Molecular and Cellular Mechanisms of Cardiac Arrhythmias

then slowly repolarize, which is denoted as the T wave on the ECG, leading to cardiac relaxation and completion of one cardiac cycle. The rhythmic activity of the human heart is clearly apparent by ultrasound after only five weeks of gesta-Department of Cardiology Children's Hospital tion. It is remarkable to think how flawlessly the heart performs in the course of one's life, beating ‫07ف‬ times Boston, Massachusetts 02115 † Department of Medicine per minute over a course of 75 to 90 years. One's heart must beat, therefore, ‫63ف‬ million times a year, a remark-Division of Cardiology Eccles Program in Human Molecular Biology able feat considering that there is virtually no tolerance for failure. Cardiac Arrhythmias and Sudden Death Although most hearts beat with remarkable fidelity and resilience, under certain circumstances the rhythm of the heart can fail. This is known as a cardiac arrhythmia. Introduction When the heartbeat is too slow (bradyarrhythmia or The heart is a pump (Figure 1). Through coordinated bradycardia) blood pressure cannot be maintained, contraction of the atria, blood is pumped into the ventri-leading to loss of consciousness and death. Bradyar-cles. The ventricles, which do most of the work of the rhythmias often result from disease or death of pace-heart, contract synchronously to pump blood to the rest maker and other specialized conducting cells and can of the body. The right ventricle pumps deoxygenated be effectively treated with artificial, electronic pacemak-blood to the lungs for gas exchange. The left ventricle, ers. Similarly, if the heart rhythm is too rapid (tachyar-the most forceful chamber of the heart, pumps blood rhythmia or tachycardia), blood pressure cannot be to the rest of the organs, including the brain, kidneys, maintained, leading to syncope and sudden death. The liver, skeletal muscle, and other vital organs. Through most dangerous tachyarrhythmias are focused in the this pumping action, the left ventricle maintains sys-ventricles and are known as ventricular tachycardia, tor-temic blood pressure at approximately 120 over 80 milli-sades de pointes ventricular tachycardia, and ventricu-meters of mercury. The brain is particularly sensitive to lar fibrillation (Figure 1). Torsades de pointes means blood pressure and flow. If the coordinated contraction twisting around the point, an allusion to the alternating of ventricles is stopped for only a few seconds, blood axis of the QRS complex around the isoelectric line of the pressure drops and consciousness is lost. This sudden ECG during this arrhythmia (Lazzara, 1997) (Figure …