Introduction

Using a non-invasive ultrasound technique, it is possible to measure the thickness of the carotid artery wall. What use is such a measurement? Previous studies have shown that increased carotid wall thickness is representative of atherosclerosis in other arteries, and the occurrence of cardiovascular disease. A new study has now looked for a possible connection between carotid artery thickening and heart attack (myocardial infarction or MI) or stroke in people over 65 who don't have existing cardiovascular disease¹.

Method

The study was sponsored by the US National Heart, Lung, and Blood Institute. Ultrasonograms of the carotid arteries were done in over 4,000 people over 65 who had no sign of cardiovascular disease. The inner and middle layer of two parts of the carotid artery - the common carotid and the internal carotid - were measured, and the participants divided into 5 equal groups (or quintiles) according to these measurements.

Over the next 6 years, the occurrence of MIs and strokes in the participants was determined by questions at annual examinations and 6-monthly telephone interviews, and from Medicare hospital records. Medical records were then studied to provide a precise diagnosis. The rates per 1,000 patient-years of MI, stroke, and either event (i.e. a new MI or a new stroke) were calculated for each quintile group. Traditional risk factors - age, sex, race, atrial fibrillation, hypertension, diabetes, smoking, low-density lipoprotein (LDL) and high-density lipoprotein cholesterol levels - were incorporated into a second analysis.

Results

At the outset, the participants had an average age of 72.5 years; 61% were female, 15% black, 40% had hypertension, 13% had diabetes, 12% were smokers, 3% had atrial fibrillation, and their average LDL-cholesterol was 130 mg/dL (3.36 mmol/L). After 6 years, 267 MIs and 284 strokes had occurred; 496 participants had had either a new MI or a new stroke - i.e. 55 patients had both conditions, but only the event that occurred first was counted for the combined analysis.

The rates for the combined events (a new MI or stroke) for each quintile group of carotid thickness are shown in the following figure. It can be seen that the rates increase along with the artery thickness.

There is clearly an increased risk of an adverse cardiovascular event in subjects with thicker carotid artery walls. This risk was found to be unrelated to sex or age, and was present over and above other risk factors such as raised blood pressure, smoking, and diabetes.

Comment

Thickening of the carotid arteries, as shown by ultrasound, has already been found to equate with other risk factors and with the existence of cardiovascular disease. The present study supplies a missing link - people aged 65 years and above without cardiovascular disease are nevertheless at increased risk of MI or stroke if they have increased thickening of the middle and inner layers of their carotid arteries. In other words, carotid ultrasound examination provides an additional test for the risk of a serious cardiovascular event, which is at least as powerful as other tests - e.g. cholesterol levels, blood pressure, ECG, assessment of diabetes, smoking, etc. The authors of the study believe that this examination may help to identify apparently healthy elderly people who are at increased risk of having a serious cardiovascular incident.