Patients often tell me “I was diagnosed with an irregular heart rhythm.” But, as we learned in last week’s blog, “irregularity” is a fairly broad category. It may be something benign—and actually normal—like having PACs or PVCs. Or the irregularity can be something that needs to be taken quite seriously, like atrial fibrillation.
Atrial fibrillation is an irregular heart rhythm caused when the atria (the upper chambers) develop completely disorganized electrical activity and the impulses come so rapidly that the mechanical function can’t keep up, so the atria just quiver—or fibrillate. There are two immediate implications of this. First, the heart loses the accessory benefits of the atria contracting, which generally contributes about 10-30% of the cardiac output, as the atria serve to “prime” the ventricles. Secondly, the electrical impulses arrive at the ventricle in a haphazard pattern. A third potential consequence, seen in many but not all people with atrial fibrillation, is that the heart rate is rapid.
In atrial fibrillation, the atria fire about 400 times a minute. But, before they can reach the ventricle, they arrive at the AV node (the electrical gateway that I mentioned in last week’s blog), where most of those impulses are blocked or filtered out. Anywhere from 10-40% or so get through, depending on how well the AV node can conduct. Therefore, the heart rate may be between 40-160 (or even faster, particularly in young people). The impulses don’t arrive at the ventricle in a predictable pattern, though. Sometimes 2 or 3 might come through very quickly, one right after the other, while at other times several get blocked, so that there is a longer gap between heart beats. This is what causes the irregularity in atrial fibrillation.
I should note one other aspect of the heart’s rapidity and irregularity. When you take the pulse of a person in atrial fibrillation (or the blood pressure cuff measures it), the rate is often underestimated. Some of those quick heart beats that occur closely together may not be detectable. Patients often report that their heart rate is normal, but on an EKG it can be quite rapid.
Atrial fibrillation has several clinical consequences. First, people have symptoms that can be unpleasant, related to either the loss of atrial mechanical function, the irregularity, or the fast heart rate. The most common is to feel palpitations—the subjective sensation that the heartbeat doesn’t feel right. People may also become short of breath, develop chest pain or get light-headed. Rarely it can cause people to faint.
The second consequence, separate from whether a person feels symptoms or not, is that the heart function can be affected and lead to CHF. Again, this can be due to the loss of atrial mechanical function or the irregularity of the heartbeat, but is most often due to the heart beating too fast. The heart gets overworked, as if the person is exercising constantly.
The third consequence is that the risk of stroke is elevated. Strokes can occur even if a person has no symptoms. The risk of stroke is perhaps the most dangerous aspect of atrial fibrillation. In next week’s blog, we’ll explore this risk—and how we attempt to minimize it.
Greg Koshkarian, MD, FACC