Stressing out the heart

MANY patients with coronary artery disease (CAD), also known as ischaemic heart, may have no symptoms, and when investigated, they have a normal electrocardiogram (ECG) and chest X-ray at rest.

However, with exercise, like climbing up stairs or slopes, symptoms of ischaemia (oxygen starvation) like chest tightness or breathlessness may develop. This would raise the suspicion of the presence of a severe stenosis (narrowing/blockage) in a coronary artery.

An exercise stress test can reveal this in a controlled clinic setting, either through exercise on a treadmill or stationery bicycle, or it can also be artificially induced with stimulant drugs to achieve the desired physical burden on the heart.

The most popular treadmill stress test protocol is the Bruce protocol. It is made up of progressive three minutely incremental treadmill speeds combined with increasing inclination angles.

Baseline observations are made and the ECG is recorded and monitored throughout the protocol. The patient’s BP is checked periodically to make sure that it is neither too high (which poses a hazard) nor too low (which might indicate severe underlying CAD or poor heart function).

The patient’s symptoms are monitored closely, in particular, the point at which fatigue or chest pain begins and becomes limiting.

The onset of angina (dull central chest tightness or breathlessness) with ECG evidence may suggest regional blood supply deficiency to a part of the heart (ischaemia). The stress-induced appearance of extra electrical beats (ventricular ectopics) would also raise concerns about the presence of disease.

All changes should then normalise with a resolution of symptoms during the rest phase.

A test that reveals any suspicious abnormalities is called positive and a test that is clearly normal is called negative. A test which has only mild or unconvincing ECG changes is usually deemed equivocal and requires more sophisticated tests for clarification.

When a patient is unable to complete the required level of exercise due to lack of fitness or some other physical disability, the test is usually deemed inconclusive and may require an alternative investigation.

There are several indications for stopping a stress test apart from the patient’s symptoms, which are more technical and only meaningful to the clinician present, e.g. severe ST segment depression (>3 mm) i.e. severe ischaemia, and ST upwards deflection (ST elevation), a rare and serious sign of heart rhythm problems.

Usually, after exercise, monitoring and recording is continued for between five and 15 minutes depending on the speed of recovery and the patients overall well-being.

The sensitivity of the exercise stress test (ability to rule out the presence of a narrowing) is about 75%. Thus, in 75% of patients whose test is negative (normal), the disease can be safely ruled out. By the same token, about 25% of people with a normal stress test may have a stenosis (narrowing) that may have been missed.

The specificity of a treadmill stress test is 70%. Thus, 70% of patients with an abnormal test are likely to have a stenosis and 30% with normal arteries will falsely be labelled as having significant narrowing. This is a problem encountered more frequently in women.

Thus, the diagnosis of coronary artery disease should not be based solely on this test result alone. An evaluation of the patient’s symptoms and background risk profile is of greater importance. To elaborate on the details of this evaluation would be beyond the scope of this article. In a nutshell, an individual with multiple cardiac risk factors (e.g. male, diabetes, family history, smoking, hypertension and high cholesterol) with classical heart pain (exercise-induced chest discomfort which is relieved by rest) is very likely to have significant coronary disease and is unlikely to benefit from the result of the stress test to make the diagnosis.

On the other hand, an individual with no risk factors and non-cardiac sounding pain is most unlikely to have coronary disease and hence a stress test is not recommended as there is considerable potential for a false diagnosis of coronary artery disease.

Routine stress testing in people with no symptoms (with the possible exception of diabetics who may not be able to sense chest pain) is not recommended.

The limited sensitivity and specificity of the test have led doctors to look for other techniques to improve the accuracy of stress testing by assessing other parameters that occur during ischaemia. Also, certain patients have abnormal rest ECGs that make it impossible to assess the ECG changes during exercise.

Stress echocardiography is the most popular alternative to the treadmill stress exercise method and involves obtaining baseline ultrasound images of the heart at rest. This allows evaluation of features that may change beyond the ECG alone.

Attention is given to the size, movement of the walls of the heart, and contraction. The set of scans is then repeated promptly at the peak/end of stress when ischaemia should manifest so that the cardiologist can look for signs of reduced wall movement or reduced contraction.

The presence of abnormalities detected on the scans that were not obvious at rest increases the likelihood of the presence of a severe narrowing in the coronary artery supplying that territory. Accompanying ECG changes will further strengthen the diagnosis. This technique has a predictive accuracy of about 85%

Dobutamine is the most common chemical agent used when “stressing” the heart without requiring the patient to exercise. It is particularly useful if a patient is not physically able to exercise (e.g. previous stroke, arthritis) and can be used as an alternative to stress echocardiography.

A chemical agent is injected into a vein at progressively increasing rates to mimic exercise. In patients with previous heart attacks, this technique of heart stimulation allows continuous scanning of the heart and may be able to predict whether nonfunctional areas may potentially improve once normal blood flow is restored. This information can be used to decide whether procedures to improve heart blood supply like angioplasty or bypass surgery will be able to improve the heart function.

All these investigative methods are very safe when conducted on appropriately selected individuals with proper supervision. The rate of death or heart attack is about one in 10,000 tests (0.01%). Full resuscitation facilities should be available in all stress test settings and informed consent should be given prior to all such tests.

Ideally one should be fasted three to four hours prior to the test to reduce the likelihood of nausea that may occur with heavy exertion after a heavy meal. The entire period of preparation and recovery should not take more than an hour with treadmill-based protocols and two hours with a chemical stress test.

Comfortable clothing and appropriate footwear would obviously be practical for a treadmill test. Certain drugs like beta-blockers and digoxin should be discontinued prior to a stress test (based on the physician’s advice) because they may affect the heartbeat response to stress and produce false ECG changes.

In appropriately selected patients after a good assessment of the background risk and nature of symptoms, stress testing is a modestly effective tool in identifying the presence of underlying coronary artery disease, but more importantly, an effective tool to rule out severe/life-threatening disease.