Brief Answer:
Evaluation for underlying cause is needed

Detailed Answer:
Hi, thanks for posting the concern in the HCM.

As you have stated above, I can see that you are a known diabetic and known hypertensive patient. There is nothing called "mild" diabetes. Either you have it or you don't have it. If you don't have it, it is needed to diagnose whether you are a prediabetic. And if you have it, it is necessary to identify if you have good control. I will come to that part later.
Now, the principal concern is that you have got a sinus tachycardia and you have been taking Tab Atenolol for that. If you miss a dose, the heart rate shoots up.
You want to know the cause and if it is a long term problem.

To start with, I would say Sinus tachycardia is a benign form of cardiac arrhythmia, which can be present even when you don't have a structural heart disease. There is a SA node in our heart. That is the trigger switch. It generates the cardiac impulse. Then that spreads to other areas of heart. It generates impulse normally at a rate of 100 bpm in the absence of any neural or hormonal control. But, our autonomic nervous system controls this and reduces the rate to what you see as the normal heart rate. Now, there are different conditions in which this impulse generation rate would abnormally increase or decrease. E.g. in case anxiety, fever, stress, exercise the rate increases. But, the increase is inappropriate if that persists for long, even at rest, without any hemodynamic reason. This condition is known as inappropriate sinus tachycardia. In this case the autonomic control is poor or inapropriate and so medicine is needed from outside to induce such control. Atenolol is a beta blocker, which is such a drug.
Therefore, whenever you have this medicine you are in good control of your heart rate. Whenever you don't take this medicine your heart rate shoots up. Now, there is an important issue to remember. You have diabetes and its good control is very much needed to improve or at least prevent worsening of autonomic functions.

Now, you know what you have. The second question is whether it is long term. Yes the problem can be long term depending upon the underlying cause. Therefore, it is very important to try to find out if any identifiable cause is present. Therefore, a battery of investigations will be needed-

Complete blood count, CRP, Urea, Creatinine, Electrolytes, Serum Ca, fasting TSH, fasting Lipid profile, LFT
Urine for routine examination
Echocardiography
24 hr Holter monitoring

I think you should also consult a local cardiologist for further evaluation.

Now, a few preventive medicine tips for you-
Medicine is not the ultimate treatment for DM and HTN. Dietary and lifestyle modifications are considered the best treatment modality. A salt restricted oil restricted high fibre diabetic diet and regular exercise can help you to achieve good control of blood pressure and sugar.
Regular monitoring for complications is also needed. You also need annual eye check-up, evaluation for microalbuminuria (test for 24 hr urine protein) and cardiology checkup at regular interval.
3 monthly check of Serum HbA1c to know glycemic control is also needed.

If you find this answer helpful, please close the thread and rate my answer.
Regards,
Dr. Kaushik Sarkar

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