Human Physiology

Lesson 31 : Heart

Electrocardiogram

Electrocardiography (ECG): It is graphic recording of the electrical events of heart and a diagnostic tool to identify and pin point defects in heart functioning especially electrical events. During myocardial contraction, electrical potentials of the order of millivolts are generated in Sino Atrial node that spreads into other parts of conduction system and entire myocardium in specified manner. These potentials also spreads to body surface which can be detected by ECG machine and recorded on a moving paper in the form of ECG waves (Fig 31b). Magnitude of these waves and distances of occurrence of these waves get disturbed in various types of heart problems and this forms basis of diagnosis of several heart diseases. Typical ECG has following components.

P. Wave: represents atrial depolarization.
QRS complex: represents ventricular depolarization.
T wave: represents ventricular repolarization.
PR interval: time required for impulse to reach from atrium to ventricles.
QT interval: time required for activity of ventricles.

One requires a thorough knowledge of cardiac physiology to understand these changes. Cardiologist is a specialized medical practitioner who is a specialized medical practitioner in heart diseases, their diagnosis and treatment.

Recording of clinical ECG in human patient requires placement of electrodes on both forelimbs and hind limbs as shown below (Fig 31c) and connecting them to a sutable ECG machine.