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Exercise ECG testing: Performing the test and interpreting the ECG results

Exercise ECG testing: Performing the test and interpreting the ECG results
Literature review current through: Jan 2024.
This topic last updated: Jan 16, 2020.

INTRODUCTION — The exercise electrocardiogram (ECG) is a well-validated procedure for establishing the diagnosis and prognosis of coronary heart disease, as well as assessing exercise capacity (ie, functional capacity). The exercise ECG indirectly detects myocardial ischemia, which is the physiologic consequence of a mismatch between myocardial oxygen delivery (coronary blood flow) and myocardial oxygen demand (myocardial work). (See "Approach to the patient with suspected angina pectoris".)

A well-recognized sequence of events is precipitated by an imbalance between myocardial oxygen supply and demand (figure 1). This sequence helps explain why measures of ischemia obtained with echocardiographic or radionuclide imaging are more sensitive indicators than ECG measures; they occur at a lower intensity of ischemia, and thereby precede ECG changes. Similarly, silent ischemia that is identified electrocardiographically often occurs at a lower ischemic threshold than that needed for the symptomatic expression of ischemia.

This topic provides a brief overview of exercise ECG testing including methodology, indications, and contraindications, and discusses the ECG manifestations of exercise-induced ischemia. A discussion on how to select the optimal cardiac stress test in a given patient is presented elsewhere. (See "Selecting the optimal cardiac stress test".)

INDICATIONS — There are many patients in whom exercise ECG testing can be used for diagnostic or prognostic purposes, including those with [1]:

Symptoms suggesting myocardial ischemia

Acute chest pain in whom acute coronary syndrome (ACS) and myocardial infarction have been excluded

Recent ACS treated without coronary angiography

Known coronary heart disease (CHD) and change in clinical status

Prior coronary revascularization

Abnormal or equivocal findings on coronary computed tomography angiography

Valvular heart disease

Certain cardiac arrhythmias

An indication for cardiac assessment prior to non-cardiac surgery

An extensive discussion of the indications for exercise ECG testing (and stress testing in general) is presented separately. (See "Selecting the optimal cardiac stress test", section on 'Indications for stress testing'.)

CONTRAINDICATIONS — Exercise testing is not without potential risks or discomfort to the patient, although the procedure is generally quite safe for most patients. Serious complications (ie, acute myocardial infarction, sudden cardiac death) occur approximately once for every 10,000 tests performed [2]. Thus, an assessment of the benefits and risks of testing must be made in each patient prior to the procedure. (See 'Life-threatening complications' below.)

Professional society guidelines have identified a collection of agreed upon absolute and relative cardiovascular contraindications to exercise testing (table 1) [3,4]. Absolute contraindications include the following:

Acute myocardial infarction (within two days)

Ongoing unstable angina

Uncontrolled arrhythmias with hemodynamic compromise

Symptomatic severe valvular stenosis

Decompensated heart failure

Active endocarditis

Acute myocarditis or pericarditis

Acute aortic dissection

Acute pulmonary embolism, pulmonary infarction, or deep venous thrombosis

Physical disability that precludes safe and adequate testing

There are also relative contraindications that can be superseded if the benefits outweigh the risks (table 1) [2,3]. Relative contraindications should be weighed versus the potential benefits in each individual patient prior to making the decision to proceed with exercise ECG testing. Exercise testing is generally safe in patients with mild to moderate hypokalemia or hyperkalemia [5].

There have been concerns about the safety of exercise testing in patients with a recent percutaneous coronary intervention. There were no acute coronary events in a report of 261 patients who underwent exercise testing soon after PCI with stent placement [6]. However, exercise testing is rarely necessary before 30 days after a stent procedure; when performed it is usually done to evaluate the hemodynamic severity of residual coronary heart disease.

LIMITATIONS TO EXERCISE ECG TESTING — There are two broad groups of patients in whom exercise ECG testing is not the most appropriate diagnostic test [3]:

Patients who are unable to exercise sufficiently due to leg claudication, arthritis, deconditioning, pulmonary disease, or other conditions – An exercise ECG test that fails to achieve 85 percent of the patient's predicted maximal heart rate may reduce sensitivity to detect ischemic heart disease even if the test is otherwise negative (ie, absence of symptoms of angina or normal ECG findings) [7]. There are potential problems with solely using the patient's predicted maximal heart rate, including significant variability in the predicted maximal heart rate and concurrent use of medications that can slow the heart rate, as well as excessive early heart rate response that can lead to the erroneous conclusion of having achieved an adequate exercise test. Patients who are unable to exercise sufficiently should be considered for pharmacologic stress testing (algorithm 1). (See 'Test endpoints' below and "Selecting the optimal cardiac stress test", section on 'Pharmacologic'.)

