What is the "natural pacemaker of the heart"?
Bundle of His.
SA node.
Purkinje fibers.
AV node.
The Correct Answer is B
Choice A rationale
The Bundle of His is not the natural pacemaker but a component of the cardiac conduction pathway transmitting impulses from the AV node to the ventricles. It provides electrical continuity through the interventricular septum, ensuring synchronous depolarization. However, its intrinsic automaticity rate (20–40 beats/min) is insufficient for maintaining effective cardiac output, functioning only as a secondary pacemaker during atrioventricular nodal or sinoatrial failure.
Choice B rationale
The sinoatrial (SA) node is the natural pacemaker of the heart, located at the junction of the superior vena cava and right atrium. It generates spontaneous depolarizations through automaticity, firing at 60–100 impulses/min under normal physiologic conditions. SA nodal cells possess leaky sodium channels and calcium-dependent action potentials that determine intrinsic rhythm, initiating electrical conduction through atrial myocardium and establishing the baseline sinus rhythm that drives cardiac output.
Choice C rationale
Purkinje fibers function as specialized conductive fibers located in the subendocardial layer of ventricles, transmitting impulses rapidly to facilitate synchronized ventricular contraction. Although capable of spontaneous depolarization at 20–40 beats/min, they are tertiary pacemakers. They assume pacemaking only when higher pacemakers fail. Their primary function is propagation rather than initiation of impulses, ensuring efficient myocardial contraction and ejection fraction maintenance.
Choice D rationale
The atrioventricular (AV) node serves as a relay station slowing conduction from atria to ventricles, allowing ventricular filling during diastole. It has intrinsic automaticity at 40–60 beats/min, functioning as a secondary pacemaker only during SA nodal failure. The delay at the AV node ensures atrial contraction precedes ventricular systole, optimizing stroke volume. However, its primary role is conduction delay, not initiation of the intrinsic cardiac rhythm.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is B
Explanation
Choice A rationale
An Implantable Cardiac Defibrillator (ICD) is an electronic device designed to monitor the heart's electrical activity for dangerous rhythms and deliver therapy. Monitoring respirations is the function of a respiratory monitor or pulse oximetry, not the primary and specialized function of the ICD device.
Choice B rationale
The principal function of an ICD is to continuously monitor the heart rhythm, detect life-threatening ventricular tachyarrhythmias (VT or VF), and automatically deliver a high-energy electrical shock (defibrillation/cardioversion) to terminate the dysrhythmia and restore a perfusing rhythm.
Choice C rationale
While an ICD does monitor the heart rate to detect tachycardia and bradycardia, simply monitoring the heart rate is a secondary function. Its defining therapeutic role is the delivery of electrical therapy to treat life-threatening ventricular dysrhythmias, a capability standard pacemakers lack.
Choice D rationale
An ICD does not directly regulate systemic blood pressure; this is achieved through autonomic nervous system regulation, fluid volume management, or vasoactive medications. While treating a lethal dysrhythmia will secondarily stabilize blood pressure, its primary mechanism is electrical, not hemodynamic.
Correct Answer is B
Explanation
Choice A rationale
Using the highest energy setting available is generally not the initial approach for synchronized cardioversion, especially for rhythms like atrial flutter, which often respond to lower energy levels (e.g., 50-100 J biphasic). Starting high increases the risk of myocardial damage and post-shock dysrhythmias. Energy delivery should be titrated based on the patient's response and the specific arrhythmia being treated.
Choice B rationale
Ensuring the synchronizer switch is on is the most crucial step in synchronized cardioversion. This setting detects the patient's R wave and delivers the shock precisely during ventricular depolarization, which is a period of relative refractoriness. This timing prevents the shock from falling during the vulnerable T wave (repolarization), which could induce lethal ventricular fibrillation.
Choice C rationale
Applying firm pressure to the pads is important to ensure optimal contact and minimize transthoracic impedance, allowing the maximum amount of energy to reach the myocardium. While essential for effective shock delivery, it is secondary to the synchronizer setting, which ensures the safety and correct timing of the shock.
Choice D rationale
Administering a shock during expiration is sometimes recommended for defibrillation to minimize air in the lungs, which can increase impedance. However, for synchronized cardioversion, the timing relative to the R wave is the absolute priority; the respiratory cycle is a minor consideration compared to the risk of inducing ventricular fibrillation if the synchronizer is off. —.
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