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Step 1: Heart Rate

The heart rate can be determined via paper speed and the distance between 2 R waves. There are 2 paper speeds: 25 and 50 mm/s.

With the paper speed of 50 mm/s, one minute equals a strip length of 3,000 mm or 600 large squares (1 large square equals 5 mm):

Heart rate (beats/min) = 600/number of large squares between 2 R waves.

It is easier to determine the heart rate with the aid of an ECG ruler.

Heart rate Term Examples
< 60/min Bradycardia


Image : Sinus bradycardia as seen in lead 2. HR about 50. By James Heilman, MD, License: CC BY-SA 3.0

60–100/min Normal heart rate


Image : ECG-RRinterval. By Created by Agateller (Anthony Atkielski), converted to svg by atom. derivative work: Kychot (talk), License: Copyrighted free use

> 100/min Tachycardia


Image : Sinus tachycardia. DD atrial flutter. By MoodyGroove, License: CC BY-SA 3.0

Step 2: Heart Rhythm

When interpreting the heart rhythm, you should look for P waves, which is a sign of atrial excitation. When every P wave is followed by a QRS complex, the ECG shows sinus rhythm.


Image: Schematic diagram of normal sinus rhythm for a human heart as seen on ECG. By Agateller (Anthony Atkielski)/atom, License: Public Domain

If the P waves are irregular, sinus arrhythmia is likely present. If the P waves are missing altogether, the following differential diagnoses should be considered:

  • Atrial fibrillation: The fibrillation is characterized by low-amplitude, high-frequency atrial fibrillatory waves.
Atrial fibrillation (red arrow) and sinus rhythm (blue arrow)

Image: Scheme of atrial fibrillation (top) and sinus rhythm (bottom). The purple arrow indicates a P wave, which is lost in atrial fibrillation. By J. Heuser, License: CC BY-SA 3.0

  • Atrial flutter: The flutter waves are configured in a saw-tooth pattern.
Atrial flutter with variable block

Image: Atrial flutter with a variable block (between 3 and 4 to 1). By James Heilman, MD, License: CC BY-SA 3.0

  • Sinus arrest with escape rhythm: The retrograde atrial stimulation is caused by centers other than the sinus node. In this instance, bradycardia occurs with small QRS complexes but without P waves (i.e., synchronization of the QRS complexes and P waves).

Step 3: Electrical Heart Axis

The electrical heart axis is determined using the Cabrera circle, which is complex, or by examining the waves of the QRS complex (in limb leads I, II and III).

Image : Cabrera circle. By Andthu, License: CC BY-SA 3.0

Since the 2nd method is easier, memorize the following ‘rules of thumb’:

  • Right heart axis deviation: Leads I and II show negative deflection, whereas lead III has a positive deflection.

Image : ECG showing the right axis deviation. By Michael Rosengarten, BEng, MD.McGill, License: CC BY-SA 3.0

  • Right heart axis: Lead I has negative deflection and leads II and III show positive deflection.
  • Vertical cardiac axis: All leads show positive deflection, R in III > R in I.
  • Normal cardiac axis: All leads show positive deflection, R in I > R in III.
  • Left heart axis: Lead III has negative deflection, while leads I and II have positive deflection.
  • Left heart axis deviation: Leads II and III show negative deflection, whereas lead I has positive deflection.

Step 4: The PR Interval

The normal PR interval ranges between 120 and 200 ms (0.12–0.2s). A PR interval > 200 ms suggests a first-degree AV block.

