ECG

Electrocardiography is an important diagnostic tool in cardiology. External electrodes are used to measure the electrical conduction signals of the heart and record them as characteristic lines on graph paper (an electrocardiogram; ECG). The interpretation of the amplitude and duration of these lines allows for the assessment of normal cardiac physiology as well as the detection of cardiac arrhythmias, conduction system abnormalities, or ischemia. This article provides an overview of the most essential components of the ECG.

Overview ^[1][2]

• Definition: An ECG represents a recording of the electrical activity of the heart that is captured via external electrodes and transcribed onto graph paper as ECG leads (for more information on the electrical activity of the heart, see “Conducting system of the heart”).
• Electrodes
  • Physical conductive pads attached to the patient's chest and limbs
  • Detect the direction of the depolarization vectors
• Leads : graphical representation of the depolarization vectors of the heart
  • Six precordial leads (V1–V6) capture the electrical activity of the heart in a horizontal plane.
  • Six limb leads (I, II, III, aVL, aVF, aVR ) capture the electrical activity of the heart in a vertical plane.
    • Input from three of the limb electrodes is combined to form the six limb leads.
    • The fourth electrode is neutral.

Electrode placement ^[1][2]

• Four limb electrodes
  • Left arm
  • Right arm
  • Left leg
  • Right leg (neutral electrode)
• Six chest electrodes
  • V1: fourth intercostal space, right parasternal region
  • V2: fourth intercostal space, left parasternal region
  • V3: midway between V2 and V4
  • V4: fifth intercostal space, left midclavicular line
  • V5: fifth intercostal space, left anterior axillary line
  • V6: fifth intercostal space, left midaxillary line

Anatomical relationships of leads ^[1][2]

See also “Localization of the myocardial infarction on ECG.”

• Limb leads
  • I: left ventricle, lateral wall
  • II, III: inferior surface of the heart
  • aVL: left ventricle, high part of the lateral wall
  • aVR: right heart and basal septum ^[3]
• Precordial leads
  • V1 and V2: ventricular septum and right ventricle
  • V3 and V4: anterior wall of the left ventricle
  • V5 and V6: lateral wall of the left ventricle and apex of the heart
  • V7–V9: posterior wall of the left ventricle ^[4]
  • V1R–V6R: right ventricle

ECG paper ^[1]

• ECG paper speed
  • In the US, the ECG paper speed is generally 25 mm/s.
  • Alternatively, a paper speed of 50 mm/s can be used.
• Machine calibration: 1 mV = 1 cm (i.e., 1 mV of electrical activity results in a 1 cm vertical deflection on the grid paper)
• Rhythm strip: a prolonged 10-second recording of a lead (usually lead II)
• ECG grid paper
  • Small squares of 1 mm²
    • Horizontally: 1 mm = 0.04 s (0.02 s for a paper speed of 50 mm/s)
    • Vertically: 1 mm = 0.1 mV
  • Large squares of 5 mm²
    • Horizontally: 5 mm = 5 x 0.04 s = 0.2 s (0.1 s for a paper speed of 50 mm/s)
    • Vertically: 5 mm = 5 x 0.1 mV = 0.5 mV

It is easy to misinterpret an ECG if the paper speed and calibration are not taken into account.

ECG components ^[2][5]

Overview

• Wave: a deflection of the ECG line due to any change in the electrical activity of the heart (e.g., P wave, T wave)
  • Positive (upward) deflection: the electrical impulse is moving toward the electrode
  • Negative (downward) deflection: the electrical impulse is moving away from the electrode
  • Equiphasic (equally upward and downward) deflection: the electrical impulse is moving perpendicular to the electrode
  • Some waves form complexes (e.g., QRS complex).
• Segment: the line between two different waves, excluding the waves (e.g., ST segment)
• Interval: includes a segment and one (or more) waves (e.g., PR interval)

Key components ^[6]

• P wave: atrial depolarization originating in the sinoatrial node (SA node)
• PR interval: depolarization originating in the SA node and traveling through the atria, the AV node, and the His-Purkinje system
• QRS complex: ventricular depolarization
• ST segment: the duration between ventricular depolarization and repolarization
• T wave: ventricular repolarization
• QT interval: total time of ventricular depolarization and repolarization
• U wave: occurs after the T wave; exact origin unknown ^[7]

Approach to ECG interpretation ^[2]

