Stress Testing

Dynamic

Dynamic exercise is the modality of choice in myocardial perfusion imaging as image quality is usually superior and addition information is acquired about the patient general fitness.

The two common forms of exercise are treadmill and bicycle.
Patients are advised to stop medication that may interfere with heart rate response during exercise e.g. beta blockers.

Treadmill exercise is usually performed using the Bruce protocol where speed and elevation of the treadmill are increased at 3 minute intervals.

Bicycle exercise is an alternative. A typical protocol will involve the patient pedalling at a constant 50rpm with the workload increasing in steps of 25W every 3 minutes.

Blood pressure and 12 ECG are monitored during the stress test and in the recovery phase.

The aim is to achieve at least 85% of the target heart rate (220 minus patient’s age). Although the test may be terminated earlier if there are symptoms or changes in blood pressure / ECG.

The radioisotope is injected at peak stress and exercise is carried on for at least another 1 minute post tracer injection.

Vasodilator

When the patient is unable to perform dynamic exercise, vasodilators are the pharmacological agent of choice. It causes flow heterogeneity between areas with normal blood supply and that of stenosis. Rarely it may induce ischaemia due to steal phenomena.

Vasodilator stress is also preferred in patients with un interpretable ECG e.g. left bundle branch block or cardiac pacemaker.

Patients need to be caffeine-free for at least 24 hours prior to vasodilator stress.

The vasodilators commonly used are regadenoson, adenosine and dipyridamole, all of which are administered intravenously.

Regadenoson is administered as single dose while the dose of adenosine and dipyridamole is derived from the patient’s weight.

ECG and blood pressure are monitored during and after vasodilator administration.

Regadenoson is given as a single IV injection of 400 micrograms in 5mL over at least 10 seconds. This is followed by a 10mL saline flush and injection of the tracer 20 to 30 seconds later. Regadenoson injection can be combined with low-level exercise. This can help improve test tolerability and scan quality.

Regadenoson is also commonly used to “rescue” dynamic exercise tests, which would otherwise be suboptimal.

Common side effects of regadenoson are breathlessness, abdominal discomfort and headache. They are short-lasting although addominal symptoms and headache may time up to 30 minutes to resolve completely.

Adenosine is usually given as an infusion over 6 minutes although an abbreviated 5-minute protocol has been found to be equally effective. The tracer is injected between the third and fourth minute, and the infusion is continued for at least a further 2 minutes post-tracer injection. Low level dynamic exercise can be used in conjunction with the adenosine infusion.

Common side effects of adenosine are breathlessness, chest discomfort and flushing. These resolve within a few minutes after discontinuation of the infusion, and therefore aminophylline is seldom used to treat adenosine-induced symptoms.

Dipyridamole is given as an infusion over 4 minutes. Three to four minutes after the end of the infusion the radioisotope is given. Low-level exercise can also be used following dipyridamole.

Dipyridamole side effects are similar to those of adenosine. Side effects are less frequent but tend to last longer and therefore aminophylline is commonly administered after tracer injection to alleviate symptoms.

Inotropic

When the patient is unable to undergo dynamic exercise testing and has a contraindication to vasodilator stress, inotropic stress may be considered.

Dobutamine is used as the stress agent the dose is calculated from the patient’s weight.

Dobutamine is given as an infusion with the infusion rate increasing at 3 minute intervals until target heart rate is achieved. Sometimes atropine is given to augment the effects of the dobutamine. The radioisotope is given when target heart rate is reached.

Common side effects of dobutamine are nausea and hypotension.