  • First-degree AV block: When PR interval > 200 ms, each P wave is followed by a QRS complex.
  • Second-degree AV block (Mobitz I or Wenckelbach): The PR interval steadily increases until the impulse transmission fails (skipped heartbeat, and missing QRS complex).
  • Second-degree AV block (Mobitz II): A constant PR interval with sudden failure of conduction to the chambers (missing QRS complex), frequent 2:1 conduction (2 P waves followed by a single QRS complex), or 3:1 conduction (3 P waves followed by a QRS complex).
  • Third-degree AV block: The atria and ventricles act independent of each other (AV dissociation).
  AV block
Type I


Image : Heart block. By Npatchett, License: CC BY-SA 4.0

Type II Wenckebach block
Type II Mobitz block
Type III

Step 5: The QRS Complex

The normal QRS complex consists of a small negative Q wave (amplitude < ¼ of an R wave) as well as small R and S waves. The physiological QRS duration is 60–100 ms (0.06–0.1 s).


Image : Schematic representation of the QRS complex. By Häggström, Mikael (2014), License: Public Domain

Broad and deformed QRS complexes can occur in the case of:

  • Ventricular extrasystoles (VES; no preceding P wave)
  • Conduction system disorders
Conduction System Disorders
Right bundle branch block (RBBB)
  • Broad, frequently M-shaped QRS complexes in leads V1 and V2
  • Complete RBBB: QRS > 120 ms
  • Incomplete RBBB: QRS = 100–120 ms


Image : Trifascicular block consisting of first degree AV block and right bundle branch block and left axis deviation. By Steven Fruitsmaak, License: CC BY-SA 3.0

Left bundle branch block (LBBB)
  • Broad, frequently M-shaped QRS complexes in leads V5 and V6
  • Complete LBBB: QRS > 120 ms
  • Incomplete LBBB: QRS = 100–120 ms
  • Attention! Exception: hemiblocks may occur


Image : A left bundle branch block. By James Heilman, MD, License: CC BY-SA 3.0

Step 6: Repolarization

Repolarization includes the ST segment and the T wave (repolarization of chambers). The standard ST segment is an isoelectric line. Elevations and depressions of the ST segment are, therefore, pathological abnormalities (specifically > 1 mm in the limb leads and > 2 mm in the chest leads).


Image: Schematic diagram of normal sinus rhythm for a human heart as seen on ECG. By Agateller (Anthony Atkielski), License: Public Domain

The most important causes for such type of ST elevation are acute myocardial infarction (AMI) and acute pericarditis. In cases of AMI with ST-segment elevation (STEMI), the ST-segment takes off from the descending limb of the R wave, whereas in cases of pericarditis, it takes off from the ascending limb of the S wave.


Image: A 12-lead ECG showing a STEMI. Elevation of the ST segment can be seen in some leads. By James Heilman, MD, License: CC BY-SA 4.0

Note: An indication for STEMI is ST-segment elevation with poor R wave progression in at least 2 limb leads (amplitude > 0.1 mm) or 2 adjacent breast leads (amplitude > 0.2 mm). Horizontal or descending ST-segment depressions with a downward slope > 1 mm are considered pathological, and point to acute myocardial ischemia. Downsloping depressions also occur in digitalis therapy.

Repolarization abnormalities manifest as changes in T wave configuration. Possible pathological causes of repolarization abnormalities include:

  • Tent-shaped T waves as signs of hyperkalemia, but the sensitivity of an ECG for hyperkalemia is poor.
  • Inverted T waves: The causes for inverted T waves vary, including acute myocardial infarction and pulmonary embolism. Therefore, these findings should always be analyzed in conjunction with the remainder of the ECG as well as the patient’s other clinical signs.

N.B. Inverted T waves are not considered pathological per se. They are obligatory in lead aVR and are also found in leads III, V1 and V2, without manifesting any sign of disease.