• When interpreting an ECG, it is important to keep in mind the patient's clinical picture and, if possible, compare the current ECG with previous ones.
• A thorough ECG interpretation algorithm should assess:
  1. Heart rhythm (best seen in lead II)
  2. Heart rate (any lead)
  3. Cardiac axis (leads I and aVF)
  4. P-wave morphology and size (best seen in lead II)
  5. PR-interval duration (best seen in lead II)
  6. QRS-complex morphology and duration (assessed in all leads individually)
  7. ST-segment morphology (assessed in all leads individually)
  8. T-wave morphology (assessed in all leads individually)
  9. QT-interval duration (lead aVL)
  10. U-wave morphology (leads V2–V4)

Determination of the heart rhythm ^[1]

• The heart rhythm is assessed by evaluating the frequency and regularity of the P waves and the QRS complexes, as well as the relationship between the two.
• A 10-second rhythm strip is required to assess the heart rhythm.
• Any abnormalities of the heart rhythm should prompt further evaluation (see “Cardiac arrhythmias”).

Sinus rhythm

• Definition: a physiological heart rhythm and age-appropriate heart rate set by the SA node
• Sinus rhythm features
  • P wave
    • Positive in leads II, III, and aVF
    • Negative in lead aVR
    • Followed by a QRS complex
  • Regular PR interval
  • QRS complex: preceded by a P wave
• Respiratory sinus arrhythmia: a variation of the heart rate during respiration, which is normal and common in young adults ^[8]

Determination of the heart rate ^[1]

• The ventricular rate can be calculated by using the frequency of the QRS complexes, which correlate with ventricular systoles.
• The atrial rate, which correlates with atrial systole, can be calculated by using the frequency of the P waves (e.g., when assessing supraventricular arrhythmias).
• In clinical settings, the heart rate can be measured with an ECG ruler.

Heart rate (HR) estimation methods

• Regular QRS rhythm
  • HR = 300/number of large (5 mm²) boxes between two successive QRS complexes (e.g., if you count 5 large boxes between one R wave and the next, the HR is approx. 300 ÷ 5 = 60/min)
  • HR = 150/R-R interval in cm (e.g., if there are 2 cm in between two consecutive R waves, HR = 150/2 = 75/min)
  • HR = 60/R-R interval in seconds (e.g., if there is a 0.5 s interval between two successive R waves, HR = 60/0.5 = 120/min)
• Irregular QRS rhythm: HR = 6 x total number of QRS complexes on a standard 10-second ECG rhythm strip (e.g., if you count 10 QRS complexes on a standard 10-second ECG rhythm strip, the HR is approx. 6 x 10 = 60/min)

Normal resting heart rate according to age

Definition ^[1]

• The electrical axis of the heart represents the mean direction of ventricular depolarization in a frontal plane.
• The normal cardiac axis in an adult is between -30° and +90°.

Methods for determining the cardiac axis ^[1]

There are several methods to determine the cardiac axis using the QRS complex polarity. The axis is calculated according to the hexaxial reference system (Cabrera circle).

• Isoelectric (equiphasic) QRS complex method
  1. Determine the lead in which the QRS complexes are isoelectric (equally positive and negative).
  2. Assess the two leads perpendicular to this lead on the Cabrera circle.
  3. The cardiac axis corresponds to the perpendicular lead with positive QRS complexes.
• Leads I and aVF method
  1. Determine the QRS complex polarity in leads I and aVF.
     • Positive QRS complex: the area above the isoelectric line and under the curve is larger than the area under the isoelectric line above the curve
     • Negative QRS complex: the area under the isoelectric line and above the curve is larger than the area above the isoelectric line and under the curve
  2. The cardiac axis can be approximated by evaluating the combinations of the QRS complex polarities in leads I and aVF. ^[10]
     • Positive in both leads I and aVF: normal axis (0°–90°)
     • Positive in lead I and negative in aVF: left axis deviation (-90°–-30°) or normal axis (-30°–0°)
     • Negative in lead I and positive in aVF: right axis deviation (90°–180°)
     • Negative in both leads I and aVF: extreme right axis deviation (-180°–-90°)
  3. Lead II can be used for a more accurate determination of the cardiac axis if the QRS complex is positive in lead I and negative in aVF.
     • Negative QRS complex in lead II: left axis deviation
     • Positive or isoelectric QRS complex in lead II: normal axis

