• Cardiovascular System
  • Physiology

Electrocardiogram Physiology, Waves, Intervals, Axis and Segments

  • Reading time: 7 minutes, 16 seconds
  • 137 Views
  • Revised on: 2020-08-06

The electrocardiogram (ECG or EKG) is a diagnostic tool that is routinely used to assess the electrical and muscular functions of the heart. It is a recording of the small extracellular signals that are produced by the movement of action potentials through heart cells known as cardiac myocytes.

As cardiac cells depolarize and repolarize, electrical currents spread throughout the body because the tissues surrounding the heart are able to conduct electrical currents generated by the heart.

When these electrical currents are measured by an array of electrodes placed at specific locations on the body surface, the recorded tracing is called an electrocardiogram

When the impulse is conducted, the inside of the myocardial cell becomes positive and the outside becomes negative. If electrodes are placed over the chest wall and the electrical impulse travels toward it, the recording of the electrode shows a positive or upward deflection. When the impulse moves away from the electrode, it shows a negative deflection.

How to obtain an electrocardiogram

By placing the electrodes in different standard parts of the chest wall, arms, and legs, the electrical activity of the heart can be captured in various “viewpoints”.

To obtain a standard 12-lead electrocardiograph, two electrodes are placed on the upper extremities, two on the lower extremities, and six on standard locations across the chest.

The electrodes that are placed on the extremities generate the six limb leads (three standard and three augmented), and the chest electrodes produce the six precordial leads.

Typically, electrodes are placed on the arms and legs and at six chest positions which we shall look at on which landmarks are they placed. The electrical change is recorded by an electrocardiograph.

What is a lead?

For a better understanding of the electrocardiogram, it is recommended that you understand the basic terminologies that are used. One of them is a ‘’lead’’.

In a lead, one electrode is treated as the positive side of a voltmeter and one or more electrodes as the negative side. Therefore, lead records the voltage difference between positive and negative electrodes.

A pair of electrodes form a lead.

An electrocardiogram looks at the heart from two planes, the frontal plane and the transverse plane.

The frontal plane is defined by the six limb leads and the perpendicular transverse plane is defined by the six precordial leads.

Each lead is an axis in one of the two planes, onto which the heart projects its electrical activity.

Placement of the leads

As mentioned above we obtain a 12 lead electrocardiogram from limb leads and chest leads. Therefore having a strong understanding of how these leads are placed is key. Let's look at the placement of these leads.

Limb leads

A 12-lead ECG is taken by having the patient relax in a supine position and connecting four electrodes to the limbs. These leads are known as bipolar limb leads. This means that the electrocardiogram is recorded from two electrodes located on different sides of the heart.

The trunk and limbs are viewed as an equilateral triangle known as Einthoven’s triangle with one vertex on the groin and the other two on the shoulder joints.

Clinically you may find these leads connected either to the wrist or the shoulder. This is because, electrical attachment to an arm is electrically equivalent to a connection at the shoulder joint, and an attachment to either leg is equivalent to a connection at the groin.

By convention, the left leg represents the groin.

The fourth electrode, connected to the right leg, is used for electrical grounding. The three limb leads represent the difference between two of the limb electrodes:

limb leads

The four limb leads are attached to the two arms and legs

  • Red = Right Arm;
  • Yellow = Left Arm;
  • Green = Left Leg;
  • Black = Right Leg:

By using these leads, positions I, II, III, aVR, AVL, and aVF are derived.

Lead I

Lead I is constructed by placing a positive connection to the left arm and a negative connection to the right arm. This lead, therefore, defines an axis in the frontal plane at 0 degrees.

Lead II

Lead II is constructed by placing a positive electrode to the left leg and a negative to right arm.  It defines an axis in the frontal plane at 60 degrees.

Lead III

This lead is formed by placing a positive electrode to the left leg and negative electrode to the left arm. This lead defines an axis in the frontal plane at 120 degrees

Besides these three leads, there are also another three leads that are formed from the limb leads connection. These leads are known as augmented unipolar limb leads; aVR, aVL, and Avf.

In the naming of these leads, a stands for augmented, and the V represents unipolar.

Electronic reconstruction of the three limb connection creates a virtual electrical reference point in the middle of the heart. This is known as Wilson’s central terminal. This point constitutes the negative connection for the augmented “unipolar” limb leads and also for the chest leads.

ecg leads

The three augmented unipolar limb leads compare one limb electrode to the average of the other two:

aVR

This is formed by a positive connection to the right arm and a negative connection is Wilson’s central terminal at the center of the heart.

This limb defines an axis in the frontal plane that is −150 degrees.

aVL

This lead is formed by placing a positive electrode to the left arm as the negative electrode is Wilson’s central terminal. The axis defined by this limb lead in the frontal plane is −30 degrees.

aVF

Lastly, this limb is formed by a positive electrode to the left leg [foot] and Wilson’s central terminal as the negative. The axis defined by this limb lead in the frontal plane is +90 degrees.

If you have been keen on noticing the axis, the positive and negative ends of these six leads define axes every 30 degrees in the frontal plane.

Chest/Precordial Leads

The chest leads lie in the transverse plane of the body and they are perpendicular to the plane of the frontal leads.

The positive connection is one of six different locations on the chest wall, and Wilson’s central electrode forms the negative connection.

electrocardiogram chest leads

Just like the nomenclature in the augmented limb leads, the V stands for unipolar.

The six chest leads are then attached as follows:

V1 Red: fourth intercostal space, right sternal border.

V2 Yellow: fourth intercostal space, left sternal border.

V3 Green: midway between V2 and V4.

V4 Brown: fifth intercostal space, left mid-clavicular line.

V5 Black: level with V4, left an anterior axillary line.

V6 Violet: level with V4, left mid-axillary line.

Typically, the electrical activity during each contraction of the heart is recorded as a series of positive and negative waves, each part of the wave represent impulse conduction along with different parts of the heart.

Electrocardiogram Waves

The fluctuations in extracellular voltage recorded by each lead vary from fractions of a millivolt to several millivolts. These fluctuations are called waves and are named with the letters of the alphabet

The repeating waves of the electrocardiogram represent the sequence of depolarization and repolarization of the atria and ventricles.

electtocardiogram waves

The ECG measures volts (y-axis) per unit time (x-axis) and it is generally recorded on paper at a speed of 25 mm/sec and with a vertical calibration of 1 mV/cm. Therefore, every 25 millimeters in the horizontal direction represent 1 second, and each 5-millimeter segment, marked by the dark vertical lines, represents 0.20 second. The 0.20-second intervals are then broken into five smaller intervals by thin lines, each of which represents 0.04 seconds.

y-axis (volts): one big box = 0.5 mV and x-axis (time): one big box = 0.2 sec (200 msec)

An ECG consists of a set of waves: P wave, a QRS complex, and a T wave

The P wave

The P wave represents left and right atrial depolarization as an impulse started by the SA node travels throughout the atria.

The period after the P wave represents the time in which the atrial cells are depolarized and the impulse is traveling within the AV node, where conduction velocity is greatly reduced.

The P wave signals that the atria are going to be in systole and that the atrial myocardium is about to contract. it is usually 0.08 to 0.12 seconds or 3 small squares.

The first one-third of the P wave corresponds to right atrial activation, the last one third corresponds to left atrial activation; the middle 1/3 is a combination of the two.

In most leads, the right and left atrial waveforms to move in the same direction, forming a monophasic P wave.

However, in lead V1 the right and left atrial waveforms to move