FRAMINGHAM, MASSACHUSETTS. It is estimated that atrial fibrillation (AF) now affects over 2 million individuals in the USA, some 4 million in Europe, and 8 million in China. By 2050 the prevalence of AF in the USA alone is expected to approach 16 million – truly a growth of epidemic proportions! As long as the cause(s) of the epidemic is largely unknown it is difficult to see how a program can be established to effectively prevent its continued growth. Nevertheless, it is important to determine the major risk factors for the development of AF in the general population. This is what the developers of the Framingham risk score for cardiovascular disease have now done.
Their study involved 4764 participants in the Framingham Heart Study enrolled between June 1968 and September 1987. The participants were monitored regularly for the first event of AF over a 10-year period following enrolment. About 55% of the study cohort were women, age range was 45 to 95 years, and at enrolment fewer than 5% had heart disease as defined by significant cardiac murmur, heart failure or previous myocardial infarction (heart attack). During the 10-year follow-up 457 (10%) participants developed AF with the incidence being almost twice as high among men than among women. After analyzing the data obtained during over 8,000 medical examinations, the Framingham researchers concluded that there are seven measurable factors which materially affect the risk of developing AF. These are:
- Age (risk increased with age)
- Body mass index (risk increased with a BMI greater than 30)
- Systolic blood pressure (risk increased if over 160 mm Hg)
- Being treated for hypertension (increased risk)
- PR interval (risk increased if over 160 ms)
- Age at first sign of heart disease (the earlier the greater the risk)
- Age at heart failure (the earlier the greater the risk)
 PR interval is the interval between the onset of the P-wave and the beginning of the QRS complex. It ranges between 120 ms and 200 ms with a higher value indicating vagal dominance.
 Age at which a significant cardiac murmur is first detected.
The age score ranges from –3 for women between the ages of 45 and 49 years to +8 for both men and women above the age of 85 years. A BMI greater than 30 (obesity) carries a risk score of 1, as does systolic blood pressure above 160 mm Hg, being treated for hypertension, and having a PR interval between 160 and 199 ms. A PR interval of 200 ms or higher is given a risk score of 2. The age at which heart disease is first diagnosed is given a score between 0 and 5, with the highest score given for age 45 to 54 years. Having heart failure diagnosed between the ages of 45 to 54 years carries a risk score of 10, while being diagnosed at age 75 years or later carries a risk score of 0.
Thus, a woman between the ages of 55 and 59 years with no other risk factors would have a less than 1% risk of developing AF over the next 10 years. On the other hand, an obese man between the ages of 55 and 59 years with high systolic blood pressure and being treated for hypertension would have a 30% risk of developing AF if he was also diagnosed with heart disease or had experienced a heart attack. The scoring system was validated with a white, middle-aged to elderly cohort so may not be applicable to a different population such as younger adults.
The researchers point out that echocardiographic measurements such as left atrial diameter, left ventricular wall thickness, and left ventricular fractional shortening also affect the risk of developing AF, but not nearly to the same extent as do the seven major risk factors identified.Schnabel, RB, et al. Development of a risk score for atrial fibrillation (Framingham Heart Study): a community-based cohort study. The Lancet, Vol. 373, February 28, 2009, pp. 739-45
It is, as usual, unfortunate that no attempt was made to separate the risk of developing lone AF from that of developing the heart disease-associated from of AF. If this had been done it is likely that such factors as sleep apnea, GERD, and excessive physical exercise would also have emerged as important risk factors. It is interesting that the researchers associate a PR interval of 200 ms or greater as a sign of vagal dominance. Perhaps this readily obtainable ECG component could be used to distinguish between adrenergic and vagal AF among lone afibbers?