Hello! Welcome and thank you for asking on HCM! I understand your concern, and would like to explain that your fast and hard heart beatings feeling needs to be further investigated. To differentiate between a sinus tachycardia or a cardiac arrhythmia, it is necessary a careful medical check up (cardiac auscultation, etc.) a resting ECG, and if suspicions are raised an ambulatory ECG monitoring and cardiac ultrasound. It is true that smoking cessation is frequently associated with autonomic fluctuation mainly in the first two weeks (physical abstinence), but it will pass soon. So, don t worry too much! As, you have experienced such feelings prior to smoking stop (though to a lesser degree), means that another additional factor should be sought. Several medical tests are necessary to for a comprehensive differential diagnosis. Besides the cardiac tests above mentioned, others like: - complete blood count (to exclude anemia), - thyroid function tests (for ruling out a possible disorder), - fasting glucose, - blood electrolytes, etc. are helpful in clinical decision making. Regarding the life style, a healthy profile is strongly recommended: daily physical activity (at least 4 km walking), avoiding smoking contacts, outdoor recreational activities, avoiding excessive alcoholic drinks, a healthy diet (with poor fat content, and rich veg fibers, without spicy foods, etc) like mediterranean diet is recommended. Periodic blood lipid profile should be checked (6 - 12 months) and blood pressure values should be checked too. You need to discuss with your attending doctor about the above mentioned issues. Hope to have been helpful to you! In case of further uncertainties do not hesitate to contact me! Greetings! Dr. Iliri