Patients with ECG changes at rest that can interfere with interpretation of the exercise test – ECG abnormalities that interfere with the diagnosis of ischemia include:

Ventricular preexcitation (Wolff-Parkinson-White pattern)

Ventricular paced rhythm

Left bundle branch block

Greater than 1 mm ST depression at rest

Digoxin use with associated ST-T abnormalities

Left ventricular hypertrophy with ST-T abnormalities

Hypokalemia with ST-T abnormalities

Conversely, ischemia can be accurately assessed in patients with right bundle branch block or those with less than 1 mm of ST depression at rest, who undergo diagnostic exercise ECG testing in the appropriate clinical setting. (See "Stress testing in patients with left bundle branch block or a paced ventricular rhythm".)

The sensitivity and specificity of exercise ECG testing have been derived from studies correlating the ECG response to exercise with angiographic CHD. Patient gender, age, coronary risk factors, and the characteristics of the chest pain are also important determinants of the pretest probability of CHD, which impact the post-test likelihood of disease, and therefore of the diagnostic accuracy of exercise ECG testing. These issues are discussed separately. (See "Stress testing for the diagnosis of obstructive coronary heart disease", section on 'Estimating pretest probability'.)

TYPES OF EXERCISE — The two most common modalities for clinical exercise testing are the motor-driven treadmill and the stationary cycle ergometer. While treadmill exercise is more commonly performed, the choice of exercise modality varies according to local expertise and available equipment. Cycle ergometry can be used in patients for whom weight bearing is problematic, and can be performed using a stationary bicycle or arm ergometry [2]. Bicycle ergometry is generally preferred to arm ergometry, as the smaller muscle groups involved in arm ergometry generally result in a lower maximum workload.

Most clinical indications for exercise testing require an incremental protocol, which progresses from low workloads to higher workloads until either a predetermined end point is reached (target heart rate or workload) or signs or symptoms develop that preclude further exercise. The most efficient protocols for treadmill or bicycle testing are patient specific as the protocol is chosen to match the physical working capacity of the individual so that a maximal effort is reached in 6 to 12 minutes. Protocols that are either too short or too long in duration may not truly reflect the patient's functional capacity.

Comparison of types of exercise — Each type of exercise has advantages and disadvantages, and many exercise testing laboratories in hospitals and clinics have two or three options available:

Treadmill exercise protocols are more flexible than bicycle protocols because both speed and inclination can be varied independently.

Bicycle and arm ergometry are generally less expensive, less noisy, more portable, and require less space.

The ECG data are cleaner, especially at high workload, with bicycle ergometry because of less upper body motion. Blood pressures are also more easily obtained during bicycle exercise, since the upper body is more stable.

Older, more-frail individuals, and those with gait, balance, and orthopedic abnormalities may perform better with bicycle or arm ergometry than on the treadmill. On the negative side, individuals who are not experienced cyclists may experience quadriceps muscle fatigue before achieving a maximal cardiovascular effort, and the smaller muscle groups involved in arm ergometry generally result in a considerably lower maximum workload.

Common exercise protocols — The specific speed and grade parameters for three standard treadmill testing protocols along with estimated oxygen costs for each workload are shown in the following table (table 2).

Bruce protocol — The Bruce protocol is generally preferred for office-based exercise testing largely because it has been extensively validated [8]. The protocol is divided into successive three-minute stages, each of which requires the patient to walk faster and at a steeper grade (table 2). The determinants of the end of the protocol are discussed below. (See 'Test endpoints' below.)

Modified Bruce protocol — The modified Bruce protocol can be used in sedentary patients in whom the standard Bruce protocol may be too strenuous or for risk stratification of patients after an acute coronary syndrome (ACS) in whom vigorous exercise in the immediate post-ACS period is not advised. The modified protocol adds two low-workload stages to the beginning of the standard Bruce protocol, both of which require less effort than Stage 1.

Naughton protocol — The Naughton protocol is often used in post-MI exercise testing to classify patients into high-risk and low-risk categories and to determine optimal treatment strategies [9]. This protocol is also used for functional exercise testing with gas analysis techniques to measure oxygen uptake and VO2max. (See "Cardiopulmonary exercise testing in cardiovascular disease" and "Exercise capacity and VO2 in heart failure".)

Other treadmill protocols — Other treadmill exercise protocols have been developed that provide a stress test intermediate in difficulty between the Bruce and Naughton protocols; a popular protocol uses two-minute stages and increases by 2 METs per stage and may be appropriate for a broader range of patients than either the Bruce or Naughton [10].

In the Cornell protocol, each stage of the Bruce protocol is divided into two smaller and shorter stages. Although this was done to provide more data points for the computerized ECG analyses, the protocol is also more applicable to patients with limited exercise tolerance because of the smaller workload increments. While once popular, this protocol is now rarely used.

