An event monitor is a portable device used to record your heart’s electrical activity when you have symptoms. It records the same information as an electrocardiogram (ECG), but for longer durations of time. Most of these devices can transmit the recorded information directly to your healthcare provider. This allows him or her to analyze the electrical activity of your heart while you are having symptoms.
Normally, a special group of cells begin the electrical signal to start your heartbeat. These cells are in the sinoatrial (SA) node. This node is in the right atrium, the upper right chamber of the heart. The signal quickly travels down the heart’s conducting system on the way to the ventricles, the two lower chambers of the heart. As it travels, the signal triggers nearby parts of the heart to contract. This helps the heart contract in a coordinated way.
ECGs and event monitors are used to help analyze this electrical signaling through the heart. These tests are very helpful in diagnosing a variety of abnormal heart rhythms and medical conditions. A standard ECG only records the heart signal for a few seconds, and it is not portable.
An event monitor is very similar to something called a Holter monitor. This is another portable device used to analyze the heart’s signaling. Holter monitors record continuously, usually for about 24 to 48 hours. An event monitor does not record continuously. Instead, it records when you activate it. Some event monitors will automatically start recording if an abnormal heart rhythm is detected. Event monitors can be worn for a month or longer.
There are two main types of event monitors: symptom event monitors and memory looping monitors. When you activate a symptom event monitor, for the next few minutes, it records the information from the heart’s electrical signal. A memory looping monitor does the same thing. However, it also records the information from a few minutes before the device was activated, so data from before, during and after the symptom will be captured.
Why might I need to use an event monitor?
Sometimes a healthcare provider may suspect that you have an abnormal heart rhythm based on your medical history, even if your ECG looks normal. Certain abnormal heart rhythms happen infrequently and temporarily. A random ECG is unlikely to pick up your abnormal heart rhythm if this is the case. An event monitor may be a better option for you. That way, you can record your heart’s electrical activity when you are having symptoms from your abnormal rhythm. Wearing the event monitor can help determine whether you have an abnormal heart rhythm. If you do have an abnormal rhythm, the event monitor can help determine what type.
You may need to wear an event monitor if your heartbeat is abnormally fast, abnormally slow, or irregular. If you are already being treated for an abnormal heart rhythm, your event monitor may be used to see how well it is working.
You may need an event monitor to evaluate certain kinds of temporary symptoms, such as palpitations. You might feel that your heart is beating too hard or skipping a beat. Dizziness and fainting are other symptoms that might be signs that you need an event monitor.
What are the risks for using an event monitor?
Event monitors are completely safe. They do not cause any pain. Sometimes the sticky patches used to attach the sensors to your chest can cause skin irritation.
How do I prepare for using an event monitor?
Your healthcare provider will show you how to use your event monitor. There are different types of event monitors that all work in different ways.
Cardiac memory loop monitors have sensors that attach to your chest using sticky patches. Wires connect these sensors to a monitor, which you can usually put on your belt or in your pocket. Before you put your sensors on your chest, your skin should be free of oils, creams, and sweat. Clean your skin before putting them on. You may need to shave the area before applying. A technician will show you how to place the electrodes.
Cardiac event recorders may not have sensors that attach to your chest, such as post-event recorders. Some models are handheld, and others attach to your wrist. For some of these models, you need to push the button on your wrist when you feel symptoms. In other models, you need to hold the recorder up to your chest in order to record.
What happens while using an event monitor?
In general:
- If you have a cardiac loop monitor, change your sensors as instructed.
- When you have a symptom, push the button to start recording. (Some start recording automatically when an abnormal rhythm is detected.)
- After you do this, stop moving. This will help the device get a good recording. The device should record for several minutes.
- For some event monitors, you will need to send your recordings over the phone to your healthcare provider.
- Someone will review your recording. In some cases, you may need to go see your healthcare provider.
- Follow all instructions about exercise. Sweat can make the sensors come off.
- If you can, avoid items that can disrupt the event monitor. These include magnets, metal detectors, microwave ovens, electric blankets, electric razors, electric toothbrushes, cell phones, and iPods. You will receive specific instruction at the time the monitor is placed.
