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I can understand you are worried about your heart investigations. At-large there is nothing they found on later EKG (electrocardiography) report. And in both the reports (attachment removed to protect patient identity) heart markers troponin-I is normal and in range meaning, there was nothing abnormal found in the heart. In such a situation, cardiac investigation results are correlated by a cardiologist with the physical examination of the patient. This gives the whole picture of the heart.
Sinus tachycardia means the heart is pumping at the normal pattern but at a fast pace. If there is increased heart rate due to some structural abnormality or conduction defects in the heart, it does not cause sinus tachycardia. In the later tests where you mentioned, they found nothing wrong, means there was no correlation between the symptoms you felt with the heart's health and there was all fine to go with. Otherwise, they must have gone for some medications or therapy for the heart to correct any defects.
Possible left atrial enlargement, but they could not confirm that or at least it looks like from the data provided here in this query. Nonspecific T wave abnormality, they could not specify if there was the T-wave abnormality and that was linked to your health issues. If suspicious they may have gone for more investigations to rule out any issues. But they did not mean there was not much to explore and in the later evaluation, they found all fine. In such a situation and also because the reports are from long time ago, it is better to go back for a thorough heart evaluation. Your cardiologist may go for extensive heart studies including but not limited to physical examination, EKG, echocardiography, exercise stress test, cardiac markers, if needed, etc.
For more information consult a cardiologist online --> https://www.icliniq.com/ask-a-doctor-online/cardiologist