Time for the test: Selecting the protocol
While the Bruce exercise treadmill testing protocol is the most commonly used (82% of tests)11 in healthy adults, it may not be appropriate for women or the elderly as most protocols were developed for the evaluation of men.12 A ramp method with gradual increase in grade each minute is preferred by some clinicians when patients are unable to perform a standard Bruce protocol. Another option, the Modified Bruce protocol, which is more gradual than the standard Bruce protocol; it has two 3-minute warm-up stages.12
The 3 PSMI protocols include adenosine, dipyridamole, and dobutamine. Each has a different administration routine,9 though ECG, blood pressure and pulse are taken every minute for all 3.
- Adenosine is infused with a pump over 6 minutes and technetium Tc99m is injected 3 minutes into the infusion.
- Dipyridamole is infused over 4 minutes and technetium Tc99m is injected 2 to no more than 5 minutes after the infusion. Theophylline is injected, after dipyridamole if necessary, no earlier than 1 minute after technetium Tc99m is administered to avoid interference with the uptake.
- Dobutamine is titrated with a dose increase every 3 minutes. Tc99m is injected after the first minute at the highest concentration. (Typically dobutamine is used with echocardiography.6)
Which pharmacologic stress test is best for which patient?
Adenosine
A potent vasodilator, this endogenous nucleoside is rapidly cleared (half-life, <10 seconds) along with its side effects of flushing, headache, and nausea.
Dipyridamole
This coronary vasodilator inhibits the uptake of adenosine. The same side effects as adenosine—flushing, headache, nausea—may last longer with dipyridamole (half-life,13 hours) but they are more common with adenosine and are relieved by administering theophylline.
All methylxanthines may interfere with either dipyridamole or adenosine and should be held for 24 to 48 hours prior to examination.
- Avoid both adenosine and dipyridamole in patients with asthma, severe COPD (FEV1<30%), second- or third-degree heart block, hypotension, or those who are on oral dipyridamole.
Dobutamine
This synthetic catecholamine increases heart rate, systolic blood pressure, and myocardial contractility, thereby provoking ischemia. Dobutamine (half-life 2 minutes) is not affected by methylxanthines.
- It is preferred in patients who are unable to use adenosine or dipyridamole.
- Caution is needed in patients with systolic BP less than 100 mm Hg, hypertension, ventricular ectopy, and glaucoma.
- Side effects can be reversed with beta-blockers.
Ideal endpoints and the realities that may creep in
The ideal endpoint in an exercise treadmill testing is 100% of the age-predicted maximum heart rate (220–age). Eighty-five percent of maximum heart rate is the minimum for an acceptable test.
Absolute contraindications
Absolute indicators for stopping an exercise treadmill test are either a 10 mm Hg systolic drop in blood pressure from standing baseline, moderate to severe angina symptoms, feelings of syncope, skin color changes suggestive of hypoxia or hypotension, ischemic ST changes, or the patient’s desire to stop.
Relative contraindications
Relative indicators to stop include fatigue, shortness of breath, leg pain, increased arrhythmias—particularly PVCs that increase with the exercise level—and blood pressure ≥250 systolic or 115 diastolic.6
Ideally, PSMI evaluations are terminated according to the prescribed length of infusions. They will also be terminated if a patient develops wheezing, severe or increasing chest pain or hypotension, neurological symptoms, ST-segment elevation abnormalities, or arrhythmias. To reverse the side effects of adenosine or dipyridamole, aminophylline IV will be administered (1–2 mg/kg slowly, up to 250 mg).
Heart of the matter: What a report should cover
Assuming you have ordered the stress test (and not done it yourself), a complete report should include
- ST changes
- symptoms during testing
- reason for ending the test
- estimation of exercise capacity
- blood pressure response
- the presence and frequency of arrhythmias or ectopy.
Abnormal. ST segment change is the most important ECG finding in a positive test; it’s defined as >1 mm horizontal or down sloping depression or elevation, at least 60 to 80 milliseconds after the end of the QRS complex and should prompt further workup to confirm CAD.5 An abnormal ECG during a PSMI test indicates an elevated risk of multivessel CAD and should prompt further evaluation regardless of normal myocardial perfusion imaging.13
The calculations behind the scores
Duke Treadmill Score
Duke Treadmill Score (DTS)=exercise time–(5 × ST deviation) - (4 × exercise angina), with 0=no angina during exercise, 1=nonlimiting angina, and 2=exercise-limiting angina.
The score typically ranges from –25 to +15.4
- Low risk: > +5
- Moderate risk: –10 to +4
- High risk: < –11
Elderly Alternative Treadmill Score (for patients over 65 years of age)
This score has 2 variables in common with the Duke Treadmill Score (exercise duration or the MET equivalent and millimeters of ST changes). It also has 2 different variables (drop in exercise systolic blood pressure below resting value and history of congestive heart failure [CHF] or use of digoxin [Dig]).17,18
The score is calculated as follows: 5 × (CHF/Dig [yes=1; no=0]) + exercise-induced ST depression in millimeters + change in systolic blood pressure score–METs
Systolic blood pressure score:
- 0 for an increase >40 mm Hg
- 1 for an increase of 31–40 mm Hg
- 2 for an increase of 21–30 mm Hg
- 3 for an increase of 11–20 mm Hg
- 4 for an increase of 0–10 mm Hg
- 5 for a reduction below standing systolic pre-exercise blood pressure.
A score of < –2 is low risk, –2 to 2 is moderate risk, and >2 is high risk.