Another Alarming Study With Serious Flaws

Unless you understand the limitations of the report, you might worry too much about a stroke based on your resting heart rate.

Photo by Or Hakim on Unsplash

Another day, and another alarming and attention-gathering headline. This time, it was in my Cardiology update email feed. The study’s first paragraph states the following:

Resting heart rates that are lower or higher than 60–69 bpm signal elevated risks of stroke among individuals with no prior history of stroke or atrial fibrillation (AF), according to an analysis of UK Biobank data.

In fully adjusted models…the relative risk of ischemic stroke was 15% higher when the resting heart rate was below 50 bpm and 9% higher when it was 80–89 bpm.

After reading this, my first impulse was to check my pulse or heart rate. It was 68. Whew! But seriously, let’s dig into this more.

(Note the headline referred to heart rates less than 60. However, they later use the 15% increase in stroke risk if the heart rate was less than 50. I assume that is because between 50 and 60, the increased stroke risk might not have been as statistically significant. If that is the case, then why even state 60–69?)

The lead author, Dexter Penn, stated the findings “…could indicate some underlying cardiovascular dysfunction.” However, other factors can cause resting heart rates to fall outside this range.

A low heart rate can be due to medications, particularly beta blockers. They are used for high blood pressure, heart failure, and after heart attacks. And people with these conditions have a higher risk of a stroke than the rest of the population, regardless of whether they are on or off these medications.

Heart rates are often low in athletes, especially those who do intensive cardio training, such as running, biking, and swimming. High heart rates can also be due to various medications.

Prior studies examining this issue of stroke and too high or too low heart rates have shown mixed results.

Did the US Biobank account for these other (known in the statistical world as “confounding”) factors? The researchers focused on approximately 450,000 patients in the UK Biobank, and excluded individuals with atrial fibrillation (AF) since this is a known strong risk factor for stroke.

There are other diseases, such as thyroid diseases, that can cause a low or high heart rate. It is not clear to me that these were excluded from the study. Over-the-counter cold and allergy medicines may elevate the heart rate. Again, I am unsure if these were excluded.

And then there is the issue of relative versus absolute risk reduction. Most studies quote the relative risk reduction of a drug or intervention. Let’s say you study 1000 patients who took drug “A” versus 1000 patients who took a placebo (sugar pill) to see if it helped control diabetes. Drug A may control diabetes better than a placebo in 20% of the patients. This does not mean it will help control your individual diabetes by 20%, but rather the entire group of 1000 patients.

To assess your individual risk reduction, you need to look at the absolute risk reduction, which is usually a much lower number. Let’s look at this in the above study.

If your resting heart rate was below 50, the study found a 15% reduction in the risk of ischemic stroke. This reduction was relative, and not absolute. The absolute or individual risk reduction was not stated. It is likely far lower, for example, less than 1%. (We don’t know the exact number.) Thus, if your resting heart rate is 49, should you worry about having a stroke if your increased risk is 1%?

This is not to say a low resting heart rate should not be of concern or fully evaluated. It does say that many headline-grabbing studies that do not more honestly exclude confounding factors or don’t provide absolute risk reduction figures should not cause you to be overly concerned.

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