What is an ECG (electrocardiogram) test?

What is an ECG Test?

An ECG (electrocardiogram) is a noninvasive medical test that records the electrical activity of your heart using electrodes placed on your skin, producing a graphic tracing that physicians use to diagnose heart rhythm abnormalities, detect heart attacks, and identify various cardiac conditions. 1

How the Test Works

• The ECG captures the electrical impulses generated as your heart beats, displaying this activity as waveforms on paper or a digital screen 1
• A standard ECG uses 12 leads (electrode positions) placed on your chest, arms, and legs to create different views of your heart's electrical activity 1
• The test is completely painless, takes only a few minutes to perform, and requires no special preparation 2, 3

What the ECG Measures

The ECG records several key components of your heart's electrical cycle 1:

• Heart rate and rhythm: Whether your heart beats regularly and at a normal speed (60-100 beats per minute) 4
• Electrical conduction: How quickly electrical signals travel through different parts of your heart 1
• Chamber size and thickness: Indirect evidence of enlarged or thickened heart chambers 1
• Blood flow problems: Signs of current or past heart attacks, including areas of damaged heart muscle 1

Primary Clinical Uses

The ECG is the only practical, noninvasive method for recording and analyzing abnormalities of cardiac rhythm. 1 It serves multiple critical functions:

• Immediate diagnosis of life-threatening conditions: The ECG provides instantaneous information about heart attacks, dangerous arrhythmias, and conduction problems that may not be available with any other test 1
• Detection of silent heart disease: ECG abnormalities may be the first or only sign of heart disease, particularly in elderly patients with "silent" heart attacks 1
• Monitoring drug effects and metabolic problems: The test can detect life-threatening side effects of medications or severe electrolyte disturbances before they cause symptoms 1

When You Need an ECG

The American College of Cardiology and American Heart Association identify clear situations where an ECG is essential 1:

• Chest pain, dizziness, or fainting: These symptoms may indicate heart attack, dangerous rhythm problems, or conditions that can cause sudden death 1
• Known heart disease: If you have any diagnosed heart condition, the ECG serves as a baseline and monitoring tool 1
• Before surgery: To assess cardiac risk in patients undergoing procedures 1
• Suspected heart problems: When physical examination or symptoms suggest possible cardiac disease 1

Important Limitations to Understand

The ECG records electrical activity, not structural changes directly—diagnoses of heart attacks, enlarged chambers, or other structural problems are made by inference and are therefore subject to error. 1 This means:

• The same ECG pattern can occur in different heart conditions, which is why physician interpretation is crucial 1
• Computer-generated ECG interpretations alone are not considered properly interpreted ECGs and must always be verified by a qualified physician 1, 4
• The test has varying sensitivity and specificity for different conditions—it may miss some problems or suggest problems that aren't present 1

Critical Technical Factors

For an ECG to provide accurate diagnostic information, several technical requirements must be met 4:

• Proper electrode placement is essential, as incorrect positioning can create false patterns or mask real problems 4
• Standard calibration settings (paper speed of 25 mm/second, amplitude of 10 mm/mV) must be used for accurate measurements 4
• The physician must compare your current ECG with any previous tracings to detect changes over time 1, 5

The Interpretation Process

A skilled physician must integrate your clinical information, compare the ECG with previous tracings, and recognize that the same pattern may represent different conditions in different patients. 1 This is why:

• Interpretation requires specialized training, typically gained during residency or cardiology fellowship 1
• The physician must consider your symptoms, physical examination findings, and other test results alongside the ECG 1, 5
• Serial ECGs (multiple tracings over time) are often needed to track evolving conditions like heart attacks or response to treatment 1

Common Pitfalls to Avoid

• Never rely solely on computer interpretation: Computers frequently make errors, especially in detecting rhythm disturbances, ischemia, or heart attacks 1, 4
• Artifact can mimic disease: Muscle tremor, poor electrode contact, or equipment problems can create false abnormalities 4
• Normal ECG doesn't exclude heart disease: Approximately 5% of patients with normal ECGs discharged from emergency departments are later found to have heart attacks or unstable angina 6