The action potential includes a depolarization (activation) followed by repolarization (recovery). The rapid activation of the ventricles yields a narrow QRS complex (defined as QRS duration <120 ms). Impulse transmission is rapid through the bundle of His and the Purkinje fibers, such that virtually all ventricular myocardium is activated (depolarized) simultaneously. These structures transmit the atrial pulse to the ventricles. The AV system consists of the AV node, the bundle of His and the Purkinje fibers. The sinus node and the electrical conduction system. Figure 1 illustrates the sinus node and the components of the conduction system. When contractile myocardium receives the action potential, it is activated and contracts. Conduction cells form bundles of fibers that spread the action potential rapidly and sequentially to the contractile myocardium. To coordinate these two tasks, the heart has an intrinsic pacemaker–i.e the sinus node–and an electrical conduction system composed of specialized myocardial cells. Sequential activation implies that the atria are activated first and they fill the ventricles with adequate volumes of blood, before ventricular contraction commences.
Rapid activation is important in order to activate as much myocardium simultaneously as possible the more myocardium contracting at the same time, the more efficient the pumping mechanism. Principles of myocardial excitability and the conduction systemĪchieving an effective pumping mechanism requires the atria and the ventricles to be activated rapidly and sequentially. The first part consists of a brief rehearsal of the basics of cardiac automaticity, action potentials, and pacemaker cells. More advanced devices (ICD, implantable cardioverter defibrillator CRT, cardiac resynchronization therapy) are discussed in subsequent chapters. This section is devoted to artificial pacemakers. Give the wide use of pacemakers, and the trend towards increased use of cardiac devices in general, it is crucial to be familiar with these devices. Additional breakthroughs have been achieved in recent years, with the leadless pacemaker being the most promising improvement (1). Artificial pacemakers have benefitted immensely from advances in engineering, notably with the advent of transistors, programmable circuits, lithium batteries, and Internet-connected devices. Cardiac pacing has evolved from a hazardous experiment in the 1930s, to a routine, safe and sophisticated treatment used worldwide. The artificial pacemaker is one of the great medical inventions of the 20th century.
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