Hand with drugs

Q: "I've been on a beta-blocker (atenolol) for my high blood pressure. Lately I find that I can't keep up with the profiles my instructor presents in the Spinning classes I take. When I push to the target energy zones, I feel like I am overexerting myself. I'm exhausted. Can you help me understand how this medication might be affecting my heart rate?"
- J.T.

Melissa writes:
An excellent question indeed. I'm glad you've noticed that something isn't quite as you expect, and that you've empowered yourself to get to the 'heart' of what's going on. (Bad joke...)

Beta-blockers are one kind of well over 15 types of different medications used to treat hypertension (high blood pressure), some of which you may also have heard of: angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), calcium-channel blockers, diuretics, and many more. Each class of drugs works on different receptors in different locations (heart, blood vessels, lungs, skeletal muscles, etc.), and thus has different effects. I will limit my discussion here to Beta-blockers, and defer the others for a later article.

Beta-blockers have received a great deal of clinical attention due to their ability to improve outcomes in patients suffering from hypertension, ischemic heart disease, congestive heart failure and arrhythmias. They are well-studied for safety and cardioprotective abilities, and -- of no small import -- they are relatively cheap.

Pardon this overly simplistic (and likely overly boring) physiology lesson to help everyone understand how these medications work. There are three main kinds of what is referred to as beta-adrenergic receptors that receive signaling from your body's sympathetic nervous system (often associated with the "flight or fight" response). Beta-1 receptors are mainly located in the heart and kidney; Beta-2 receptors are located in blood vessels, lungs, liver, and the gut; Beta-3 receptors are located in fat tissue. Beta-blockers act on all three types of receptors and reduce the effects of the signals these receptors usually receive, though different drugs vary in their affinity for the particular receptor types (1). But that's basically how they work.

By blocking the beta receptors in the heart, these medications decrease cardiac output — more specifically, the heart's contractility (a fancy word that refers to a change in the performance of the heart's large chambers, the ventricles, without a change in the amount of blood being pumped), and the heart rate (by slowing electrical conduction). Think: we're blocking the "flight or fight response," so we get the opposite of what we would want to happen when running from a bear -- and when exercising.

When we decrease contractility and heart rate, the heart”™s demand for oxygen decreases. We have problems when oxygen demand exceeds oxygen supply — so by lowering the demand, we have fewer problems. When we exercise, contractility and heart rate would typically increase dramatically. Under the influence of these medications, however, there is a built-in brake on the system.

What does this mean, as a practical matter? Most people find that they cannot elevate their heart rate. At the same level of perceived exertion, their heart rates are much lower than they would usually expect. What we know is that the heart”™s response to challenge is directly blunted. Actual “maximum heart rate” is blunted (thus, if you are using percentages of an estimated theoretical maximum heart rate, your old numbers will be based on an invalid number). VO2 max is blunted. “Exercise tolerance,” or endurance, is blunted. (2) There is also mixed evidence that beta blockers also directly cause premature muscle fatigue -- such that it might be your muscles saying "stop" as opposed to your heart.

Either way, your heart rate is unreliable for athletic training purposes. Even to reach your former “recovery” rates might actually be a LOT of cardiovascular challenge, thus defeating the purpose of specific training zones.

There is no precise way to quantify the effects of beta-blockers on your heart rate.You might consider re-evaluating the role of Rate of Perceived Exertion (RPE) in your life— at least temporarily. Since you already train with a heart rate monitor, you clearly appreciate that perceived exertion has its limits. But in this case, just as in cases for people who are overtrained, perceived exertion has more utility.

While rested, use a chart like the one posted here (I find that a 0-10 scale is easier for HRM users to “return to”) while you train. Wear your heart rate monitor and keep track of the heart rates you hit at each of the RPE marks. Record your observations, and repeat until you have 3-5 data sets. If you can demonstrate consistency (i.e., you are always at 138 bpm when you perceive your exertion to be a “6,” you can equate that 138 bpm heart rate with “75% MHR” and reset your target zones. 138 may not actually be 75% of your MHR — but it doesn”™t matter. You have your new relative bench marks.

You also might consider speaking with your doctor about whether beta-blockers are the right class of drugs for you. The cardiologists with whom I work are routinely switching patients from beta-blockers who complain that their athletic training is affected too much for their comfort. There might be a reason why he or she recommended this specific type of drug (as opposed to, say, an ACE inhibitor, which has a different mechanism that does not affect heart rate), but it”™s also important that your doctor knows about your concerns about your training (i.e., something that is important to you). Have the conversation, and work together to find a middle ground. A doctor-patient relationship should be a partnership, collaborating to find a solution to meet your goals in the context of your values. That”™s how I intend to practice, at least.

(1) UptoDate 2009  http://uptodate.com

(2) Eston, R, Connolly, D. The use of ratings of perceived exertion for exercise prescription in patients receiving beta-blocker therapy. SportsMed, 1996 Mar, 21(3): 176-90

Melissa Marotta is a second-year medical student at University of Vermont College of Medicine. She is a STAR 3 Spinning Instructor and Certified Personal Trainer (ACE).


Email Melissa at melissa.marotta@uvm.edu

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