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«Introduction Step 1: Heart Rate»

Author(s) I.A.C. van der Bilt, MD
Moderator I.A.C. van der Bilt, MD
some notes about authorship


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How do I begin to read an ECG?

A short ECG registration of normal heart rhythm (sinus rhythm)
An example of a normal ECG. Click on the Image for an enlargement

Click on the ECG to see an enlargement. Where do you start when interpreting an ECG?

  • on the top left are the patient's information, name, sex and date of birth
  • at the right of that are below each other the Frequency, the conduction times (PQ,QRS,QT/QTc), and the heart axis (P-top axis, QRS axis and T-top axis)
  • further to the right is the interpretation of the ECG written (this often misses in a 'fresh' ECG, but later the interpretation of the cardiologist or computer will be added)
  • down left is the 'paper speed' (25 mm/s on the horizontal axis), the sensitivity (10mm/mV) and the filter's frequency (40Hz, filters noise from eg. lights)
  • finally there is a calibration on the ECG, on the beginning of every lead is a vertical block that shows with what amplitude a 1 mV signal is drawn. So the height and depth of these signals are a measurement for the voltage. If this is not the set at 10 mm, there is something wrong with the machine setting.
  • further we have the ECG leads themselves of course, these will be discussed below.

Note that the lay-out is different for every machine, but most machines will show the information above somewhere.

What does the ECG register?

The electrocardiogram
An electrocardiogram (ECG or EKG) is a registration of the heart's electric activity.

Just like skeletal muscles, the heart is electrically stimulated to contract. This stimulation is also called activation or excitation. Cardiac muscles are electrically charged at rest. The inside of the cell is negatively charged relative to the outside (resting potential). If the cardiac muscle cells are electrically stimulated they depolarize (the resting potential changes from negative to positive) and contract. The electrical activity of a single cell can be registered as the action potential. As the impulse spreads through the heart, the electric field changes continually in size and direction. The ECG is a graphical visualisation of these electric signals in the heart.

The ECG represents the sum of the action potentials of millions of cardiomyocytes

This movie shows the contraction of a single (rabbit) heart cell. The glass electrode measures the electrical current in the heart cell (with thepatch-clamp method). The electrical signal is written in blue and shows the actionpotential. Courtesy of Arie Verkerk and Antoni van Ginneken, AMC, Amsterdam, The Netherlands.
The heart consists of approximately 300 trillion cells
In rest the heart cells are negatively charged. Through the depolarization by surrounding cells they become positively charged and they contract.
During the depolarization sodium-ions stream inwards the cell. Subsequently the calcium-ions stream into the cell. These calcium-ions give the actual muscular contraction. Finally the potassium-ions stream out of the cell. During the repolarisation the ion concentration is corrected. On the ECG, an action potential wave coming towards the electrode is shown as a positive (upwards) signal. Here the ECG electrode is represented as an eye.

The individual action potentials of the individual cardiomyocytes are averaged. The final result which is shown on the ECG is actually the average of trillions of microscopic electronical signals.

The electric discharge of the heart

Conduction system en.png

The sinal node (SA node) contains pacemakercells which determine the heart frequency. First the atria depolarise and contract, after that the ventricles The electrical signal between the atria and the ventricles goes from the sinus node, via the atria to the AV-node (atrioventricular transition) to the His bundle and subsequently to the right and left bundle branch, which end in a dense network of Purkinje fibers.

The different ECG waves

The origin of the different waves on the ECG
The QRS complex is formed by the sum of the electric avtivity of the inner (endocardial) and the outer (epicardial) cardiomyocytes
Example of the different QRS configuations

The P wave is the result of the atrial depolarization. This depolarization starts in the SA (sino-atrial) node. The signal produces by pacemakercells in the SA node is conducted by the conduction system to the right and left atria. Normal atrial repolarisation is not visible on the ECG (but can be visible during atrial infarction and pericarditis).

The QRS complex is the average of the depolarization waves of the inner (endocardial) and outer (epicardial) cardiomyocytes. As the endocardial cardiomyocytes depolarize slightly earlier than the outer layers, a typical QRS pattern occurs (figure).

The T wave represents the repolarisation of the ventricles. There is no cardiac muscle activity during the T wave.

One heart beat consists of an atrial depolarization --> atrial contraction --> p-wave, ventricular depolarization --> ventricular contraction --> ORS-complex and the resting phase (including the repolarization during the T-wave) between two heart beats.

Have a look at this excellent [animation of the heart cycle]

The origin of the U wave is unknown. This wave possibly results from "afterdepolarizations" of the ventricles.

The letters "Q", "R" and "S" are used to describe the QRS complex:

  • Q: the first negative deflection after the p-wave. If the first deflection is not negative, the Q is absent.
  • R: the positive deflection
  • S: the negative deflection after the R-wave
  • small print letters (q, r, s) are used to describe deflections of small amplitude. For example: qRS = small q, tall R, deep S.
  • R`: is used to describe a second R-wave (as in a right bundlebranch block)

See figure for some examples of this.

The history of the ECG

A concise history of the ECG is presented in a different chapter.

