Anxiety or Panic Attack: This is often characterized by a sudden onset of fear or discomfort that reaches a peak within minutes, and includes symptoms such as palpitations, sweating, trembling, and a feeling of choking, which could be described as "heart shaking."

Atrial Fibrillation: A common type of arrhythmia that can cause an irregular, often rapid heart rate, leading to symptoms like heart shaking or palpitations.
Mitral Valve Prolapse: A condition where the two valve flaps of the mitral valve do not close smoothly or evenly but instead bulge (prolapse) upward into the left atrium when the heart contracts, which can cause palpitations.
Hypertension: High blood pressure can lead to heart palpitations or a feeling of the heart shaking due to the increased workload on the heart.

Myocardial Infarction (Heart Attack): Although less common, a heart attack can present with atypical symptoms including palpitations or a feeling of heart shaking, especially in women or diabetic patients.
Pulmonary Embolism: A blockage of an artery in the lungs that can cause sudden and unexplained palpitations among other symptoms, and is a medical emergency.
Cardiac Tamponade: Fluid accumulation in the sac around the heart (pericardium) that can compress the heart and impede its function, leading to palpitations and potentially life-threatening complications.

Hyperthyroidism: An overactive thyroid gland can cause an increase in heart rate and lead to palpitations or a sensation of heart shaking.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of palpitations, hypertension, and other symptoms due to the excessive release of catecholamines.
Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC): A rare heart condition where the muscle in the right ventricle is replaced by fat and scar tissue, potentially leading to life-threatening arrhythmias and palpitations.