Cardiac arrhytmias

Occasionally, disturbances in the normal sequence of cardiac events can lead to an irregular heart rhythm, called an arrhythmia. These disturbances vary in degree of seriousness. Bradycardia and tachycardia are two types of arrythmias. Bradycardia is defined as a RHR lower than 60 beats/min, whereas tachycardia is defined as a resting rate greater than 100 beats/min. With these arrythmias, the sinus rhythm is normal, but the rate is altered. In extreme cases, bradycardia or tachycardia can affect maintenance of sufficient blood pressure. Symptoms of both arrythmias include fatique, dizziness, light- headedness, and fainting. Tachycardia can sometimes be sensed as palpitations or a “racing” pulse.

Other arrythmias also occur. For example, premature ventricular contractions(PVCs), which result in the feeling of skipped or extra beats, are relatively common and result from impulses originating outside the SA node. Atrial flutter, in which the atria contract at rates of 200 to 400 beats/min, and atrial fibrillation, in which the atria contract in a rapid and uncoordinated manner, are more serious arrythmias that cause ventricular filling problems. Ventricular tachycardia, defined as three or more consecutive premature ventricular contractions, is a very serious arrhythmia that can lead to ventricular fibrillation, in which contraction of the ventricular tissue is uncoordinated. When this happens, the heart is extremely inefficient, with the result that little or no blood is pumped out of the heart. Most cardiac deaths result from ventricular fibrillation. Use of a defibrillator to shock the heart back into a normal sinus rhythm must occur within minutes if the victim is to survive. Chances of survival are greater if emergency treatment, including defibrillation, is provided quickly.

Interestingly, most highly trained endurance athletes develop low RHRs, an advantageous adaptation, as a result of training. Also, the heart rate naturally increases during physical activity to meet the increased demands of exercising muscle for blood flow. These adaptations should not be confused with pathological bradycardia or tachycardia, which are abnormal alterations in the RHR that usually indicate a pathological problem.