Exercise testing and ancillary techniques to screen for coronary heart disease

Abstract In screening for asymptomatic CHD, risk factor analysis alone has a sensitivity of only approximately 20%. In the studies reviewed, sensitivity of the maximal exercise ECG ranged from 30% to 67%. In other words, only 30%–67% (for an average of 55%) of those who developed end-points for CHD had an abnormal ECG response when tested. The recent studies by Allen et al. and from the Seattle Heart Watch show markedly lower sensitivity and predictive values for the exercise ECG compared to prior investigations. In Allen's study, roughly 10% of the subjects were lost to follow-up, hence the incidence of CHD may well be underestimated. Furthermore, risk factor data was not given for abnormal responders or for those who developed CHD. The Seattle Heart Watch is difficult to interpret in light of preceding studies done by Bruce and others. There was no predictive value for an abnormal exercise ECG in the subgroup that had no risk factors. Unfortunately, not all of the population had a serum cholesterol measurement, so the risk factor data were not complete. Early investigations paid less attention to risk factors. Had previous studies subdivided the population according to the presence and absence of risk factors, perhaps results similar to the Seattle Heart Watch would have been obtained. These latter two studies also took into consideration exercise test responses other than ST segment depression, and thereby improved sensitivity and specificity. Further studies are needed to substantiate these findings. Since an abnormal exercise ECG is neither 100% sensitive nor specific, an abnormal test does not absolutely predict the presence of CHD or its future development, nor does a normal test rule out disease or the future occurrence of coronary events. Some of those who are going to develop coronary disease will change from a normal to an abnormal response when retested, so possibly serial testing will increase sensitivity. However, McHenry has reported that a change from a negative to a positive test is no more predictive than an initially abnormal test 52 and an individual has been reported who changed from a normal to an abnormal test but was free of angiographically significant disease. 53 The predictive value of the abnormal maximal exercise ECG ranged from 5% to 46% in the studies reviewed. That is, 5%–46% (for an average of 21%) of the abnormal responders developed CHD over the follow-up period. Thus, over one-half of the abnormal responders were false positives. The possibility exists that some of these individuals have coronary disease that has yet to manifest itself, but angiographic studies have supported this high false positive rate when using the exercise test in asymptomatic populations. This is explained by the fact that the predictive value of an abnormal exercise test response is directly related to the prevalence of CHD in the population tested. Thus, the usefulness of the test is enhanced in populations with increased risk factors or markers for CHD. Exercise testing may prove to have another value in asymptomatic populations in addition to its screening capabilities. Bruce and colleagues examined the motivational effects of maximal exercise testing for modifying risk factors and health habits. 54 A questionnaire was sent to nearly 3000 men, 35–65 yr of age who had undergone symptom-limited treadmill testing at least 1 yr earlier. Individuals were asked if the treadmill test motivated them to stop smoking (if a smoker), increase daily exercise, purposely lose weight, reduce the amount of dietary fat, or take medication for hypertension. There was a 69% response to this questionnaire, and 63% of the responders indicated that they had modified one or more risk factors and health habits and that they attributed this change to the exercise test. In fact, a greater percentage of patients with abnormal functional aerobic impairment compared to normal subjects reported changing risk factors or health habits. Various techniques have been recommended to improve the performance of a maximal exercise test as a screening tool. All of these techniques require further research to optimize their methodologies and establish their limitations, particularly in regard to their application in asymptomatic populations. These secondary noninvasive studies may be able to improve both the sensitivity and specificity of exercise testing and particularly increase the predictive value of an abnormal response. Radionuclide left ventricular angiography is an exciting new technique that can noninvasively obtain the same prognostic information previously available only from invasive cardiac catheterization. Cardiokymography and cardiac fluoroscopy also show promise to enhance the use of exercise testing for screening purposes. HDL and conventional risk factor analysis have also been recommended as effective ancillary screening techniques. Of particular interest is the fact that wall motion abnormalities can precede and occur without ECG changes, as shown by both animal 55 and human studies. 56 Development of a technique that accurately and reproducibly detects exercise-induced regional wall motion abnormalities could revolutionize identification of latent CHD. When considering the use of procedures for a screening tool, one must ask if the procedure fulfills a number of criteria. Before we can recommend the routine application of maximal exercise testing or any other method to screen latent CHD, it must be demonstrated that acceptable methods of effective treatment are available and that detection and treatment can substantially reduce morbidity, mortality, or both. One study suggests that in asymptomatic patients with three vessel disease, coronary artery bypass surgery confers improved survival. 57 More research is necessary to demonstrate that any form of secondary prevention, medical or surgical, may improve the prognosis for asymptomatic men with CHD. Hypothetically, if a method of secondary prevention were proven and available today, we would offer the following three-step approach to screening for asymptomatic CHD in men over 35 yr of age. First, a history for angina, risk factor analysis (including an HDL level), and a resting ECG should be performed. It must be remembered that the sensitivity and predictive value of these resting techniques are low. If any of the data collected place the individual at risk, the second step should be a maximal exercise test. If this test is interpreted as abnormal based on ST segment shifts and perhaps other abnormal responses, the third step should be utilization of cardiac radionuclide imaging, cardiac fluoroscopy, or cardiokymography. The lack of data on the diagnostic power of these tools in asymptomatic individuals prevents more specific recommendations at this time. Good clinical judgment must be used to avoid the iatrogenic complication of producing cardiac cripples by mislabeling well people.