Age (ethnicity) n V1 V2 V3 V4 V5 V6
1 week–1 year 210 92% 74% 27% 20% 0.5% 0%
1 year–2 years 154 96% 85% 39% 10% 0.7% 0%
2 years–5 years 202 98% 50% 22% 7% 1% 0%
5 years–8 years 94 91% 25% 14% 5% 1% 1%
8 years–16 years 90 62% 7% 2% 0% 0% 0%
12 years–13 years 209 47% 7% 0% 0% 0% 0%
13 years–14 years 260 35% 4.6% 0.8% 0% 0% 0%
16 years–19 years (Whites) 50 32% 0% 0% 0% 0% 0%
16 years–19 years (Blacks) 310 46% 7% 2.9% 1.3% 0% 0%
20 years–30 years (Whites) 285 41% 0% 0% 0% 0% 0%
20 years–30 years (Blacks) 295 37% 0% 0% 0% 0% 0%
12 years–13 years 174 69% 11% 1.2% 0% 0% 0%
13 years–14 years 154 52% 8.4% 1.4% 0% 0% 0%
16 years–19 years (Whites) 50 66% 0% 0% 0% 0% 0%
16 years–19 years (Blacks) 310 73% 9% 1.3% 0.6% 0% 0%
20 years–30 years (Whites) 280 55% 0% 0% 0% 0% 0%
20 years–30 years (Blacks) 330 55% 2.4% 1% 0% 0% 0%

Table: “Numbers from Lepeschkin E in.” by Antaloczy, Z (1979). Modern Electrocardiology. Amsterdam: Excerpta Medica. p. 401.

Step 7: The R/S Ratio

Usually, the height of the R wave in the breast leads increases, while the S wave decreases and the S wave is completely missing in V6. The R/S ratio is considered as the area where R is taller than S (usually between V2 and V3, or V3 and V4). If this is not the case, the situation is referred to as poor R wave progression, which may be an indication of myocardial infarction or left ventricular hypertrophy.

How to Interpret an ECG: An Overview

Interpretation of an ECG in seven easy steps

  1. Heart frequency
  2. Heart rhythm
  3. Electrical heart axis
  4. PR interval
  5. QRS complex
  6. Repolarization
  7. R/S ratio

While these steps represent a good start, these guidelines are not considered complete. Rather, these steps simplify the approach to reading and interpretation of an ECG in clinical practice.

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11 thoughts on “How to Interpret an ECG in Seven Steps

  • Mahammad Aming

    Thank you for precise explanation

  • Nene Samba

    Thanks this is very helpful. Doe my physiology laboratory project

  • Maryam Ghafghazi

    it is perdect

  • Murthy Kanniappan

    Sorry did not complete the post
    As hypo/hyperlcaemia would have its effect on ST segment duration.

    Overall I liked the content as it is precise and to the point.

    1. Stanley Oiseth

      Although this specific article does not address ECG changes which can occur with abnormal calcium levels, you are correct that both hypocalcemia and hypercalcemia would have effects on the ST segment duration, with hypocalcemia making it longer while hypercalcemia would make it shorter.

      Best regards,
      Stan from Lecturio

  • Murthy Kanniappan

    Please note that the Tenting of T wave occurs in Hyperkalaemia.
    May be typographical error.It is given as hypercalcaemia in the article

    1. Stanley Oiseth

      You are correct and we have changed it. Hyperkalemia causes tenting of the T-wave, but is only seen in less than half of patients with elevated potassium. Thanks for pointing out the error.


  • Tom Petersen

    I thought bradycardia was defined as a HR less than normal and therefor <60 BPM rather than <50 BPM as defined here.

    1. Stanley Oiseth

      You are correct and we have changed it. Thanks for pointing this out, and sorry for the delay in answering.

      Some background may be helpful here. Although the conventional definition of bradycardia is a heart rate less than 60 beats per minute with a normal P wave vector on the surface ECG, several professional societies, including the Am. Coll. of Cardiology and the Am. Heart Assoc, define bradycardia as a rate less than 50. A rate less than 50 beats per minute may in fact be a more pragmatic definition, as most people with sinus rates in the 50s are asymptomatic. Many well-trained athletes will have rates in the low 50s or even the high 40s. It is important to note that the rate at which a patient is labeled as having bradycardia is somewhat age dependent, and the rate should always be correlated with any new symptoms.

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