Physicians encounter ECGs in their clinical routine every day. Additionally, ECGs are frequently the topic of exams, which is reason enough for us to provide an analysis algorithm that will aid students in interpreting an ECG. Learn the seven steps to interpret an ECG and test your knowledge by taking the ECG quiz.

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Step 1: Heart Rate

The heart rate can be determined via the paper speed and the distance between two R waves. There are two paper speeds: 25 or 50 mm/s. With regard to 50 mm/s, one minute equals a strip length of 3000 mm or 600 large squares (one large square equals 5 mm):

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

It is easier to determine the heart rate with the aid of an ECG ruler that simply lets you read the rate on its scale.

Heart rate Term
< 50 beats/min bradycardia
50 – 100 beats/min normal heart rate
> 100 beats/min tachycardia


Step 2: Heart Rhythm

In regards to the heart rhythm, you should take care to find out if the P waves are a sign of atrial excitation. If every P wave is followed by a QRS complex, you are dealing with a sinus rhythm. If the P waves are irregular, a sinus arrhythmia is present. If the P waves are missing altogether, the following differential diagnoses are to be considered:

  • Atrial fibrillation: The fibrillation is characterized by low-amplitude, high-frequency atrial fibrillatory waves.
  • Atrial flutter: The flutter waves are configured in a saw-tooth pattern.
  • Sinus arrest with escape rhythm: Retrograde atrial stimulation is caused by centers other that the sinus node. Here, bradycardia occurs with small QRS complexes but without P waves (the QRS complexes and P waves are synchronized).

Step 3: Electrical Heart Axis

The electrical heart axis can either be determined using the Cabrera circle, which is complicated, or by examining the waves of the QRS complex (limb leads I, II and III). As the second method is easier, memorize the following rules of thumb:

  • Right heart axis deviation: leads I and II have negative deflection and there is positive deflection in lead III.
  • Right heart axis: lead I has negative deflection and leads II and III have positive deflection.
  • Vertical cardiac axis: all leads have positive deflection, R in III > R in I.
  • Normal cardiac axis: all leads have positive deflection, R in I > R in III.
  • Left heart axis: lead III has negative deflection, leads I and II have positive deflection.
  • Left heart axis deviation: leads II and III have negative deflection, lead I has positive deflection.

Step 4: The PR Interval

The normal PR interval is between 120 – 200 ms (0.12 – 0.2s). If the PR-interval remains > 200ms, a first degree AV block is present.

  AV block
Type I PR interval >200 ms, each P wave is followed by a QRS complex
Type II Wenckebach block The PR interval steadily increases until failure in impulse transmission occurs (dropped beat, missing QRS complex)
Type II Mobitz block Constant PR interval with sudden failure of conduction to the chambers (missing QRS complex), frequent 2:1 conduction (two P waves followed by one QRS complex) or 3:1 conduction (three P waves followed by a QRS complex)
Type III The atria and ventricles act independently of each other (AV dissociation)


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 a small R and S wave. The physiological QRS duration is 60 – 100 ms (0.06 – 0.1 s). 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
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


Step 6: Repolarization

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

As the most important causes for such an ST elevation, you should memorize 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.

Note: An indication for STEMI are ST segment elevations with poor R wave progression in at least two limb leads (amplitude > 0.1 mm) or two adjacent breast leads (amplitude > 0.2 mm).

ST segment depressions > 1 mm that are downsloping, horizontal or descending are considered pathological and point to acute ischemia. Downsloping depressions can also be found with regard to digitalis therapy.

Repolarization abnormalities manifest themselves in T wave configuration changes. Possible pathologies include:

  • Tent-shaped T waves as signs of hypercalcemia;
  • Negative T waves: The causes for negative T waves are varied—from acute myocardial infarction, all the way to pulmonary embolism. Therefore, the findings should always be analyzed in conjunction with the rest of the ECG as well as the patient’s other clinical signs.
Keep in mind: Inverted T waves are not considered pathological per se. They are obligatory in lead aVR and can also be found in leads III, V1 and V2 without it being a sign of disease.

Step 7: The R/S Ratio

Usually, the R wave height in the breast leads increases while the S wave decreases and is completely missing in V6. The R/S ratio is considered to be 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. This may be an indication of myocardial infarction or left ventricular hypertrophy.

An Overview of All Seven Steps: How to Interpret an ECG

In order to be able to get a good first impression of an ECG, these seven steps are sufficient. The following will provide you with a short list of all the steps:

How to interpret an ECG in seven steps:

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

When taking all of this into consideration, you should not forget: Interpreting an ECG is a complex task. It is not without reason that numerous papers and books have been written about the subject. To interpret an ECG in seven steps can only be an approximate attempt.

Therefore, the above guidelines are not to be considered complete, either. They are rather supposed to simplify the approach to reading an ECG, which will help you make a systematic interpretation in your clinical routine and during the exam.

Practice Makes Perfect

In closing, here is a small quiz. The correct answers are below.


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