Exercise Tolerance Test (ETT)
Exercise tolerance test - aka Exercise ECG testing
- Confirm the diagnosis of angina
- Give an indication of the severity of CAD – thus allowing you to asses risk. This can be done either in a patient with CAD, or in somebody with CAD who has had a previous MI.
It is possible to have a normal resting ECG, even if there is considerable narrowing of the coronary arteries.
The test has:
- Specificity of 80%
- Sensitivity of 70%
The Bruce protocol was established in the 1960’s, and this says that:
- The patient should at an incline of 10% at 1.7mph. This is gradually increased – both the incline and the speed. The gradient is increased by 2& each time, and the speed by roughly 0.8mph.
- You continue to increase these until the patient reaches their target heart rate. The target rate is:
- Men – 220 – age
- Women – 210 – age
- The target heart rate is approximately 85% of the maximum heart rate – the whole aim of the test is to reach this target heart rate! – note that the changes in the ECG can appear during the resting period after exercise, eve if they didn’t appear in the ECG during the original exercise!
- Beta-blockers should be stopped the day before the test – as these can prevent the target heart rate being reached
- Digoxin should be stopped a week before the test – as this can alter the ST segment, and make interpretation for the test difficult.
- The Bruce protocol technically states that patients should exercise for 21 minutes, however they rarely exercise for this long. Normally the test is stopped once the target heart rate is reached.
Measurements during the test
- The patent is hooked up to a 12 lead ECG – a reading is taken before the test, and also recorded throughout the duration of the test
- Blood pressure should be taken at the beginning of the test, and at the beginning of each new stage of the test
- Systolic BP often rises – it is not unusual for it to go above 220mmHg
- Diastolic BP often falls slightly
- A normal test does not necessarily rule out CAD – however, if you have CAD and have a negative test, then your prognosis is still good
20% of those with a positive test result actually don’t have CAD. These kinds of results are much more common in young people, thus it is controversial to test young, asymptomatic patients.
- Also note that a disproportionately small amount of women are tested – and as well as this, women are more likely to have atypical symptoms of CAD.
Those who have a strongly positive test (which is ST depression within 6 minutes) and those most suitable for coronary angiography.
The test is probably most useful as a prognostic tool – and not as useful as a diagnostic tool. If you already know the diagnosis, then a positive test at a low workload is a poor prognostic sign.
- Death or MI occurs in 0.01% of patients. You should select patients carefully to minimise this risk
- VT or VF can occur in 1 in 0.02%
- Acute MI in the last 4-6 days
- Unstable angina – with pain at rest in the last 48 hours
- Uncontrolled heart failure
- Systemic infection , myocarditis, pericarditis
- Uncontrolled hypertension (systolic >220, diastolic >120)
- Severe aortic stenosis – this can cause sudden death!
- Recent aortic surgery
- ST depression – should be >1mm. The deeper the depression, and the lower the HR at which is occurs generally indicate the severity of the disease
- T wave elevation – where this occurs to >1mm and there are no Q waves (in that particular lead) then this can be a bad prognostic sign
Reasons to stop the test
- ST depression >3mm
- Target heart rate reached
- ST elevation of >1mm in a lead with no Q waves
- New atrial fibrillation
- Development of new BBB
- Cardiac arrest
- The most common reason the test is stopped is fatigue and breathlessness as the patient is unaccustomed to exercise!
- ST depression – Ischaemia
- ST elevation - Infarction