By Aaron B. Holley, MD

Beta-blockers are excellent drugs. They're cheap and effective; feature prominently in hypertension guidelines; and remain a sine qua non for coronary artery disease, myocardial infarction, and heart failure treatment. They've been around forever, and we know they work. Good luck finding an adult medicine patient who isn't on one.

Beta-blockers act by slowing resting heart rate (and blunting the heart rate response to exercise. The latter is a pernicious cause of activity intolerance that often goes unchecked. Even when the adverse effects of beta-blockers are appreciated, providers are loath to alter dosing, much less stop the drug. After all, beta-blockers are an integral part of guideline-directed medical therapy (GDMT), and GDMT saves lives.

Balancing Heart Rate and Stroke Volume Effects

The pulmonologist sees beta-blockers differently. To augment cardiac output and optimize oxygen uptake (VO2) during exercise, we need the heart rate response. In fact, the heart rate response contributes more to cardiac output than augmenting stroke volume (SV) and more to VO2 than the increase in arteriovenous (AV) oxygen difference. An inability to increase the heart rate commensurate with physiologic work is called chronotropic incompetence (CI). That's what beta-blockers do ─ they cause CI.

Physiology dictates that CI will cause activity intolerance. That said, it's hard to quantify the impact from beta-blockers at the individual patient level. Data suggest the heart rate effect is profound. A study in patients without heart failure found that 22% of participants on beta-blockers had CI, and the investigators used a conservative CI definition (≤ 62% of heart rate reserve used). A recent report found that stopping beta-blockers in patients with heart failure allowed for an extra 30 beats/min at max exercise.

Wasserman and Whipp's textbook, the last word on all things exercise, presents a sample subject who undergoes two separate cardiopulmonary exercise tests (CPETs). Before the first, he's given a placebo, and before the second, he gets an intravenous beta-blocker. He's a 23-year-old otherwise healthy male — the perfect test case for isolating beta-blocker impact without confounding by comorbid diseases, other medications, or deconditioning. His max heart rate dropped by 30 beats/min after the beta-blocker, identical to what we saw in the Cardiology study (with the heart rate increasing by 30 beats/min following withdrawal). Case closed. Stop the beta-blockers on your patients so they can meet their exercise goals and get healthy!

Such pithy enthusiasm discounts physiology's complexities. When blunting our patient's heart rate response with beta-blockers, we also increase diastolic filling time, which increases SV. For the 23-year-old in Wasserman and Whipp's physiology textbook, the beta-blocker increased O2 pulse (the product of SV and AV difference). Presumably, this is mediated by the increased SV. There was a net reduction in VO2 peak, but it was nominal, suggesting that the drop in heart rate was largely offset by the increase in O2 pulse. For the patients in the Cardiology study, the entire group had a small increase in VO2 peak with beta-blocker withdrawal, but the effect differed by left ventricular function. Across different studies, the beta-blocker effect on heart rate is consistent but the change in overall exercise capacity is not.

Patient Variability in Beta-Blocker Response

In addition to left ventricular function, there are other factors likely to drive variability at the patient level. We've treated the response to beta-blockers as a class effect — an obvious oversimplification. The impact on exercise and the heart will vary by dose and drug (eg, atenolol vs metoprolol vs carvedilol, and so on). Beta-blockers can also affect the lungs, and we're still debating how cautious to be in the presence of asthma or chronic obstructive pulmonary disease.

In a world of infinite time, resources, and expertise, we'd CPET everyone before and after beta-blocker use. Our current reality requires the unthinkable: We'll have to talk to each other and our patients. For example, heart failure guidelines recommend titrating drugs to match the dose from trials that proved efficacy. These doses are quite high. Simple discussion with the cardiologist and the patient may allow for an adjustment back down with careful monitoring and close attention to activity tolerance. With any luck, you'll preserve the benefits from GDMT while optimizing your patient's ability to meet their exercise goals.

Aaron B. Holley, MD, is a professor of medicine at Uniformed Services University in Bethesda, Maryland, and a pulmonary/sleep and critical care medicine physician at MedStar Washington Hospital Center in Washington, DC.