Exercise ECG Testing

Synonyms include exercise tolerance testing, treadmill testing, ETT.

The evaluation of chest pain can be very difficult. It is possible to have a normal resting ECG with considerable narrowing of the coronary arteries. Exercise testing was developed in the 1950s with the Bruce protocol published in 1963. It is now a well established technique.

Exercise ECG testing can be used in the following circumstances:

• Assessing a clinical diagnosis of angina
• Risk stratification after myocardial infarction
• Risk stratification in patients with hypertrophic cardiomyopathy
• Evaluation of revascularisation procedures or drug treatment
• Evaluation of exercise tolerance and cardiac function
• Assessment of cardiopulmonary function in patients with dilated cardiomyopathy or heart failure
• Assessment of treatment for arrhythmia
• Assessment of asymptomatic people in high risk occupations like airline pilots.

• Exercise testing has a sensitivity of 78% and a specificity of 70% in detecting coronary artery disease. Hence a negative test does not adequately rule out disease. A positive test is much more likely to be false in a young person than an older person unless there is a very good history as they are at much lower risk. Therefore testing of young, asymptomatic people is controversial. There is a suggestion that not enough women and the elderly are being tested.¹ Atypical chest pain is more common in women and risk factors that apply to men are not directly applicable.²
• Prognostic testing can be useful. A positive test at a low workload is a poor prognostic sign and it indicates the need for urgent treatment.
• Bayes' theorem of diagnostic probability states that the predictive value of an abnormal test varies according to the probability of the disease in the population being studied. Therefore, exercise testing is usually performed in patients with a moderate probability of coronary artery disease, rather than in those with a very low or high probability.

• The Bruce protocol is very widely used and has been extensively validated. There are 7 stages of 3 minutes each so that a complete test takes 21 minutes.
• The level of exercise is estimated in METs. 1 MET or metabolic equivalent is the amount of energy expended at rest or 3.5ml oxygen per kilogram per minute.
• In stage 1 the patient walks at 1.7 mph (2.7 km) up a 10% incline. Energy consumption is estimated to be 4.8 METs during this stage. The speed and incline increase with each stage. The information from an ETT can be used to advise the patient on what activities and exercise levels are reasonable.
• A modified Bruce protocol is used for exercise testing within one week of myocardial infarction and for those who are old and frail or expected to have poor exercise tolerance for other reasons. It starts at a lower work level and so takes longer to achieve the required heart rate. This would make the patient more susceptible to fatigue before achieving the required rate but it seems that judicious use of either the Bruce or modified Bruce protocols gives satisfactory results.³
• Ideally, for an adequate test the patient should achieve 85% of maximum heart rate. Maximum heart rate is calculated as 220- age in years for men and 210-age for women.
• Beta blockers are usually stopped the day before if possible as they can prevent an adequate heart rate being achieved. Not everyone insists in this. Digoxin is stopped a week before as the effect on the ST segment can make interpretation difficult.

• The patient is connected to the exercise ECG machine and a standard resting ECG is performed.
• This is repeated with the patient standing as some changes such as T wave inversion can occur simply on standing.
• A record during hyperventilation may also be made to note changes purely from hyperventilation. During the test the machine provides a continuous record of the heart rate and the 12 lead ECG is recorded intermittently.
• Blood pressure is measured before starting and at the end of each stage of exercise.
  • Blood pressure may fall or be stable during the early stages.
  • Systolic blood pressure should increase as the exercise level rises. Up to 225 mm Hg is normal, although athletes can have higher levels.
  • Diastolic pressure usually falls slightly.
• The aim of the exercise is for the patient to achieve the target heart rate of 85% of maximum.

The incidence of serious complications including death or acute myocardial infarction is low if patients are adequately selected.

• Death or myocardial infarction occurs in about 1 in 10,000 tests or 0.01%.
• Ventricular tachycardia or ventricular fibrillation may occur in about 1 in 5,000.

Full CPR equipment must be present and test supervisors must be properly trained in resuscitation.

The following are contraindications to performing the test:

• Acute myocardial infarction in the previous 4 to 6 days
• Unstable angina with rest pain in the previous 48 hours
• Uncontrolled heart failure
• Acute myocarditis or pericarditis
• Acute systemic infection
• Deep vein thrombosis is likely to shift and cause pulmonary embolism
• Uncontrolled hypertension with a systolic blood pressure >220 mm Hg or diastolic >120 mm Hg
• Severe aortic stenosis can cause sudden death on exercise
• Severe hypertrophic obstructive cardiomyopathy
• Untreated life threatening arrhythmia
• Dissecting aneurysm
• Recent aortic surgery

In asymptomatic aortic stenosis, a limited ETT may be of value to reassure the patient that a certain degree of exercise can be tolerated but it cannot be used to assess coronary artery disease.⁴

• An abnormal ST segment response is when there is horizontal (planar) or down-sloping depression of >1 mm. Some people take 0.5mm or 2mm as their endpoint. The lower figure will have more sensitivity but less specificity whilst 2 mm of depression will have less sensitivity but more specificity.
• T wave elevation of >1 mm in leads without Q waves is also abnormal. It suggests severe coronary artery disease and poor prognosis.
• T wave changes such as inversion and pseudo-normalization when an inverted T wave becomes upright are non-specific changes.
• A highly specific sign of ischaemia is inversion of the U wave but U waves are often difficult to identify when present, especially at high heart rates and so this finding is not sensitive.
• Extrasystoles induced by exercise are of no significance for coronary artery disease.