- When you need to use an electronic device, keep it at least 6 inches away from the monitor.
You will also need to keep a diary while using your event monitor. Record any symptoms when they happened, and note what you were doing at the time. You may need to wear your event monitor for several days or up to a month.
What happens after using an event monitor?
Ask your doctor about what you can expect after you use an event monitor.
After a few readings, you may be able to stop wearing your event monitor. Your healthcare provider may use those readings to start your treatment. In some cases, more testing maybe needed. Follow-up tests might include:
- Exercise stress test, to see how the heart responds to exercise
- Tilt-table test, if you have had fainting
- Electrophysiological testing, to get more information about the heart’s electrical signal
- Echocardiogram, evaluate the structure and pumping function of the heart
Next steps
Before you agree to the test or the procedure make sure you know:
- The name of the test or procedure
- The reason you are having the test or procedure
- What results to expect and what they mean
- The risks and benefits of the test or procedure
- What the possible side effects or complications are
- When and where you are to have the test or procedure
- Who will do the test or procedure and what that person’s qualifications are
- What would happen if you did not have the test or procedure
- Any alternative tests or procedures to think about
- When and how will you get the results
- Who to call after the test or procedure if you have questions or problems
- How much will you have to pay for the test or procedure
Exercise tolerance testing is an important diagnostic and prognostic tool for assessing patients with suspected or known ischaemic heart disease. During exercise, coronary blood flow must increase to meet the higher metabolic demands of the myocardium. Limiting the coronary blood flow may result in electrocardiographic changes. This article reviews the electrocardiographic responses that occur with exercise, both in normal subjects and in those with ischaemic heart disease.
ST segment depression (horizontal or downsloping) is the most reliable indicator of exercise-induced ischaemia
Exercise tolerance testing (also known as exercise testing or exercise stress testing) is used routinely in evaluating patients who present with chest pain, in patients who have chest pain on exertion, and in patients with known ischaemic heart disease.
Assessment of chest pain in patients with intermediate probability for coronary artery disease
Arrhythmia provocation
Assessment of symptoms (for example, presyncope) occurring during or after exercise
Risk stratification after myocardial infarction
Risk stratification in patients with hypertrophic cardiomyopathy
Evaluation of revascularisation or drug treatment
Evaluation of exercise tolerance and cardiac function
Assessment of cardiopulmonary function in patients with dilated cardiomyopathy or heart failure
Assessment of treatment for arrhythmia
Exercise testing has a sensitivity of 78% and a specificity of 70% for detecting coronary artery disease. It cannot therefore be used to rule in or rule out ischaemic heart disease unless the probability of coronary artery disease is taken into account. For example, in a low risk population, such as men aged under 30 years and women aged under 40, a positive test result is more likely to be a false positive than true, and negative results add little new information. In a high risk population, such as those aged over 50 with typical angina symptoms, a negative result cannot rule out ischaemic heart disease, though the results may be of some prognostic value.
Exercise testing is therefore of greatest diagnostic value in patients with an intermediate risk of coronary artery disease.
The Bruce protocol is the most widely adopted protocol and has been extensively validated. The protocol has seven stages, each lasting three minutes, resulting in 21 minutes' exercise for a complete test. In stage 1 the patient walks at 1.7 mph (2.7 km) up a 10% incline. Energy expenditure is estimated to be 4.8 METs (metabolic equivalents) during this stage. The speed and incline increase with each stage. A modified Bruce protocol is used for exercise testing within one week of myocardial infarction.
Assessment of workload is measured by metabolic equivalents (METs)
Workload is a reflection of oxygen consumption and hence energy use
1 MET is 3.5 ml oxygen/kg per minute, which is the oxygen consumption of an average individual at rest
To carry out the activities of daily living an exercise intensity of at least 5 METs is required
β Blockers should be discontinued the day before the test, and dixogin (which may cause false positive results, with ST segment abnormalities) should be stopped one week before testing.
