Electrocardiograph, known as EKG in America, from the German Elektrokardiogramm is an interpretation of the electrical activity of the heart over time captured and externally recorded by electrodes placed on the skin. It is noninvasive and performed with an electrocardiographic medical equipment device. The word is derived from the Greek—electro, because it is related to electrical activity, cardio for heart, and graph, meaning to write.

The electrocardiograph was invented in 1903 by Willem Einthoven who won a Nobel Prize for his discovery. His medical equipment is a direct descendant of the equipment used today. Rather than using today's self-adhesive electrodes Einthoven's subjects at the turn of the century would immerse each of their limbs into containers of salt solutions from which the EKG was recorded.

The EKG today is one of the most important diagnostic medical equipment devices. It is commonly used in almost every kind of clinical setting, including exam rooms, recovery rooms, cardiology clinics, and emergency rooms. EKG is the best method for determining cardiac murmurs, seizures, perceived cardiac and dysrhythmias. The EKG is a well-known feature in movies and television shows about medical procedures and hospitals because of its instantly recognizable external display of heartbeat. It's become a cliché, but the cliché is based in the reality that EKG medical equipment is ubiquitous.

With the development of silicon chip technology and smaller, lightweight electrodes, EKG devices have become smaller and more portable than ever before. EKG equipment is now almost always made with built in computers, handling massive amounts of information at a time. Many EKG systems still print out their graphs on paper, but more often today, doctors are looking at on-screen and even online versions of the graphs.

Cardiac Science a modern company which manufactures EKG medical equipment also invented the modern stress test and the first fully automatic bedside defibrillator. Defibrillators go hand in hand with the technology of EKG devices, although they are used in completely different ways. While an EKG is a monitoring, diagnostic machine, defibrillators are used in emergency situations only. While a patient can be monitored with an EKG for a routine test, a defibrillator in action is always a serious event.

Both defibrillators and EKG medical equipment work due to the fact that the smooth muscles of the coronary system are in motion in coronary circulation. The heart acts as a double pump. The function of the right side of the heart is to collect de-oxygenated blood in the right atrium from the body (via superior and inferior vena cavae) and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up. This is gas exchange. This works because of muscle contraction. Muscle contractions can be measured for their electrical signal, which comes from brain and nerve activity, and contractions can be induced by electric shock. In the case of the EKG monitor, the signals are just viewed and analyzed. In the defibrillator, the electric signal is used to restart a heart that has stopped beating.

Different waves signal different problems in the heartbeat, when viewed in an EKG. A normal EKG should have a regular curve to it, based on different graphing techniques. Abnormalities do not always mean a disease, but when judged with a patient's history in mind, repeat abnormalities and even rare ones can aid in a diagnosis by a physician.