BASEL, SWITZERLAND. Premature atrial complexes (PACs), also known as atrial extra systoles or atrial premature beats, are extremely common and can be found on 24-hour Holter monitoring in over 60% of normal adults. They are usually entirely benign and do not require treatment unless they are very frequent or result in uncomfortable palpitations. PACs originate from foci of "rogue" heart cells that decide to take on a beat of their own. Depending on when the PAC "fires" it may not be transmitted to the ventricles at all, but in some cases it may cause a pause in the normal heart rhythm, which may or may not be followed by a more forceful ventricular contraction.

PACs can be precipitated by stress, fatigue, fever, thyrotoxicosis, tobacco, caffeine, and certain other stimulants and drugs including cold medications and weight-loss preparations. PACs may also be a sign of underlying heart disease such as ischemia (angina pectoris), heart failure or myopericarditis. PACs can initiate atrial fibrillation (AF), atrial flutter and/or supraventricular tachycardia. Research has shown that these arrhythmias originate from the same focal points that generate PACs. PACs can be distinguished fairly easily on an electrocardiogram; they are characterized by a smaller and earlier than expected P wave. The P wave originates in the sino-atrial node and is the electrical impulse that initiates the heart beat.

It is known that a high level of PACs is associated with an increased risk of developing AF and suffering a stroke. What is not known are, what is a normal level of PACs and what are the factors associated with an increased level? A group of researchers from Basel University now provides answers to these questions. Their study, part of the Swiss Study on Air Pollution and Lung Disease in Adults, involved 1742 participants aged 50 years or older who underwent 24-hour Holter monitoring. The main findings of the study were:

• 99% of study participants had at least one PAC over the 24-hour monitoring period.
• The number of PACs increased with age with participants between the age of 50 and 55 years experiencing an average of 0.8/hr as compared to those over the age of 70 years who experienced an average of 2.6 PACs/hr.
• Compared with individuals with a PAC frequency below 1.27/hr, individuals with a frequency at or above 1.27/hr were older, had a higher systolic blood pressure, a higher prevalence of cardiovascular disease, a higher level of BNP (brain natriuretic peptide), and a lower education level.
• Individuals with higher PAC frequency also experienced significantly more PVCs (premature ventricular complexes) and had greater heart rate variability.
• There was an intriguing correlation between height and PAC frequency with taller individuals having significantly more PACs than shorter individuals. The researchers speculate that tallness may be associated with a larger left atrium, but also suggest that taller individuals may have elevated electric activity.
• Moderate physical activity and higher HDL cholesterol levels were both associated with less PACs. The researchers speculate that HDL levels may influence the composition of cell membranes, a major determinant of cell excitability.
• There was no correlation between PAC frequency and hypertension, or between PAC frequency and body mass index.

Conen, D, et al. Premature atrial contractions in the general population: frequency and risk factors. Circulation, October 9, 2012 [Epub ahead of print]

Editor's comment: It is interesting to note that individuals without AF generally experienced maybe one or two PACs an hour, whilst some afibbers report experiencing dozens or even hundreds of PACs and PVCs an hour. Fortunately, the incidence of PACs and PVCs can often be substantially reduced by ensuring optimum magnesium and potassium status.