Electrocardiography is the graphic recording of the heart’s electrical activity detected through
electrodes placed on the surface of the body. Electrical activity is shown as series of waveforms on a visual display, a strip recorder. Electrocardiogram (ECG) is a graphic record of this activity. Electrodes applied to the body detect the magnitude and direction of electrical currents produced in the
heart. The electrocardiograph converts the electrical impulses it receives into a series of waveforms which represent cardiac depolarization ( that is the reduction of a membrane potential to a less negative value) and repolarization ( that is the return of the cells to a resting or polarized state).
ECG waveforms reflect the direction of electrical flow in relation to a positive electrode. Current flowing toward the positive electrode produces an upward (positive) waveform; current flowing away from the positive electrode produces downward (negative) waveform. Current flowing perpendicular to the positive pole produces a biphasic (both positive and negative) waveform. Absence of electrical acitivity ios represented by a straight line called isoelectric line.
The cardiac cycle is depicted as a series of waveforms, the P,Q, R, S, and T waves. The P wave represents atrial depolarization and contraction. The impulse is from the sinus node (the dominant pacemaker of the heart).
The PR interval represents the time required for the sinusimpulse to travel to the AV (atrioventricular) node (the second pacemaker of the heart) and into the bundle branches. A delay in the PR wave which is interval greater than 0.20 seconds indicate a delay in conduction from the SA node to the ventricles.
The QRS complex represents ventricular depolarization and contraction. QRS complexes greater than 0.10 seconds indicate delays in transmitting the impulse through ventricular conduction system.
The ST segment signifies the beginning of ventricular repolarization. It is the period from the end of the QRS complex to the beginning of the T wave. It should be isoelectric. Displaced ST segment is one which is either elevated or depressed.
The T wave represents ventricular repolarization. Abnormality in T wave may indicate myocardial ischemia or injury, or electrolyte imbalances.
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. It represents the total time of ventricular depolarization and repolarization. Prolonged QT interval indicate a prolonged relative refractory period and a greater risk of dysrhythmias. Shortened QT intervals may result from medications or electrolyte imbalances.
The U wave is not normally seen. It is thought to signify repolarization of the terminal Purkinje fibers. If present, the U wave follows the same direction as the T wave. It is most commonly seen in hypokalemia.