Bicycle testing — For bicycle testing, exercise should begin with the patient pedaling against minimal resistance for several minutes. The work rate should then be increased in increments of 15 to 25 watts (90 to 150 kpm [kilopond meters/min]) every one to two minutes depending upon the subject's heart rate response; the goal is a duration of 8 to 12 minutes. Extremely sedentary or debilitated subjects may require even smaller increments.

EXERCISE TEST PROCEDURE

Pre-test instructions — Patients should be instructed not to eat, drink, or smoke for at least three hours prior to the examination, as this permits the patient to achieve a higher workload. The patients should bring comfortable exercise clothing and walking shoes to the testing facility. Before testing, the patient should be provided with education regarding the test, including the benefits and risks of exercise testing, as well as provide consent to proceed.

When the test is ordered, patients should be queried about the use of several categories of medications, including:

Beta blockers, non-dihydropyridine calcium channel blockers (ie, diltiazem and verapamil), and certain antiarrhythmic medications (eg, amiodarone, sotalol), as these drugs will reduce the maximal heart rate that is achieved.

Digoxin, which may also reduce the maximal heart rate that is achieved and is associated with reduced specificity for angiographic CHD. (See "Stress testing: The effect of medications and methylxanthines", section on 'Digoxin'.)

Nitrates, which can reduce the ischemic response to exercise in patients with coronary disease.

Patients should be given specific instructions by the ordering clinician on whether or not to take their usual medications. However, no consensus exists on withholding or continuing beta-blockers (or other medications which may reduce the ischemic response) before exercise testing. As an example, although beta blockers have been shown to markedly reduce maximal exercise heart rate, no differences in exercise test performance were found in a consecutive group of men taking versus not taking beta blockers while being evaluated for possible CHD [11]. Thus, in our practice, we do not have patients hold their usual medications for exercise testing, although this decision should be individualized. (See "Stress testing: The effect of medications and methylxanthines".)

Patient interview and examination — A brief interview by a clinician or qualified health professional should be performed prior to testing to evaluate for any contraindications (table 1) and to gather information that will facilitate interpreting the test. (See 'Contraindications' above.)

All patient medications must be identified, and it must be determined in advance if the patient has taken any cardiac medications that would alter the hemodynamic response to exercise and therefore potentially affect the results of the exercise ECG test. (See 'Pre-test instructions' above and "Stress testing: The effect of medications and methylxanthines".)

A limited cardiac examination should be performed, with attention given to detecting heart murmurs (particularly aortic stenosis), evidence of heart failure, and pulmonary findings such as wheezing. The presence of an arrhythmia, confirmed by the physical examination or the resting ECG, should be documented since it may have an impact on exercise. Important examples are atrial fibrillation or atrial flutter which, if not appropriately treated with an AV nodal blocking agent, may result in excessively high heart rates during exercise. Uncontrolled ventricular or supraventricular tachyarrhythmia is a contraindication to exercise testing (table 1). (See 'Contraindications' above.)

Establishing the target heart rate — In order to optimize the diagnostic utility of the exercise ECG test, patients should strive to achieve the target heart rate, generally defined as 85 percent of his or her predicted maximal heart rate. Failure to achieve this goal in the absence of chest discomfort or ECG evidence of ischemia reduces the diagnostic power of the test to rule out ischemic heart disease [7,12]. However, optimally, the exercise test should be symptom limited (see below) rather than stopped for achievement of a target heart rate, as in some patients, a symptom that is being investigated may not occur until a higher heart rate is reached.

The maximal HR decreases as a function of age, and traditionally has been predicted by the following equation [13]:

 Maximal HR = 220 – age (in years)

This formula is derived from populations, and an individual's maximum heart rate may vary by as much as 10 percent. For example, a meta-analysis of 18,712 subjects found that this equation underestimates the maximal heart rate in older subjects [14]. The following equation was more accurate for predicting maximal HR in healthy adults:

 Maximal HR = 208 – 0.7 x age (in years)

Conversely, maximal HR in women appears to be overestimated by the traditional formula of 220 – age (in years) [15,16].

In a study of 5437 asymptomatic women age 35 years or older without active cardiovascular disease and who were able to walk on a treadmill to a moderate level, the maximum heart rate was more accurately predicted by the following formula [15]:

 Maximal HR = 206 – 0.88 x age (in years)

In a retrospective cohort of 31,090 persons (49 percent female) age 40 years or older (mean 55 years) referred for exercise testing, the maximum heart rate in women was more accurately predicted by the following formula [16]:

Maximal HR = 197 – 0.8 x age (in years)

For simplicity and consistency, we suggest predicting the patient’s maximal HR using the equation 220 minus age (in years).

Electrocardiogram — A resting 12-lead ECG should be obtained and reviewed by qualified personnel before exercise begins. The resting ECG should be reviewed for any contraindications to exercise testing before proceeding with the test. Additionally, if any ECG abnormalities are identified that will impair the accurate assessment of ischemia, the test should be performed with the addition of an imaging modality (algorithm 1). (See 'Limitations to exercise ECG testing' above and "Selecting the optimal cardiac stress test".)

