The pacemaker of heart is?

DHS 2018 shift 1st

Applied Anatomy

Easy

Appeared in: DHS 2018 shift 1st

Explanation

The Sinoatrial (SA) node is known as the heart's natural pacemaker.
It has the highest intrinsic rate of spontaneous depolarization, typically firing at 60-100 beats per minute in a healthy adult.
This inherent speed means it initiates the cardiac impulse before any other part of the conduction system can, thereby setting the heart rate.
The impulse generated by the SA node spreads through the atria, causing them to contract, and then travels to the AV node.

Option B: The AV node has a slower intrinsic firing rate (40-60 bpm) than the SA node. Its primary role is to delay the electrical impulse slightly to allow the ventricles to fill with blood before they contract.
Option C: The Bundle of His is a crucial part of the conduction pathway that transmits impulses from the AV node to the ventricles, but it is not a primary pacemaker. Its intrinsic rate is very slow.
Option D: Purkinje fibers have the slowest intrinsic firing rate of all potential pacemaker sites (20-40 bpm). Their main function is to rapidly distribute the electrical impulse throughout the ventricular muscle to ensure a coordinated and efficient contraction.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Cardiac Conduction System and Primary Pacemaker as background academic context rather than a clinical decision trigger.
Understanding the heart's pacemaker hierarchy is fundamental for interpreting ECGs and diagnosing arrhythmias. For example, if the P wave (representing SA node firing and atrial depolarization) is absent and the heart rate is slow (40-60 bpm), it may indicate a junctional rhythm where the AV node has taken over as the pacemaker.
When the SA node malfunctions (a condition known as sick sinus syndrome), it can lead to severe bradycardia (slow heart rate) or alternating periods of slow and fast heart rates. This often necessitates the implantation of an artificial electronic pacemaker to regulate the heart's rhythm.
What if? If a patient develops a complete heart block, the electrical connection between the atria and ventricles is severed. In this case, the Purkinje fibers would likely take over as the pacemaker for the ventricles, resulting in a very slow idioventricular rhythm (20-40 bpm). This is a medical emergency requiring immediate intervention, often with a temporary or permanent pacemaker.

How to Approach the Question

First, identify the core concept of the question, which is the 'pacemaker' of the heart.
Recall the components of the heart's electrical conduction system: SA node, AV node, Bundle of His, and Purkinje fibers.
Remember the principle of automaticity and the hierarchy of intrinsic firing rates. The component with the fastest rate is the primary pacemaker.
Compare the intrinsic rates: SA node (60-100 bpm) > AV node (40-60 bpm) > Purkinje fibers (20-40 bpm).
Conclude that the SA node, having the fastest rate, is the primary pacemaker of the heart.
Select the option corresponding to the SA node.

Concept Tested & Keywords

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E6 Pharmacology Nursing Lilley 11e Part 2 p. 84-86

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