Oximetric quantitation of intracardiac left-to-right shunting: Limitations of the Qp Qs ratio

For over 40 years, analysis of the blood oxygen content in the right heart chambers has been used to detect and quantitate intracardiac left-to-right shunting.¹ Using the Fick principle, pulmonary and systemic blood flows are calculated and then their ratio ( Qp Qs) is determined. Subsequently, numerous studies-including indicator dilution,² radionuclide angiocardiography^3,4 and Doppler echocardiography^5,6-have used this ratio as the gold standard for measuring intracardiac left-to-right shunting, but its widespread use as such suggests an insufficient understanding of its potential inaccuracies. We performed this study to assess and quantitate the limitations of the Qp Qs ratio in patients with intracardiac left-to-right shunting.