ECG Basics

How to evaluate a certain ECG strip? First, we have to know and obey some rules.

1st Heart rhythm evaluation

It is important to evaluate, if there is or is not present the sinus rhythm. And if there is not, then what kind of rhythm there is. It is essential to find any P waves and their relationship to QRS complexes. P wave absence is typical for non-sinus rhythms, negative P wave occurs in some forms of junction rhythm. P waves should be in the same distance from following QRS complexes.

Rhythm disorders or arrhythmias are caused by disrupted electric impulse formation or by disorders of its conduction. We can divide them into:

a) Replacement rhythms (non-sinus)

In these arrhythmias, the SA node is unable to correctly form the electric impulses and it is replaced by other myocardial cells located either in atria, or in AV node or even in ventricles.

- Atrial rhythm

The electric impulses are emitted in heart atria, but outside the SA node. P waves are present but they have abnormal shape. When electric impulses are formed in heart atria outside the SA node, there will be differently shaped P waves found in the ECG strip. QRS complexes have normal shapes, they are narrow and the frequency is usually also normal.

- Junctional rhythm

In this case, the electric signal is formed in the AV node. The P waves are inverted or totally missing, but QRS complexes have normal narrow shapes. The frequency is usually lower (approximately 30-50 beats per minute) as the AV node cells are usually unable to maintain high enough frequency.

- Ventricular rhythm

When there is ventricular rhythm, the electric impulses are formed in ventricles. There are wide QRS complexes and the heart rate is very slow (about 30 beats per minute and less). This is a common rhythm in third-degree AV block.

b) Extrasystoles

There are supraventricular and ventricular extrasystoles. Both types are very common and usually non-threatening (see below).

c) Tachycardias (except of sinus tachycardia)

Tachycardias have frequency above 90 bpm. In non-sinus tachycardias, the electric impulses are not properly formed in the SA node. They are either formed elsewhere, or they chaotically cycle.

- Supraventricular tachycardias

This group of arrhythmias does not include sinus tachycardia as it is not an arrhythmia. These arrhythmias include atrial tachycardia, atrial flutter, AVNRT, AVRT and atrial fibrillation. Atrial tachycardia starts in atria outside the SA node. The frequency is terribly high, but fortunately, the AV node prevents quicker transmission to the ventricles and maximal heart rate is less than 200 bpm. QRS complexes are narrow and there are visible P waves (if they are not hidden in T waves because of frequent QRS complexes). Atrial flutter also has a very high frequency of P waves with a “saw tooth” shape. Atrial flutter is regular and the QRS complexes are narrow. AVNRT and AVRT are regular supraventricular tachycardias that are typical by disordered conduction of electric impulses that cycle between atria and ventricle or within the AV node. Their frequency is about over 150 bpm, the QRS complexes are narrow and there are no visible P waves. In atrial fibrillation, the electric impulses come from the atria but they are produced chaotically causing an irregular rhythm. The QRS complexes are narrow and there are no classic P waves visible.

- Junctional tachycardia

In this case, the tachycardia comes from the AV node. P waves are not visible as they are hidden within QRS complexes. QRS complexes are again narrow and have normal shape.

- Ventricular tachycardia

In this case, the tachycardia is caused by ventricular myocardial cells. The QRS complexes are wide. Ventricular tachycardia may be monomorphic and polymorphic, sustained or no-sustained. Ventricular tachycardia can progress into ventricular fibrillation, possibly the most dangerous arrhythmia of them all.

2nd Heart rate regularity

The regularity may be evaluated from the distance of QRS complexes as they should be approximately equal. Irregularities are typical for supraventricular and ventricular extrasystoles, for atrial fibrillation and ventricular fibrillation.

Note: When there are some extrasystoles present in otherwise regular rhythm, we evaluate the rhythm as regular.

3rd Frequency

The frequency helps us to recognize bradycardia and tachycardia. The heart rate should be between 50-60 beats per minute. Of course, heart beat outside this interval does not have to be pathological. When we diagnose a tachycardia or bradycardia, it is necessary to also evaluate the present rhythm (see above).

Frequency can be counted using an easy formula. In classic paper speed of the ECG record, one small square is 40ms and one large square (equals to 5 small squares) is 200ms. We have to count the number of large squares between two average neighbor QRS complexes and divide 300 by this number. The outcome is the current heart rate per minute.

4th ECG axis

The ECG axis is also known as the electrical axis or heart axis. The axis is typically intermediate but it can also be (semi-)vertical or (semi-)horizontal.

It is the average direction of conduction of the electric impulse through the ventricular myocardium and it is approximately equal to the heart anatomical axis. It should be physiologically between plus 90 degrees and minus 30 degrees.