Patients rarely exercise for the full 21 minutes of the Bruce protocol but completion of 9 to12 minutes of exercise or reaching 85% of the maximum predicted heart rate is usually satisfactory.
The test may be stopped for the following reasons:

ECG criteria

• Severe ST depression of >3 mm
• ST elevation >1 mm in non-Q wave lead
• Frequent ventricular extrasystoles unless the test is to assess a ventricular arrhythmia
• Onset of ventricular tachycardia
• New atrial fibrillation or supraventricular tachycardia
• Development of new bundle branch block
• Progression of heart block to second or third degree
• Cardiac arrest!

Symptoms and signs

• Patient too exhausted to continue
• Severe chest pain, dyspnoea, or dizziness
• Fall in systolic blood pressure beyond 20 mm Hg
• Rise in blood pressure above systolic of 300 mm Hg or diastolic of 130 mm Hg
• Unsteadiness
• The most common reason for stopping a test is fatigue and breathlessness as a result of the unaccustomed exercise.

Normal changes that are not a reason to stop the test include:

• P wave increases in height
• R wave decreases in height
• J point becomes depressed (the J point is an isoelectric point between the T wave and the next P)
• ST segment becomes sharply up-sloping
• Q-T interval shortens
• T wave decreases in height

The test is terminated when diagnostic criteria have been met or when the patient's condition prevents continuation. After the exercise has ended the recording continues for up to 15 minutes. ST segment changes or arrhythmias may occur during the recovery period even if they were not present during exercise. These changes usually carry the same significance as those occurring during exercise.

This is a non-invasive screening test rather than a "gold standard".
The following findings suggest high probability of coronary artery disease

• Horizontal ST segment depression of <2 mm
• Down-sloping ST segment depression
• Early positive findings within 6 minutes
• Persistence of ST depression for more than 6 minutes after stopping
• ST segment depression in 5 or more leads
• Hypotension with exercise

Limitations

ST segment depression has been noted in up to 20% of normal people on ambulatory ECG monitoring. There are many causes of ST segment changes to confuse the result of exercise testing apart from coronary artery disease . If the resting ECG is abnormal, the usefulness of an exercise test is reduced or it may not be justified.
Repolarisation and conduction abnormalities including left ventricular hypertrophy, left bundle branch block, pre-excitation and the effects of digoxin prevent accurate interpretation of the ECG during exercise. Other tests such as adenosine or dobutamine scintigraphy or angiography are required for these patients.

Diagnostic testing

Any abnormal ECG changes must be interpreted in the light of the probability of coronary artery disease and physiological response to exercise. A normal test result or one that indicates a low probability of coronary artery disease is where 85% of the maximum predicted heart rate is achieved with a physiological response in blood pressure and no associated ST segment depression. A high probability of coronary artery disease exists where there is substantial ST depression at low work rate with typical angina-like pain and a drop in blood pressure. Deeper and more widespread ST depression generally indicates more severe or extensive disease.
False positive results are common in women, reflecting the lower incidence of coronary artery disease in this group.

Prognostic testing

Exercise testing in patients who have just had a myocardial infarction is indicated only if a revascularisation procedure is contemplated. A less strenuous modified Bruce protocol is used. Testing provides prognostic information. Patients with low exercise capacity and hypotension induced by exercise have a poor prognosis. Asymptomatic ST depression after myocardial infarction is associated with a more than 10-fold increase in mortality compared with a normal exercise test.
Conversely, patients who reach stage 3 of a modified Bruce protocol with a blood pressure response of >30 mm Hg have an annual mortality of <2%. Exercise testing can also add prognostic information in patients after percutaneous transluminal coronary angiography (PCTA) or coronary artery bypass graft (CABG).
Drivers of LGVs and PCVs have to achieve test results clearly specified by the DVLA before they are considered fit to resume driving.

Screening

Exercise testing of asymptomatic patients is controversial because of the high false positive rate. Angina remains the most reliable indicator of the need for investigation. In some asymptomatic groups with particular occupations such as airline pilots there is a place for regular exercise testing, although more stringent criteria for an abnormal test result such as ST segment depression of >2 mm should be applied.
Part of the Framington study involved exercise testing of asymptomatic men and women.⁵ Among asymptomatic men, ST-segment depression, failure to reach target heart rate and exercise capacity during ETT provided prognostic information over and above the Framingham risk score, particularly among those at highest risk. The evaluation of the results in women was limited by the sample size and the few CHD events in women.
The testing of asymptomatic adults without high risk scores is controversial at best. An expert American group concluded that a wealth of data indicate that exercise testing can be used to assess and refine prognosis, particularly when emphasis is placed on measures other than ECG such as exercise capacity, chronotropic response, heart rate recovery, and ventricular ectopics.⁶ There are no RCTs on the clinical value of screening exercise testing. It is not known whether a strategy of routine screening exercise testing in selected subjects reduces the risk for premature mortality or major cardiac morbidity. A large-scale trial is required.

• An abnormal exercise test is followed by coronary angiography to assess the precise location and degree of any coronary stenosis. This may be followed by coronary artery bypass grafting or coronary angioplasty may be performed at the same time.
• If an inadequate test was performed this may be followed by a thallium scan. This can be very useful in the elderly⁷ where COPD or arthritis may prevent adequate exercise ability. In the elderly⁸ thallium scanning can produce slightly more reliable results than ETT.
• Stress echocardiography⁹ is also used at times.

There are three newer techniques that may be more widely used in the future.¹⁰ One is cardiac computerised tomography (CCT) but this provides an even higher dose of radiation that plain x-rays. Another is single photon emission computer tomography (SPECT) that gives a functional rather than an anatomical picture. The third and possibly the most important is cardiac MRI scanning. The advantage of MRI is that it gives a far more detailed 3-dimensional picture than x-rays and without the hazards of exposure to radiation.