Hello. Thank you for your question and welcome to HCM. I carefully read your query. Normal heart rate is standardized for the worldwide population to 60 to 100 beats per minute. Heart rate always increases during physical activity or psychological stress. It also is a subject to various hormonal peak and nadir levels within a day, and caffeine-containing beverages can also speed it up. The more concerning thing is the left atrial enlargement, rather than tachycardia itself. Because every state that makes the atria enlarge, make them subject to generating various supraventricular arrhythmias, varying from atrial tachycardia to atrial fibrillation. The atria are not characterized by abundant muscle content, therefore they cannot compensate by muscular hypertrophy, but they immediately dilate when a constant stress is imposed on them. This dilation stretches the heart muscle and, therefore, supraventricular arrhythmias can occur. In a young woman such as yourself, the main suspicion for this phenomenon is the thyroid hormone dysfunction. Thus, I would recommend you a thyroid function profile: TSH, fT3, fT4. With proper treatment of the culprit condition, it is almost secure that these above mentioned arrhythmias are prevented. I hope I was helpful with my answer. Take care. Regards, Dr. Meriton