ACSM’s Guidelines For Exercise Testing & Prescription Chapter 6 Interpretation Of Clinical Test Data
World Health Organization : “The primary purpose of an exercise test is to determine the responses of the individual to [exercise] efforts at given levels and from this information to estimate probable performance in specific life and occupational situations.”
Bayes Theorem states : “The post-test probability of a patient having disease is determined by the disease probability before the test and by the probability that the test will provide a true result.”
Bayes Theorem speaks to the issue that an exercise test must have two qualities : sensitivity and specificity.
Sensitivity • Sensitivity refers to the percent of patients tested who have CAD and who demonstrate a positive (abnormal) test outcome - i.e. - how often the test will identify a sick individual (~ 70%).
Specificity • Specificity refers to the percent of healthy people without CAD who also demonstrate a negative (normal) stress test - i.e. - how often the test is able to identify a healthy individual (~80%).
False Negative Tests • Failure to reach ischemic threshold • Monitoring an insufficient # of leads • Failure to recognize non-EKG signs & symptoms associated w/ CAD • Collateralization that masks angiographically significant CAD • Technical & observer error
False Positive Tests • Drugs - digitalis preparations • Wolff-Parkinson-White syndrome • Left Bundle Branch Block • Cardiac hypertrophy • Hypokalemia • Mitral valve prolapse • Coronary spasm - Prinzmetal angina
Predictive Value • The predictive value of exercise testing is a measure of how accurately a test result correctly identifies the presence or absence of CAD in the test population.
Using exercise tests to screen large numbers of asymptomatic people is of questionable value. • Testing individuals with symptoms is a much more practical reason for an exercise test.
Symptoms which make Bayesian senseand which increase the predictive value of an exercise test would be : • Angina, chest discomfort/pressure • Anginal equivalents • Recurrent palpitations w/ exercise • SOB w/ exertion that is unexpected • Dizziness, syncope, vertigo, diplopia w/ ex.
Imaging Stress Tests • Generally speaking, echocardiogaphic studies will slightly improve the sensitivity & specificity of simple GXT’s .
Exercise GXT’s with technetium-Tc99m sestamibi imaging or thallium chloride-201 imaging tend to improve the sensitivity and specificity of the test to ~ 85+%.
The sensitivity and specificity of any test is improved by the number of diseased vessels.