Table of Contents
1. Indications for Radionuclide Myocardial Perfusion Imaging (rMPI) with Stress Testing
2. Diagnostic Utility of rMPI
3. Exercise Stress Testing
a. Contraindications to Exercise Stress Testing
b. Indications for Terminating Exercise Stress Testing
c. Exercise Protocols
d. Monitoring During Exercise Stress Testing
4. Beyond ST Depression
5. Pharmacologic Stress Testing
a. Adenosine
b. Regadenoson
c. Dobutamine
6. The Basic Stress Testing Routine at the MUSC ART
7. Patients with LBBB or a Paced Ventricular Rhythm
8. Some Notes on SPECT
9.
Indications for rMPI with Stress Testing
The vast majority of “stress” testing (both exercise and pharmacologic) is performed on adults with symptoms of known or probable ischemic heart disease. Candidates for stress testing may have stable symptoms of chest pain, may be stabilized by medical therapy following symptoms of unstable chest pain, or may have already had a myocardial infarction or a revascularization procedure. The clinical suggestion of CAD based on patient history findings, ECG tracings, and symptoms of chest pain must be established and used as a guide to determine if stress testing may be useful according to the Bayes theorem, which states that the diagnostic power of exercise stress testing is maximal when the pretest probability of CAD is intermediate (30-70%) based on age, sex, and the nature of the chest pain. When the diagnosis of CAD is certain, based on age, sex, description of chest pain, and history of prior myocardial infarction, a clinical need may arise for risk or prognostic assessment to reach a decision regarding possible coronary angiography and further medical management. Myocardial infarction is a common first presentation of ischemic heart disease. This subset of patients also may require prognostic and/or risk or assessment.
Stable patients with an acute coronary syndrome (myocardial infarction or unstable angina) may undergo a submaximal exercise test prior to discharge unless they have undergone percutaneous coronary intervention or coronary artery bypass graft surgery and been fully revascularized (eg, single vessel disease successfully treated with PCI). The submaximal exercise test uses one of the following end points:
· A peak heart rate of 120 to 130 beats per minute or 70 % (not 85%) of the maximal predicted heart rate for age
· A peak work level of 5 METs
· Mild angina or dyspnea
· ≥2 mm of ST segment depression
· Exertional hypotension
· Three or more consecutive ventricular premature beats
Diagnostic Utility of rMPI
A meta-analysis compared the performance of the following tests in patients with an intermediate pretest risk of CHD (25 to 75 percent): exercise ECG testing, planar thallium imaging, SPECT perfusion imaging, stress echocardiography, and positron emission tomography (PET), each of which was followed by coronary angiography if the test was positive. The following values for sensitivity and specificity were noted:
· Exercise ECG testing — 68 and 77 percent in 132 studies of over 24,000 patients
· Planar thallium rMPI (including both exercise and pharmacologic testing) — 79 and 73 percent in six studies of 510 patients.
· Thallium SPECT rMPI (including both exercise and pharmacologic testing) — 88 and 77 percent in 10 studies of 1174 patients
· Stress echocardiography — 76 and 88 percent in six studies of 510 patients
· PET scanning — 91 and 82 percent in three studies of 206 patients.
In a second meta-analysis, 44 articles met criteria for determining the sensitivity and specificity (compared to coronary angiography) of exercise SPECT rMPI and exercise echocardiography for the diagnosis of CHD. The two tests had similar sensitivity (85 and 87 percent), but the specificity was significantly lower (ie, more false positives) with exercise rMPI (77 versus 64 percent).
Exercise Stress Testing
All treadmill stress tests should by symptoms limited. In patients who are able to exercise and can…
· achieve an adequate heart rate (defined as ≥85 % of their age-predicted maximum where the maximal HR = 220 - age in years) and
· achieve an adequate workload (defined as ≥80 percent functional aerobic capacity). Adequate Functional capacity is >10 METs or normal for age:
Women 13.7-(0.13 x age) = METs.
Men 18 – (0.15 x age) = METs or 14.7 – (0.11 x age) = METs.
A less accurate and less desirable estimate of ‘adequate stress’ is provided by the “double product” (ie the product of peak systolic blood pressure and heart rate with adequate usually defined as ≥20,000).
The treadmill or bicycle (Europe) exercise is the preferred form of stress, because it provides the most information concerning patient symptoms, cardiovascular function, and prognosis.
