A Basic Overview of the ECG Waveform

An Electrocardiogram or ECG is a test of the electrical activity of the heart, generally performed by placing stickers on the patient's chest, then attaching wires, called 'Leads" to the stickers which connect the patient to the machine. The wires sense the electrical activity of the heart and transmit the information to the ECG machine, which converts the information into lines on a screen, or a paper printout.

With training and practice, it is possible to read these lines and identify problems with the heart, either with blood flow, in the case of a heart attack, or conduction of the electrical impulse itself. While it takes training and experience to correctly read and ECG, laymen's knowledge will help patients understand what their doctor is explaining, and what the test can, and cannot, determine.

In order to understand what the lines mean, it is necessary to understand the basic, correct, electrical conduction system of the heart. The heart is divided into 4 chambers, the bottom two larger than the top two, and the left side larger than the right. The top two chambers are called the right and left atrium, and the bottom two called the right and left ventricle. The electrical pattern of the heart will follow a preferred pathway, like a well-worn trail, and this regular pattern of activity is what displays on a healthy ECG

At the top of the heart, the normal pacemaker, which is supposed to start the heart electrical cycle, is called the SA, or Sino-Atrial node. The SA node generates the initial electrical impulse, which causes the cells in the atriums to depolarize, and then contract. It is important to remember that the presence of the impulse does not mean the heart actually contracts, or that blood is pumped. There are several mechanical problems that can interfere with this process that the ECG will not identify. After the impulse reaches the end of the atrium, moving down the heart, it is stopped at something called the A-V, or Atrial-Ventricular Node.

The AV node stops the impulse from continuing to the ventricles right away, generally delaying it 120 to 200ms long, in a healthy heart, from the start of the impulse at the SA node. This allows the atriums to fully contract, and allow the ventricles to fully fill. This "Atrial Kick" is generally considered to allow a 30% increase in the total blood flow through the heart. If something happens to the SA node, the AV node can also generate regular electrical impulses causing the heart to beat, though usually at a slower rate. This is called a Junctional Escape Rhythm, and is intended to maintain blood flow if the SA node is disabled or malfunctioning.

After the delay of the AV node, the electrical impulse is allowed to continue into the Ventricles along an area called the Bundles of His, and into the Purkinje Fibers. This causes the cells in the ventricles to contract, and pump blood out of the heart. The Purkinje Fibers can also generate electrical impulses, causing the heart to beat. This is intended as a last defense against electrical standstill, called Asystole, in the heart, though the ability of the cells of the ventricles to generate the impulses can actually lead to several severe lift threatening consequences. If the Purkinje Fibers are causing the heart to beat on their own, this is called the Ventricular Escape Rhythm. It is generally the last effective pacemaker available to the heart.

After the cells have fully depolarized, they have to reset, to allow the process to repeat. This process, called repolarization, occurs first in the atrium and then the ventricles. The ventricular repolarization is the last wave a healthy ECG.

Now that the basic electrical process of the heart has been explained, the various wave forms on the ECG can be examined. Each wave in the entire process of a single beat is given a letter designation, starting with p, and ending with t. This means there are P, Q, R, S, and T waves. Each of these represents a distinct part of the electrical cycle, though the waves themselves are generally talked about in groups, the P- wave, the QRS complex, and the T-wave.

The P wave represents the depolarization of the atrium, and in a healthy heart is an upward wave, before the QRS complex. The wave can also be upside down, called and Inverted P-wave, which can indicate a conduction problem in the atrium. From the start of the P-wave to the start of the QRS complex is called the P-R Interval. This is a measure of the delay in the A-V node.

After the P was is the QRS complex. In a healthy heart, there is no Q wave, which is the first negative deflection after the P wave on the ECG. The Q wave sometimes developed after some sort of conduction disturbance after a heart attack. Most hearts will follow the P wave with an R wave, which is an upward deflection after the P-wave. The S-wave is a downward deflection, after the R-wave. The length of time it takes for the QRS complex is a measure of the conduction pathways through the heart. In a healthy heart the conduction time is generally 40ms to 80ms. Longer times can show a problem with the conduction pathway, or an electrical beat generated outside the normal conduction pathway.

Following the QRS complex, the last wave is the T-wave, which is ventricular repolarization, which is an upward deflection after the QRS complex. Measure the time from the start of the QRS interval to the end of the T-wave is called the Q-T interval and is the total time for the ventricle fully contract, and relax and be ready to beat again.

Understanding these waves is a step towards understanding the possible conduction problems with the heart, and helps you understand the test you may undergo from your doctor. You should never attempt to diagnosis your own heart problems and should contact your personal doctor with any problems.