DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

370 CARDIAC PACING AND DEFIBRILLATIONSinoatrial(SA) NodeAortaLeftAtriumLeft BundleBranchRightAtriumLeftVentricleRightVentriclePurkinje FiberAtrioventricular(AV)NodeRight BundleBranchFigure 8.1 The human heart has four chambers, two on the left side and two on the right side. Each side is further divided into a receivingchamber (atrium) and a pumping chamber (ventricle). One-way valves separate the atria from the ventricles. The right side of the heartpumps blood to the lungs (via the pulmonary artery), and the left side pumps blood to the rest of the organs (via the aorta).node acts as a natural pacemaker, setting the rate at which the heart beats. At rest, the sinusnode fires 60 to 80 times per minute. When metabolic demand increases, for example dueto physical activity or emotional stress, the sinus node accelerates to make the heart beatfaster and increase cardiac output.The depolarization of the sinus node creates a depolarization wave that spreads rapidlyacross the right atrium and the left atrium, causing them to contract. The atria are insulatedelectrically from the ventricles by the atrioventricular (AV) groove that runs around theoutside of the heart. Electrical activity from the atria is transferred to the ventricles via asecond electrical structure of the heart called the atrioventricular node or AV node, locateddeep in the center of the heart. The AV node is connected to the bundle of His, a bundle ofspecialized cells in the heart that can rapidly conduct the electrical signal to the musclecells of the ventricles. The bundle of His branches downstream into a right bundle branch(to the right ventricle) and a left bundle branch (to the left ventricle). The fibers eventuallybranch out to the distant ventricular tissues and are at that point referred to as Purkinjefibers.When the complete conduction system works properly, the atria contract about 200 to300 ms ahead of ventricular contraction. This allows extra filling of the ventricles beforethey pump the blood through the lungs and peripheral circulation. Another important functionof this system is that it allows all portions of the ventricles to contract almost simultaneously,which is essential for effective pressure generation in the ventricular chambers.As such, the heart must maintain a more or less steady rhythm in order to pump properly.Excessively slow heart rhythms make the cardiac output insufficient, causing symptoms offatigue, weakness, lightheadedness, and loss of consciousness. On the other hand, if theheart beats too quickly, it does not get enough time in between beats to fill up with blood.