This is a fairly complex issue...you are talking about a number of different issues that may be occuring here. Rather than figure out exactly what is going on with your system, which I will leave to your cardiologist, let me instead address a number of possible different exercise-related consequences of your condition and medication intake.
Typically, the heart rate will speed up as exercise level increases...this is one of the ways the body supplies more oxygen to aerobically working muscles as the workrate goes up. Therefore, in the healthy individual, a lower heartrate means less blood and therefore less oxygen being pumped to the working muscles, which are the skeletal muscles being used in the movement and the heart muscle, which must also increase its oxygen supply...one of the causes of myocardial infarction is blocked coronary arteries inhibiting blood flow to the heart, which then leads to too little oxygen when a person's heartrate goes up, either in exercise or some other form of stress.
When a person hits the so-called anaerobic threshold,(lactate threshold, which is slightly different and based on a metabolic rather than ventilatory breakpoint, for you scientists out there, is the preferred measurement now) it means that too little oxygen is being delivered to the muscles to meet all the work being done aerobically and you have now started accumulating lactate. This is a somewhat simplistic explanation, but theoretically, if your HR is high you should be pumping enough blood to the muscles...the limiting factor then becomes the muscles themselves and the adaptations that have occured with training. Given your exercise history, I am assuming you have a high level of adaptation. If we go on this assumption and assume that your limiting factor is systemic ie delivery of blood to the muscles, then I am guessing that you may have incomplete ventricular filling because of the high heart rate, ie your heart chamber is not filling with the normal amount of blood before it pumps it out. Therefore what is called your stroke volume is low.
If this is indeed the case, the question then becomes, does the medication improve that during exercise? Slowing down the heartrate MAY allow the chambers to fill properly, but that is also assuming there are no other problems. The problem with the metroprolol is that its HR-lowering actions, combined with the fact that it also relaxes the blood vessels (one of the reasons it is used to lower blood pressure) will lead to a decrease in your blood pressure. As blood pressure is one of the driving forces of blood into your heart, this may counteract at least some of the positive HR lowering effects of the drug on your performance. Couple this with the fact you are at altitude, and people typically tend to dehydrate, which lowers blood volume, and you may not have as big an improvement in performance as you expect. I think, if anything, that it is going to take some experimentation on dosage to see where the proper balance is when it comes to exercise, particularly at altitude. That is something for you to discuss with your cardiologist.
Hope this helps.
