Performance of a capnodynamic method estimating effective pulmonary blood flow during transient and sustained hypercapnia

Abstract

The capnodynamic method is a minimally invasive method continuously calculating effective pulmonary blood flow (CO_EPBF), equivalent to cardiac output when intra pulmonary shunt flow is low. The capnodynamic equation joined with a ventilator pattern containing cyclic reoccurring expiratory holds, provides breath to breath hemodynamic monitoring in the anesthetized patient. Its performance however, might be affected by changes in the mixed venous content of carbon dioxide (C_vCO₂). The aim of the current study was to evaluate CO_EPBF during rapid measurable changes in mixed venous carbon dioxide partial pressure (P_vCO₂) following ischemia-reperfusion and during sustained hypercapnia in a porcine model. Sixteen pigs were submitted to either ischemia-reperfusion (n = 8) after the release of an aortic balloon inflated during 30 min or to prolonged hypercapnia (n = 8) induced by adding an instrumental dead space. Reference cardiac output (CO) was measured by an ultrasonic flow probe placed around the pulmonary artery trunk (CO_TS). Hemodynamic measurements were obtained at baseline, end of ischemia and during the first 5 min of reperfusion as well as during prolonged hypercapnia at high and low CO states. Ischemia-reperfusion resulted in large changes in P_vCO₂, hemodynamics and lactate. Bias (limits of agreement) was 0.7 (-0.4 to 1.8) L/min with a mean error of 28% at baseline. CO_EPBF was impaired during reperfusion but agreement was restored within 5 min. During prolonged hypercapnia, agreement remained good during changes in CO. The mean polar angle was -4.19° (-8.8° to 0.42°). Capnodynamic CO_EPBF is affected but recovers rapidly after transient large changes in P_vCO₂ and preserves good agreement and trending ability during states of prolonged hypercapnia at different levels of CO.

Keywords: Animal model; Capnodynamic; Carbon dioxide; Cardiac output; Effective pulmonary blood flow; Intraoperative monitoring.

Fig. 1

Timeline showing a mean (SD) values for CO_EPBF, CO_TS and CO_PAC from baseline (BL) to end of ischemia (EOI) approximately 30 min later and at minute 1, 3 and 5 after reperfusion, and b continuous values from one animal for CO_EPBF and CO_TS (not possible with CO_PAC) from 2 min before balloon release (vertical broken line) and up to 8 min after reperfusion

Fig. 2

Bland–Altman plot showing 40 paired values for CO_EPBF vs CO_TS during dead space induced hypercapnia at baseline (blue dots), low CO (light blue triangles) and high CO (purple quadrants). Bias is represented with a whole blue line with corresponding CI (blue dotted lines) and levels of agreement (LoA) are shown with broken grey lines with corresponding CI (grey dotted lines)

Fig. 3

Polar plot for CO_EPBF with CO_TS as the reference method during dead space induced hypercapnia. The radial length is the mean of the pairwise delta values of the reference method and the test method (L/min) and is shown with a whole black line with corresponding CI (dotted lines). Data spread closely to the polar axis (orange dotted lines) indicate good trending