Heart Failure: This is the most likely diagnosis because a dilated Inferior Vena Cava (IVC) is often associated with right-sided heart failure, which can lead to fluid overload and subsequent dilation of the IVC. The mention of possible cardiac arrhythmia further supports this diagnosis, as arrhythmias can both cause and be a result of heart failure.

Constrictive Pericarditis: This condition, where the pericardium becomes stiff and restricts the heart's ability to fill, can lead to a dilated IVC due to increased pressure in the heart. It can also cause arrhythmias due to the mechanical restriction of the heart.
Cardiac Tamponade: Although less likely than heart failure, cardiac tamponade (fluid accumulation in the pericardial sac) can compress the heart, impede its filling, and lead to a dilated IVC. Arrhythmias can occur due to the compression and the body's attempt to compensate.
Tricuspid Regurgitation: This valvular heart disease can cause right-sided heart failure, leading to IVC dilation. Arrhythmias, particularly atrial fibrillation, can be associated with tricuspid regurgitation.

Pulmonary Embolism (PE): Although less common, a large PE can cause acute right ventricular failure, leading to a dilated IVC and arrhythmias. Missing this diagnosis can be fatal, making it crucial to consider despite its lower likelihood.
Cardiac Sarcoidosis: This condition can cause both heart failure and arrhythmias. It might not be the first consideration but is critical not to miss due to its potential for severe outcomes.

Restrictive Cardiomyopathy: A condition where the heart muscle becomes stiff, making it difficult for the heart to fill. It can cause a dilated IVC and arrhythmias but is less common than other forms of cardiomyopathy.
Cor Pulmonale: Right-sided heart failure caused by lung disease, which can lead to a dilated IVC. While not extremely rare, it's less likely than left-sided heart failure causes and requires specific pulmonary conditions to be considered.