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 whether the capnodynamic method represents systemic blood flow (Qs) or pulmonary blood flow (Qp). Clinically, separating Qs from Qp 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 Qs or Qp in an animal model of aorto-pulmonary left-to-right shunt. METHODS: An artificial aorto-pulmonary shunt was constructed in 10 mechanically ventilated pigs. Measurements of hemodynamic parameters including EPBF as well as Qs and Qp were performed at different fractions of shunt flow. Simultaneous recordings of EPBF, Qs, and Qp 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 Qs was 0.24 l/min, with limits of agreement -0.74 (95% CI, -1.51 to -0.40) to 1.22 (95% CI, 0.88 to 1.99) l/min and mean percentage error 30%. Corresponding values for EPBF and Qp 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 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 R2 = 0.69 and 0.76; conditional R2 = 0.86 and 0.89). Lin's concordance correlation coefficients for EPBF versus 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. CONCLUSIONS: In this experimental model of left-to-right shunt, EPBF more closely reflected Qs than Qp. This alignment with systemic output is clinically relevant for monitoring and hemodynamic care, as Qs is one of the determinants of oxygen delivery.