The resting ECG is usually obtained both supine and standing, since patient position can influence the QRS and T wave axes. Prior to the initiation of exercise, an additional resting ECG should be obtained following a simple modification of the standard 12-lead ECG where the arm and leg electrodes are moved to the torso. This is done to minimize the effect of motion artifact on the ECG.

It is important that the arm electrodes be placed at the base of the shoulder just inside the border of the deltoid muscles and 1 to 2 cm below the clavicles. More medially placed electrodes are associated with false positive and false negative diagnostic errors for myocardial infarction [17].

The leg electrodes should be positioned below the umbilicus and above the anterior superior iliac crest.

The precordial leads, V1 to V6, remain in their standard positions.

ECGs obtained during exercise should be compared with the resting standing ECG, while ECGs obtained during recovery should be compared with the resting ECG in the same position. (See 'Recovery period' below.)

During the exercise test, the patient may hold the handrails lightly to maintain balance but should be instructed not to grip the handrails tightly, as doing so reduces the workload experienced by the patient and may also introduce ECG artifact. Data should be obtained at the end of each stage and at any time an abnormality is detected on the monitor. Similarly, during recovery from exercise, the ECG should be recorded every two minutes for 7 to 10 minutes until the heart rate slows below 100 beats per minute or the ECG waveform returns to the control baseline pattern. In addition, continuous monitoring of the ECG waveform in selected leads should be performed throughout the exercise period and during recovery to assess cardiac rate, rhythm, and ST segment responses. Ventricular arrhythmias can occur during the recovery period, and are associated with an increased risk of death during follow-up [18].

The interpretation of the exercise ECG focuses on the presence or absence of ischemic ECG changes along with any induced arrhythmias. These abnormalities are discussed in greater detail below. (See 'ECG abnormalities during exercise' below.)

Blood pressure measurements — The blood pressure (BP) should be measured at rest (supine and standing) and during the last minute of each exercise stage. For ease of measurement, the arm should be straightened and the hand placed on the shoulder or in the axilla of the person taking the pressure. The systolic blood pressure should rise with each stage of exercise until peak is achieved, while the diastolic pressure falls or remains unchanged.

Symptoms and perceived exertion — The development of dyspnea out of proportion to the level of exercise or chest discomfort is an important finding, especially if the patient is undergoing diagnostic testing. Careful observation of the patient's facial expression and color, the ECG, and the blood pressure will usually enable the chest discomfort to be classified as anginal or nonanginal.

It is not necessary to stop exercise at the onset of chest discomfort if the intensity is mild, the blood pressure and heart rate are appropriate, and the ECG does not show significant ST segment abnormalities. Indications for terminating the test include increasing pain intensity, a fall in systolic pressure, marked ST segment depression or elevation, and increasing frequency of ventricular ectopic beats. (See 'Test endpoints' below.)

In addition to monitoring and recording the presence of chest discomfort or dyspnea, the patient's perceived level of exertion during the last five seconds of each exercise minute using defined scales, such as the rating of perceived exertion (RPE or Borg) scale (table 3) [3,19].

Test endpoints — The optimal duration of an individual exercise test is one that is carried out until the patient feels that he/she cannot exercise further. This is called a symptom-limited maximal exercise test. However, the decision to stop an exercise test can be patient-determined, protocol-determined, or provider-determined (table 4 and table 5) [3,19].

Patient-determined endpoints — The patient's request to stop should always be a serious consideration for terminating the test. If the request to stop is prompted by significant symptoms (ie, limiting chest pain or dyspnea, dizziness or lightheadedness, leg discomfort, fatigue, etc), the test should be promptly terminated. At times, however, when the patient seems poorly motivated or is not accustomed to exercise and is clearly in no distress, the supervising health professional may encourage the patient to continue exercising until more limiting signs or symptoms are noted; this may sometimes be accomplished by adjusting the speed and incline of the treadmill to allow the patient to continue.

Provider-determined endpoints — In addition to patient-related symptoms, the provider may decide to terminate the test based on a variety of patient- or ECG-derived factors (table 4). Though patients may not volunteer symptom, if the provider determines that the patient does not look well (eg, unsteady gait, confusion, pallor, etc) the test should be stopped. Significant exercise-induced hypertension and exercise-induced hypotension are other patient-related signs that would prompt termination of the test.

The provider may also decide to stop the exercise test based on the presence of new significant findings on the ECG (table 4), including the presence of:

Marked ST depression in the absence of Q waves in the same lead

New bundle branch block which cannot be distinguished from ventricular tachycardia (VT)

New high grade (ie, Mobitz 2 second degree or complete) AV block

Sustained VT or ventricular fibrillation

Increasing frequency of ventricular ectopy

Onset of supraventricular tachyarrhythmia

ECG endpoints may be absolute or relative. Clinical judgment, experience, and knowing the patient are all important considerations in deciding when to stop.

