Treadmill exercise test (TMT)

Treadmill exercise test (TMT) or exercise stress test is one of the commonest forms of stress tests used to induce provocable myocardial ischemia for diagnostic evaluation of coronary artery disease. Myocardium is the muscle of the heart and ischemia is the technical term for reduced supply of oxygenated blood. Other forms of stress test are dobutamine stress echocardiography and adenosine stress test. In treadmill exercise test, the subject walks on a treadmill as per the protocol of the test. One of the commonest protocol used is known as Bruce protocol. Every stage of Bruce protocol consists of three minutes. The starting speed is 1.7 miles per hour (mph) and the gradient is 10%. 0.7-08 mph increment in speed and 2% increment in gradient is given for each consecutive stage. Gradient is the ratio between the length of the treadmill and the height to which it is slanting. If a 100 cm treadmill belt is used, a ten percent gradient would mean that the front end of the belt will be kept 10 cm higher than the back end to produce a slant. Usual Bruce protocol has upto 7 stages, but most often when it is done in a post myocardial infarction patient (person who has had a heart attack), upto 10 minutes of exercise is taken as sufficient.

The heart rate and blood pressure are monitored continuously along with 12 lead ECG (electrocardiogram: electrical signals arising from the heart) with computerised ST (a segment in the electrocardiogram) and arrhythmia (irregulaties in heart rhythm) detection algorithms. Test is stopped if the subject develops limiting angina (chest pain due to decreased blood supply to the heart muscle), syncope (fall due to giddiness or light headedness), breathlessness, fatigue or when the target heart rate is achieved. Significant ST depression of 3 or more millimeters is also an indication for termination of the treadmill test. Occasionally the test has to be terminated because of serious cardiac arrhythmia (life threatening irregular rhythm of the heart). A defibrillator (life saving electronic device used to give high voltage countershock to the heart to restore normal heart rhythm) and qualified personal and equipment for resuscitation should be available during the test.

Total time of exercise without causing any symptom or changes in the electrocardiogram would indicate the level of fitness of the individual. The peak heart rates and blood pressure attained are also important. Poorly conditioned persons may have a rapid increase in heart rate with each stage of exercise while a well trained athlete can have a slow increase in heart rate with progressive exercise.

Significant ST depression developing at lower workloads (less than 5 MET) especially if associated with typical symptoms is highly suggestive of inducible ischemia (decreased supply of oxygenated blood, usually due to blockages in the blood vessels supplying the heart known as coronary arteries). One MET or metabolic equivalent is the oxygen consumption at basal metabolic rate. Downsloping ST depression is most significant while horizontal ST depression is of lesser significance. Upsloping ST depression is of least significance. ST depression persisting more than 5 minutes into recovery is also highly significant. The leads in which ST depression occurs do not indicate the region of ischemia usually. But if ST elevation occurs, it is supposed to be indicative. ST elevation occurring in leads with Q waves (initial negative wave on the electrocardiogram) indicate viable myocardium (heart muscle) in the infarcted territory.