Contraindications to Exercise Stress Testing
Some reasonable contraindications to exercise that are not specifically listed above:
· Marked ST segment depression (= to or >3 mmm)
· Ischemic ST segment elevation of >1 mm in leads without pathological Q waves
· Frequent appearance of non-sustained ventricular tachyarrythmia
· CNS symptoms
· Peripheral hypoperfusion
· Any factors that will impair the ability to monitor the EKG (ex. LBBB) or BP
· Severe pulmonary hypertension
Indications for Terminating Exercise Stress Testing
The optimal duration of an individual exercise test is one that is carried out until the patient feels that he/she cannot exercise further. This is called a symptom-limited maximal exercise test. However, the decision to stop an exercise test can be patient-determined, protocol-determined, or physician-determined.
Some reasons to stop a stress test that aren’t listed above:
· Ischemic ST segment elevation of >1 mm in leads without pathological Q waves.
· CNS symptoms
· Peripheral hypoperfusion
· Technical difficulties in monitoring EKG or BP (a kind of equipment failure)
Exercise Protocols
The Bruce protocol is generally preferred for office-based exercise testing largely because it has been carefully validated. The protocol is divided into successive three minute stages, each of which requires the patient to walk faster and at a steeper grade. Stage I is at an incline of 10 percent and a speed of 1.7 miles per hour; stage II progresses to an incline of 12 percent and a speed of 2.5 miles per hour. The modified Bruce protocol can be used for risk stratification of patients after an acute coronary syndrome (myocardial infarction or unstable angina) and in sedentary patients in whom the standard Bruce protocol may be too strenuous. The modified protocol adds two low-workload stages, both of which require less effort than Stage 1, to the beginning of the standard Bruce protocol.
The Cornell protocol was developed for use with computerized ST/HR slope determination, a possibly improved method of quantitative exercise electrocardiography. The ACC/AHA guidelines concluded that the ST/HR slope (the rate-related change in exercise-induced ST segment depression) has not yet been validated, but that it could prove useful in patients with borderline or equivocal ST responses, such as ST segment depression associated with a very high exercise heart rate. In the Cornell protocol, each stage of the Bruce protocol is divided into two smaller and shorter stages. Although this was done to provide more data points for the computerized ECG analyses, the protocol is also more applicable to patients with limited exercise tolerance because of the smaller workload increments.
The Naughton protocol is often used in post-MI exercise testing to classify patients into high-risk and low-risk categories and to determine optimal treatment strategies. This protocol is also used for functional exercise testing with gas analysis techniques to measure oxygen uptake and VO2max.
Regardless of the protocol used, patients should be instructed not to eat, drink, or smoke for at least three hours prior to the examination, as this permits the patient to achieve a higher workload. A brief interview by a physician or qualified health professional should be performed prior to testing to rule out contraindications and to gather information that will facilitate interpreting the test. All patient medications must be identified since certain drugs will reduce the maximal heart rate that is achieved (eg, beta-blockers, verapamil, diltiazem, and amiodarone), while other drugs, particularly digoxin, are associated with a false-positive ECG response to exercise. In addition, diuretic-induced hypokalemia can interfere with the interpretation of the ST segment and T waves, and recent use of nitrates can minimize the ischemic response to exercise in patients with coronary disease. In general, patients undergoing exercise testing for diagnostic purposes should not take anti-ischemic medications or drugs that slow the heart rate. However, anti-ischemic medications should be continued if the purpose of the test is to establish prognosis or adequacy of anti-ischemic therapy. A limited cardiac examination should be performed, with attention given to detecting heart murmurs (particularly aortic stenosis), evidence of heart failure, and pulmonary findings such as wheezing. Detecting mitral valve prolapse is also important, since this valve lesion may be associated with a false positive ECG response to exercise.
Monitoring During Exercise Stress Testing
The most popular lead system for exercise ECG testing is a simple modification of the standard 12-lead ECG with the arm and leg electrodes moved to the torso. It is important that the arm electrodes be placed at the base of the shoulder just inside the border of the deltoid muscles and 1 to 2 cm below the clavicles. More medially placed electrodes are associated with false positive and false negative diagnostic errors for myocardial infarction. The leg electrodes should be positioned below the umbilicus and above the anterior superior iliac crest.
