Introduction
A capnodynamic equation can be used to calculate effective pulmonary blood flow (COEPBF) i.e. cardiac output (CO) minus shunt.(1)  An ischemic injury with subsequent reperfusion increases the concentration of carbon dioxide temporarily, a situation that theoretically could affect the agreement of a capnodynamic method.
The aim of the current study was to evaluate COEPBF during rapid changes in PvCO2 in an ischemia and reperfusion porcine model. 

Methods
A ventilatory pattern containing cyclic reoccurring expiratory holds was used to create the required alterations of alveolar carbon dioxide concentration and the mathematical model used to calculate COEPBF assumed a steady state in PvCO2 levels.
COEPBF was compared to a reference method for CO, an ultrasonic flow probe around the pulmonary trunk and a pulmonary artery catheter in a porcine model (N=8). 
A 10 Fr Reliant catheter (Medtronic) was placed in the aorta below the diaphragm via the femoral artery and inflated until blood flow in the contralateral femoral artery was abolished according to ultrasound doppler. Hemodynamic measurements, lactate levels and PvCO2 were obtained at baseline before balloon inflation, at minute 27 of ischemia and after deflation at minute one, three and five during reperfusion. 

Results and discussion
The ischemic model resulted in significant changes in lactate and PvCO2 levels from baseline 1.5 mmol/L (1.2-2.4) and 8,5 kPa (7.2-11.4) to 8.2 mmol/L (7.2-9.0) and 5.2 kPa (4.1-6.2) at minute 27 of ischemia and 10.4 mmol/L (7.8-11.4) and 10.7 kPa (9.0-12.3) at minute one after deflation. Bland Altman plot at baseline showed bias 0.7 L/min, limits of agreement (LoA) -0.2 – 1.6 L/min and a percentage error (PE) 22%.  At minute five bias was 0.02 L/min, LoA -1.8 – 1.8 L/min and PE 36%. 

Conclusion
COEPBF recuperates fast after rapid changes in PvCO2 resuming acceptable levels of agreement in 5 minutes.

Evaluation of a capnodynamic method for monitoring effective pulmonary blood flow in an ischemia and reperfusion porcine model