What are the basics of an electrocardiogram (ECG)?

ECG Basics

What is an ECG?

An electrocardiogram (ECG) records the electrical activity of the heart from electrodes placed on the body surface, capturing potential differences that reflect depolarization and repolarization of myocardial cells during each cardiac cycle. 1

• The ECG represents the instantaneous uncanceled electrical forces of the heart at any point in time, with signal strength and direction affected by torso geometry and tissue impedances 1
• Pairs of electrodes (or combinations serving as one electrode) create leads, which are the tracings that result from recording these electrical potentials 1
• The standard 12-lead ECG provides different views of the heart's electrical activity from multiple angles 2

How the ECG Signal Works

Signal Characteristics

• The fundamental frequency for the QRS complex at the body surface is 10 Hz, with most diagnostic information contained below 100 Hz in adults 1
• QRS complexes in infants contain important components as high as 250 Hz 1
• T waves have a fundamental frequency of approximately 1-2 Hz 1
• High-frequency components define rapidly changing parts of the signal, including Q waves and notched components within the QRS complex 1

Signal Processing

Modern digital electrocardiographs convert the analog ECG signal to digital form at the front end (typically the lead cable module) before further processing. 1

• The digital ECG must eliminate low-frequency noise from baseline wander, movement, and respiration, plus high-frequency noise from muscle artifact and electromagnetic interference 1
• Digital filters with linear phase characteristics avoid distortion introduced by classic analog filters 1
• Individual templates are constructed for each lead from dominant complexes, from which amplitude and duration measurements are made 1

Common Pitfall: Inadequate high-frequency response results in systematic underestimation of signal amplitude and smoothing of notches and Q waves, while inadequate low-frequency response causes important distortions of repolarization 1

Systematic ECG Interpretation Approach

Step 1: Rate and Rhythm Assessment

• Calculate heart rate by counting 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 2
• Identify the underlying rhythm (sinus, atrial, junctional, ventricular) based on P wave morphology and relationship to QRS complexes 2

Step 2: Interval Measurements

• PR interval: Normal is 120-200 ms (3-5 small squares) to assess AV conduction 2
• QRS duration: Normal is <120 ms (<3 small squares) to evaluate ventricular conduction 2
• QT interval: Calculate and correct for heart rate (QTc normal <450 ms for men, <460 ms for women) to assess repolarization 2

Step 3: Axis Determination

• Examine leads I and aVF to quickly determine the quadrant of the axis 2
• Normal axis is between -30° and +90° 2
• Left axis deviation: positive deflection in lead I and negative deflection in aVF 2
• Right axis deviation: negative deflection in lead I and positive deflection in aVF 2

Step 4: Waveform Analysis

P Waves:

• Normal morphology: upright in I, II, aVF; biphasic in V1 2
• Normal duration <120 ms and amplitude <2.5 mm 2

QRS Complexes:

• Analyze for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) suggesting myocardial infarction 2
• Assess R wave progression across precordial leads, with R wave amplitude increasing from V1 to V4 then decreasing toward V6 2

ST Segments and T Waves:

• Examine ST segments for elevation or depression measured at the J point 2
• T wave morphology is normally upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2 2

Step 5: Additional Features

• Look for U waves, which may indicate hypokalemia or bradycardia 2
• Assess for signs of chamber enlargement using P wave abnormalities and QRS voltage criteria 2
• Identify conduction abnormalities such as PR prolongation and QRS widening 2

Clinical Utility

The ECG is indispensable for diagnosing acute coronary syndromes, intraventricular conduction disturbances, and arrhythmias, and is the most commonly performed cardiovascular diagnostic test. 1

• The ECG can detect electrolyte abnormalities, particularly potassium and calcium 1
• It permits detection of genetically mediated electrical or structural cardiac abnormalities 1
• The ECG is routinely used to monitor patients on antiarrhythmic drugs and in preoperative assessment 1

Important Caveats

• Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 2
• Certain medications can affect ECG findings, including antiarrhythmics and psychotropics 2
• Computerized ECG interpretations should always be verified by a qualified physician 2
• Technical factors like electrode placement and filtering can significantly affect ECG appearance and interpretation 2
• Measurement error has an important effect on the accuracy of ECG diagnostic statements 1