12Lead ECG

EKG Course

EKG Course – Understanding Electrocardiogram Complexes

Interpreting an electrocardiogram (EKG) is arguably the most important skill used by health professionals in the critical care setting. Not only does it save valuable time which can mean life or death for patients with cardiovascular disease, but it also provides a cost-effective means of monitoring the effectiveness of treatment. For effective EKG interpretation, one must first interpret its various components, which will be discussed in this short but concise EKG course.

Atrial depolarization: the P-wave


In a normal heart an action potential generated in the SA-node will lead to depolarization of the atrial syncytium. The electrical activity associated with the depolarization of the atrial syncytium is visualized as a smooth complex on the ECG and is called the P wave. Normally, the P wave is an upright or positively deflected ECG complex.

The PQ or PR interval

PQ or PR Interval

Once a cardiac impulse from the atria reaches the AV-node its speed of conduction is delayed. At this time no electrical activity is visible on the ECG. The electrical activity flowing through the minute and well-isolated AV-nodal fibers is too weak to be recorded by the ECG leads. Consequently, the phase corresponding to the passage of the cardiac impulse through the AV-node shows an isoelectric (flat) line on the ECG. This isoelectric segment combined with the P wave is called the PQ interval (or PR interval). A normal PQ interval or PR interval ranges from 120 milliseconds to 200 milliseconds.

The QRS complex

QRS Complex

About 120 to 200 milliseconds after the generation of an action potential in the SA-node, the cardiac impulse reaches the ventricles through the AV-nodal/His bundle system. The cardiac impulse is conducted to the ventricular mass through the bundle branches and the Purkinje fibers.

The QRS complex represents ventricular depolarization. Normally, the QRS complex is seen as a “sharp” complex, resulting from the rapid depolarization of the ventricles via the fast conducting Purkinje fibers of the bundle branches. Compared to the intraatrial conduction speed, the conduction speed of the bundle branches is high, averaging about 4 meters per second.

The duration of the QRS complex should not exceed 100 ms. A prolonged duration of the QRS complex signifies a conduction delay in the ventricles. Causes of intraventricular conduction delay include:

The ST segment

ST Segment

Following complete depolarization, the entire ventricular mass is negatively charged with each region having the same magnitude of charge. Thus, the electrodes measure no potential difference (voltage) between the different parts of the ventricles and this registers in the ECG as a flat line with 0 voltage (isoelectric line). This flat segment is called the ST segment. The point where the QRS complex ends and the ST segment begins is called the J point.

The T wave and ST-T segment

T wave and ST-T Segment

The ECG segment representing ventricular repolarization is called the ST-T segment and consists of the ST segment and the T wave. The T wave represents the major part of the electrical activity during ventricular repolarization and is seen as a broad, relatively slowly ascending and subsequently descending wave.

Any deviation from normal findings on the EKG suggests the possibility of cardiovascular disease. To learn EKG analysis further, a very comprehensive EKG course is available at Medical-ELearning.com, which offers the best online medical training to date.