Cardiac axis deviation

• Physiology ^[6]
  • The P wave represents atrial depolarization, which originates in the SA node.
  • Because of the slower depolarization magnitude and a slight delay in left atria (LA) depolarization compared to the right atria (RA) , the P wave has a lower amplitude and a more curved shape compared to QRS complex waves.
• Morphology ^[6]
  • Present in all leads
  • Duration: < 0.12 s (in all leads) ^[12]
  • Amplitude: < 0.25 mV (in all leads) ^[13]
  • Polarity
    • Positive in leads I, II, and aVF
    • Negative in lead aVR
    • Biphasic in lead V1: negative deflection < 1 mm ^[12]

• Physiology ^[5]
  • PR interval
    • Measured from the start of the P wave to the start of the QRS complex, which may be a Q wave or an R wave
    • Includes the P wave and the PR segment
    • Represents the depolarization originating in the SA node and traveling through the atria, the AV node, and the His-Purkinje system
    • Mainly assessed in lead II ^[15]
  • PR segment
    • Reflects the transmission of the electrical impulse through the AV node
    • Considered the reference isoelectric line for the remainder of the ECG components
• Morphology
  • Duration: 0.12–0.20 s ^[2]
  • Amplitude and polarity: P wave followed by an isoelectric line (see “P wave”)
  • Precedes each QRS complex

Overview

Physiology ^[5]

• The QRS complex represents ventricular depolarization and corresponds to ventricular systole.
• Because of a faster depolarization magnitude of the ventricles , the waves of the QRS complex have a sharp appearance.

QRS complex components ^[5]

• Q wave
  • The first negative deflection of the QRS complex
  • Represents depolarization of the interventricular septum (left-to-right direction) via the bundle of His
  • Q waves are a normal finding in leads I, III, aVL, V5, and V6 (see “Pathological Q waves” below).
• R wave
  • Any positive deflection in the QRS complex
  • Represents depolarization of the left ventricular myocardium (right-to-left direction)
• S wave
  • A negative deflection after an R wave
  • Depolarization of the upper lateral ventricular wall via Purkinje fibers (left-to-right direction)
• Intrinsicoid deflection ^[18]
  • The interval between the onset of the Q wave and the peak of the R wave
  • Delayed intrinsicoid deflection (> 0.05 s) is associated with an increased risk of heart failure. ^[19]

Morphology ^[5]

• Duration
  • QRS complex: < 0.1 s
  • Q wave: < 0.04 s
  • R wave > S wave
• Amplitude
  • Q wave: < 0.2 mV
  • R wave: progressively increases from lead V1 to V5
  • S wave: progressively decreases from lead V1 to V5

“From V1 to V5, there's sunSet and sunRise”: From leads V1 to V5, S wave Sets while R wave Rises.

QRS complex abnormalities

Abnormalities of QRS-complex waves

A new pathological Q wave is most likely an indication of myocardial infarction.

Bundle branch blocks

• Incomplete bundle branch block: QRS duration of 0.1–0.12 s
• Complete bundle branch block: QRS duration ≥ 0.12 s

New-onset left bundle branch block with concurrent angina should be treated immediately as acute coronary syndrome (ACS).

“WiLLiaM MoRRoW:” In LBBB the QRS looks like a W in V1 and an M in V6 (WiLLiaM), in RBBB the QRS looks like an M in V1 and a W in V6 (MoRRoW).

Ventricular hypertrophy

“R1ght 5ignS:” R in V1 and S in V5 are the dominant waves seen in right ventricular hypertrophy.

Overview ^[5]

• Physiology: represents the interval between ventricular depolarization and repolarization ^[27]
• Morphology
  • Horizontal isoelectric line, but may slope upward slightly before the T wave
  • Extends from the J point (end of the S wave) to the start of the T wave

Abnormalities of the ST segment

Brugada syndrome ^[34]

