Essential Concepts: What Is an ECG?

The electrocardiogram ( ECG or EKG ) is a special type of graph that represents changes in cardiac electrical activity from one instant to the next. Specifically, the ECG provides a time-voltage chart of the heartbeat.

10 seconds of ECG data (lead II) from healthy young adult. Note variation in rate due to breathing.

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The ECG is a key component of clinical diagnosis and management of inpatients and outpatients because it often provides critical information. Therefore, a major focus of this book is on recognizing and understanding the “signature” ECG findings in life-threatening conditions such as acute myocardial ischemia and infarction, hypertension, severe hyperkalemia or hypokalemia, hypothermia, certain types of drug toxicity that may induce cardiac arrest, pericardial (cardiac) tamponade, abnormal heart rhythms (arrhythmias) among many others.

The general study of ECGs, including its clinical applications, technologic aspects, and basic science underpinnings, comprises the field of electrocardiography , or more generally, electrocardiology . The broader field of cardiac electrophysiology includes electrocardiography (recordings from the surface of the body), intra-cardiac recordings and cardiac implantable electronic devices (pacemakers and defibrillators; Chapter 22 ), ablation therapy , as well as basic studies of the electrical properties of cardiac cells and tissues .

The device used to obtain and display the conventional (12-lead) ECG is called the electrocardiograph , or more informally, the ECG machine or device. It records cardiac electrical currents (voltages or potentials) by means of sensors, called electrodes , selectively positioned on the surface of the body. ^a

a As discussed in Chapter 3 , stated more precisely, ECG “leads” record the time-varying differences in electrical potential between pairs or configurations of electrodes.

Students and clinicians are often understandably confused by the basic terminology that labels the graphical recording as the electrocardio gram and the recording device as the electrocardio graph ! We will point out other potentially confusing ECG semantics as we go along.

Contemporary ECGs used in day-to-day clinical medicine are usually recorded with disposable paste-on (adhesive) silver–silver chloride electrodes. For the standard ECG recording, electrodes are placed on the lower arms, lower legs, and across the chest wall (precordium). In settings such as emergency departments, cardiac and intensive care units (CCUs and ICUs), and ambulatory (e.g., Holter and long-term) monitoring, only one or two “rhythm strip” leads may be recorded, usually by means of a few chest and abdominal electrodes.

We are also well into a burgeoning era of ECG recorders being directly marketed to consumers. These products include hand-held devices that record single lead or multi-lead ECGs. Users can then transmit the recordings to personal laptop or desktop computers and send them electronically to medical caregivers. Complementary “medical wearable” products include “smartwatches” that record and transmit single lead ECGs (typically 30 seconds). Discussion of the uses and limitations of these devices, as well as of their underlying technologies, goes beyond the scope of this introductory book.