What is an electrocardiogram (ECG) and how is it used in clinical practice?

Electrocardiogram (ECG): Fundamentals and Clinical Applications

The electrocardiogram (ECG) is the most commonly conducted cardiovascular diagnostic procedure and a fundamental tool of clinical practice, indispensable for diagnosing acute coronary syndromes and the most accurate means of diagnosing intraventricular conduction disturbances and arrhythmias. 1

Definition and Basic Principles

• The ECG provides a record of the electrical activity of the heart, serving as an independent marker of myocardial disease and reflecting electrophysiologic, anatomic, metabolic, and hemodynamic alterations 1
• First introduced by Willem Einthoven with the string galvanometer in 1901, electrocardiography has undergone significant technical advances to become the most widely employed procedure for evaluating cardiovascular function 1
• The ECG is painless, without direct risk to the patient, reproducible, and relatively inexpensive compared to many other procedures 1

Technical Aspects and Recording Methods

• Modern ECGs are typically recorded by digital, automated machines equipped with software that measures intervals and amplitudes, provides interpretation, and compares to previous tracings 1
• Computerized ECG interpretations should always be verified by a qualified physician, as automated systems may have different technical specifications resulting in significant differences in measurements and diagnostic statements 1, 2
• A standard 12-lead ECG includes limb leads (I, II, III, aVR, aVL, aVF) and precordial leads (V1-V6) to provide a comprehensive view of cardiac electrical activity from different angles 2

Systematic Approach to ECG Interpretation

Heart Rate and Rhythm Assessment

• Calculate heart rate by counting the number of large squares between consecutive R waves or by counting QRS complexes in a 6-second strip and multiplying by 10 2
• Evaluate rhythm regularity by examining R-R intervals for consistency, which is crucial for identifying normal sinus rhythm versus arrhythmias 2

Interval and Segment Analysis

• Measure PR interval (normal 120-200 ms or 3-5 small squares) to assess atrioventricular conduction 2
• Evaluate QRS duration (normal <120 ms or <3 small squares) to assess ventricular conduction 2
• Calculate QT interval and correct for heart rate (QTc, normal <450 ms for men, <460 ms for women) to assess ventricular repolarization 2
• Examine ST segments for elevation or depression that may indicate ischemia, injury, or other pathology 2

Electrical Axis Determination

• Examine leads I and aVF to quickly determine the quadrant of the axis (normal axis is between -30° and +90°) 2
• Left axis deviation is characterized by a positive deflection in lead I and a negative deflection in aVF 2
• Right axis deviation is characterized by a negative deflection in lead I and a positive deflection in aVF 2

Waveform Analysis

• Analyze P wave morphology (normal: upright in I, II, aVF; biphasic in V1) to assess atrial conduction 2
• Evaluate QRS complex morphology for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) suggesting myocardial infarction 2
• Assess T wave morphology (normally upright in leads I, II, V3-V6, inverted in aVR) 2
• Look for U waves, which may indicate hypokalemia, bradycardia, or other conditions 2

Clinical Applications

Primary Indications (Class I)

• Patients presenting with chest pain, dizziness, or syncope; symptoms that may predict sudden death or myocardial infarction 1
• Detection of life-threatening side effects of drugs or severe metabolic/electrolyte disturbances 1
• Diagnosis of "asymptomatic" myocardial infarction, particularly in elderly patients 1

Other Important Applications

• Diagnosis and prompt initiation of therapy in patients with acute coronary syndromes 1
• Recognition of electrolyte abnormalities, particularly serum potassium and calcium 1
• Detection of genetically mediated electrical or structural cardiac abnormalities 1
• Monitoring patients treated with antiarrhythmic and other drugs 1
• Preoperative assessment of patients undergoing noncardiac surgery 1
• Screening individuals in high-risk occupations and for participation in sports 1

Special Considerations

• Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 2
• Certain medications can significantly affect ECG findings (e.g., antiarrhythmics, psychotropics) 2
• Technical factors like electrode placement and filtering can affect ECG appearance and interpretation 2
• The same ECG pattern may be recorded in patients with different structural and pathophysiologic states, explaining the sometimes low specificity of the ECG for determining etiology and anatomy of cardiovascular disease 1

Importance of Physician Interpretation

• Before an ECG can have proper diagnostic accuracy, a skilled physician must integrate clinical data, provide a differential diagnosis, and compare with previous tracings 1
• An incorrect ECG diagnosis can have significant undesirable medical and legal consequences 1
• The physician should consider the clinical question being asked, the additional information to be derived, and the usefulness of the new information in establishing a diagnosis or making management decisions 1

Recent Advances in ECG Technology

• Application of new signal processing techniques and expansion in the use of additional leads allows clinicians to extract more information from cardiac electrical activity 3
• Computational analysis of ECG data (ECG signal processing) can reveal specific patterns or heart cycle trends otherwise unnoticeable by medical experts 4
• Understanding the technology inherent in ECG recording allows for better appreciation of both benefits and limitations of electrocardiography 3