Protocol-determined endpoints — While protocol-determined end points have been defined, most exercise testing should continue until the patient experiences symptoms, achieves his or her maximal level of exertion, or has another significant provider-determined end point. The primary exception to this is for submaximal exercise testing. (See 'Submaximal testing' below.)

Although exercise testing may be terminated when the patient achieves a certain percentage (usually 85 to 90 percent) of his or her predicted maximal heart rate, there are potential problems with using attainment of a percent of maximal heart rate as a test end point. These include:

Inter-patient variability in maximal heart rate

Concurrent use of beta blockers or other drugs that can slow the heart rate

Excessive early heart rate response leading to early attainment of the maximal heart rate prior to symptom development or maximal exercise capacity

In most circumstances the test should not be stopped just because the patient has achieved maximal particular heart rate, and exercise should continue until the patient has reached his or her maximal capability or another clinically important end point (table 5) [3].

Submaximal testing — Stable patients with an acute coronary syndrome (myocardial infarction or unstable angina) may undergo a submaximal exercise test prior to discharge unless they have undergone percutaneous coronary intervention or coronary artery bypass graft surgery and been fully revascularized (eg, single vessel disease successfully treated with percutaneous coronary intervention). (See "Risk stratification after acute ST-elevation myocardial infarction" and "Risk stratification after non-ST elevation acute coronary syndrome".)

Submaximal exercise testing most commonly uses one of the following end points for termination of the test:

A peak heart rate of 120 to 130 beats per minute or 70 percent of the maximal predicted heart rate for age

A peak work level of 5 METs

Mild angina or dyspnea

≥2 mm of ST segment depression

Exertional hypotension

Three or more consecutive ventricular premature beats

Recovery period — The ECG should be recorded after a brief cool-down (15 to 30 seconds), while the patient is still on the treadmill or sitting on the bicycle. An exception to this is exercise echocardiography, in which the echocardiographic images are obtained immediately after peak exercise, without any recovery exercise.

If significant ECG abnormalities did not develop during exercise, and the test is being done to diagnose ischemia, the patient should return to the supine position for the remainder of the recovery period. The increased venous return in the supine position may precipitate ischemic abnormalities not seen when upright on the treadmill. ST segment changes limited to the recovery period are as predictive of underlying coronary disease as changes seen during exercise [20,21].

If, however, the patient develops ischemic ECG abnormalities during exercise, it may be safer to have the patient sit during recovery to minimize the risk of increasing ischemia and ventricular arrhythmias. (See 'ECG abnormalities during recovery' below.)

The ECG should be recorded every two minutes for 7 to 10 minutes until the heart rate falls below 100 beats per minute or the ECG waveform returns to the control baseline pattern. These ECGs should be compared with the resting ECG in the same position (supine or standing). In addition, continuous monitoring of the ECG waveform in selected leads should be performed during recovery to assess cardiac rate, rhythm, and ST segment responses. Ventricular arrhythmias can occur during the recovery period, and are associated with an increased risk of death during follow-up [18].

Life-threatening complications — Although rare, exercise testing can be associated with serious complications. The frequency of serious adverse cardiac events (ie, myocardial infarction, sustained ventricular arrhythmia, and death) has been estimated to be approximately 1 in 10,000 patients [2].

In an effort to minimize the risk of any complications, and to have the capacity to diagnose and manage life-threatening complications, there are several requirements and protocols that should be followed (table 6). Additionally, periodic in-service training should be given to the exercise testing staff to review emergency treatment procedures, so that any serious or life-threatening complications can be recognized early and managed appropriately.

Exercise test summary report — At the conclusion of an exercise test, a summary report should be prepared by the provider supervising the exercise test and reviewed by the supervising clinician (if the test is performed by a different provider). The standard report should contain a minimum of the following:

Patient name

Referring provider

Indication for test

Date performed

Protocol performed

Duration of exercise

Baseline and maximum heart rate, blood pressure, and workload achieved

Description of abnormal blood pressure response if present

Symptoms

Reason(s) for stopping the test

ECG changes and/or arrhythmias – peak ST segment depression

Functional capacity (percentile rating based on estimated VO2max, or "above," "average," "below average" function)

Additional information, such as the Duke treadmill score, may also be reported. Information about how the workload achieved compares with that expected for age and sex is also useful. (See "Prognostic features of stress testing in patients with known or suspected coronary disease", section on 'Duke treadmill score'.)

Ideally, the results should be summarized as "normal" or "abnormal" as opposed to "positive" or "negative" to convey the most information to the ordering provider [2].The summary report should then be placed in the patient's medical record, with an additional copy sent to the referring clinician.

