A 32-year-old woman presents with recurrent episodes of sudden-onset palpitations, chest tightness, and fear of dying lasting 15–20 minutes, occurring unpredictably 2–3 times per week for the past 3 months. She reports no chest pain, dyspnea, or syncope during attacks. Physical examination and resting ECG are normal. Which investigation is most appropriate to exclude cardiac etiology before confirming panic disorder?