In the last few weeks, the Skeptical Cardiologist has had a run on with patients with premature atrial contractions (PACs).
I've discussed in detail premature ventricular contractions (PVCs) and . They are the most common cause of people feeling that their heart is skipping a beat or fluttering briefly, what we call palpitations.
Premature beats, which can be either PVCs or PACs, in addition to causing palpitations, are the most common cause of an irregular pulse detected by a blood pressure device or a health care worker.
What Causes PACs?
Like PVCs, PACs occur when electrically-active tissue in the heart decides to fire off (or depolarize) before it has received the signal from the normal pacemaker of the heart, the sinus node. In the case of PACs, the rogue tissue is in one of the atria, the upper chambers of the heart.
In the ECG recording below, the PAC (labeled APC) occurs earlier than expected (prematurely). The normal (sinus) beats occur at regular intervals and are all preceded by p waves of normal configuration -- the normal electrical signature of atrial contraction. The larger spike that follows the p wave (the QRS complex) represents ventricular depolarization and is unchanged from the normal sinus beats because activation of the ventricle is normal with PACs.
These early beats, in and of themselves, are felt to be benign.
PACs Are Very Common
PACs are extremely common when we monitor ECG rhythm for an extended period, even in young, totally normal individuals. They are more common, in fact, than PVCs.
For example, in , a study found: "Rare, occasional, frequent and very frequent isolated atrial ectopy occurred in 72.9%, 2.6%, 2.3% and 0.3%, respectively. The same categories of isolated ventricular ectopy occurred in 40.9%, 7.9%, 3.3% and 0.0%."
Frequency of isolated ectopy was classified as a percentage of the total beats on the Holter monitor: rare (≤0.1%), occasional (>0.1 to 1.0%), frequent (>1.0 to 10%), and very frequent (>10%).
Thus, the majority of the time, we will see some PACs in normal subjects who we monitor for 24 hours by ECG.
It was also common to see two PACs in a row (an atrial couplet or pair). Atrial couplets occurred in 14.5% of these aviators.
The highlighted box from the three-lead Holter monitor recording below shows an atrial couplet.
The QRS complex of the premature atrial complex is usually preceded by a visible P wave that has a slightly different shape or different PR interval from the P wave seen with sinus beats. The PR interval of the PAC may be either longer or shorter than the PR interval of the normal beats. In some cases, the P wave may be subtly hidden in the T wave of the preceding beat.
When three or more premature atrial beats occur in a row, we start calling this nonsustained supraventricular tachycardia.
Nonsustained supraventricular and ventricular tachycardia (duration three to 10 beats) occurred in 4.3% (13/303) and 0.7% (2/303), respectively, of those normal male aviators.
More Common as We Age
One found that in normal individuals over age 50 years, 99% had at least one PAC during 24-hour Holter monitoring. The PAC prevalence strongly increased with age from about one per hour in those ages 50 to 55 years to 2.6 per hour among those 70 and older.
Another analyzed 24 hour Holter recordings at 5-year intervals and found the frequency of PACs (and PVCs) increased significantly in all age groups over that time span, as this table from the paper in Age and Ageing shows:
PACs and Atrial Fibrillation
Not uncommonly, when a patient has PACs, especially if they are frequent, computer ECG interpretations mistakenly diagnose atrial fibrillation. This happens regularly even with a full, medical-grade 12-lead ECG. Fortunately, such ECGS are still over-read by cardiologists who usually make the correct diagnosis.
The computerized algorithms used by single-lead mobile ECG devices like Apple Watch 4 and AliveCor's Kardia similarly are frequently confused by premature beats, especially PACs. I wrote about this in detail in my post on PVCs and PACs .
Sometimes the devices will diagnose "possible atrial fibrillation" in a patient with frequent PACs in sinus rhythm and sometimes "unclassified."
In addition, patients with very frequent PACs a higher tendency to develop atrial fibrillation and a higher risk of cardiovascular complications.
Various Names of the Extra Beats
Whereas a consensus has been achieved (for the most part) on the term for early beats from the ventricles (premature ventricular contractions or PVCs), the term for PACs varies from one cardiologist to another and one paper to another.
If I enter in "atrial premature" into my electronic medical record problem list search, multiple naming options appear (all with the same ICD code of I49.1).
In addition, you may encounter the terms atrial ectopy, premature atrial beats or various combinations of "supraventricular" with either contraction, beats, or ectopy.
The two most popular acronyms are APCs and PACs, and I am guilty of using these interchangeably and seemingly randomly.
Markers for Atrial Cardiomyopathy?
Through most of my cardiology life, I had considered PACs to be totally benign. And certainly, in and of themselves, they cause no problems other than occasional palpitations. However, in the last decade have shown consistent associations between frequent PACs and stroke, death, and atrial fibrillation.
Some researchers have suggested the concept of to explain this association. A diseased atrium could be the reason for both PACs and atrial fibrillation (as well as stroke and death), as opposed to atrial fibrillation being the primary cause of increased cardiovascular events.
Clearly, PACs, stroke, and CV disease share common risk factors such as age and obstructive sleep apnea, making cause and effect difficult to sort out. Could PACs and atrial fibrillation represent different phenotypes of atrial cardiomyopathy?
These data on frequent PACs raise a whole host of questions that remain unanswered:
- Is there a frequency of PACs, say >100 per 24 hours, that is useful in predicting adverse outcomes?
- Are there clinically-measurable predictors of which patients with frequent PACs are most likely to have poor outcomes?
- Does treatment of PACs, say with anticoagulation therapy or suppression, in the absence of atrial fibrillation reduce risk of CV events?
The Bottom Line on PACs
Premature atrial contractions are very common in normal individuals and increase with aging. They can cause palpitations and an irregular pulse but are benign in and of themselves.
Frequent PACs (more than 1% of total heart beats) are a marker of increased risk of atrial fibrillation, stroke, and death. The concept of a diseased atrium (atrial cardiomyopathy) causing both atrial dysrhythmias and raising the risk of stroke and death helps to explain these associations.
More research is needed to answer the important clinical questions related to the independent significance of frequent PACs and what treatments might be warranted.
, is a private practice noninvasive cardiologist and medical director of echocardiography at St. Luke's Hospital in St. Louis. He blogs on nutrition, cardiac testing, quackery, and other things worthy of skepticism at , where a version of this post first appeared.