The diagnostic tool that is most useful in establishing a cardiac cause for respiratory symptoms is

• A. Echocardiography
• B. Chest X-ray
• C. Shunt study
• D. ECG

Answer: (C)

A shunt study is most useful because it directly measures blood flow between the systemic and pulmonary circuits, revealing shunt physiology that links a cardiac problem to respiratory symptoms. By quantifying the shunt (the Qp:Qs ratio), it shows whether there is significant left-to-right or right-to-left mixing. A notable left-to-right shunt leads to pulmonary overcirculation and symptoms such as tachypnea, wheezing, and edema, pointing to a cardiac rather than a primary pulmonary cause. A right-to-left shunt explains hypoxemia that isn’t fully accounted for by lung disease. This direct assessment of shunt presence and magnitude provides concrete evidence of a cardiac contribution to respiratory distress.

Chest X-ray can raise suspicion with findings like cardiomegaly or edema but doesn’t quantify shunting. ECG shows rhythm or conduction issues but not shunt physiology. Echocardiography is invaluable for anatomy and estimating flows, but the shunt study gives explicit, direct measurement of the shunt fraction, which is why it’s most useful for establishing a cardiac cause of respiratory symptoms.