By: Anthony Pearson, MD January 6, 2020

In the past few weeks, this skeptical cardiologist has communicated with patients with premature atrial beats (PACs).

I have discussed ventricular premature beats (PVC) in detail here and here. They are the most common reason people feel the heart beating or trembling briefly (we call it palpitations).

Premature beats may be PVC or PAC. In addition to causing palpitations, they are the most common cause of irregular pulse detected by blood pressure equipment or medical staff.

Like PVC, PAC occurs when the electrically active tissue in the heart decides to emit (or depolarize) before receiving a signal from the normal pacemaker (sinus node) of the heart. In the case of PAC, the rogue tissue is located in one of the atria, the upper chamber of the heart.

In the ECG record below, PAC (labeled APC) occurred earlier than expected (too early). Normal (sinus) beats occur at regular time intervals, and they were previously normally configured p-waves—the normal electrical signals of atrial contractions. The larger spike (QRS complex) after the p wave represents ventricular depolarization, and there is no change compared to normal sinus beats because the ventricular activation of PAC is normal.

These early beats, by themselves, are considered benign.

When we monitor the ECG rhythm for a long time, PAC is very common, even in completely normal young people. In fact, they are more common than PVC.

For example, in a selected group of male pilots, a study found: "The incidence of rare, occasional, frequent, and very frequent solitary atrial ectopic disease is 72.9%, 2.6%, 2.3%, and 0.3%, respectively The same category of isolated ventricular ectopic disease occurred in 40.9%, 7.9%, 3.3% and 0.0%."

The frequency of isolated ectopic is classified as the percentage of total beats on Holter: rare (≤0.1%), occasional (>0.1 to 1.0%), frequent (>1.0 to 10%) and very frequent (>10%)) .

Therefore, in most cases, we will see some PACs in normal subjects monitored by ECG for 24 hours.

It is also common to see two PACs (atrial pair) in a row. 14.5% of these pilots had atrial couplets.

The highlighted box in the three-lead Holter monitor record below shows an atrial couplet.

The QRS complex of atrial premature beats is usually preceded by a visible P wave, and the shape or PR interval of this P wave is slightly different from that of a sinus beat. The PR interval of PAC may be longer or shorter than the PR interval of a normal heartbeat. In some cases, the P wave may be cleverly hidden in the previous T wave.

When three or more atrial premature beats occur consecutively, we begin to call this non-sustained supraventricular tachycardia.

Non-sustained supraventricular tachycardia and supraventricular tachycardia (duration 3 to 10 times) occurred in 4.3% (13/303) and 0.7% (2/303) of normal male pilots, respectively.

It is more common with age

A study found that 99% of normal people over 50 years old have at least one PAC during the 24-hour Holter monitoring period. The prevalence of PAC increases significantly with age, from 1 time per hour for 50 to 55 years old to 2.6 times per hour for 70 years and older.

Another study analyzed 24-hour Holter recordings at 5-year intervals and found that the frequency of PAC (and PVC) increased significantly in all age groups during this time, as shown in the table in the paper on age and aging:

It is not uncommon that when a patient has PAC, especially when PAC occurs frequently, the computerized ECG interpretation will incorrectly diagnose atrial fibrillation. Even with a complete medical grade 12-lead ECG, this situation often occurs. Fortunately, cardiologists who usually make the correct diagnosis still overread such electrocardiograms.

The computerized algorithms used by single-lead mobile ECG devices such as Apple Watch 4 and AliveCor's Kardia are also often confused by premature beats, especially PAC. I covered this in detail in my post on PVC and PAC here.

Sometimes these devices will diagnose "possible atrial fibrillation" in patients with frequent sinus rhythm PAC and sometimes "unclassified" patients.

In addition, patients with frequent PAC have a higher tendency to develop atrial fibrillation and a higher risk of cardiovascular complications.

Various names for extra beats

Although consensus has been reached on the terminology of early ventricular beats (premature ventricular beats or PVC) (in most cases), the terminology of PAC varies from cardiologist to paper and from paper to paper.

If I enter "atrial premature beats" in the electronic medical record question list search, multiple naming options will appear (all options have the same I49.1 ICD code).

In addition, you may experience various combinations of atrial ectopic, premature atrial contractions or "supraventricular" with contractions, pulsations, or ectopics.

The two most popular acronyms are APC and PAC, and I feel guilty about these interchangeable and seemingly random uses.

For most of my cardiology, I have always believed that PAC is completely benign. Of course, on their own, they won't cause any problems except for occasional heart palpitations. However, research in the past decade has shown that there is a consistent association between frequent PAC and stroke, death, and atrial fibrillation.

Some researchers have proposed the concept of "atrial cardiomyopathy" to explain this association. Diseased atria may be the cause of PAC and atrial fibrillation (as well as stroke and death), rather than atrial fibrillation being the main cause of increased cardiovascular events.

Obviously, PAC, stroke, and cardiovascular disease share common risk factors, such as age and obstructive sleep apnea, so it is difficult to clarify the causal relationship. Can PAC and atrial fibrillation represent different phenotypes of atrial cardiomyopathy?

These data on frequent PACs raise a series of unanswered questions:

The bottom line of PAC

Premature atrial contractions are common in normal people and increase with age. They can cause palpitations and irregular pulse, but they are benign in themselves.

Frequent PAC (more than 1% of total heartbeats) is a sign of increased risk of atrial fibrillation, stroke, and death. The concept that a diseased atrium (atrial cardiomyopathy) causes atrial arrhythmias and increases the risk of stroke and death helps explain these associations.

More research is needed to answer important clinical questions related to the independent significance of frequent PAC and which treatments may be needed.

Anthony Pearson, MD, is a private practice noninvasive cardiologist and medical director of echocardiography at St. Luke's Hospital in St. Louis. He blogged about nutrition, heart testing, scams, and other questionable things at The Skeptical Cardiologist, where a version of this article first appeared.

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