What is an electrocardiogram (ECG)?

From the Guidelines

An electrocardiogram (ECG or EKG) is a crucial medical test for recording the electrical activity of the heart, and it should be performed as a standard of care in patients suffering from acute chest pain if the cause of the pain is not sufficiently clear from the patient's history and physical examination. The ECG works by detecting electrical signals produced when the heart contracts, and during the test, electrodes are placed on the skin of the chest, arms, and legs, which connect to a machine that translates these electrical signals into waveforms displayed on paper or a monitor 1.

Importance of ECG in Diagnosing Heart Conditions

The standard ECG has 12 leads that provide different views of the heart's electrical activity, making it essential for diagnosing various heart conditions, including arrhythmias (irregular heartbeats), heart attacks, and structural abnormalities. The test is quick, painless, and non-invasive, typically taking only 5-10 minutes to complete. Patients usually don't need special preparation, though they may be asked to avoid caffeine or certain medications beforehand.

Interpretation of ECG Results

ECGs are interpreted by looking at specific waves (P, Q, R, S, T) that represent different phases of the heart's electrical cycle, with abnormalities in these waves indicating potential heart problems. For instance, the presence of ST-segment elevation is a diagnostic sign of acute myocardial infarction in about 80–90% of cases, while ST-depressions indicate myocardial ischemia but have a poor power to identify an ongoing myocardial infarction 1.

Clinical Guidelines and Recommendations

According to the 2018 AHA/ACC guideline for the management of adults with congenital heart disease, the ECG is an essential part of a complete cardiovascular evaluation of a patient with ACHD, and it is recommended to obtain an ECG at baseline for comparison to any subsequently obtained ECG 1. Similarly, the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay emphasizes the importance of a resting ECG as an initial evaluation of those with known or suspected bradycardia or conduction disorder 1.

Key Points to Consider

• ECG is a standard of care in acute chest pain: It should be performed if the cause of pain is unclear from history and physical examination.
• ECG interpretation: Abnormalities in the P, Q, R, S, T waves indicate potential heart problems.
• Clinical guidelines: ECG is essential in evaluating patients with congenital heart disease, bradycardia, and cardiac conduction delay.
• Recent studies: The most recent guidelines and studies, such as those from 2018 and 2019, emphasize the critical role of ECG in diagnosing and managing heart conditions 1.

From the Research

ECG Overview

• The electrocardiogram (ECG) is a widely used imaging tool for diagnosing and managing patients with acute coronary syndrome symptoms 2.
• ECG readings should be considered alongside clinical assessments of patients to ensure accurate diagnoses.

ECG Patterns and Interpretations

• ST segment elevation in patients with active symptoms typically indicates acute occlusion of an epicardial artery with ongoing transmural ischemia 2.
• ST depression in leads other than V1-V3 usually indicates subendocardial ischemia secondary to subocclusion of the epicardial artery, distal embolization to small arteries, or spasm supply/demand mismatch 2.
• Comparison to previous ECGs and subsequent ECGs can aid in diagnosis and interpretation in difficult cases 2.

Clinical Features and ECG

• Certain clinical features, such as chest pain, nausea, and past history of diabetes mellitus, myocardial infarction, or angina pectoris, can help identify patients with acute cardiac ischemia (ACI) 3.
• ECG abnormalities, including ST-segment elevation, Q waves, and T-wave abnormalities, are more frequently seen in patients with ACI 3.
• Normal ECGs can be seen in patients with ACI, including 20% of patients with acute myocardial infarction (AMI) and 37% of patients with unstable angina pectoris (UAP) 3.

Troponin and ECG

• Cardiac troponin (cTn) assays are the gold-standard marker for acute myocardial necrosis, but elevated cTn can also be observed in patients without clinical signs of AMI 4.
• High-sensitivity cardiac troponin I concentrations can be used to differentiate myocardial injury and infarction, but troponin concentrations at presentation are insufficient to distinguish type 1 myocardial infarction from other causes of myocardial injury or infarction 5.
• The use of high-sensitivity troponin (hs-cTn) can safely rule out myocardial infarction (MI) in patients presenting with acute chest pain in the emergency department 6.