The ECG electrodes

The limb leads
The chest leads

Electric activity going through the heart, can be measured by external (skin)electrodes. The electrocardiogram (ECG) registers these activities from these electrodes which have been attached on different places on the body. In total, twelve leads are calculated using ten electrodes.

The ten electrodes are:

  • the extremity electrodes:
    • LA - left arm
    • RA - right arm
    • N - neutral, on the right leg (= electrical earth or point zero to which the electrical current is measured)
    • F - foot, on the left leg

It makes no difference whether the electrodes are attached proximal or distal on the extremities. However, it is best to be uniform in this. (eg. do not attach an electrode on the left shoulder and one on the right wrist).

  • the chest electrodes:
    • V1 - placed in the 4th intercostal space, right of the sternum
    • V2 - placed in the 4th intercostal space, left of the sternum
    • V3 - placed between V2 and V4
    • V4 - placed 5th intercostal space in the nippleline. Official recommendations are to place V4 under the breast in women.[1]
    • V5 - placed between V4 and V6
    • V6 - placed in the midaxillary line on the same height as V4 (horizontal line from V4, so not necessarily in the 5th intercostal space)

Using these 10 electrodes, 12 leads can be derived. There are 6 extremity leads and 6 precordial leads.

The Extremity Leads


The extremity leads are:

  • I from the right to the left arm
  • II from the right arm to the left leg
  • III from the left arm to the left leg

An easy rule to remember: lead I + lead III = lead II This is done with the use of the height or depth, independent of the wave (QRS, P of T). Example: if in lead I, the QrS complex is 3 mm in height and in lead III 9mm, the height of the QRS-complex in lead II is 12mm.

Other extremity leads are:

  • AVL points to the left arm
  • AVR points to the right arm
  • AVF points to the feet

The capital A stands for "augmented" and V for "voltage".

(aVR + aVL + aVF = 0)

The Chest Leads

The precordial, or chestleads, (V1,V2,V3,V4,V5 and V6) 'observe' the depolarization wave in the frontal plane

Example: V1 is close to the right ventricle and the right atrium. Signals in these areas of the heart have the largest signal in this lead. V6 is the closest to the lateral wall of the left ventricle.

Lead reversals

Right and left arm lead reversal can be distinguished from the (much rarer) dextrocardia by looking at the precordial R wave progression.
Right arm and left leg lead reversal. Lead II now measures the signal between the left and right leg, which is remote from the heart.
A patient with dextrocardia (and previous inferior myocardial infarction)

Sometimes an ECG is not made properly. Mistakes do happen and leads can be switched. Always remain careful to check this or you might come to the wrong conclusions. One of the most common mistakes is to switch the right and left arm. This will result in negative complexes in I, indicating a right axis deviation!

Common mistakes are reversal of:

  • right leg and right arm:
    • Hardly any signal in lead II.
  • right and left arm electrodes;
    • reversal of leads II and III
    • reversal of leads aVR and aVL
  • left arm and left leg:
    • reversal of leads I and II
    • reversal of leads aVR and aVF
    • inversion of lead III
  • right arm and left leg:
    • inversion of leads I, II and III
    • reversal of leads aVR and aVF

It is possible to distinguish lead reversal and dextrocardia by watching the precordial leads. Dextrocardia will not show any R wave progression in leads V1-V6, whereas lead reversal will.


artifacts (disturbances) can have many causes. Common causes are:

  • Movement
  • Electrical interference

Special Leads

Leads V7,V8 and V9 can be helpful in the diagnosis of posterior myocardial infarction

In case of an inferior wall infarct, extra leads may be used:

  1. In a right side ECG, V1 and V2 remain on the same place.V3 to V6 are placed on the same place but mirrored on the chest. So V4 is in the middle of the right clavicle. On the ECG it should be marked that it is a Right sided ECG. V4R (V4 but right sided) is a sensitive lead to diagnose right ventricular infarction.
  2. Leads V7-V8-V9 can be used to diagnose a posteriorinfarct. It is means that after V6, leads are placed towards the back. See the chapterIschemia for other ways of diagnosing posterior infarction.


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  3. Hoffa M, Ludwig C. 1850. Einige neue versuche uber herzbewegung. Zeitschrift Rationelle Medizin, 9: 107-144

  4. Waller AD. A demonstration on man of electromotive changes accompanying the heart's beat. J Physiol (London) 1887;8:229-234

  5. Einthoven W. Le telecardiogramme. Arch Int de Physiol 1906;4:132-164

  6. Einthoven W. Über die Form des menschlichen Electrocardiogramms. Pfügers Archiv maart 1895, pagina 101-123

  7. Marey EJ. Des variations electriques des muscles et du couer en particulier etudies au moyen de l'electrometre de M Lippman. Compres Rendus Hebdomadaires des Seances de l'Acadamie des sciences 1876;82:975-977

  8. Márquez MF, Colín L, Guevara M, Iturralde P, and Hermosillo AG. Common electrocardiographic artifacts mimicking arrhythmias in ambulatory monitoring. Am Heart J. 2002 Aug;144(2):187-97. DOI:10.1067/mhj.2002.124047 | PubMed ID:12177632 | HubMed [Marquez]
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All Medline abstracts: PubMed | HubMed