An ECG test (EKG or electrocardiogram) is a recording of the electrical activity of the heart from different vantage points and the displaying of it in a graphic form. It is a fairly inexpensive test but provides a lot of bang for the buck. It reveals valuable information about the heart when it is healthy and when it is not. It can also provide clues to some conditions which don’t directly involve the heart but which can affect it.
The graphical information the ECG displays coincides with the electrical events from one heartbeat to the next in real time. Each cardiac cycle (heart beat and the time before the next beat) correlates with specific waveforms on the EKG tracing. They are the P wave, PR interval, QRS complex, ST segment and T wave. Each has a typical appearance on a normal tracing but their shape and/or duration are different when there is a problem(s). Of note is the fact that Q waves – the deflections of the QRS complexes that point downward – are often not present in some leads under normal circumstances.
The P wave on the ECG tracing correlates with electrical stimulation and contraction of the left and right atria. The QRS complex represents the travel of electrical impulses from the AV node to the Purkinje fibers and myocardial cells which results in contraction of the right and left ventricles. The PR interval corresponds to the time in between contraction of the atria and ventricles. It is also the time for an electrical impulse generated in the SA node to pass through the AV node before reaching the ventricles. T waves correspond with the reestablishment of charge differences between the inside and the outside of heart cells (repolarization) in preparation for the next triggered heartbeat. Elevation or depression of the ST segment is a common sign of a heart attack or ischemia. But can also be due to other factors.
The ECG test machine gathers information about the heart through leads. ECG leads are different angles from which the ECG machine views and records electrical activity moving through the heart. Each lead has a positive pole and a negative pole aligned in totally opposite directions. In other words, the angle between the two is 180 degrees. Electrodes –sensors attached to the skin connected to wires from the machine – are the source of input from the different locations of the body (leads) to the machine.
Limb leads are positions along the flat plane of the chest. Chest leads are in a plane around the chest looking down on the top of the head. They extend from a point just to the right of the breast bone to the left side of the chest wall. A standard ECG test consists of six limb leads and six chest leads. Each lead in each plane is 30° apart. Thus, the test provides views of electrical activity moving through the heart from several vantage points.
The lead concept is akin to instant replay in the National Football League. The officiating crew often has to view a play from several angles to make the correct call. The difference here is the ECG test machine makes the correct call in real time. Much like the referees though, the person who conducts the test must be precise and the machine must be functional.
The electrodes (terminal sensors) are the positive poles of each lead. As such they interpret electrical activity approaching them as positive. In doing so they cause upward deflections on the ECG tracing as the two large and small chambers of the heart pump blood out and as changes in the electrical charges occur across the surfaces of the two large chambers immediately before the next heartbeat. When electrodes sense net electrical movement away from them during these events they produce negative deflections in their respective leads on the ECG tracing.
The direction of movement of electrical activity through the heart in the frontal plane during the QRS complex is the ECG axis. In a normal heart it is toward the left lower quadrant of the frontal plane of the chest. It ranges from 0° to 90°.
The ECG test can show whether a heart attack is in progress, acute or remote. With the wealth of valuable data it provides it also reveals the following:
- The heart rate (how fast the heart beats)
- The heart rhythm – the regularity or irregularity of the heartbeat
- The presence of an arrhythmia
- A disturbance in electrical conduction (heart block)
- Left ventricular hypertrophy (LVH) – thickening of the muscle wall of the left ventricle
- Right ventricular hypertrophy (RVH)– thickening of the muscle wall of the right ventricle
- Evidence of enlargement of the left atrium
- Evidence of enlargement of the right atrium
- Evidence of an aneurysm of the heart muscle
- Evidence of pericarditis
- Evidence of a pericardial effusion
- Evidence of ischemia
- Evidence of myocarditis
- Evidence of a large pulmonary embolism
- Evidence of hyperkalemia or hypokalemia
- Evidence of hypercalcemia or hypocalcemia
The ECG test alone is not the cornerstone for detecting some of the above abnormal conditions. But can serve as a signpost for actions to establish or confirm a diagnosis. It is also a means of assessing the course of some conditions and their response to treatment. It is a valuable tool for health care providers and an asset to patients.