Primary Prevention of Cardio-Vascular Disease in Ethnic South Asians

• Adjusting Framingham Risk Estimate
• Issues to Consider When Adjusting Risk
• Consider Adjusting Risk for Additional Risk Factors by a Factor of 1.5-2.0.

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• South Asians
• The Importance of Conventional Risk Factors in South Asians- The INTER-HEART Study

NZ Guidelines on Assessment and Management of Cardiovascular Risk

NZ Guidelines and Adjusting Calculated Risk

The guidelines provide for adjustment upwards of risk because of presence of factors not included in the Framingham calculator. The guidelines state:

People in the following groups should be moved up one risk category (5%) as their cardiovascular risk may be underestimated in the Framingham risk equation:

• people with a strong family history of proven coronary heart disease or ischaemic stroke
• Mäori
• Pacific peoples
• people from the Indian subcontinent
• people with both diabetes and microalbuminuria
• people who have had type 2 diabetes for at least 10 years or who have an HbA1c consistently over 8% (where risk factor thresholds are given these should be interpreted as approximate guides to clinical practice only)
• people who meet the definition of the metabolic syndrome.

These adjustments should be made once only for people who have more than one criterion (the maximum adjustment is 5%).

This method of adjustment of risk seems to be quite attractive at initial assessment. This recommendation assumes that the risk estimate derived from the Framingham equation is reasonably accurate, in other words this estimate accounts for most of the risk and presence of other factors or will not markedly increase overall risk. To avoid over-emphasis on additional risk factors the guidelines allow for a maximum upward adjustment to estimated risk of up to five percent. It recommends this adjustment for the factors outlined above and does not recommend routine measurement of some emerging risk factors such as highly sensitive CRP, lipoprotein (a), or homocysteine.

The effect of a maximal upwards adjustment of risk by five percent is as follows:

• If the calculated risk is 5% over five years, then it is possible to increase this estimate to 10%. In this instance, the additional risk factors are judged to have increased the patient's relative risk by a factor of 2.0.
• If the calculated risk is 10% over five years, then it is possible to increase this estimate to 15%. In this instance, the additional risk factors are judged to have increased the patient's relative risk by a factor of 1.5.
• If the calculated risk is 15% over five years, then it is possible to increase this estimate to 20%. In this instance, the additional risk factors are judged to have increased the patient's relative risk by a factor of only 1.3.

In effect, the recommendations for adjusting total risk results imply that the impact of additional risk factors is less in higher risk patients compared with lower risk patients. Whether this was an intended effect of the guidelines or not is not certain, but viewed in this way an alternative method of adjustment may be considered in some instances.

The method of adjustment becomes important because of the 15 percent threshold set for drug treatment of risk factors. It seems likely that patients who we might think are at increased risk because we have measured some emerging risk factors such as CRP or apolipoproteins may still end up falling below the drug treatment threshold if the adjustment is limited to an increase in estimated risk by five percent. Adjustment by a factor of 1.5 to 2.0 may be reasonable.

There seems to be accumulating evidence that South Asians have a 1.5 to two fold or greater risk than Caucasians with the same risk factors. The INTERHEART Study confirmed that first infarction occurs earlier in many ethnic groups. I suspect that we may be justified in adjusting the estimated risk for some patients by a factor of 1.5 or 2.0 rather than simply just increasing the estimated risk by a maximum of five percent.

Careful review of the risk assessment diagrams in the NZ Guidelines makes for interesting observations. For example, non-diabetic and non-smoking males aged 40 or 50 years would only get pharmacological therapy if they have extremely high blood pressure or very severe dyslipidaemia. Even if there was an upward adjustment of risk by five percent, only those with severe hypertension or dyslipidaemia would qualify for pharmacological therapy. Thus, the NZ Guidelines simply will not prevent first ischaemic events in non-smokers and non-diabetic patients in the forty to fifty age group, and will be of little help in raising the age of first infarction in South Asians or other ethnic groups. Therefore, I think we will have to try to identify patients whose risk is underestimated by the Framingham based risk assessment tools- we should begin to measure emerging risk factors to try to identify patients whose risk would otherwise be underestimated. We should consider also being guided by a "risk factor" approach to management of dyslipidaemia as recommended by the USA's ATP III guidelines.

Hitesh Patel, Cardiologist
3 October, 2004