Which symptom is primarily indicative of pericarditis as compared to other cardiac conditions?

The symptom that is primarily indicative of pericarditis is the one associated with relief when sitting up. Patients with pericarditis often experience pleuritic chest pain that worsens when lying down and improves when sitting up or leaning forward. This is a distinguishing feature of pericarditis, as many other cardiac conditions do not exhibit this positional dependence.

In contrast, other symptoms such as chest tightness, palpitations, and dizziness can occur in various cardiac conditions, including myocardial infarction and congestive heart failure, making them less specific. For example, chest tightness can be a symptom of angina, while palpitations might arise from arrhythmias or anxiety. Dizziness is also a nonspecific symptom that can be caused by numerous factors, including dehydration, arrhythmias, or hypotension.

Therefore, the positional nature of the symptom, specifically the improvement of pain with sitting up, strongly supports a diagnosis of pericarditis, setting it apart from other potential cardiac issues.