There are two scenarios and time correlations that link tachycardia and low blood pressure. If the blood pressure drops first such as in orthostatic hypotension/POTS ( when you first get up from sitting or lying down) the heart rate increases to compensate for the low blood pressure to prevent passing out. If the heart rate increases first and gets so fast (> 180), the heart cannot fill with blood sufficiently between beats and the blood pressure drops as a result of low cardiac output. In The second situation, keeping the heart rate from escalating so high will prevent the drop in blood pressure and a medication like a beta blocker can be safe and useful. it would be good to know what her resting heart rate is when she is not in tachycardia. If it’s 65 to 70 or above, a dose appropriate beta blocker may be very helpful in controlling her symptoms until the cause of this is determined. Regardless of her cardiologist reputation, I think a second cardiologist’s opinion ASAP is warranted.