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 , abnormalities, or . This article provides an overview of the most essential components of the ECG.
- 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 “ ”).
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 
Four limb electrodes
- Left arm
- Right arm
- Left leg
- Right leg (neutral electrode)
- Six chest electrodes
Anatomical relationships of leads 
See also “.”
- Limb leads
- Precordial leads
|Anatomical relationships of ECG leads|
|Limb leads||Precordial leads||Corresponding heart structure|
|Inferior leads|| || || |
|Lateral leads|| || |
|Anteroseptal leads|| || || |
ECG paper 
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
- Small squares of 1 mm2
It is easy to misinterpret an ECG if the paper speed and calibration are not taken into account.
ECG components 
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 
- 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
- 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 
Approach to ECG interpretation 
- 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:
- Heart rhythm (best seen in lead II)
- Heart rate (any lead)
- Cardiac axis (leads I and aVF)
- P-wave morphology and size (best seen in lead II)
- PR-interval duration (best seen in lead II)
- QRS-complex morphology and duration (assessed in all leads individually)
- ST-segment morphology (assessed in all leads individually)
- T-wave morphology (assessed in all leads individually)
- QT-interval duration (lead aVL)
- U-wave morphology (leads V2–V4)
Determination of the heart rhythm 
- 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 “ ”).
- Definition: a physiological heart rhythm and age-appropriate set by the SA node
- Sinus rhythm features
- Respiratory sinus arrhythmia: a variation of the heart rate during respiration, which is normal and common in young adults 
Determination of the heart rate 
- 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 ).
- 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 
|Age||Bradycardia||Normal heart rate||Tachycardia|
|Newborns (0–1 month)|| |
|Infants (1–11 months)|| |
|Children (1–2 years)|| |
|Children (3–4 years)|| |
|Children (5–6 years)|| |
|Children (7–9 years)|| |
Children (> 10 years)
|Adult athletes|| |
- 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 
- Isoelectric (equiphasic) QRS complex method
Leads I and aVF method
- Determine the QRS complex polarity in leads I and aVF.
- The cardiac axis can be approximated by evaluating the combinations of the QRS complex polarities in leads I and aVF. 
- 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.
|Deviation of the cardiac axis |
|Axis||QRS polarity||Degrees||Common causes|
|Lead I||Lead aVF|
|Left axis deviation|| || || |
|Normal|| || || || |
|Right axis deviation|| || || || |
|Extreme right axis deviation|| || || |
- Physiology 
- Present in all leads
- Duration: < 0.12 s (in all leads) 
- Amplitude: < 0.25 mV (in all leads) 
- Positive in leads I, II, and aVF
- Negative in lead aVR
- Biphasic in lead V1: negative deflection < 1 mm 
|Abnormalities of the P wave |
|P pulmonale|| |
|P mitrale|| |
|P biatriale|| |
- PR interval
- Reflects the transmission of the electrical impulse through the AV node
- Considered the reference isoelectric line for the remainder of the ECG components
|Abnormalities of the PR interval |
|Duration|| || |
| || |
|Relationship to QRS|