Authors: Hedov J et al.
Anesthesiology, February 25, 2026.
Capnodynamic Cardiac Output Assessment in a Porcine Model of Aorto-Pulmonary Shunt.
This experimental study examined whether capnodynamic monitoring reflects systemic blood flow or pulmonary blood flow when a left-to-right cardiac shunt is present. Capnodynamic monitoring is a minimally invasive technique that estimates effective pulmonary blood flow (EPBF) using changes in exhaled carbon dioxide during controlled ventilation. In patients without significant pulmonary shunting, EPBF approximates cardiac output. However, in congenital heart disease with intracardiac or extracardiac shunts, it has remained unclear whether this technique reflects pulmonary circulation (Qp) or systemic circulation (Qs).
To investigate this question, researchers used a porcine model in which an artificial aorto-pulmonary shunt was surgically created. Ten mechanically ventilated pigs were studied under controlled experimental conditions. Hemodynamic measurements were obtained while varying the degree of shunt flow. At each stage, investigators simultaneously measured:
• Effective pulmonary blood flow estimated by the capnodynamic method (EPBF)
• Systemic blood flow (Qs)
• Pulmonary blood flow (Qp)
The goal was to determine which circulation EPBF most closely tracked when a left-to-right shunt was present.
When the shunt was open, the agreement between EPBF and systemic blood flow was relatively strong. The mean difference between EPBF and systemic flow was small (0.24 L/min) with a percentage error of about 30%. Concordance analysis showed a correlation coefficient of 0.79 between EPBF and systemic blood flow, indicating good agreement.
In contrast, agreement between EPBF and pulmonary blood flow was substantially weaker. The mean bias was −1.28 L/min with a higher percentage error of 38%, and the concordance coefficient was only 0.43.
Mixed-effects statistical modeling further confirmed these findings. Changes in EPBF were positively associated with both systemic and pulmonary blood flow, but the relationship was significantly stronger with systemic circulation. A 1 L/min increase in EPBF corresponded to a 1 L/min increase in systemic blood flow, whereas pulmonary flow increased by a larger but less tightly correlated amount.
The results indicate that in this model of left-to-right shunting, the capnodynamic method reflects systemic blood flow more closely than pulmonary blood flow. This finding has potential clinical importance because systemic blood flow is the key determinant of oxygen delivery to tissues. If capnodynamic monitoring primarily reflects systemic output even in the presence of a shunt, it may still provide clinically useful hemodynamic information in patients with congenital heart disease.
Although promising, the authors emphasize that the study was performed in an animal model under controlled conditions. Human physiology, variable shunt sizes, and complex congenital heart lesions could influence how accurately capnodynamic monitoring reflects systemic output in clinical practice.
Key Points
• Capnodynamic monitoring estimates effective pulmonary blood flow using exhaled CO₂ analysis during ventilation.
• In patients without pulmonary shunting, EPBF approximates cardiac output.
• In this porcine model of an aorto-pulmonary left-to-right shunt, EPBF correlated more closely with systemic blood flow (Qs) than pulmonary blood flow (Qp).
• Agreement between EPBF and systemic output showed a small bias and reasonable error (~30%).
• These findings suggest capnodynamic monitoring may still reflect systemic cardiac output in the presence of left-to-right shunts.
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