What is the key symptom that suggests orthopnea in a patient?

The key symptom that suggests orthopnea is shortness of breath while lying down. Orthopnea is a condition typically associated with heart failure and other respiratory conditions, where a person experiences difficulty breathing when supine (lying flat). This occurs because lying down can increase venous return to the heart, leading to pulmonary congestion in patients with heart failure, ultimately exacerbating dyspnea.

In contrast, chest pain when standing, difficulty breathing during exercise, and a persistent cough at night do not specifically indicate orthopnea. While each of these symptoms can be associated with various clinical conditions, they do not directly relate to the positional aspect of breathing difficulties that characterizes orthopnea. Thus, recognizing shortness of breath specifically while lying down is key to identifying this symptom.