The assessment of the heart axis can be made by relationship of QRS amplitudes (highest spikes) direction. I am used to evaluate aVL and aVF leads.

According to the results, we distinguish five basic types of the cardiac axis:
ECG axis

Horizontal axis is present in LAFB and LBBB; vertical axis may accompany RBBB and overload of the right ventricle.

5th P wave evaluation

It is important to know if the P wave is present at all. We should confirm its presence at least in one lead; the best visible P waves are usually in lead II.

P waves may have some shape abnormalities present in various pathologies. Inverted P waves may be present when the electric impulse spreads from the AV node towards atrial myocardium, which is typical for so-called junctional rhythm. Excessively high P wave (P-pulmonale) is present in hypertrophy of the right atrium and bifid P wave (P-mitrale) occurs in hypertrophy of the left atrium.

6th PR interval evaluation

The PR (PQ) interval is the distance between the start of P wave and start of Q wave. Normally, the PR (PQ) interval should be between 120-200ms. Shorter PR interval is present for example in sinus tachycardia and in WPW syndrome. Prolonged PR interval typically occurs in first-degree AV block.

Note: PR segment is the distance between the end of P wave and the start of Q wave.

7th QRS evaluation

It is important to properly evaluate the shape and length of QRS complexes as they may be influenced by certain pathologies. QRS should be shorter than 100ms. Normal narrow QRS complexes mean that the electric impulses are properly conducted through the AV node, thus excluding ventricular tachycardia, ventricular fibrillation, ventricular rhythm in third-degree AV block and pacemaker electrode placed in the ventricle. Wide QRS complex either indicates that the electric signal has arisen somewhere in the ventricle (for example ventricular extrasystole), or that it comes normally from the AV node by with disrupted ventricular conduction (LBBB, RBBB, etc.).

QRS complexes can have may patters, the way how to properly describe a certain QRS complex can be found in this article about QRS patterns.

Common abnormalities causing specific changes of QRS pattern in certain leads include:

(see relevant articles)

8th ST interval evaluation

We are most interested in ST segment horizontal elevations and depressions. The depressions may be related to cardiac ischemia including non-STEMI infarction and ST elevations may be caused by pericarditis or by STEMI infarction. Ascending ST elevations often accompany LBBB, left ventricular hypertrophy, ventricular aneurysm and early repolarization.

9th T wave evaluation

Negative T wave is an unspecific finding that may be related to a number of pathological conditions including cardiac ischemia, infarction, ventricular hypertrophy, bundle branch blocks, etc.

10th QT interval evaluation

QT interval is the distance between Q wave and the end of the next T wave. The distance depends on actual heart rate, so we used the QTc, which is a “normalized” QT interval calculated for standard frequency. The QTc should be between 350-450 milliseconds. Clinically most significant is the prolongation of QT interval (see the related text).

11th aVR lead warning

Never make any conclusions from the lead aVR unless you are really skilled. This lead often shows various abnormalities, pathological Q waves, bizarre QRS complexes, etc. Lead aVR certainly has some significance but only for experienced cardiologists.


We should be able to distinguish supraventricular and ventricular extrasystoles (see related texts). The most important thing is not to be fooled by presence of multiple supraventricular extrasystoles as they may mimic an irregular rhythm and be mistaken for atrial fibrillation.

13th AV blocks

There are three types AV blocks – first-degree AV block, second-degree AV block and third-degree AV block (see related articles). Third-degree AV block is the most serious as it may cause severe bradycardia. This block must be always thought about in finding of severe bradycardia, especially when wide QRS complexes are present.

14th Other findings

This short part should contain other possible abnormalities that can be found in an ECG strip.


We can find high pointed T waves and shortened ST segment. In much severe cases, the QRS complexes turn wide and there is increased risk of malignant arrhythmias.


Hypokalemia can cause sinus bradycardia, the T waves flatten and so-called U waves may occur.

Wolff-Parkinson-White syndrome

This syndrome is caused by an extra conduction path that can in addition to AV node transmit electric impulses from atria to ventricles. PR interval is shorter and QRS complexes are slightly deformed by a so-called delta wave (see related article with images). The extra conduction path may also cause a pathological cycling of an electric impulse causing a paroxysm of regular supraventricular tachycardia.


ECG of patient with a pacemaker often has special “stimulation peaks” that are caused by electric stimulation. When the pacemaker's electrode is in the atrium, the peak is followed by a P wave and normal narrow QRS complex; electrode located in a ventricle forms a wide QRS complex that follows the stimulation peak. More information can be found in the related article.