• Definition: a rare autosomal dominant genetic mutation that leads to abnormal cardiac conduction and sudden death
• Etiology: The most common identified mutation affects cardiac voltage-gated sodium channels.
• Epidemiology
  • Most common in Asian men
  • Symptoms mostly manifest in adulthood.
• Clinical features
  • Often an incidental finding, as most patients are asymptomatic
  • Syncope
  • Polymorphic ventricular tachycardia
  • Ventricular fibrillation
• Diagnostics
  • ECG findings
    • Brugada pattern: pseudo-RBBB with ST elevation in leads V1–V3
    • J waves in leads V1–V3 ^[33]
  • Rule out underlying heart disease (e.g., cardiac stress test and echocardiography).
• Treatment
  • General measures
    • Avoid certain medications (e.g., certain antiarrhythmics, psychotropics, anesthetic agents). ^[35]
    • Avoid excessive alcohol intake, cocaine, and large meals.
    • Treat fever with antipyretics.
  • Implantable cardiac defibrillator (ICD)
  • Screen all first-degree relatives annually with clinical examination and ECG.
• Complications
  • Sudden cardiac death
  • Increased risk of atrial fibrillation ^[36]

Overview ^[5]

• Physiology: : The T wave represents ventricular repolarization.
• Morphology
  • Shape: asymmetrical, with the downward slope steeper than the initial upward slope
  • Amplitude: < 10 mm (between 1/8 and 2/3 of the R wave)
  • Polarity: physiologically concordant to the QRS complex (positive if the QRS complex is positive or negative if the QRS complex is negative)

Abnormalities of the T wave

If electrical conduction of the heart is abnormal (e.g., bundle branch block), the ST segment and T wave cannot be reliably evaluated because of abnormal repolarization.

Overview ^[28]

• Represents the entire duration of ventricular depolarization and repolarization
• Measured from the beginning of the QRS complex to the end of the T wave
• Measured in the lead with the longest QT interval

Corrected QT interval (QTc) ^[28]

• General
  • Correction for the QT interval is necessary, as it varies with heart rate.
  • QTc = QT interval/√R-R interval (seconds)
• Duration
  • Adult men: QTc = 390–450 ms
  • Adult women: QTc = 390–460 ms

Generally, the QT interval should not be more than half of the R-R interval.

Abnormalities of the QT interval

A prolonged QT interval is associated with sudden cardiac death, usually due to acute ventricular arrhythmias. ^[39]

• General ^[5]
  • Small deflection after the T wave
  • Polarity is the same as the T wave
  • Best seen in leads V2 to V4, but not always visible
  • Normal finding in athletes
• Etiology
  • Exact origin is unknown
  • Thought to be due to delayed repolarization of the midmyocardial cells (myocardium) and the His-Purkinje system
• Causes of prominent U waves ^[5]
  • Hypokalemia
  • Hypercalcemia
  • Bradycardia

Ambulatory ECG monitoring ^[41]

• Description: ECG devices can be used in the outpatient setting to monitor and record the cardiac rhythm over a prolonged period of time.
• Types
  • Continuous: Holter monitor ^[42]
    • A continuous, ambulatory, battery-operated ECG recorder worn for 24–48 hours
    • Common metrics
      • Average, minimum, and maximum heart rate
      • Heart-rate variability
      • Episodes and duration of arrhythmias
      • QRS late potentials
      • ST-segment changes
      • Analysis of the P and T waves
    • Limitations
      • The short duration of monitoring results in a low diagnostic yield.
      • Devices are not waterproof.
      • The patient needs to document symptoms separately.
  • Intermittent
    • Event recorder
      • A device used in the evaluation of arrhythmias or syncope to record the patient's heart rhythm during symptomatic episodes
      • Devices are triggered to record data either by the patient (when experiencing symptoms) or automatically (when an arrhythmia is detected)
    • Loop recorder
      • A type of event recorder that can be triggered either automatically or manually by the patient
      • Records the patient's heart rhythm up to an hour prior to the arrhythmic event as well as during the event
      • External recorders: worn externally for short periods of time (4–6 weeks)
      • Implantable loop recorders: can be used for up to 36 months (e.g., for patients with more infrequent episodes)
  • Pacemakers or implanted cardioverter defibrillators
  • Patient-led monitoring (e.g., via a smartwatch)
• Indications
  • Daily or near-daily symptoms of:
    • Syncope
    • Palpitations
    • Chest pain
  • Assessment of the effects of new rate-control medication (e.g., metoprolol) or pacemaker function
  • Screening for ventricular ectopy in high-risk patients (e.g., patients with cardiomyopathy or acute coronary syndrome)

Other clinical applications of ECG

• Diagnosis of ACS (see also “ECG changes in STEMI”)
• Cardiac arrhythmias
• Cardiac tamponade
• Pericarditis
• Long QT syndrome
• Cardiac stress test
• Sick sinus syndrome
• Pericardial effusion

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