Introduction to ECG

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Assistant Editor(s)-in-Chief: Rim Halaby


Electrocardiography (ECG or EKG from the German Elektrokardiogramm) is a transthoracic (across the thorax or chest) interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body. The recording produced by this noninvasive procedure is termed electrocardiogram (also ECG or EKG). An ECG test records the electrical activity of the heart. ECG is used to measure the rate and regularity of heartbeats, as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart, such as a pacemaker. A 12-lead EKG is one in which 12 different electrical signals are recorded at approximately the same time and will often be used as a one-off recording of an ECG, traditionally printed out as a paper copy.

  • An EKG is a simple test that records the heart's electrical activity. The test shows how fast the heart is beating and its rhythm (steady or irregular).
  • An EKG also records the strength and timing of electrical signals as they pass through each part of the heart.
  • This test is used to detect and study many heart problems, such as heart attacks, arrhythmias (irregular heartbeats), and heart failure.
  • EKG results also can suggest other disorders that affect heart function.
  • The two most common types of portable EKGs are Holter and event monitors.

ECG Graph Paper

  • The output of an ECG recorder is a graph (or sometimes several graphs, representing each of the leads) with time represented on the x-axis and voltage represented on the y-axis.
  • At a paper speed of 25 mm/s:
    • One small block of ECG paper translates into 40 ms.
    • Five small blocks make up one large block, which translates into 200 ms.
    • Hence, there are five large blocks per second.
  • As for the amplitude:
    • 1 small box corresponds to 0.1 mV
Below is an image showing the significance of the small and large squares on an ECG graph paper.

ECG graph paper

Electrical Activity of the Heart

  • Electrical changes occur in the heart leading to depolarization and consequently cardiac muscle contraction. The depolarization starts spontaneously at the level of the sinoatrial node (SA node).
  • A normal rhythm starts at the SA node, and it is called sinus rhythm.
  • Later on, the depolarization spreads in the following direction:
    • SA node, atria à AV node (slow), bundle of his (fast), right and left bundles (fast) and then slowly spreads in ventricles due to “purkinje fibers”.
  • To note is that the conduction of the depolarization is slowest at the level of AV node causing a delay in the spreading of the electrical activity.

Waves and Intervals

  • A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex and a T wave.
  • The baseline voltage of the electrocardiogram is known as the isoelectric line.
Below is an image showing a typical ECG tracing.

Different parts of a normal ECG tracing

  • P wave
    • It corresponds to atrial depolarization
  • QRS wave
    • It corresponds to ventricular depolarization.
    • QRS is larger than the P wave because there is more muscular mass in the ventricles than that in the atria
    • Normal duration of QRS: 120 ms (3 small squares)
    • The formation of the QRS complex:
      • If the first deflection is downwards, it is called “Q wave”
      • Any upward deflection is called “R wave”, whether it is preceded by a Q wave or not
      • A downward deflection following an R wave is called “S wave”
  • T wave
    • It corresponds to ventricular repolarization (there is no discernable wave for atrial repolarization, it is buried within the QRS complex)
  • RR interval
    • The RR interval represents the distance between one R wave and another
    • A regular rhythm has identical RR throughout the ECG
  • PR interval
    • The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.
    • The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node and entering the ventricles. The PR interval is, therefore, a good estimate of AV node function.
    • Normal duration of the PR interval: 120-200 ms (3-5 small squares)
  • PR segment
    • The PR segment connects the P wave and the QRS complex.
    • The PR interval is more clinically relevant.
  • ST segment
    • The ST segment connects the QRS complex and the T wave. The ST segment represents the period when the ventricles are depolarized. It is isoelectric.

The ECG Electrodes

  • Electrical activity going through the heart can be measured by external (skin)electrodes. The electrocardiogram (ECG) registers these activities from electrodes which have been attached onto different places on the body. In total, twelve leads are calculated using ten electrodes.
  • The ten electrodes are:
  • The four 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
Below is an image showing the position of the four extremity electrodes.

The position of the four extremity electrodes

  • The six 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 nipple line. Official recommendations are to place V4 under the breast in women
    • 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)
Below is an image showing the position of the six chest electrodes.

The position of the six chest electrodes

  • With the use of these 10 electrodes, 12 leads can be derived. There are 6 extremity leads and 6 precordial leads.


  • The term "lead" in electrocardiography causes some confusion because it is used to refer to two different things. The word lead may be used to refer to the electrical cable attaching the electrodes to the ECG recorder. Alternatively (and some would say properly, in the context of electrocardiography), the word lead may refer to the tracing of the difference in voltage between two of the electrodes and is what is actually produced by the ECG recorder. For example "lead I" is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" is the voltage between the right arm and the feet.
  • The leads look at the heart from different directions and thus produce different ECG patterns.
  • There are 6 limb leads and 6 V leads.

Limb Leads Look at the Heart in a Vertical Plane:

  • Limbs I and VL: look at the left lateral side of the heart
  • Limbs II, III and VF: look at the inferior side of the heart
  • Limb VR: looks at the right atrium

V Leads Look at the Heart in a Horizontal Plane:

  • The depolarization first occurs in the septum (from left to right) before spreading to the ventricles.
  • The V leads reflect the depolarization of the following areas of the heart:
    • V1 and V2: look at the right ventricle
    • V3 and V4: look at the septum and the anterior wall of the left ventricle
    • V5 and V6: look at the anterior and lateral wall of the left ventricle
  • The septal depolarization corresponds to an initial positive deflection (R wave) in V1 and V2 and negative deflection (Q wave) in V3 and V4.
  • Because the left ventricular mass is larger than that of the right ventricle, the vector corresponding to the ventricular mass depolarization is directed towards the left. Thus, the ventricular depolarization corresponds to a negative deflection (S wave) in V1 and V2 and a positive deflection (R wave) in V3 and V4.
  • The progression of V1 to V6 is important to notice: The QRS complex is predominantly deflected downwards in V1 and gradually becomes positively deflected to show a predominantly upwards deflection in V6
  • The transition of progression corresponds to an R wave equal to the S wave. This is an indication of the position of the interventricular septum.
    • For example, if there is right ventricular hypertrophy, the septum will be more deviated towards the left and the transition of progression will become located in V5, V6 or even V7 instead of V3 or V4.

Shape of the QRS complex

  • The electrical depolarization waves in the heart occur in all directions; however, the direction of the main electrical vector corresponding to the sum of all the depolarization is roughly from the right upper side of the heart towards the left lower side of the heart.
  • Each of the 12 leads register a distinct form of an ECG pattern depending on the angle with which it looks to the depolarization wave:
    • When a depolarization spreads towards a lead there is a positive upward deflection and R >S
    • When a depolarization spreads away from the lead there is a negative downward deflection and R <S
    • When a depolarization spreads in a direction at a right angle from the lead, R=S
Below is an image showing different shapes of QRS depending on the position of the leads.

Deflection of waves

Below is an image showing different shapes of QRS.

Different QRS shapes

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