The resting ECG is sometimes obtained both supine and standing, since patient position can influence the QRS and T wave axes. ECGs obtained during exercise should be compared with the resting standing ECG, while ECGs obtained during recovery should be compared with the resting ECG in the same position. The presence of an arrhythmia, confirmed by the resting ECG, should be documented since it may have an impact on exercise. Important examples are atrial fibrillation or atrial flutter which, if not appropriately treated with an AV nodal blocking agent, may result in excessively high heart rates during exercise.
During the exercise test, data should be obtained at the end of each stage and at any time an abnormality is detected clinically (eg, chest pain) or on the monitor. Similarly, during recovery from exercise, the ECG should be recorded every two minutes for 7 to 10 minutes until the heart rate slows below 100 beats per minute or the ECG waveform returns to the control baseline pattern. In addition, continuous monitoring of the ECG waveform in selected leads should be performed throughout the exercise period and during recovery to assess cardiac rate, rhythm, and ST segment responses. During the test, you can change the leads that are continuously displayed by clicking on their labels. For example, rather than monitoring leads I, II, and III, you can monitor leads aVR, II, and V5. By doing so, you’ll be very unlikely to miss any evolving St segment changes. Ventricular arrhythmias can occur during the recovery period, and their occurrence during recovery is associated with an increased risk of death during follow-up.
The ECG should be recorded after a brief cool-down, while the patient is still on the treadmill or sitting on the bicycle. If significant ECG abnormalities did not develop during exercise, and the test is being done to diagnose ischemia, the patient should return to the supine position for the remainder of the recovery period. The increased venous return in the supine position may precipitate ischemic abnormalities not seen when upright on the treadmill. ST segment changes limited to the recovery period are as predictive of underlying coronary disease as changes seen during exercise. If, however, the patient develops ischemic ECG abnormalities during exercise, it may be safer to have the patient sit during recovery to minimize the risk of increasing ischemia and ventricular arrhythmias. Other abnormalities that occur during recovery also have prognostic importance:
· A slower than expected fall in heart rate at one minute (≤12 to ≤18 beats/min; see below)
· A delayed fall in systolic pressure
· The development of frequent ventricular ectopy
The blood pressure should be measured at rest (supine and standing) and during the last minute of each exercise stage. For ease of measurement, the arm should be straightened and the hand placed on the shoulder or in the axilla of the person taking the pressure. The systolic blood pressure should rise with each stage of exercise until peak is achieved, while the diastolic pressure falls or remains unchanged.
In addition to monitoring and recording the presence of chest discomfort or dyspnea, the American Heart Association recommends recording the patient's perceived level of exertion during the last five seconds of each exercise minute using defined scales, such as the rating of perceived exertion (RPE or Borg) scale.
Report exercise capacity in estimated metabolic equivalents (METs) of exercise. A MET refers to the resting volume oxygen consumption per minute (VO2) for a 70-kg, 40-year-old man. One MET is equivalent to 3.5 mL/min/kg of body weight.
Once you’ve reviewed the ECGs, write your interpretation somewhere on them. On the resting ECG at the top right corner (adjacent to the resting ECG interpretation) is a reasonable place. Be sure to report the rhythm and rate (eg, sinus tachy. To 167), any arrhythmia, and any ischemic or other noteworthy changes. Some attendings may be interested in additional indices that can be calculated from the exercise stress data (see below), but others are not.
Beyond ST Depression
Exercise-induced ST segment elevation is uncommon except in leads showing previous Q wave infarctions, but it does occur during stress testing in two groups of patients.
· Patients with severe and often multivessel CHD may develop transmural ischemia because of a marked decrease in coronary blood flow to a segment of myocardium during exercise. In contrast to ST depression, the leads showing ST elevation in these patients localize the coronary artery responsible for the ischemia. This difference was demonstrated in a study of 452 patients with single vessel coronary disease undergoing exercise testing. ST depression occurred most commonly in leads V5 or V6 regardless of which coronary artery was involved. In contrast, anterior ST elevation indicated left anterior descending coronary disease in 93 percent of cases, and inferior ST elevation indicated a lesion in or proximal to the posterior descending artery in 86 percent of cases.
· Variant or Prinzmetal's angina is characterized by episodic chest pain occurring mostly at rest, ST segment elevation during pain, and coronary artery spasm. Exercise-induced ST elevation occurs in 10 to 30 percent of patients with variant angina.