ECG

Last updated: March 22, 2022

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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
  • Anterior wall of both ventricles
  • Ventricular septum

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/RR interval in cm (e.g., if there are 2 cm in between two consecutive R waves, HR = 150/2 = 75/min)
    • HR = 60/RR 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]
P biatriale
  • 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]