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Exercise ECG Testing
What does it involve?
The individual is connected to the ECG electrodes and a standard 12 lead ECG is taken at
rest. Loose clothing and comfortable shoes are recommended. The subject then starts to
walk on a treadmill, initially at a gentle pace, and the heart is continually monitored.
Periodic blood pressure readings are taken. Gradually the speed and incline of the
treadmill are increased according to a standard protocol called the Bruce Protocol which is
widely used and has been thoroughly validated. For a diagnostic test, the subject should
achieve 85% of their maximum predicted heart rate. For men this is calculated as 220 –
age, and for women 210 – age.
Why is the ECG monitored during exercise?
Under conditions of stress, such as exercise, the heart muscle requires that more blood
and oxygen be supplied through the coronary arteries. If these arteries are diseased then
the heart can be partially starved of blood so that the heart muscle becomes starved of
blood or “ischaemic”. This can cause the chest pains known as Angina. Ischaemia leads to
typical changes on the ECG trace. An ECG test taken at rest can be normal even with
severe coronary artery disease, and exercise ECG testing is designed to reveal
abnormalities that would otherwise be missed.
How safe is it?
Complications are extremely rare if subjects are carefully selected. Heart attack or death
has a reported frequency of 1 in 10,000 cases (0.01%) and serious heart rhythm problems
1 in 5,000 cases. Although complications are rare, trained staff and resuscitation
equipment is always available to deal with emergencies.
Who shouldn’t have an Exercise ECG?
There are some medical conditions where exercise testing should be avoided. It is
important therefore that patients are assessed by an experienced clinician before
undergoing a treadmill test. In addition, some patients are not able to manage the treadmill
perhaps due to arthritis or other musculo-skeletal problems. Under these circumstances
alternative tests are more appropriate. Mobility problems particularly in the frail and elderly
can make the test undesirable. Abnormalities of the resting ECG such as Left Bundle
Branch Block can make the test very difficult to interpret.
What does the doctor look for?
A continuous ECG and regular blood pressure measurements are recorded and the
physician will look for both normal and abnormal changes induced by exercise. Abnormal
changes induced by ischaemia typically occur in the ST segments and T waves of the
ECG. However the physician will also assess exercise capacity, heart rate changes
(chronotropic response), and look for heart rhythm abnormalities during exercise. The
doctor will also monitor how the heart recovers once the exercise test is stopped.
When is the test stopped?
Once the maximum heart rate has been achieved without any adverse changes, typically
at about 9-12 minutes, the test may deemed negative and stopped. Similarly, once
diagnostic ECG changes have occurred, signifying a positive result, the test is usually
stopped. Other reasons for stopping the test early are a significant fall in blood pressure,
an excessive rise in blood pressure and symptoms such as dizziness, chest pain, and
more commonly, fatigue. If the exercise test is sub-optimal (for example if it has to be
stopped early before diagnostic criteria have been reached), the consultant will assess the
need for further investigations based on the likelihood of heart disease, the clinical history
and examination findings and the level of exercise achieved.
How useful is Exercise ECG testing?
For diagnosis. A negative test, where 85% of the predicted heart rate is achieved with no
diagnostic ECG changes and no fall in blood pressure indicates a low probability of
coronary artery disease.
A positive test where there are typical ST segment ECG changes with angina type chest
pain indicates a high probability of coronary disease. Under these circumstances further
investigation is usually indicated, see below.
For prognosis. Exercise testing can provide good prognostic information. Prognosis
describes the outlook for the individual in the future and the risk of further cardiac events,
in particular death, heart attacks or further angina pains. Patients who reach stage 3 of the
Bruce protocol with no ECG changes and no fall in blood pressure have a good prognosis.
Conversely, ST segment depression at low workload is associated with poor prognosis
and patients are advised to proceed to further investigations usually in the form of a
coronary angiogram, see below. Patients who have heart disease who drive heavy goods
vehicles (LGVs and PCVs) have to achieve results specified by the DVLA before being
allowed to resume driving.
Screening. Most studies of exercise testing have been population studies. Population
studies estimate the risk of certain groups of people based on their age, sex and risk
factors for coronary artery disease. However, they do not accurately determine the risk of
the individual. Exercise testing in people with no symptoms and minimal risk factors has
not been extensively studied. However there is evidence to suggest that ST segment
changes, poor exercise tolerance, failure to reach target heart rate, ventricular ectopy, and
poor recovery all provide evidence of risk over and above that of the expected cardiac risk
of the individual. Some people with occupations such as airline pilots, professional divers
and HGV drivers require regular testing even in the absence of symptoms.
What are the limitations of Exercise ECG testing?
Exercise testing, like most medical tests, is not 100% reliable. Exercise testing has a
sensitivity of approximately 78% and a specificity of approximately 70% in detecting
coronary artery disease. This means that it will fail to detect some people who have heart
disease, and diagnose some people who will ultimately prove not to have heart disease.
Bayes theorem of diagnostic probability can be applied to exercise testing. It states that
the predictive value of an abnormal test varies depending on the probability of the disease
in the population being studied. This means that the more likely that the disease is
present, for example where an individual has several risk factors, the more accurate the
test becomes. Young women with minimal risk factors are most likely to have false positive
results. This reflects the low incidence of coronary artery disease in this group. This
illustrates the importance of careful consideration by the specialist of whether the test is
suitable for the individual, and how the results should be interpreted in the context of the
subject’s existing risk.
What happens next?
The Exercise ECG result needs to be considered in the context of existing risk factors and
pre existing medical conditions. Where a test is positive, further investigation may be
recommended to assess the site and extent of any coronary artery disease. This is usually
achieved with a coronary angiogram. Ultimately, a revascularisation procedure may be
required to improve the blood supply to the heart muscle, relieve symptoms and reduce
the risk of future cardiac events. The revascularisation procedure might be a coronary
artery bypass grafting operation or “CABG” but often revascularisation can be achieved
through the less invasive percutaneous coronary intervention PCI. PCI is a “keyhole”
procedure that usually involves the implantation of a Stent. A Stent is a small, tubular
metal mesh a bit like the spring in a ball point pen. It is implanted into the artery through a
thin plastic tube called a catheter which is inserted via the arm or leg under local
anaesthetic. The Stent is used to hold open the narrowing in the artery thus restoring
blood flow to the heart muscle.
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