Published on:
March 6, 2026
The electrocardiogram — commonly known as an ECG — remains one of the most widely used and valuable diagnostic tools in modern medicine. Whether you are experiencing symptoms such as chest pain, palpitations, or breathlessness, or you simply want a proactive check on your heart health, an ECG provides a window into your heart's electrical activity that few other tests can match.
Yet not all ECGs are the same. The two most common forms — the resting ECG and the exercise ECG (sometimes called a stress ECG or exercise tolerance test) — serve different clinical purposes and have distinct strengths and limitations. Over the past decade, the way clinicians use these tests has evolved considerably, driven by advances in cardiac imaging, a better understanding of diagnostic accuracy, and updated guidelines from bodies such as the European Society of Cardiology (ESC) and the National Institute for Health and Care Excellence (NICE).
In this article, we explore what each test involves, what conditions they can detect, how the clinical approach has changed in recent years, and when you might need one — or both — types of ECG. If you are considering an ECG at our clinic, this guide will help you understand what to expect.
A resting ECG is performed while the patient lies still on an examination couch. Small adhesive electrodes are placed on the chest, arms, and legs, and the test typically takes no more than five to ten minutes. It records the electrical signals that coordinate each heartbeat, producing a characteristic tracing that a clinician can interpret almost immediately.
Despite its simplicity, a resting ECG can reveal a remarkable amount of information. It is the frontline test for detecting cardiac arrhythmias — abnormal heart rhythms such as atrial fibrillation, supraventricular tachycardia, and heart block. It can identify structural changes in the heart muscle, including signs of left ventricular hypertrophy (thickening of the heart wall) and certain conduction abnormalities like bundle branch block.
A resting ECG is also essential in the acute setting. When a patient presents with chest pain, the ECG can show ST-segment elevation or depression, which may indicate a heart attack (myocardial infarction) in progress or recent ischaemic damage. It can detect pericarditis, electrolyte imbalances affecting the heart, and the effects of certain medications on cardiac conduction.
However, the resting ECG has a significant limitation: it captures only a snapshot in time. Many cardiac conditions — particularly stable coronary artery disease — may produce no abnormalities at rest because the heart is not under sufficient demand. This is precisely where the exercise ECG has traditionally played its role.
An exercise ECG, also known as a stress test or exercise tolerance test (ETT), monitors the heart's electrical activity while the patient exercises — usually on a treadmill or stationary bicycle. The workload is gradually increased in stages according to a standardised protocol (the Bruce protocol being the most commonly used), and the ECG is recorded continuously throughout exercise and into the recovery period.
The principle behind the exercise ECG is straightforward: by increasing the heart's workload, any underlying coronary artery narrowing that limits blood flow to the heart muscle becomes more apparent. As the heart rate rises and myocardial oxygen demand increases, areas of the heart supplied by narrowed arteries may become ischaemic, producing characteristic ST-segment changes on the ECG tracing.
Beyond detecting ischaemia, the exercise ECG provides valuable functional information. It assesses exercise capacity — measured in metabolic equivalents (METs) — which is itself a powerful predictor of cardiovascular outcomes. Research has consistently demonstrated that patients who can achieve higher workloads (for example, 10 METs or above) without ST-segment changes have an excellent prognosis, regardless of other risk factors. As documented in a study referenced in World Journal of Cardiology, such patients rarely show significant ischaemia on subsequent imaging, suggesting that further testing may be unnecessary in these cases.
The test also monitors blood pressure response, heart rate recovery, and the onset of symptoms such as chest tightness or breathlessness during exercise. A blunted heart rate response or slow heart rate recovery after exercise can indicate autonomic dysfunction and is associated with increased cardiovascular risk. If you have concerns about your cardiovascular health, it is worth having a baseline assessment including blood pressure monitoring alongside any ECG testing.
Perhaps the most significant development in cardiac diagnostics over the past decade has been the reduced reliance on the exercise ECG for diagnosing coronary artery disease. For decades, the treadmill stress test was the standard initial investigation for patients with suspected stable angina. However, accumulating evidence has shown that its diagnostic accuracy — particularly in certain populations — is more limited than previously thought.
The exercise ECG has a sensitivity of approximately 50–70% and a specificity of around 60–75% for detecting obstructive coronary artery disease. This means that a significant proportion of patients with genuine coronary disease may receive a falsely reassuring "normal" result, while others without significant disease may be told their test is abnormal. The problem is particularly acute in women, where false positive rates are notably higher, partly due to hormonal influences on ST-segment morphology and a higher prevalence of microvascular disease that the exercise ECG cannot reliably detect.
A 2012 study comparing exercise ECG with stress perfusion cardiac MRI in women found that MRI achieved a sensitivity of 85% versus just 50% for the exercise ECG, with specificity of 94% versus 73%. These findings have contributed to a broader shift in guidelines.
In 2016, NICE updated its guidelines for the assessment of chest pain of recent onset (CG95, later updated as NG203 in 2021), removing the exercise ECG from the recommended diagnostic pathway for investigating stable chest pain suspected to be angina. Instead, NICE now recommends CT coronary angiography (CTCA) as the first-line investigation for patients with typical or atypical chest pain and an intermediate likelihood of coronary artery disease.
