The standard ECG provides 12 different vector views of the
heart's electrical activity as reflected by electrical potential
differences between positive and negative electrodes placed on
the limbs and chest wall. Six of these views are vertical (using
frontal leads I, II, and III and limb leads aVR, aVL, and aVF),
and 6 are horizontal (using precordial leads V1, V2, V3, V4, V5,
and V6). The 12-lead ECG is crucial for establishing many
cardiac diagnoses, especially arrhythmias and myocardial
ischemia (see Table 4: Cardiovascular Tests and Procedures:
Interpretation of Abnormal ECGs). It can also identify atrial
enlargement, ventricular hypertrophy (see Table 5:
Cardiovascular Tests and Procedures: Criteria for ECG Diagnosis
of Left Ventricular Hypertrophy), and conditions that predispose
to syncope or sudden death (eg, Wolff-Parkinson-White syndrome,
long QT syndrome, Brugada syndrome). A useful ECG tutorial is
available from the University of Utah. Standard ECG Components
By convention, the ECG tracing is div