The heart rate is normally determined by intrinsic impulses generated in the SA node. The rhythm is determined by the route of impulse transmission through the conducting system. The heart rate is usually measured as the pulse,but to determine the rhythm,an electrocardiogram(ECG)is required. A cardiac arrhythmia is any disorder of the heart rate or rhythm,and is the result of abnormal generation or conduction of impulses. The normal cardiac cycle gives to normal sinus rhythm,which has a rate of between 60 and 100 BPM.
Sinus bradycardia.This is normal sinus rhythm below 60 BPM. This may occur during sleep and is common in athletes. It is an abnormality when it follows myocardial infarction or accompanies raised intracranial pressure.
Sinus tachycardia. This is normal sinus rhythm above 100 BPM when the individual is at rest . This accompanies exercise and anxiety,but is an indicator of some disorders,eg.fever,hyperthyroidism,some cardiac conditions.
Asystole. This occurs when there is no electrical activity in the ventricles and therefore no cardiac output. The ECG shows a flat line. Ventricular fibrillation and asystole cause sudden and complete loss of cardiac output.
Fibrillation. This is the contraction of the cardiac muscle fibres in a disorderly sequence. The chambers do not contract as a coordinated unit and the pumping action is disrupted.
