Disorders of the Sinoatrial Node - When a Hero Falls
So the last time we left off with an introduction to arrhythmias. Now it is time to see what can go wrong with this precise process of conduction.
The first order of business is to see what can go wrong with the pacemaker - the boss - the Sino atrial node.
Now before we understand what can go wrong with the Sino atrial node, let’s understand a little bit more about this node.
The SA node is a small collection of fusiform cells that envelops the SA nodal artery. It is found at the junction of the right atrium and the superior vena cava in the sulcus terminalis on the epicardial surface. Electrically and microscopically, it is different from the cardiac muscles. About 60 % of the time, it is supplied by the right circumflex coronary artery and 40% of the time it is supplied by the left circumflex artery.
Location of pacemaker tissue.
Reproduced from Harrison’s Principles of Internal Medicine 20th edition.
PATHOPHYSIOLOGY
In order for the SA node to malfunction one of two things must happen.
The electrical activity that causes automatic firing of the node must be damaged - decreased automaticity.
The trasmission of the impulse to the rest of the conduction system must be impaired - exit block.
In lay terms, this is known as failure of impulse initiation and failure of impulse conduction. This functional failure in the SA node can be caused by various diseases. Broadly, these can be classified into extrinsic and intrinsic causes.
CAUSES
The extrinsic causes are generally reversible and the intrinsic causes are usually degenerative and are a result of fibrosis.
Causes of sinoatrial nodal disease.
Reproduced from Harrison’s Principles of Internal Medicine 20th edition.
CLINICAL FEATURES
Patients with sinus node disease can present with symptoms of tachycardia, bradycardia or thromboembolism. In patients with tachycardia, there is a phenomenon called overdrive suppression that can lead to a sinus pause and result in syncope. Thromboembolism occurs in tachycardia variants of sinus node disease as a result of turbulent flow in the ventricles.
DIAGNOSIS
The most important tool for diagnosing SA node disease is the ECG. There are four possible ECG presentations of sinus node disease.
Sinus Bradycardia - heart rate of less than 40 in the awake state in the absence of physical conditioning is considered abnormal.
Sinus pause or sinus arrest - here the SA node just decides to stop conducting. No P waves are seen.
Tachycardia Bradycardia syndrome - there is usually a supra ventricular tachycardia followed by bradycardia due to overdrive suppression of the SA node.
Sinus exit block - in this case, the initiation of the impulse is normal but it is not transmitted to the rest of the heart.
ECG showing sinus pause. Notice how after five beats there is a long pause. Here the SA node fails to fire despite there already being an underlying bradycardia.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
ECG showing 2:1 sinus exit block. Notice that the heart rate is normal and inbetween that there is a dropped P wave signifying a missed beat.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
This ECG shows the tachycardia bradycardia syndrome in which tachycardia (in this case atrial fibrillation) leads to bradycardia due to overdrive suppression of the SA node.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
Continuous ECG monitoring can be done on a Holter machine.
Another feature of SA node disease is called chronotropic incompetence. This is defined as the inability to increase the heart rate in response to exercise.
Calculation of the intrinsic heart rate can also be helpful in identifying SA nodal involvement.
Confirmation of diagnosis is done by electrophysiological studies which calculate the Sinus Node Recover Time (SNRT) and Sino Atrial Conduction Time (SACT).
TREATMENT
The final treatment of SA node disease permanent pacing. However, in acute settings, some drugs can be used. Digitalis, isoprotanil, atropine and theophylline are approved for treatment of sinus Bradycardia.
This gives us a brief overview of diseases that may cause the SA node to malfunction, how to identify them and treat them. Among these diseases, there is a disease of particular interest called Sick Sinus Syndrome. The next lecture will discuss this entity.
Author: Narendran Sairam (Facebook)
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