Brief Answer:
Calcium channel blocker

Detailed Answer:
A class of blood pressure medications from the family of 'Non dihydropyridine calcium channel blockers' such as diltiazem or verapamil are effective in slowing down the heart rate in addition to lowering blood pressure.

You are correct in knowing that beta blockers like metoprolol alone are to be strictly avoided in pheochromocytoma. But generally endocrinologists give adequate doses of alpha blockers like hytrin then proceed to add a beta blocker type medication a couple weeks later after assessing certain parameters.

So I dont know if you were managed according to these broad principles.

Sometimes when patients are referred to me for suspected pheochromocytoma, they are already on beta blocker type medications such as metoprolol, and then we have to somehow manage the situation.

The key thing in your individual situation is also the sinus tachycardia. People who have pheochromocytoma typically do not have a sustained tacky. They tend to experience episodes of high blood pressure, headache, sweating and fast heart rate.
So a thorough evaluation for persistent tachycardia is also relevant in your instance, before initiating treatment with the medications I mentioned above.
For example, a simple non fasting blood test for TSH will help rule out an over active thyroid as one of the potential causes

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Brief Answer:
Follow up

Detailed Answer:
Holter is fine but a cardiologist needs to evaluate you systematically.

Further work up for the elevated metanephrines entails a complex strategy best performed by an endocrinologist. This entails:

1 Assessing degree of metanephrine elevation

2 Postural changes that might have falsely raised metanephrines

3 Lab test method used to test metanephrines

4 Possible drug interference resulting in false positive test result

5 Further confirmatory testing for metanephrines and catecholamines

6 Other modalities to look for paraganglioma type tumors if blood test indicate the presence of this.

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