ECG: overview

Introduction

• An electrocardiogram (ECG), is a recording of the electrical potential generated within the heart during the cardiac cycle.
• It is a measurement of voltage (amplitude), against time.
• The electrical potential generated during the cardiac cycle is measured by connecting electrodes (attached to specific body sites), to a galvanometer and the changes in electrical potential with time are recorded on to graph paper.
• The cardiac cycle is usually initiated by an electrical impulse from the sinus node and a wave of depolarization and subsequent repolarization spread throughout the cardiac myocardium.
• The ECG machine measures the sum of this electrical activity, as detected by the surface electrodes, against time and, providing that the electrodes are placed correctly, an examination of the subsequent ECG tracing will provide useful information on cardiac structure and arrhythmias.
• Under certain circumstances the ECG may be useful in the assessment of some electrolyte and acid-base disorders.