By convention, the maximum predicted heart rate is calculated as 220 (210 for women) minus the patient's age
A satisfactory heart rate response is achieved on reaching 85% of the maximum predicted heart rate
Attainment of maximum heart rate is a good prognostic sign
The patient is first connected to the exercise electrocardiogram machine. Resting electrocardiograms, both sitting and standing, are recorded as electrocardiographic changes, particularly T wave inversion, may occur as the patient stands up to start walking on the treadmill. A short period of electrocardiographic recording during hyperventilation is also valuable for identifying changes resulting from hyperventilation rather than from coronary ischaemia.
Acute myocardial infarction (within 4-6 days)
Unstable angina (rest pain in previous 48 hours)
Uncontrolled heart failure
Acute myocarditis or pericarditis
Acute systemic infection
Deep vein thrombosis
Uncontrolled hypertension (systolic blood pressure >220 mm Hg, diastolic >120 mm Hg)
Severe aortic stenosis
Severe hypertrophic obstructive cardiomyopathy
Untreated life threatening arrhythmia
Dissecting aneurysm
Recent aortic surgery
During the test the electrocardiogram machine provides a continuous record of the heart rate, and the 12 lead electrocardiogram is recorded intermittently. Blood pressure must be measured before the exercise begins and at the end of each exercise stage. Blood pressure may fall or remain static during the initial stage of exercise. This is the result of an anxious patient relaxing. As the test progresses, however, systolic blood pressure should rise as exercise increases. A level of up to 225 mm Hg is normal in adults, although athletes can have higher levels. Diastolic blood pressure tends to fall slightly. The aim of the exercise is for the patient to achieve their maximum predicted heart rate.
If patients are carefully selected for exercise testing, the rate of serious complications (death or acute myocardial infarction) is about 1 in 10 000 tests (0.01%). The incidence of ventricular tachycardia or fibrillation is about 1 in 5000. Full cardiopulmonary resuscitation facilities must be available, and test supervisors must be trained in cardiopulmonary resuscitation.
The specificity of ST segment depression as the main indicator of myocardial ischaemia is limited. ST segment depression has been estimated to occur in up to 20% of normal individuals on ambulatory electrocardiographic monitoring. There are many causes of ST segment changes apart from coronary artery disease, which confound the result of exercise testing. If the resting electrocardiogram is abnormal, the usefulness of an exercise test is reduced or may even be precluded. Repolarisation and conduction abnormalities—for example, left ventricular hypertrophy, left bundle branch block, pre-excitation, and effects of digoxin—preclude accurate interpretation of the electrocardiogram during exercise, and as a result, other forms of exercise test (for example, adenosine or dobutamine scintigraphy) or angiography are required to evaluate this group of patients.
The J point (the point of inflection at the junction of the S wave and ST segment) becomes depressed during exercise, with maximum depression at peak exercise. The normal ST segment during exercise therefore slopes sharply upwards.
By convention, ST segment depression is measured relative to the isoelectric baseline (between the T and P waves) at a point 60-80 ms after the J point. There is intraobserver variation in the measurement of this ST segment depression, and therefore a computerised analysis that accompanies the exercise test can assist but not replace the clinical evaluation of the test.
P wave increases in height
R wave decreases in height
J point becomes depressed
ST segment becomes sharply upsloping
Q-T interval shortens
T wave decreases in height
The standard criterion for an abnormal ST segment response is horizontal (planar) or downsloping depression of >1 mm. If 0.5 mm of depression is taken as the standard, the sensitivity of the test increases and the specificity decreases (vice versa if 2 mm of depression is selected as the standard).
Severe ST segment depression (>3 mm)
ST segment elevation >1 mm in non-Q wave lead
Frequent ventricular extrasystoles (unless the test is to assessment ventricular arrhythmia)
Onset of ventricular tachycardia
New atrial fibrillation or supraventricular tachycardia
Development of new bundle branch block (if the test is primarily to detect underlying coronary disease)
New second or third degree heart block
Cardiac arrest
Patient requests stopping because of severe fatigue
Severe chest pain, dyspnoea, or dizziness
Fall in systolic blood pressure (>20 mm Hg)
Rise in blood pressure (systolic >300 mm Hg, diastolic >130 mm Hg)
Ataxia
Other recognised abnormal responses to exercise include ST elevation of >1 mm, particularly in the absence of Q waves. This suggests severe coronary artery disease and is a sign of poor prognosis. T wave changes such as inversion and pseudo-normalisation (an inverted T wave that becomes upright) are non-specific changes.
