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Both atenolol (Tenormin™) and labetalol are beta blockers. The big difference is that atenolol is selective. It works only on B1 receptors to lower blood pressure. Normal starting dosage is 25-50 mg once a day with most people taking 100 mg once a day. Doses above 100 mg/day are unlikely to produce any further blood pressure lowering effect and another drug should be tried.
Labetalol on the other hand is not cardioselective, so it blocks B1 receptors as well as B2 and alpha receptors. Normal starting dosage is 100 mg twice a day, increasing to 200-400 mg twice a day until blood pressure is controlled. It does not lower the heart rate as much as atenolol does.
Taking a second dose is not cumulative, just as it wasn't for atenolol. If a drug lowers your SBP 20 mm Hg with one dose, it doesn't lower it another 20 mm with the second dose and yet another 20 mm the third dose.
Generally, the starting dose for labetalol is 100 mg twice a day rather than 200 mg twice a day. Usually 200 mg twice a day is a maintenance dose. It takes several days of taking the drug as directed to tell what the ultimate effect will be on routine blood pressure levels.
I would not be concerned about a heart rate of 91. Anything under 100 is just fine. No doubt anxiety about the change in medication and your readings were part of the increase in rate.
We want blood pressure to be under 120/80, so the options were increasing the atenolol from 50 to 100 mg per day, or switching to a non-selective beta blocker like labetalol, or adding a second drug to your atenolol, which would be an increased expense.
If you aren't having symptoms of lightheadedness, dizziness or fainting, there is nothing to be concerned about. If you are lightheaded, it is best to be very careful when you first sit up or stand up. Dangle your feet over the side of the bed for a second or two before getting up.
It isn't going to hurt you one way or the other whether you choose to take the atenolol or labetalol over the weekend. I would speak with your regular cardiologist or his nurse on Monday to discuss your concerns.
All the best,