This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles.
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This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles. |
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LESSON 3bTHE CARDIAC CYCLE AND THE CONDUCTION SYSTEM
The rhythmic contraction of the heart is called heartbeat. A cardiac cycle is defined as a complete cardiac movement, including systole, intervening pause, and diastole. The cardiac cycle begins with depolarization of the SA node and atrial contraction. Each cycle requires a certain length of time for its completion. Pressure, volume, electrical, and sound changes occur during each cycle. Heart sounds are created primarily from turbulence in blood flow created by valves closing, not from contraction. Two separate networks of cardiac fibers regulate atrial and ventricular contraction. The heart is innervated by the autonomic nervous system, which neither initiates contraction nor affects the cardiac cycle. The conduction system is composed of specialized muscle tissue and initiates and conducts depolarization waves through the myocardium. Muscle fibers of the ventricular walls are arranged in whirls that squeeze blood out of the ventricles when they contract. The contraction phase is systole, while the filling phase is diastole. Atria contract while ventricles relax, and the ventricles contract while atria relax. THE CARDIAC CONDUCTION SYSTEM In order for the heart to pump efficiently, the individual myocardial fibers must contract and relax in a coordinated, rhythmic fashion. This characteristic of the healthy heartbeat is called synchrony. Synchrony is maintained by the heart's own intrinsic electrical system, which originates and transmits electrical impulses through a specialized conduction pathway. Just as the pulse is evidence of the heart's mechanical activity, the electrocardiogram or ECG, is evidence of its electrical activity. In the absence of synchrony, myocardial fibers contract in a random, uncontrolled fashion called ventricular fibrillation and the heart can no longer pump effectively. If no oxygenated blood reaches distant tissues, the patient dies.
Anatomy The cardiac conduction system consists of highly specialized cells that histologically resemble nerve tissue. When the anatomic components of this system are affected by disease, abnormalities in the heart's electrical activity called arrhythmias can result.
Five distinct anatomical structures comprise the cardiac conduction system:
The heart muscle grossly resembles skeletal muscle, yet it is structurally different. Cardiac muscle cells are interconnected to form a syncytium, a multinucleate mass of protoplasm produced by the merging of cells. This permits electrical excitation waves to pass rapidly from one cardiac cell to the next. Cardiac muscle is controlled by physiologic mechanisms under involuntary control, and mediated by specific nerves. Myocardial tissue has four main characteristics that integrate the heart's electrical and mechanical activity:
In order for the heart to pump efficiently, the myocardial muscle fibers must contract and relax in a coordinated, rhythmic fashion, in synchrony. When conductive tissue is damaged or deprived of oxygen, certain abnormal ventricular contractions may occur. Ansynchronous, random, uncontrolled contraction of the ventricles is called ventricular fibrillation. In v. fibrillation, the ventricle flutters and the blood cannot move out of the LV to oxygenate distant tissues. In cardiopulmonary rescue, a defibrillator is applied in the hope of shocking the heart back into a more normal rhythm. If a defibrillator cannot be used quickly, death follows. Automaticity The property of cells of the conduction system to initiate pacing of electrical impulses independent of the autonomic nervous system is called automaticity. The SA node is the normal pacemaker. If the SA node is isolated from all neural or hormonal control, this specialized tissue can generate impulses at rates higher than 100 per minute. Under autonomic control, the SA node paces the heart at a normal rate of 60 to 100 impulses per minute. Other parts of the conduction system- the AV junctional tissue and the His-Purkinje network- also have the property of automaticity. The SA node is the pacemaker, if it initiates impulses at a faster rate than other areas and if the impulse is rapidly propagated throughout the conduction system. For instance, when AV node function is impaired, or heart block occurs at this point in the system, other cells in the ventricles may become secondary pacemakers-maintaining the vital heartbeat, though usually at a different rate. Thus, redundant automaticity is a protective mechanism that keeps the heart pumping even in the absence of normal impulses originating from the SA node. However, the ability of other cells along the conduction pathway to initiate impulses can create problems. For instance, when conductive tissue is damaged or deprived of oxygen (due to ischemia), it becomes irritable, and may cause certain kinds of abnormal ventricular contractions. Ventricular tachycardia is a particularly dangerous form of rapid heart rate that can easily convert to ventricular fibrillation -the uncoordinated, random contraction of individual myocardial fibers that stops any effective pumping action.
The AV node is the only normal conduction pathway through the atrioventricular septum. When the excitation impulse reaches the AV node, it is delayed there for 0.08 to 0.16 second because of slow conduction along the delicate junctional fibers that connect the atrial myocardium with AV nodal tissue. During this delay, atrial contraction is largely completed, so that when the impulse reaches the ventricles, ventricular filling is complete. After passing through the AV node, the impulse reaches the bundle of His and again moves faster, passing through the right and left bundle branches and to the terminal Purkinje fibers in 0.03 to 0.05 second. The Purkinje fibers penetrate the ventricular wall from the endocardial surface, and only for a part of its thickness. From the Purkinje fibers, the excitation impulse then continues through myocardial cells outside the specialized conduction pathway, and a final 0.03 second is required to reach the epicardial surface. This rapid, simultaneous spread of excitation through the ventricles produces a coordinated contraction of both ventricles, thus ensuring efficient pumping of blood to the pulmonary and systemic circulations. Summary:
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