Have you checked you Hemoglobin level,
Thyroid profile. Low hemoglobin can cause these symptoms and so can thyroid issues.
Also is this bounding pulse and throat tightness associated with fast pulse rate, uncomfortable sensation of heart beating - palpitations.
If the symptoms are post the first echo you could repeat an Echo
Also if associated palpitaions are there you can consult a cardiac electrophysiologist in your city.
Next Steps
A Cardiac electrophysiologist is a heart beat specialist. sometimes episodic pulse problems are not picked up by one time ECG. Since it is causing much discomfort to you, there are options available for long term monitoring like Holter test and so on.