Stress Testing by Treadmill
Treadmill protocol
Exercise capacity is reported in terms of estimated metabolic equivalents of task (METs). The MET unit reflects the resting volume oxygen consumption per minute (VO2) for a 70-kg, 40-year-old man, with 1 MET equivalent to 3.5 mL/min/kg of body weight.
In the standard Bruce protocol, the starting point (ie, stage 1) is 1.7 mph at a 10% grade (5 METs). Stage 2 is 2.5 mph at a 12% grade (7 METs). Stage 3 is 3.4 mph at a 14% grade (9 METs). This protocol includes 3-minute periods to allow achievement of a steady state before workload is increased.
The modified Bruce protocol has 2 warmup stages, each lasting 3 minutes. The first is at 1.7 mph and a 0% grade, and the second is at 1.7 mph and a 5% grade. This protocol it is most often used in older individuals or those whose exercise capacity is limited by cardiac disease.
The Bruce protocol has larger increments between stages than do other protocols, such as the Naughton, Weber, and Asymptomatic Cardiac Ischemia Pilot (ACIP) study protocols, all of which start with less than 2 METs at 2 mph and increase in 1- to 1.5-MET increments between stages.
Other exercise protocols include bicycle and arm ergometry, both of which are used less often in North America than treadmill stress testing is. The bicycle ergometer has the advantage of requiring less space than a treadmill. It is quieter, permits sensitive precordial measurements without much motion artifact, and is generally safer because the risk of falling from the machine is lower.
Indications for termination of exercise testing
The American College of Cardiology (ACC)/American Heart Association (AHA) guidelines also specify indications for termination of exercise testing. Absolute indications for termination of testing include the following:
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Drop in systolic blood pressure (SBP) of more than 10 mm Hg from baseline, despite an increase in workload, when accompanied by other evidence of ischemia
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Moderate-to-severe angina
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Increasing nervous system symptoms (eg, ataxia, dizziness, near-syncope)
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Signs of poor perfusion (cyanosis or pallor)
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Technical difficulties in monitoring electrocardiographic (ECG) tracings or SBP
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Subject’s desire to stop
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Sustained ventricular tachycardia
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ST elevation (> 1 mm) in leads without diagnostic Q waves (other than V1 or aVR)
Relative indications for termination include the following:
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Drop in SBP of 10 mm Hg or more from baseline, despite an increase in workload, in the absence of other evidence of ischemia
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ST or QRS changes such as excessive ST depression (horizontal or downsloping ST-segment depression >2 mm) or marked axis shift
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Arrhythmias other than sustained ventricular tachycardia, including multifocal premature ventricular contractions (PVCs), triplets of PVCs, supraventricular tachycardia, heart block, or bradyarrhythmias
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Fatigue, shortness of breath, wheezing, leg cramps, or claudication
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Development of bundle branch block or intraventricular conduction delay that cannot be distinguished from ventricular tachycardia
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Increasing chest pain
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Hypertensive response (SBP of 250 mm Hg, diastolic blood pressure [DBP] higher than 115 mm Hg, or both)
Interpretation of test findings
Interpretation should include exercise capacity and clinical, hemodynamic, and ECG response. The occurrence of ischemic chest pain consistent with angina is important, particularly if it forces termination of the test. The classic criteria for visual interpretation of positive stress test findings include the following:
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J point – This is defined as the junction of the point of onset of the ST-T wave; it is normally at or near the isoelectric baseline of the ECG
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ST80 – This is defined as the point that is 80 msec from the J point
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Depression of 0.1 mV (1 mm) or more
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ST-segment slope within the range of ± 1 mV/sec in 3 consecutive beats
Noncoronary causes of ST-segment depression include the following:
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Severe hypertension
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Severe aortic stenosis
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Cardiomyopathy
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Anemia
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Hypokalemia
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Severe hypoxia
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Digitalis
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Sudden excessive exercise
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Glucose load
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Left ventricular hypertrophy
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Hyperventilation
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Intraventricular conduction delay
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Preexcitation syndrome (Wolff-Parkinson-White [WPW] syndrome)
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Severe volume overload (aortic, mitral regurgitation)
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Supraventricular tachyarrhythmias
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Normal radionuclide uptake (dipyridamole-Cardiolite).
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Normal wall motion with radionuclide uptake.
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Inferior-wall myocardial infarct and fixed defect.
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Motion abnormalities in inferior wall consistent with inferior-wall myocardial infarction.
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Inferobasal fixed defect and lateral wall ischemia.
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Wall motion abnormalities in inferobasal region.
