If a patient complains of shortness of breath while lying down, what is the most likely cause?

When a patient experiences shortness of breath while lying down, it is strongly indicative of orthopnea, which is often associated with congestive heart failure (CHF). In CHF, fluid can accumulate in the lungs when the patient is in a supine position, leading to increased respiratory distress. This occurs because gravity plays a role in fluid distribution; when sitting or standing, more fluid may pool in the lower extremities, but when lying down, that fluid redistributes into the lung areas, exacerbating the feeling of breathlessness.

Pneumonia, while it can cause difficulty breathing, is typically associated with infection and may not exclusively worsen in a supine position. Asthma attacks can arise from many triggers and generally do not have a specific positional component. Chronic bronchitis is characterized by chronic cough and mucus production, but it does not typically cause positional shortness of breath related to lying down. Thus, the combination of symptoms and the underlying mechanisms of congestive heart failure make it the most plausible cause of shortness of breath in this context.