A highly specific sign for ischaemia is inversion of the U wave. As U waves are often difficult to identify, especially at high heart rates, this finding is not sensitive. The presence of extrasystoles that have been induced by exercise is neither sensitive nor specific for coronary artery disease.
In clinical practice, patients rarely exercise for the full duration (21 minutes) of the Bruce protocol. However, completion of 9-12 minutes of exercise or reaching 85% of the maximum predicted changes in heart rate is usually satisfactory. An exercise test should end when diagnostic criteria have been reached or when the patient's symptoms and signs dictate.
The most common reason for stopping an exercise test is fatigue and breathlessness as a result of the unaccustomed exercise
After the exercise has stopped, recording continues for up to 15 minutes. ST segment changes (or arrhythmias) may occur during the recovery period that were not apparent during exercise. Such changes generally carry the same significance as those occurring during exercise.
Any abnormal electrocardiographic changes must be interpreted in the light of the probability of coronary artery disease and physiological response to exercise. A normal test result or a result that indicates a low probability of coronary artery disease is one in which 85% of the maximum predicted heart rate is achieved with a physiological response in blood pressure and no associated ST segment depression.
Horizontal ST segment depression of ⩽ 2 mm
Downsloping ST segment depression
Early positive response within six minutes
Persistence of ST depression for more than six minutes into recovery
ST segment depression in five or more leads
Exertional hypotension
A test that indicates a high probability of coronary artery disease is one in which there is substantial ST depression at low work rate associated with typical angina-like pain and a drop in blood pressure. Deeper and more widespread ST depression generally indicates more severe or extensive disease.
False positive results are common in women, reflecting the lower incidence of coronary artery disease in this group.
Exercise testing in patients who have just had a myocardial infarction is indicated only in those in whom a revascularisation procedure is contemplated; a less strenuous protocol is used. Testing provides prognostic information. Patients with low exercise capacity and hypotension induced by exercise have a poor prognosis. Asymptomatic ST segment depression after myocardial infarction is associated with a more than 10-fold increase in mortality compared with a normal exercise test. Conversely, patients who reach stage 3 of a modified Bruce protocol with a blood pressure response of >30 mm Hg have an annual mortality of <2%. Exercise testing can also add prognostic information in patients after percutaneous transluminal coronary angiography or coronary artery bypass graft.
Bayes's theorem of diagnostic probability states that the predictive value of an abnormal exercise test will vary according to the probability of coronary artery disease in the population under study
Exercise testing is therefore usually performed in patients with an moderate probablility of coronary artery disease, rather than in those with a very low or high probability
Exercise testing of asymptomatic patients is controversial because of the high false positive rate in such individuals. Angina remains the most reliable indicator of the need for further investigation.
In certain asymptomatic groups with particular occupations (for example, pilots) there is a role for regular exercise testing, though more stringent criteria for an abnormal test result (such as ST segment depression of >2 mm) should be applied. In the United Kingdom, drivers of heavy goods vehicles and public service vehicles have to achieve test results clearly specified by the Driver and Vehicle Licensing Agency before they are considered fit to drive.
Jonathan Hill is specialist registrar in cardiology at Barts and The London NHS Trust; Adam Timmis is a consultant cardiologist at the London Chest Hospital, Barts and the London NHS Trust
The ABC of clinical electrocardiography is edited by Francis Morris, consultant in emergency medicine at the Northern General Hospital, Sheffield; June Edhouse, consultant in emergency medicine, Stepping Hill Hospital, Stockport; William J Brady, associate professor, programme director, and vice chair, department of emergency medicine, University of Virginia, Charlottesville, VA, USA; and John Camm, professor of clinical cardiology, St George's Hospital Medical School, London. The series will be published as a book in the summer.
Articles from The BMJ are provided here courtesy of BMJ Publishing Group
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