Before we leave the atria behind, let’s conclude our discussion of atrial arrhythmias this week with a discussion of some other abnormal heart rhythms arising from these chambers. The simplest and most prevalent are premature atrial complexes (PACs). We call them “complexes” because that is how we refer to individual electrical impulses seen on an EKG. Virtually everybody has PACs, some people more, some people less. Most of them we don’t feel—in fact, most people never feel any of them, though they might detect them as a “skipped” heartbeat when they are feeling their pulse. The beat feels skipped because the PAC arises prematurely (hence the name!) and so the heart hasn’t had as much time to fill up with blood. Thus, the pulse that is generated is weaker or not even noticeable, leading to the perception that the beat has been missed.
Frequent PACs can sometimes indicate that the atria are irritable, which can be seen in conditions such as hypertension, hyperthyroidism (overactive thyroid) or from the stimulation of caffeine. There is some evidence that frequent PACs increase the risk of developing atrial fibrillation in the future, though the data on that is not yet certain. If a person has an unusually large number of PACs, it is best to have a cardiac evaluation.
The other broad category of atrial arrhythmias is supraventricular tachycardia, abbreviated “SVT.” These refer to abnormal fast heart rhythms that generally run between a rate of 130-200 bpm. Various mechanisms lead to these types of arrhythmias, including re-entrant tachycardias and automatic tachycardias. While a full discussion of these mechanisms is beyond the scope of this blog, I’ll just mention that the re-entrant type occurs because the person has an extra electrical pathway that most people don’t have, allowing the electrical impulse to travel down one pathway and then back up the other, creating an impulse each time the electricity goes around the circuit.
Symptoms from SVTs most commonly include palpitations and light-headedness, but shortness of breath and chest pain can also be experienced. They can be treated with drugs that block the AV node, like beta blockers and calcium channel blockers, or by drugs that alter the formation or speed of electrical impulses, like flecainide and amiodarone. Re-entrant SVTs can potentially be cured through ablation, an invasive procedure we discussed in last week’s blog. An ablation for SVT targets a different circuit than an ablation for atrial fibrillation or atrial flutter.
While PACs and SVTs are not life-threatening, they often lead to anxiety from the symptoms they cause. Therefore, they are a common reason for people to see a cardiologist. Sometimes reassurance is all that is necessary, other times treatment is called for. Don’t hesitate to make an appointment with a cardiologist if you are experiencing palpitations—it’s always a good idea to find out what is causing them.
Greg Koshkarian, MD, FACC