AimsObjective of this study was to evaluate the feasibility of the non-invasive dye dilution method to quantify shunt size related to atrial septal defects (ASD).The diagnostic accuracy of shunt size determination in ASD’s has been suboptimal with common non-invasive methods. We have previously developed a cost-effective and time-effective non-invasive dye dilution method. In this method, the indocyanine green solution is injected into the antecubital vein and the appearance of the dye is detected with an earpiece densitometer.Methods and resultsWe studied 192 patients with an ASD. Mean pulmonary blood flow/systemic blood flow (Qp/Qs) was measured with dye dilution technique and compared with following methods: Fick’s invasive oximetry (n=49), transoesophageal echocardiography (TEE) measuring ASD size (n=143) and cardiac MR (CMR) (n=9).For the first 49 patients, Qp/Qs was 2.05±0.70 with the Fick’s invasive oximetry and 2.12±0.68 with dye dilution method with an excellent correlation between the two methods (R=0.902, p<0.001). In the second study sample, the ASD size by TEE was 15±6 mm on average, and the mean Qp/Qs 2.16±0.65 measured with dye dilution method with a good correlation between the methods (R=0.674, p<0.001). Qp/Qs measured with CMR was 1.87±0.40 resulting in a good correlation with the dye dilution method (R=0.696, p=0.037).ConclusionThe dye dilution method with earpiece densitometer recording is a clinically feasible and reliable method to assess shunt size in ASDs.