ECG ABNORMALITIES DURING EXERCISE — The most important ischemic ECG changes during exercise are related to shifts in the ST segment. However, the full spectrum of the ischemic ECG involves changes in the ST segment, the T wave, and the U wave (figure 2). These ischemic changes can be detected reliably only in patients who do not have baseline ECG abnormalities that can interfere with interpretation. (See 'Limitations to exercise ECG testing' above.)

Electrophysiology of myocardial ischemia — The electrophysiologic consequences of myocardial ischemia are responsible for the typical ST-T wave abnormalities seen on the ECG during exercise testing in patients with coronary heart disease. Reviewed briefly, the QRS complex reflects the time sequence of ventricular depolarization as the activation front spreads through the myocardium (phase 0 of the action potential) (figure 3). The ST segment coincides with the plateau or phase 2 of ventricular repolarization, and the T wave reflects the time sequence of phase 3 repolarization. Phase 4, the resting membrane potential, is responsible for the TQ segment; this segment of the ECG is usually not considered to be important, but actually has a prominent role in the ECG manifestations of ischemia [22,23]. (See 'ST segment depression' below and "Cardiac excitability, mechanisms of arrhythmia, and action of antiarrhythmic drugs".)

Action potentials become markedly altered during localized myocardial ischemia. A depiction of the ischemic alterations in typical action potentials, recorded from subepicardial and ischemic subendocardial regions of the left ventricle, is shown in the figure (figure 4). This is the usual clinical setting of exercise-induced ischemia, since the subendocardial regions supplied by critical coronary lesions are most vulnerable.

ST segment depression — ST segment depression that occurs during exercise testing is one of the most identifiable ECG signs of myocardial ischemia. The ECG portion of the exercise test is generally considered abnormal (ie, "positive" for ischemia) when there is ≥1 mm horizontal or downsloping ST segment depression in one or more leads that persists at 80 milliseconds after the J point.

Subendocardial ischemia during exercise produces ST segment depression and/or elevation. The progression of ST segment depression is as follows:

Upsloping ST depression appears first during exercise and is less specific for ischemia than horizontal ST depression (figure 2) [24].

Horizontal or downsloping ST depression of ≥1 mm (waveform 1), often associated with T wave inversion, may be seen during exercise but is most often seen during recovery from exercise. Horizontal or downsloping ST depression of ≥1 mm is more specific for ischemia [24].

The sensitivity and specificity of ST depression for the diagnosis of myocardial ischemia and obstructive coronary heart disease vary with the criterion used to define an abnormal test. Test results that absolutely distinguish those with and without disease (ie, high specificity) are often not present in many patients with disease (low sensitivity). As an example, if a "positive" ECG result were defined as 3 mm ST segment depression rather than the usual criterion of 1 mm, the probability would be very high that all individuals without disease will have negative tests. Although this approach maximizes the specificity of the stress test, it substantially reduces sensitivity since very few patients with coronary heart disease will be detected. Conversely, the application of less stringent criteria for detection of disease would increase sensitivity but decrease specificity.

Leads V4, V5, and V6 are the most sensitive leads for detecting the ST depression of subendocardial ischemia; V5 alone is often the single best lead for this purpose [25,26]. However, ST depression in the lateral precordial leads does not localize the coronary artery responsible for the ischemic change in patients with normal resting ECGs [27]. In addition, ST segment depression confined to the inferior leads II and aVF is most often a false positive response, and is of little value in diagnosing coronary disease [25]. However, greater ST segment depression involving multiple leads usually signifies extensive myocardial ischemia [27].

Upsloping ST segment depression — Depression of the J point (end of the QRS and beginning of the ST segment) is normally seen with increasing heart rates during exercise (figure 2); it is probably related to atrial repolarization that extends through the QRS rather than myocardial ischemia. In this circumstance, the ST segment rapidly (within 80 milliseconds) returns to baseline.

There are conflicting data on the predictive value of slow rising (upsloping) ST depression for the diagnosis of myocardial ischemia and obstructive coronary heart disease. Initial studies suggested that patients with this finding had an increased likelihood of coronary heart disease [28,29]. However, in a subsequent report of 1083 asymptomatic patients in whom upsloping ST depression with exercise occurred in 11.4 percent, the prognosis in these patients was not different from those with a normal exercise ECG [30].

While the incorporation of upsloping ST segment depression increases sensitivity, it also reduces specificity for the diagnosis of myocardial ischemia and obstructive coronary heart disease. We agree with the ACC/AHA guidelines, which propose the use of ≥1 mm of horizontal or downsloping ST segment depression as the criterion for an abnormal test in the setting of a normal resting ECG [3].

Horizontal or downsloping ST segment depression — Horizontal or downsloping ST segment depression of ≥1 mm is generally highly specific for ischemia. In a meta-analysis of 147 studies involving 24,074 patients who underwent both exercise ECG testing and coronary angiography, the mean sensitivity and specificity for the diagnosis of obstructive coronary heart disease were 68 percent (range 23 to 100 percent) and 77 percent (range 17 to 100 percent), respectively [24]. This wide range reflected different criteria for an abnormal exercise ECG test; however, when the criteria for an abnormal exercise ECG test were defined as ≥1 mm horizontal or downsloping ST segment depression, the mean sensitivity and specificity were approximately 60 and 90 percent, respectively.

