The QRS complex in an electrocardiogram (ECG) can vary significantly in configuration and amplitude among individuals. Typically, its duration falls between 0.06 and 0.10 seconds. However, if the QRS complex is abnormally prolonged, it may indicate a blockage in the normal conduction pathways through the ventricles, such as a blockage in the left or right bundle branch.

During the ST interval, the entire ventricular myocardium experiences depolarization. As a result, the ST segment is expected to lie on the isoelectric line. Any noticeable deviation of the ST segment from this line may suggest ischemic damage to the myocardium.

The QT interval, often referred to as the "electrical systole" of the ventricles, correlates closely with the mean action potential duration of the ventricular myocytes. Its duration is typically around 0.4 seconds, but it varies inversely with the heart rate due to the variation in the myocardial cell's action potential duration.

In most leads, the T wave aligns with the major component of the QRS complex, deflecting in the same direction from the isoelectric line. However, it is normal for certain leads to exhibit biphasic (oppositely directed) T waves. If both the T wave and QRS complex deviate in the same direction from the isoelectric line, it indicates that the repolarization process is occurring in the opposite direction to the depolarization process. Abnormal T waves, either in direction or amplitude, may indicate myocardial damage, electrolyte disturbances, or cardiac hypertrophy.

The original ECG lead system, devised by Willem Einthoven in the early 20th century, uses the resultant cardiac vector to represent the vector sum of all cardiac electrical activity at any given moment. This directional electrical force is considered to lie at the center of an equilateral triangle, known as Einthoven's triangle, with apices located in the left and right shoulders and the pubic region. Einthoven's triangle is oriented in the frontal plane of the body, and the ECG lead system detects only the projection of the resultant cardiac vector on this plane. For ease of use, electrodes are connected to the right and left forearms instead of the corresponding shoulders, as the arms are viewed as simple electrical extensions of the leads from the shoulders. Similarly, the leg is an extension of the lead system from the pubis, so the third electrode is generally connected to an ankle, typically the left one.

Specific conventions dictate how the standard limb leads are connected to the electrocardiograph. Lead I records the potential difference between the left arm and the right arm. The connections are such that when the potential at the left arm (VLA) exceeds the potential at the right arm (VRA), the tracing is deflected upward from the isoelectric line. This arrangement of connections for lead I is denoted by a plus sign at the left arm and a minus sign at the right arm. Lead II records the potential difference between the right arm and the left leg, with the tracing deflecting upward when the potential at the left leg (VLL) exceeds VRA. Finally, lead III registers the potential difference between the left arm and the left leg, and the tracing is deflected upward when VLL exceeds VLA. These connections were arbitrarily chosen to ensure that the QRS complexes appear upright in all three standard limb leads in most normal individuals.

If the frontal projection of a resultant cardiac vector is represented by an arrow, with the tail being negative and the head being positive, the potential difference VLA - VRA recorded in lead I is represented by the component of the vector projected along the horizontal line between the left arm and the right arm. This is also shown in the diagram. If the vector makes an angle (θ) of 60 degrees with the horizontal line, as depicted in the diagram, the potential difference VLA - VRA recorded in lead I is represented by the component of the vector projected along that line.
The configuration and amplitude of the QRS complex vary considerably among individuals. The duration is usually between 0.06 and 0.10 second. An abnormally prolonged QRS complex may indicate a block in the normal conduction pathways through the ventricles (such as a block of the left or right bundle branch). During the ST interval, the entire ventricular myocardium is depolarized. Therefore, the ST segment normally lies on the isoelectric line. Any appreciable deviation of the ST segment from the isoelectric line may indicate ischemic damage to the myocardium. The QT interval, sometimes referred to as the period of “electrical systole” of the ventricles, is closely correlated with the mean action potential duration of the ventricular myocytes. The duration of the QT interval is approximately 0.4 second, but it varies inversely with the heart rate, mainly because the duration of the myocardial cell’s action potential varies inversely with the heart rate (see
Fig. 16.16
In most leads, the T wave is deflected in the same direction from the isoelectric line as the major component of the QRS complex, although biphasic (i.e., oppositely directed) T waves are perfectly normal in certain leads. Deviation of the T wave and QRS complex in the same direction from the isoelectric line indicates that the repolarization process is proceeding in a direction counter to that of the depolarization process. T waves that are abnormal either in direction or in amplitude may indicate myocardial damage, electrolyte disturbances, or cardiac hypertrophy.
The original ECG lead system was devised by Willem Einthoven at the beginning of the 20th century. In this system, the vector sum of all cardiac electrical activity at any moment is called the resultant cardiac vector. This directional electrical force is considered to lie in the center of an equilateral triangle whose apices are located in the left and right shoulders and the pubic region (
Fig. 16.27 ). This triangle, called Einthoven’s triangle, is oriented in the frontal plane of the body. Hence, only the projection of the resultant cardiac vector on the frontal plane is detected by this system of leads. For convenience, the electrodes are connected to the right and left forearms rather than to the corresponding shoulders because the arms represent simple electrical extensions of leads from the shoulders. Similarly, the leg represents an extension of the lead system from the pubis, and thus the third electrode is generally connected to an ankle (usually the left one).
Certain conventions dictate the manner in which these standard limb leads are connected to the electrocardiograph. Lead I records the potential difference between the left arm and the right arm. The connections are such that when the •Fig. 16.27 Einthoven’striangle,illustratingtheelectrocardiographicconnectionsforstandardlimbleadsI,II,andIII.
potential at the left arm (VLA) exceeds the potential at the right arm (VRA), the tracing is deflected upward from the isoelectric line. In
Fig. 16.27
Fig. 16.28 , this arrangement of connections for lead I is designated by a plus sign at the left arm and by a minus sign at the right arm. Lead II records the potential difference between the right arm and the left leg, and the tracing is deflected upward when the potential at the left leg (VLL) exceeds VRA. Finally, lead III registers the potential difference between the left arm and the left leg, and the tracing is deflected upward when VLL exceeds VLA. These connections were chosen arbitrarily so that the QRS complexes are upright in all three standard limb leads in most normal individuals.
If the frontal projection of a resultant cardiac vector at some moment is represented by an arrow (tail negative, head positive), as in
Fig. 16.27 , the potential difference, VLA − VRA, recorded in lead I is represented by the component of the vector projected along the horizontal line between the left arm and the right arm, also shown in
Fig. 16.27 . If the vector makes an angle (θ) of 60 degrees with the horizontal line (as in