What is an electrocardiograph?
The modern electrocardiograph was invented by Willem Einthoven in 1903. An electrocardiograph records the electrical activity of the heart from the surface of the body and the recording is known as an electrocardiogram (ECG). It is one of the simplest and widely available investigations to assess the heart. ECG is an important part of cardiac evaluation, especially in the emergency room when the patient presents with chest pain. In spite of the availability of many modern high tech investigations, the role of a simple ECG in the diagnosis of acute myocardial infarction (heart attack) still holds good.
12 Lead ECG
Standard 12 lead ECG consists of 6 precordial (chest) leads, 3 conventional limb leads and 3 augmented limb leads. Ideally electrocardiographic data from all the 12 leads should be simultaneously acquired to be called as a 12 lead ECG. Some electrocardiographs acquire 3 channels simultaneously while still others record leads sequentially in a single channel. The advantage of simultaneous multichannel acquisition is that any beat can be analysed in all the channels. This makes analysis of complex arrhythmias (abnormal rhythm of heart) easier.
Standard limb leads are: Lead I (left arm positive, right arm negative), Lead II (left foot positive, right arm negative) and Lead III (left foot positive, left arm negative). Augmented limb leads are aVR, aVL and aVF. In augmented leads, the positive pole is the index limb while the negative terminal is derived from the other two limbs. The positive pole of aVR is the right arm, that of aVL is left arm and that of aVF the left foot. Right foot is connected the indifferent electrode during all electrocardiographic recordings.
Chest lead positions are available here.