The European Society of Cardiology has followed a similar trajectory. Its 2019 guidelines on chronic coronary syndromes recommend non-invasive functional imaging (such as stress echocardiography, stress MRI, or myocardial perfusion scintigraphy) or anatomical imaging (CTCA) over exercise ECG for diagnostic purposes. The ESC acknowledges that while the exercise ECG retains value in specific contexts, it should no longer be the default diagnostic test for suspected coronary artery disease.
This represents a paradigm shift. For decades, clinicians relied on the exercise ECG as a gateway investigation, with further imaging reserved for inconclusive or positive results. Now, the pathway often begins with imaging, and the exercise ECG plays a more targeted, supplementary role.
Despite this evolution, it would be a mistake to consider the exercise ECG obsolete. It retains important clinical applications, including:
Furthermore, in settings where advanced imaging is not readily available or where cost is a significant consideration, the exercise ECG remains a practical and informative investigation. As part of a broader health screening approach, the exercise ECG provides functional data that imaging alone cannot replicate.
The resting ECG is primarily used to identify structural and electrical abnormalities that are present at baseline. Key diagnoses include:
The exercise ECG adds a dynamic dimension, revealing problems that emerge only under physiological stress:
The decision to perform a resting or exercise ECG depends on the clinical context. A resting ECG is appropriate — and often essential — in the following scenarios:
An exercise ECG may be recommended when:
It is important to note that certain conditions can render the exercise ECG uninterpretable. Patients with a left bundle branch block, ventricular pacemaker rhythm, significant resting ST-segment abnormalities, or those taking digoxin may require alternative stress testing modalities — such as stress echocardiography or pharmacological stress MRI — to obtain reliable results. As research published in the BMJ has highlighted, the resting ECG must be interpretable for ischaemia before an exercise test can yield meaningful additional diagnostic information.
The ECG landscape is evolving rapidly, and two developments in particular are set to transform how we use this century-old technology.
Machine learning algorithms are increasingly being applied to ECG analysis, with some systems now capable of detecting conditions that even experienced cardiologists might miss on a standard tracing. AI has shown promise in identifying asymptomatic left ventricular dysfunction, predicting the likelihood of atrial fibrillation from a normal sinus rhythm ECG, and detecting subtle patterns associated with hypertrophic cardiomyopathy and other structural heart diseases. These developments may significantly enhance the diagnostic yield of the resting ECG, potentially reducing the need for more expensive downstream investigations.
Consumer devices such as the Apple Watch, Kardia Mobile, and other wearable ECG monitors now allow patients to record single-lead ECGs at home and share them with their healthcare provider. While these devices are primarily validated for detecting atrial fibrillation, their capabilities are expanding. Continuous monitoring captures intermittent arrhythmias that a standard 10-second resting ECG might miss entirely, bridging the gap between clinic-based testing and real-world cardiac activity.
However, it is essential to recognise that wearable devices have limitations. They record fewer leads than a clinical ECG, are susceptible to motion artefact, and cannot replicate the controlled conditions of an exercise ECG. For comprehensive cardiac assessment, clinic-based testing remains the gold standard.
A resting ECG can detect many cardiac conditions, including arrhythmias, heart attacks, and conduction abnormalities. However, it cannot reliably rule out coronary artery disease because narrowed arteries may not produce ECG changes at rest. If you have symptoms during exertion or significant risk factors, further testing — such as imaging or an exercise ECG — may be recommended.
Updated guidelines from NICE and the European Society of Cardiology now recommend CT coronary angiography or functional imaging as first-line investigations for suspected stable angina. This change reflects evidence that the exercise ECG has limited sensitivity and specificity compared with modern imaging, leading to both missed diagnoses and unnecessary false alarms — particularly in women.
Absolutely. The exercise ECG remains valuable for assessing exercise capacity, evaluating exercise-induced arrhythmias, risk-stratifying patients with known coronary disease, and screening certain populations such as athletes. Its ability to provide functional and prognostic information — including heart rate recovery and blood pressure response — complements what imaging alone can offer.
Wear comfortable clothing and supportive shoes suitable for walking on a treadmill. Avoid heavy meals, caffeine, and smoking for at least two hours before the test. Discuss any medications you are taking with your doctor beforehand, as some — particularly beta-blockers — may need to be temporarily adjusted. Your clinician will advise you on specific instructions based on your individual circumstances.
A resting ECG does not measure blood pressure directly. However, it can detect left ventricular hypertrophy, a common consequence of longstanding uncontrolled hypertension. If your ECG shows signs of LVH, your doctor may recommend further assessment and optimisation of your blood pressure management.
The information provided in this article is for educational purposes only and is based on NHS recommendations. It is not a substitute for professional medical advice. Always consult your doctor or a qualified healthcare provider for advice on medical conditions or treatments.
Our medical centre is at 36 Spital Square, E1 6DY, City of London.
