Why Isn't Kenley Jansen Pitching This Weekend? An Amateur's Guide to the 'Irregular Heartbeat'

Dodgers closer Kenley Jansen isn't available to pitch against the Rockies this weekend because of an irregular heartbeat. What does that mean?
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On Friday, the Dodgers commence a three-game series with the first-place Rockies in a crucial series between division rivals. Los Angeles traveled to Denver without star closer Kenley Jansen because of an irregular heartbeat, which bothered Jansen during the team’s last trip to Denver in early August. The Dodgers will obviously miss one of their best pitchers in hitter-friendly Coors Field, but having him probably isn’t worth the potential long-term damage to his health.  

Why did Jansen follow his doctors’ advice and decline to travel to Denver for the three biggest games of the season? The answer doesn’t lie in the catchall term “irregular heartbeat,” but in the medical jargon of arrhythmia and atrial fibrillation.

“Irregular heartbeat is a junk term,” Dr. Larry Allen, a cardiologist at the University of Colorado Medical School, says. “All that means is that the electrical rhythms of the heart are off, but it can mean so many different things. It just so happens that atrial fibrillation is the most common.”

Lest you’ve blocked out your high school biology lessons, the heart is comprised of two atria and two ventricles: the atria are the upper chambers which blood enters the heart, the ventricles are the two larger chambers that pump blood through the body. The left atrium receives and transfers blood into the heart through the lungs while the right atrium does so through the veins. The atria work in concert to repeat the cardiac cycle—the heart relaxes and expands while receiving blood before contracting to pump blood into the lungs and other systems of the body. The body also has a natural pacemaker, an electrical signal generated in the right atrium that causes the heart to beat.

If you’re healthy, it should be beating around 60-100 times per minute. Your heart rate will increase in strenuous activity—exercise, stress—which causes your adrenal glands to secrete more adrenaline and signal to the heart’s natural pacemaker to beat quicker.

Easy enough, right? So why isn’t Kenley Jansen’s heart acting similarly? Arrhythmia can mean your heart is beating too slowly, which is fixed by a pacemaker. Jansen’s arrhythmia is because his heart is beating too quickly due to atrial fibrillation (A-Fib). A-Fib exists in roughly 1% of the population, mostly in older patients. Sometimes it’s genetic, but it’s mostly pretty random and generally rare in young, healthy people like Jansen.

“Atrial Fibrillation is a very frustrating condition for patients and physicians because no two cases are the same,” Dr. William Cornwell, an assistant professor of cardiology at the University of Colorado says. “Some people never feel it. Others are totally symptomatic. Some people feel it all the time, others feel symptoms once a year.”

The simplest explanation of atrial fibrillation is that the upper part of the heart is beating faster than the rest of it. While the natural pacemaker in the right atrium is creating an electrical signal that responds to adrenaline (your natural heart rate), the left atrium is responding to a faster, abnormal set of signals triggered by tissue in the left atrium. Thus, while one’s resting heart rate may be 60-100 beats per minute, the atria may be beating at 300-600 beats per minute. This causes the upper part of the heart to quiver instead of squeeze, producing a feeling like your heart is leaping around your chest. The most common resulting symptoms are lightheadedness, shortness of breath and exhaustion.

There is a common linkage between atrial fibrillation and high-altitude destinations, which is why Jansen isn’t flying to Denver this weekend. His first A-Fib episode came in Colorado in 2011, and he spent two weeks on the disabled list in August after it flared up in the Dodgers’ August trip to visit the Rockies. Now, Jansen will skip this weekend’s series before rejoining the team in Cincinnati on Monday.


Air is roughly 20% oxygen, but there are fewer oxygen molecules at higher altitudes. Ever hiked in the mountains, skied at 9,000 feet or run a mountain trail? If so, you’ve probably felt a tolerable lightheadedness or shortness of breath. That is due to the lung vessels tightening because of the decrease in oxygen molecules, which means less oxygen is getting to your brain. That forces the right atrium and ventricle to work harder to pump blood so the heart can properly expand and contract. For A-Fib patients, this can trigger abnormal electrical signals in the left atrium to fire rapidly and produce the aforementioned quiver. Thus, the lightheadedness and exhaustion are exaggerated and carry significant risk to the patient’s health.

The worst possible outcome is stroke because of the increased risk of clotting within the heart: when the atria quiver at 300-600 times per minute, blood isn’t being properly squeezed and circulated through the heart, which means it coagulates and clots. If a clot reaches the head, the patient suffers a stroke because of the lack of oxygen being transferred to the brain.

“Stroke risk in athletes is a hot topic right now,” Cornwell says. “Patients who are determined to be at a higher risk of stroke, we put them on blood thinners. Otherwise the blood sits, coagulates and we worry that it would go to a stroke. The concern with athletes is that they typically don’t like to be on blood thinners because they are predisposed to bleeding.”

After Jansen’s first A-Fib episode in 2011, he had an ablation, an operation designed to burn the heart tissue producing the abnormal signals by inserting catheters into the heart. According to Allen, ablations don’t necessarily cure the arrhythmia, but they do reduce it. Jansen indicated that he’d likely undergo a similar surgery this offseason to further treat the condition.

Isn’t there medication to treat A-Fib? There is, but Jansen indicated to Bill Plunkett at the OC Register that he and his doctor agreed that he could stop taking the medication because of its sluggish side effects.

“I don’t want to make excuses,” Jansen told Plunkett. “But it’s like you’re sleepwalking out there. It makes your whole body go slow. I felt sleepy all the time.” He stopped taking the medication after surrendering four home runs in his first three appearances back from the disabled list.

“The medications basically block the stress hormone from stimulating your heart,” Cornwell says. “The problem with that is that it lowers your heart rate, blood pressure and some people feel very sluggish.”

As a result, Jansen won’t risk the trip to Denver this weekend. The problem isn’t this series, but the very feasible possibility that the Dodgers face the Rockies in the playoffs. In that case, Jansen may take the risk, he may return to his medication, or he may skip the roadtrip entirely.

The heartbeat is indeed irregular, but it’s the increased rate that is keeping Jansen off the field this weekend.