What is risk factor interaction ?

Coronary artery disease, as has been explained, is a multifactorial disease with diverse risk factors coming together and interacting to produce the pathological changes. The presence of a number of risk factors in a single individual increases the risk many times. Studies such as the Framingham and the MRFIT have clearly shown that the coexistence of multiple risk factors confers a magnified risk which is multiplicative rather than additive. It means that the various risk factors work synergistically to increase the coronary risk and mortality in an exponential manner. A smoker with modest elevations of cholesterol and blood pressure is at greater risk of coronary death than a non-smoker with severe Hypertension or marked hypercholesterolemia. A single risk factor is not sufficiently sensitive to identify all individuals at high risk of coronary artery disease. The bulk of CAD occur in individuals with only moderate elevations of a number of risk factors rather than in those who lie at the upper end of a single risk factor.

Figure: Multiplicative effect of risk factors

The above picture gives us an idea about the multiplicative effect of various risk factors. The estimated 10 yeas coronary risk show11 here is based on six risk factors - blood pressure, total cholesterol ( TC), HDL cholesterol, diabetes, smoking and presence of ventricular enlargement. As can be seen, the 10 year risk in those without any of the risk factors is less than 10 per cent. II increases to 20 percent when the systematic BP rises to 160 mm Hg and TC becomes 260 mg/dl. The risk increases further to 30 per cent in presence of additional risk factors like diabetes and low HDL cholesterol. Cigarette smoking increases the risk further to 40 per cent whereas ill presence of cardiac enlargement rises steeply to about 60 per cent. This is an important principle Lo remember for preventive measures, because the benefit obtained by improving all the risk factors together, even marginally, is definitely larger than that of strictly controlling a single factor and not addressing the other ones. The demonstration of such multiplicative risk has given rise to the concept of "comprehensive Cardio-vascular risk" or "total risk", estimating the individual's overall risk of developing Cardio-vascular disease resulting front the consortium of factors. This is particularly relevant in the Indian context because of the clustering of risk factors among ethnic Indians.