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ECG

Last updated: March 19, 2021

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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]

Anatomical relationships of leads [1][2]

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

Anatomical relationships of ECG leads
Limb leads Precordial leads Corresponding heart structure
Inferior leads
  • II
  • III
  • aVF
  • N/A
  • Inferior surface of the heart
Lateral leads
  • I
  • aVL
  • V5
  • V6
Anteroseptal leads
  • N/A
  • V1–V4

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]

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]

Sinus rhythm

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

Normal resting heart rate according to age [9]

Age Bradycardia Normal heart rate Tachycardia
Newborns (0–1 month)

< 70/min

70–190/min

> 190/min

Infants (1–11 months)

< 80/min

80–160/min

> 160/min

Children (1–2 years)

< 80/min

80–130/min

> 130/min

Children (3–4 years)

< 80/min

80–120/min

> 120/min

Children (5–6 years)

< 75/min

75–115/min

> 115/min

Children (7–9 years)

< 70/min

70–110/min

> 110/min

Children (> 10 years)

Adults

< 60/min

60–100/min

> 100/min

Adult athletes

< 40/min

40–60/min

> 60/min

Definition [1]

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.

Cardiac axis deviation

Deviation of the cardiac axis [11]
Axis QRS polarity Degrees Common causes
Lead I Lead aVF
Left axis deviation
  • +
  • -
  • -90°–-30°
Normal
  • +
  • +
  • -30°–90°
  • Normal axis
Right axis deviation
  • -
  • +
  • 90°–180°
Extreme right axis deviation
  • -
  • -
  • -180°–-90°

  • Physiology [6]
  • 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]
Abnormalities of the P wave [6][13]
Abnormality ECG findings Pathophysiology Etiology
P pulmonale
  • Amplitude: ≥ 0.25 mV in leads II, III, and aVF [13]
P mitrale
  • Duration: ≥ 0.12 sec
  • Polarity
    • Bifid in lead II: peak-to-peak interval of > 0.04 sec
    • Biphasic in lead V1: negative deflection > 1 mm [12]
Biatrial enlargement

  • Physiology [5]
  • 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
Abnormalities of the PR interval [16]
Criteria ECG findings Pathophysiology Etiology
Duration
  • ≤ 0.12 s
  • Ectopic electrical pathways → faster impulse transmission to the ventricles → shorter PR interval
  • ≥ 0.2 s
  • Delay of electrical impulse transmission at the AV node slower transmission to the ventricles → longer PR interval
  • Malfunctioning of infranodal or AV nodal cells → failure of impulse transmission to the ventricles → dropped QRS complex
Relationship to QRS
Amplitude
  • PR-segment depression

Overview

Physiology [5]

QRS complex components [5]

Morphology [5]

  • Duration
  • 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

Overview of QRS-complex wave abnormalities
Abnormality ECG findings Pathophysiology Etiology
Pathological Q waves [5][20]
  • Abnormally wide (≥ 40 ms)
  • Abnormally deep (≥ 0.2 mV or > 25% of the R wave amplitude) or detectable in V1–V3
Dominant R wave [13]
  • Tall R wave in lead V1
  • Normal in children and young adults
Poor R-wave progression [22]
  • Absence of the normal increase in the size of R waves from lead V1 to V6
  • May be a normal variant
  • Ventricular depolarization vector reduced or directed posteriorly → deviation of the depolarization vector away from electrodes → insufficient increase in the size of the R wave and deep S waves
Persistent S wave

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
Bundle branch blocks
Abnormality ECG findings Pathophysiology Etiology
Left bundle branch block (LBBB) [11][16]
  • No R wave in lead V1
  • Deep S waves (forming a characteristic W shape)
  • Wide, notched R waves in leads I, aVL, V5, V6 (forming a characteristic M shape)
  • Loss of Q waves in the lateral leads [23]
Right bundle branch block (RBBB) [11]
  • An rsr', rsR', or rSR' complex (forming a characteristic “rabbit ears” or M shape) in leads V1, V2
  • Tall secondary R wave in lead V1
  • Wide, slurred S wave in leads I, V5, V6
  • Associated feature: ST segment depression and T-wave inversion in leads V1, V2, and sometimes V3
  • Usually a normal axis
  • Normal variant in ∼ 5% of individuals [6]
Bifascicular block [24]
  • An RBBB with either of the following:
    • Left anterior fascicular block (common form)
      • Left axis deviation
      • qR pattern in lead aVL
    • Left posterior fascicular block (rare)
      • Right axis deviation
      • rS pattern in leads I and aVL
      • qR pattern in leads III and aVF

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

Ventricular hypertrophy
Abnormality ECG findings Pathophysiology Etiology
Left ventricular hypertrophy (LVH) [12]
  • Increased muscle mass (hypertrophy) → taller R waves (in leads V5, V6) and S waves (in leads V1, V2)
Right ventricular hypertrophy (RVH) [13]
  • Any of the following may suggest RVH: [25]
    • Right axis deviation
    • Dominant R wave in lead V1 (R wave > 0.6 mV or R/S > 1)
    • Deep S wave in lead V5 (> 1 mV) or V6 (> 0.3 mV)
    • Sokolow-Lyon criteria: RV1 or R2 + SV5 or S6 ≥ 1.05 mV
  • Increased muscle mass (hypertrophy) → taller R waves (in leads V1, V2) and deeper S waves (in leads V5, V6)

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

Abnormalities of the ST segment [28]
Abnormality ECG findings Etiology
ST elevation [29]
ST depression [31]
  • ≥ 0.5 mV in leads V2 and V3
  • ≥ 0.1 mV in all other leads
  • Downsloping ST depression or horizontal ST depression
  • Upsloping ST depression
  • Sagging type ST-segment depression
  • Nonspecific ST-segment depression
J wave [33]
  • Positive deflection at the J point

Brugada syndrome [34]

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

Abnormalities of the T wave [5][37]
Abnormality ECG findings [28] Pathophysiology Etiology
T-wave inversion [38]
  • Amplitude ≥ -0.1 mV
  • May be a normal finding in:
    • Leads III, aVR, or V1
    • Children
  • New-onset T-wave inversion (i.e., not present on the patient's previous ECGs)
T-wave flattening
  • Amplitude between 0.1 mV and -0.1 mV
  • T wave appears flatter than normal.
Peaked T wave
  • Tall, narrow, symmetrically peaked
Hyperacute T wave
  • Broad, asymmetrically peaked
Biphasic T wave
  • T wave consisting of an upward and downward deflection
  • The initial deflection is variable and can be either up or down.

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]

Corrected QT interval (QTc) [28]

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

Abnormalities of the QT interval

QT interval abnormalities
Condition ECG findings [28] Pathophysiology Etiology
Prolonged QT interval [39]
  • > 450 ms in men
  • > 460 ms in women
Shortened QT interval [40]
  • < 390 ms

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

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

Other clinical applications of ECG

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