Page 452 - Cardiac Nursing

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■ Figure 19-4 Nomogram of percentage normal exercise capacity ■ Figure 19-5 Nomogram of percentage normal exercise capacity for
for age in 1,388 male veterans referred for exercise testing (based on age among active and sedentary men referred for exercise testing (based
metabolic task equivalents [METs]). (With permission from Morris, on metabolic equivalents [METs]). (With permission from Morris,
C. K., Myers, J., Froelicher, V. F., et al. [1994]. Nomogram based on C.K., Myers, J., Froelicher, V. F., et al. [1994]. Nomogram based on
metabolic equivalents and age for assessing aerobic exercise capacity metabolic equivalents and age for assessing aerobic exercise capacity in
in men. Journal of the American College of Cardiology, 22, 175–182.) men. Journal of the American College of Cardiology, 22, 175–182.)
Electrocardiographic Responses 61
for evaluating a diagnostic test. Virtually every edition of exer-
In patients with CAD, exercise can cause an imbalance between cise testing guidelines that has been published suggests the appli-
myocardial oxygen supply and demand (ischemia), which can re- cation of a traditional diagnostic criterion: 1.0 mm or greater ST-
sult in an alteration (decrease or elevation relative to the baseline) segment depression that is horizontal or downsloping 60 to 80
in the ST segment of the electrocardiogram. These changes are the milliseconds after the J-point (a “positive” response). ST-segment
foundation of the exercise test clinically. Normal and abnormal depression greater than 1.0 mm that is downsloping is generally
ST-segment responses to exercise are illustrated in Figure 19-6. indicative of more severe CAD. Most (probably 90%) ischemic
4
Ever since electrocardiographic changes were first associated with ST changes occur in the lateral precordial leads. Although it has
myocardial ischemia in the 1920s, the diagnostic electrocardio- historically been thought that the diagnostic performance of the
graphic criteria and leads that exhibit abnormalities during ex- test was incomplete without all 12 leads, some evidence suggests
ercise have been the source of significant debate. Numerous that ST-segment changes isolated to the inferior leads may fre-
electrocardiographic criteria, including complex mathematical quently be false-positive responses. 62
constructs, combined scores, and ST areas during exercise and re- The significance of ST-segment elevation depends on the pres-
covery, have been proposed to optimally diagnose the presence of ence or absence of Q waves. When ST elevation occurs in the pres-
CAD. Few of these studies, however, have followed accepted rules ence of a normal resting electrocardiogram, it is usually indicative of

■ Figure 19-6 Normal and abnormal ST-segment responses to exercise and the various criteria for ST-
segment depression. (With permission from Froelicher, V. F., & Myers, J. [2006]. Exercise and the heart [5th
ed.]. Philadelphia: W.B. Saunders.)