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 lines on graph paper (i.e., 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, and/or ischemia. This article provides an overview of the most essential components of the ECG and describes its clinical application and the characteristic ECG findings for common diseases.

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.

ECG 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 lead reversal or incorrect placement ^[5]

Electrode or lead reversal or incorrect placement can alter ECG findings. The following are common findings associated with specific types of reversal or incorrect placement.

Limb lead reversal ^[5]

• Left (L) arm/right (R) arm (most common)
  • Negative P wave and negative QRS complex in lead I
  • Positive P wave and positive QRS complex in aVR
  • Right or extreme axis deviation
  • Normal R-wave progression in precordial leads
• R arm/L leg: all leads inverted (except for aVL)
• R leg/other limb: One lead appears almost flat.
• L arm/L leg: Changes may be subtle and are usually only detected when compared to previous ECGs.
  • Lead I switched with lead II
  • aVL switched with aVF
  • Lead III inverted
• L arm/L leg and R arm/R leg
  • Lead I appears flat.
  • aVL and aVR appear identical.
  • Lead II appears as an inverted lead III.

Precordial lead reversal or incorrect placement ^[5][6][7]

• Reversal
  • Usually manifests as disruption of normal P-, QRS-, and T-wave progression
  • Suspect misconnected cables if a sudden change in wave morphology returns to normal in the next lead.
• Incorrect placement
  • Changes are often subtle and difficult to detect.
  • Common misplacements: placement of V₁ and V₂ too superiorly and V₅ and V₆ too medially ^[6]
  • May appear as false reversed or poor R-wave progression, which may be mistaken for an anterior MI ^[7]

Troubleshooting ^[5]

• Compare with a prior ECG if possible.
• Verify correct ECG lead placement at the bedside and repeat the ECG if in doubt.

ECG artifact ^[5][8]

• Definition: ECG distortions not related to cardiac electrical activity
• Physiological artifact
  • Often caused by motion
  • Repetitive narrow spikes (may have a similar appearance to dysrhythmias): caused by small amplitude movements (e.g., tremors, shivering)
  • Wandering baseline: caused by large amplitude movements (e.g., patient movement, inadequate electrode contact)
• Nonphysiological artifact
  • Often caused by electrical interference
  • Appears as an indistinct or thick baseline; may make rhythm analysis difficult
• Artifact reduction methods ^[5]
  • Avoid placing electrodes over bony prominences and major muscles.
  • Shave thick hair and clean and dry the skin prior to placement.
  • Connect the ECG to a grounded outlet and turn off nearby electronics to minimize nonphysiological artifact.

Suspect artifact if ECG findings do not correlate with the clinical picture, e.g., apparent ventricular tachycardia in an asymptomatic, hemodynamically stable patient. ^[5]

ECG components ^[2][9]

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 ^[10]

• 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 ^[11]

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 I, II, 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 ^[12]

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

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. ^[14]
     • Positive in both leads I and aVF: normal axis (0° to 90°)
     • Positive in lead I and negative in aVF: left axis deviation (-90° to -30°) or normal axis (-30° to 0°)
     • Negative in lead I and positive in aVF: right axis deviation (90° to 180°)
     • Negative in both leads I and aVF: extreme right axis deviation (-180° to -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 ^[10]
  • The P wave represents atrial depolarization, which originates in the SA node.
  • The P wave has a lower amplitude and a more curved shape compared to QRS complex waves because of the slower depolarization magnitude and a slight delay in left atria (LA) depolarization compared to the right atria (RA)
• Morphology ^[10]
  • Present in all leads
  • Duration: < 0.12 s (in all leads) ^[16]
  • Amplitude: < 0.25 mV (in all leads) ^[17]
  • Polarity
    • Positive in leads I, II, and aVF
    • Negative in lead aVR
    • Biphasic in lead V₁: negative deflection < 1 mm ^[16]

• Physiology ^[9]
  • 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 ^[19]
  • 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
  • PR interval duration: 0.12–0.20 s ^[2]
  • Amplitude and polarity: P wave followed by an isoelectric line (see “P wave”)
  • Precedes each QRS complex