Abstract:
A- Normal sinus rhythm; the heart rate is about 70 beats/minute at rest, which varies with the breathing; it accelerates during inspiration and decelerates during expiration especially in depth of breathing.
B- Sinus tachycardia: It is caused by exercise. The heart rate usually above 100 beats/minute.
C- Sinus bradycardia: It is occur in athletic and vagal stimuli. The heart rate below 60 beats/ minute.
Heart block is caused by abnormality of impulse conduction in the AV node.
Premature or ectopic contraction: It is the contraction of the heart before the time that normal contraction would have been expected.
Paroxysmal tachycardia means sudden attacks of increasing heart rate that continue for a while and disappear suddenly. This phenomenon could be explained according to Reentry phenomenon in that the impulse passes in one direction to excite the cardiac muscle fibers, but if the impulse returns high up, it excites the cardiac muscle fibers again.
Rapid repolarization after discharge of infracted muscle fibers as a result of accelerated opening of K channels develops after occlusion of a coronary artery. It last only a few minutes. The infarct fibers begin to depolarize more slowly than the surrounding normal fibers. These changes cause elevation of the ST segment, and T wave inverted in ECG leads.
Hyperkalemia is very dangerous, lethal because its effect on heart. Change in ECG tall peaked T wave. As plasma K level raises, paralysis of atria and prolongation of QRS complex waves occur. The rest- membrane potential of muscle fibers decreases as the extra cellular K concentration increase. The fiber becomes unexcitable, heart stops in diastole. A decrease in the plasma K level causes prolongation of PR interval, prominent U wave, late T wave inversion. Hypokalemia is a serious condition but it is not as rapidly fatal as hyperkalemia.