The ECG produces a heartbeat rhythm involving four electrical entities. The first is the P wave (which is representative of atrial depolarization, the QRS wave (which represents ventricular depolarization), the T wave (which represents ventricular repolarization), and the rarely-seen U wave (which represents purkinje fiber repolarization). The findings are reported on a grid with large and small boxes. Each small box is 1 mm by 1 mm, while each large box is 5 mm by 5 mm and represents 0.2 seconds. The ECG can detect ischemic changes and necrotic aspects of the heart. ST depression and non-ST segment myocardial infarctions represent ischemic changes in the heart. Inversion of the T waves can also indicate ischemia. ST elevation is characteristic of necrosis and can be seen in an ST-segment elevation myocardial infarction (STEMI). The first sign of an MI is hyperacute (peaked) T waves secondary to localized hyperkalemia in the heart. ST segment elevation happens next. After several hours, the ST segment will rise and, finally, the T wave inverts and a pathologic Q wave will develop. The ST segment elevation eventually resolves but the Q wave persists. Because the ECG only measures the function and rhythm of the heart at rest and only measures the heart for a couple of minutes, it may not detect an important arrhythmia of the heart. For this reason, there are several ways of detecting the heart rhythm over a period of time, such as the Holter Monitor and the implantable loop recorder.
Holter Monitor The Holter monitor is a technique that measures the rhythm of the heart over a 24-hour period of time. The patient wears a small device attached to the body by means of several electrodes on the chest itself. It continuously records the cardiac rhythm over a single day and is linked to the patient recording the time and nature of any symptoms they might have. The symptoms are evaluated along with the rhythm strip to see if the patient has any type of cardiac arrhythmia. Like the ECG, the Holter monitor evaluates the electrical activity of the heart but in a more portable way and over a longer period of time. It is used for as long as two weeks at a time. The main purpose of the Holter monitor is to detect occasional cardiac arrhythmias. For rarer symptoms, a cardiac event monitor is used (which is used for a month). Electrodes are attached to the bony parts of the chest, using between three and eight electrodes connected to a small device hung around the neck or placed in the pocket. The device records the heart’s electrical activity over the period of time the device is worn. Unlike the 12-lead ECG, there are only 2-3 leads in a standard Holter monitor, which is operated by battery. There are 12-lead/channel Holter monitors but these are uncommon. These types of monitors sometimes can detect ischemic changes in one of the leads. Newer models can sense the patient’s activity level as it records the electrical findings of the heart. Because there are fewer leads, the Holter monitor is not as good for detecting things like ST-segment changes.
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