Electrocardiography Unit Health Article

Waveform production

When the electrodes are placed as described, the electrical activity of the heart is printed in line patterns known as waves or waveforms. Waves come in two types—positive deflection (movement above the baseline or isoelectric line) and negative deflection (movement below the isoelectric line). Positive deflection is created when electrical activity flows toward the positive electrode; a negative deflection is produced when current flows away from the positive electrode (toward the negative). No heart activity produces a baseline or isoelectric waveform. The isoelectric line is normally the beginning and ending of all waveforms.

EKGs are recorded on strips of graph paper that are fed through the machine at a constant rate (25 mm/sec or 1 in/sec) to allow for easy estimates of beats per time period and for points of comparison between the isoelectric line and the wave. At this standard feed rate, each small block of the graph paper represents 0.04 seconds, each larger dark box (having a 5 × 5 group of small boxes within it) is 0.2 seconds.

The normal heartbeat begins with an electrical impulse in the part of the heart with the fastest innate beat, the sinoatrial (SA) node. The electrical activity travels through the heart tissue, in a process known as depolarization, from the upper right of the organ to the lower left. Five major waves are produced: the P wave; the Q, R, and S waves (known as the QRS complex); and the T wave.

The P wave results from the depolarization of both atria and is a rounded, upward deflection that usually lasts about 0.10 seconds (about two small blocks of graph paper). The PR interval (PRI) is the time needed for the electrical impulse to travel from the atria to the ventricles. Normally, this lasts about three to five small squares (or 0.12 to 0.20 seconds).

The QRS complex has three recognized events and is the conduction of the impulse through the bundle of His and throughout the ventricles and atrial repolarization. The first downward deflection after PRI is the Q wave. It is followed by the largest deflection seen, the upward deflection of the R wave. Immediately after the R wave is a downward deflection called the S wave. The QRS complex generally happens in less than 0.12 seconds (three small squares) and all three waves are not always present, even in people with normal heart function.

The time interval between ventricular depolarization and repolarization is known as the ST segment and it is normally isoelectric (baseline). The full cycle is completed with the T wave, which is the result of the ventricles repolarizing. This wave is often a slightly asymmetrical, rounded positive deflection that finishes at the baseline.

Operation

To perform a resting EKG, the patient is placed on a table and the 12 electrodes are attached as described above. Sometimes, to improve connection, the areas of the skin where the electrodes will be placed are shaved or have conductive gel applied. Because some types of heart conditions are only evident when the heart is under stress, EKG analysis can also be performed with the patient on a treadmill.

Sometimes a patient's symptoms occur at unpredictable intervals and are not exercise related. Heart activity can then be followed by a special portable EKG machine known as a Holter monitor. This EKG has three electrodes and stores the information for the monitoring period (generally from 24 hours to five days).

Diagnosing heart problems

In general, there are five aspects of the EKG that can reveal potential or present heart abnormalities: the heart rate, the heart rhythm, the P wave, the PR interval, and the QRS complex.

The heart rate is determined by counting the number of QRS complexes (for ventricular rate) or P waves (for atrial rate) over six seconds (30 large boxes on the graph paper). Normal is between 60 to 100 beats per minute (bpm). Less than 60 bpm is considered a slow or brady-cardic rate and greater than 100 bpm is considered a fast, or tachycardic rate.

The heart rhythm, as revealed by the waveform pattern, can be classified either regular or irregular. To determine whether the ventricular rhythm is regular, a measurement is made from R-to-R wave. A measurement from P-to-P wave determines the regularity of the atrial rhythm. If the interval is the same between waves, the rhythm is regular, if different, the rhythm is irregular.

Numerous changes in the P wave, PR interval, and QRS complex are possible. They depend on the actual damage to the heart, such as those that accompany a heart attack (myocardial infarction or MI). First, as the heart becomes ischemic, or starved for oxygenated blood, repolarization of the ventricles becomes abnormal; this depresses the ST segment more than 1 mm below baseline and the T wave becomes inverted. Next, if there is no treatment of the ischemia, actual damage to the heart tissue will occur. This can be seen through an elevation of the ST segment of more than 1 mm above baseline.

If the heart attack actually occurs (one or more coronary arteries becomes completely blocked), at least three possible indications can appear on the EKG. First, if it hasn't already happened, the ST segment will become elevated and the T wave will invert. Changes in the ST segment will remain for up to four weeks after the attack and the T wave could remain inverted for a year. If all