenile swine weighing 25-30 kg. Interventions Collateral coronary circulation was induced in eight piglets by banding the proximal left anterior descending coronary artery for 8-10 w ks followed by total ligation. Graded hypercapnia (mean Pa-CO2, 81 torr [10 .80 kPa; Pa-CO2 = 81 torr] and 127 torr [16.93 kPa; Pa-CO2 = 127 torr]) was induced by increasing inspiratory carbon dioxide under isoflurane anesthes ia (1 minimum alveolar concentration). Measurements and Main Results: Animals were attached to instruments to meas ure pulmonary and systemic hemodynamics, regional myocardial blood flow, an d cardiac output. Regional myocardial blood flow was determined using radio labeled microspheres. Cardiac output, mean arterial pressure, and coronary perfusion pressure were unchanged at both levels of hypercapnia compared wi th baseline values. Heart rate was increased at Pa-CO2 [HI] (p < .05). Regi onal blood flow was increased at both levels of hypercapnia in the collater al-dependent and normally perfused myocardium (p < .05; as high as 56% for subendocardium and as high as 106% for subepicardium at PaCO2 ([HI])). The intercoronary blood flow ratio remained unaltered. The transmural flow rati o was reduced at PaCO2 ([HI]) (p < .05). During hypercapnia, regional lacta te extraction remained unaltered, and regional oxygen extraction was unchan ged or reduced despite the increase in oxygen consumption. Conclusions: In this swine model of chronic single-vessel coronary artery o bstruction, acute hypercapnia does not induce coronary steal from collatera l-dependent myocardium, but it does increase global coronary blood flow.