The development of horizontal or downsloping ST depression after exercise also has prognostic significance. (See "Prognostic features of stress testing in patients with known or suspected coronary disease", section on 'Flat or downsloping ST depression'.)

Resting ST segment depression — ST segment depression during exercise is not a reliable indicator of ischemia when the resting baseline ECG has ST segment depression due to a variety of abnormalities (eg, left bundle branch block, left ventricular hypertrophy, digoxin therapy). (See 'Limitations to exercise ECG testing' above.)

However, ST segment depression on a resting ECG may be seen in patients with coronary disease in the absence of these factors. In such patients, exercise ECG testing appears to have similar overall diagnostic value, with a higher sensitivity and lower specificity for the diagnosis of obstructive coronary heart disease (when compared with persons without resting ST segment depression).

Exacerbation of minor degrees of resting ST depression during exercise also has prognostic value. This was illustrated in the above review of 1083 volunteers who were apparently free from coronary disease [30]. Minor resting ST depression (<0.5 mm) that intensified to ≥1 mm of flat or downsloping ST depression during or after exercise was an independent predictor of subsequent coronary risk (relative risk 2.73). (See "Prognostic features of stress testing in patients with known or suspected coronary disease".)

ST segment elevation — Exercise-induced ST segment elevation is uncommon except in leads with a pre-existing Q wave suggesting prior transmural myocardial infarction. ST elevation in such patients is typically due to a wall motion abnormality rather than ischemia [31]. An exception occurs in patients with single vessel disease in which ST segment elevation is associated with reciprocal depression in the noninfarcted area; these changes are indicative of residual viability in the infarct-related area [32].

ST segment elevation in patients with normal resting ECGs is suggestive of transmural ischemia and can occur in two groups of patients:

Patients with "variant angina" (Prinzmetal's angina) are characterized by episodic chest pain occurring mostly at rest, typically associated with ST segment elevation, triggered by coronary artery spasm. Exercise-induced ST elevation occurs in 10 to 30 percent of these patients [33,34]. (See "Vasospastic angina".)

Patients with severe and often multivessel coronary heart disease. These patients may develop transmural ischemia because of a marked decrease in coronary blood flow to a segment of myocardium during exercise. In contrast to ST depression, the leads showing ST elevation in these patients localize the coronary artery responsible for the ischemia. This difference was demonstrated in a study of 452 patients with single vessel coronary disease undergoing exercise testing [26]. ST depression occurred most commonly in leads V5 or V6 regardless of which coronary artery was involved. In contrast, anterior ST elevation indicated left anterior descending (LAD) coronary disease in 93 percent of cases, and inferior ST elevation indicated a lesion in or proximal to the posterior descending artery in 86 percent of cases.

Exercise-induced ST segment elevation in leads without pre-existing Q waves and not in aVR may be arrhythmogenic.

ST segment elevation in aVR — The movement of the ST segment in lead aVR is generally in the opposite direction of the ST segment in other standard leads. ST segment changes occurring in lead aVR have often been overlooked in the assessment of ischemic heart disease. In a study of 454 patients who underwent exercise stress testing and then had coronary angiography within six months, 75 patients were found to have severe obstructive coronary artery disease of the left main coronary artery or the ostium of the LAD [35]. Following multivariate analysis, ST segment elevation in lead aVR was the strongest predictor of obstructive coronary artery disease involving the left main coronary artery or the ostium of the LAD (sensitivity and specificity of 75 and 81 percent, respectively).

Ectopy — Exercise-induced ventricular ectopy occurs in 7 to 20 percent of patients undergoing exercise ECG testing for known or suspected coronary heart disease. Most studies have noted an association between exercise-induced ventricular arrhythmia and increased mortality risk that may be limited to frequent ventricular ectopy during recovery. (See 'Frequent ventricular ectopy' below and "Prognostic features of stress testing in patients with known or suspected coronary disease", section on 'Ventricular arrhythmias'.)

Atrial ectopy is also frequent during exercise testing, but does not appear to be an independent predictor of adverse outcome [36]. (See "Prognostic features of stress testing in patients with known or suspected coronary disease", section on 'Atrial arrhythmias'.)

Bundle branch block — In patients with baseline right bundle branch block (RBBB) and upright ST segments and T waves in leads V5-6, exercise-induced ≥0.1 mV horizontal or downsloping ST segment depression detected in these leads is a generally accepted indicator of myocardial ischemia. Left bundle branch block (LBBB) on the baseline ECG masks the ability to identify ischemia during exercise because of the associated ST and T wave abnormalities. Stress testing in patients with LBBB or with a paced ventricular rhythm is discussed separately. (See "Stress testing in patients with left bundle branch block or a paced ventricular rhythm".)

