Capnodynamic Cardiac Output Assessment in a Porcine Model of Aorto-Pulmonary Shunt

Background: Cardiac output assessment in perioperative and intensive care settings can be challenging in patients with congenital heart disease. Capnodynamic monitoring is a minimally invasive method enabling estimation of effective pulmonary blood flow (EPBF), which corresponds to cardiac output in the absence of significant intrapulmonary shunts. In the setting of cardiac shunts, however, it is unclear if the capnodynamic method represents systemic or pulmonary blood flow. Clinically, separating systemic from pulmonary blood flow may aid in the hemodynamic care of patients with congenital heart disease. Thus, the aim of the current study was to evaluate whether EPBF represents systemic or pulmonary blood flow in an animal model of aorto-pulmonary left-to right shunt. Methods: An artificial aorto-pulmonary shunt was constructed in ten mechanically ventilated pigs. Measurements of hemodynamic parameters including EPBF, as well as systemic and pulmonary blood flow were performed at different fractions of shunt flow. Simultaneous recordings of EPBF, systemic and pulmonary blood flow were done and examined for agreement to investigate what EPBF represents in the presence of left -to right shunt. Results: With open shunt, bias between EPBF and systemic blood flow was 0.24 L/min, limits of agreement -0.74 (95%CI -1.51 to -0.40) to 1.22 (95% CI 0.88 to 1.99) L/min, mean percentage error of 30%. Corresponding values for EPBF and pulmonary blood flow were bias -1.28 L/min, limits of agreement -3.13 (95% CI -4.14 to -2.63) to 0.56 (95% CI 0.06 to 1.57) L/min, mean percentage error of 38%. Mixed-effects models with animal-level random intercepts demonstrated positive associations between EPBF and both Qs and Qp, with EPBF increases of 1.00 L/min corresponding to increases of 1.00 L/min in Qs and 1.86 L/min in Qp (marginal R 2 = 0.69 and 0.76; conditional R 2 = 0.86 and 0.89). Lin’s concordance correlation coefficient for EPBF vs systemic and pulmonary blood flow were 0.79 (95% CI 0.68 to 0.86) and 0.43 (95% CI 0.32 to 0.52) respectively with open shunt. Conclusion: In this experimental model of left-to-right shunt, EPBF more closely reflected systemic blood flow (Qs) than pulmonary blood flow (Qp). This alignment with systemic output, is clinically relevant for monitoring and hemodynamic care, as systemic blood flow is one of the determinants of oxygen delivery.