Introduction to ECG

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

Overview

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.

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

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.

• 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: 120-200 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

• 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)

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

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:
  • 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.

Shape of QRS complexes

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

Cardiac Axis

The cardiac axis is an imaginary vector corresponding to the average of the different depolarization vectors in the heart.

The limb leads look at heart in a vertical plan

• The normal depolarization vector is towards leads I, II and III. Hence, the deflections in these leads are positive in such a way that the deflection in lead II is more positive than that in leads I and III.
• The shapes of the QRS complex in leads I, II ad III are used to estimate the cardiac axis.
• The normal cardiac axis lies between the angles: -30 and +90 degrees.

• If right axis deviation is present, the axis will be tilted towards the right. Hence, the axis will be going away from lead I and thus R will be smaller than S in lead I and the axis will be going more towards lead III and thus R will be larger than S in lead III.
• If R is equal to S in lead II, then it is expected that the cardiac axis lies at a right angle from lead II. Hence, the cardiac axis would be either at an angle of -30 degrees or +150 degrees. The shape of the QRS complex, whether positively or negatively deflected, helps in determining the direction of the cardiac axis.

The V leads look at the heart in a horizontal plan

• 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: the right ventricle
  • V3 and V4: the septum
  • V5 and V6: 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.

How to begin reading an ECG

• When reading an ECG, the following sequence of parameters should be evaluated:
• Rhythm: sinus?
• Regular: Are RR intervals similar?
• Intervals: PR, QRS, QT, QTc (corrected QT)
• Axis (lead I, II, III)
• Description of P, QRS, ST, T
• Additional: Q wave, M shape, RSR’, delta shape…(To be discussed later)
• Progression from V1 to V6

External Links

http://en.ecgpedia.org/wiki/Main_Page

Template:WikiDoc Sources