The development of BBB during exercise, though not common, may be a marker of increased coronary heart disease (CHD) risk or an indicator of underlying conduction system disease.

In a review of 17,277 exercise tests, transient LBBB was noted in 70 (0.4 percent): 40 patients who developed LBBB had a history of CHD and, among the 43 patients in whom angiography was performed, 35 had significant CHD [37]. The development of LBBB was an independent predictor of death and major cardiac events at four years (adjusted relative risk 2.78 compared with 70 matched controls).

The development of bundle branch block, either RBBB or LBBB, may be rate-related. This often represents underlying conduction system disease, but is often misdiagnosed as ventricular tachycardia. (See "Left bundle branch block", section on 'Functional LBBB' and "Right bundle branch block", section on 'Functional RBBB'.)

ECG ABNORMALITIES DURING RECOVERY — In addition to the changes that are induced by ischemia during exercise, certain changes that occur during recovery also have diagnostic and prognostic significance.

ST segment depression — A minority of patients (approximately 8 percent in two studies) develop ST segment depression during recovery rather than during exercise; this change is a predictor of a greater likelihood of coronary heart disease and may have a prognostic significance similar to similar changes occurring during exercise, even in apparently healthy individuals [20,21]. In a review of 328 men who underwent a symptom-limited treadmill test and coronary angiography, abnormal ST segment responses were noted in 168 and occurred only during recovery in 26 [20]. The positive predictive value for significant angiographic coronary artery disease was similar when ST depression occurred during recovery or exercise (84 versus 87 percent). Furthermore, inclusion of ST depression during recovery significantly increased the sensitivity of the exercise test (from 50 to 59 percent) without change in predictive value.

Frequent ventricular ectopy — Exercise-induced ventricular arrhythmia that occurs during recovery is associated with increased risk of mortality. In a review of almost 30,000 patients referred to symptom-limited exercise testing who did not have a history of heart failure, arrhythmia, or valve disease, frequent ventricular ectopy occurred only during exercise in 3 percent, only during recovery in 2 percent, and in both time periods in 2 percent [18]. At a mean 5.3-year follow-up, only frequent ventricular ectopy occurring during recovery was associated with an increased risk of mortality after adjusting for confounding variables. (See "Prognostic features of stress testing in patients with known or suspected coronary disease", section on 'Ventricular arrhythmias'.)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topics (see "Patient education: ECG and stress test (The Basics)")

SUMMARY AND RECOMMENDATIONS

The exercise electrocardiogram (ECG) is a well-validated procedure for establishing the diagnosis and prognosis of coronary heart disease, as well as assessing exercise capacity (ie, functional capacity). The exercise ECG indirectly detects myocardial ischemia, which is the physiologic consequence of a mismatch between the supply and demand for blood. (See 'Introduction' above.)

Exercise ECG testing can safely be used for diagnostic or prognostic purposes in a variety of patients. However, there are some absolute and relative contraindications to exercise testing that should be considered before proceeding with the test. (See 'Indications' above and "Selecting the optimal cardiac stress test", section on 'Indications for stress testing' and 'Contraindications' above.)

There are two broad groups of patients in whom exercise ECG testing is not the most appropriate diagnostic test: patients who are unable to exercise sufficiently due to leg claudication, arthritis, deconditioning, pulmonary disease, or other conditions; and patients with ECG findings at rest that can interfere with interpretation of the exercise test. (See 'Limitations to exercise ECG testing' above.)

The procedure for performing exercise ECG testing should include appropriate pre-test preparation and instructions at the time the test is ordered as well as a brief patient interview and targeted physical examination prior to exercise. The test should be supervised by appropriate personnel, with ongoing monitoring of heart rate, blood pressure, and ECG, and frequent queries of the patient regarding his or her perceived patient exertion and the development of any symptoms. (See 'Exercise test procedure' above.)

ST segment depression that occurs during exercise testing is one of the most identifiable ECG signs of myocardial ischemia. The ECG portion of the exercise test is generally considered abnormal (ie, "positive" for ischemia) when there is ≥1 mm horizontal or downsloping ST segment depression in one or more leads that persists at 80 milliseconds after the J point. Horizontal or downsloping ST segment depression is generally more specific for ischemia than upsloping ST segment depression. (See 'ST segment depression' above.)

Exercise-induced ventricular ectopy occurs in 7 to 20 percent of patients undergoing exercise ECG testing for known or suspected coronary heart disease. Most studies have noted an association between exercise-induced ventricular arrhythmia and increased mortality risk that may be limited to frequent ventricular ectopy during recovery. (See 'Ectopy' above and 'Frequent ventricular ectopy' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Frank Yanowitz, MD, who contributed to an earlier version of this topic review.

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References

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