In a patient with acute shortness of breath and tachycardia but oxygen saturation 95% on room air, what is the most important next step?

• A. Chest X-ray
• B. Echocardiography
• C. Electrocardiography
• D. D-dimer assay

Answer: (C)

The most important next step is electrocardiography because in a patient with acute shortness of breath and tachycardia, identifying a dangerous heart rhythm or signs of ischemia is the top priority. An ECG is quick, noninvasive, and provides immediate information about rhythm (such as atrial fibrillation, SVT, or ventricular tachycardia), rate, conduction abnormalities, and ST-T changes that could indicate ischemia or infarction. This directly guides urgent treatment—definitive rhythm management, antiarrhythmics, or reperfusion considerations—often more urgently than imaging or lab tests.

Other tests have value, but they don’t steer immediate management as effectively. A chest X-ray can reveal non-cardiac or cardiac causes like edema or pneumothorax, but it won’t identify acute arrhythmias or ischemia right away. Echocardiography offers structural and functional information but takes more time and resources, and D-dimer is not diagnostic for the immediate problem and can be elevated for many reasons. Since the patient’s oxygen saturation is stable, securing the electrical and ischemic status with an ECG takes precedence to determine the safest next steps.