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Pacemakers – Sick Sinus Syndrome (SSS)

Pacemakers and Sinus Node Dysfunction

Sinus Node Dysfunction (Includes Sick Sinus Syndrome Also Known as Tachy-Brady)

One of the most common reasons for a cardiac pacemaker to be implanted is sinus node dysfunction. Sinus node dysfunction simply describes heart conditions where the sinus node is not functioning appropriately. The sinus node is also called the sinoatrial node or S.A. node for short.
Sick Sinus Syndrome
Sinus Node (Red Dot)
in Right Atrium

The Sinus Node

Situated in the right atrium, the sinus node is the heart’s natural pacemaker and integral to the heart’s conduction system. In short the sinus node is the conductor of the orchestra dictating rate and rhythm to the rest of the heart. When the sinus node ‘fires’ this sets in motion a knock on effect that results in a heartbeat.

Sinus Node Malfunction

There are a few reasons that the sinus node can start to malfunction these include:

  • Idiopathic Fibrosis (a build up of fibrous tissue in the node with no known cause)
  • Ischaemia (blockages in the coronary arteries)
  • Myocardial infarction (heart attack)
  • Myocarditis (inflamation of the heart muscle)
  • Pericarditis (swelling of the cover that surrounds your heart)
  • Rheumatic heart disease (infection)
When the sinus node does start to malfunction it can ‘fire’ too rapidly, too slowly, irregularly or a mixture of all three. This in turn causes the heart rate to become too slow, irregular, too fast or a mixture of all three.
In the scenario that the sinus node dysfunction cannot be reversed.  Cardiac medications are often necessary to prevent the heart from racing too quickly. On the flip side of the coin, cardiac pacemakers are the treatment of choice to prevent the heart rate becoming too slow (bradycardia) or experiencing long isolated pauses..

Pacemakers and Sinus Node Dysfunction

Pacemakers cannot actually treat sick sinus syndrome, what they do is prevent the heart rate from going to slowly and therefore alleviate the symptoms associated with a slow heart rate (fainting, dizziness, shortness of breath). 
Historically a single chamber (AAI) pacemaker was used.

Single Chamber AAI Pacemaker

Single Chamber Pacemaker Sick Sinus Syndrome
Single Chamber AAI
Pacemaker. One lead
positioned in the right
atrium.

A single chamber pacemaker (AAI) describes a pacemaker with one lead positioned in the right atrium.  The basis for how this type of pacemaker works is very simple. When the sinus node ‘fires’ the first part of the heart to contract is the atria. 

The pacemaker lead is able to ‘see’ if the atria contracts if it has then the pacemaker will not intervene. Having seen the atria contract the pacemaker will start a short timer (usually 1 second long). If by the end of this timer the atria has not contracted again the pacemaker will send a small electrical pulse down the lead causing it to.

This electrical wave will then move through the heart causing it to beat.

This algorithm (posh word for process) is illustrated in the picture below.

AAI Pacemaker Mode Sick Sinus Syndrome
Click to enlarge
The nature of sinus node dysfunction means this level of treatment is perfectly suitable. Through preventing the heart to pause for any longer than 1 second (60 beats per minute) the patient should stop experiencing dizziness and fainting.

This being said, there are further considerations that need to be thought over.

Considerations for Pacemaker Settings with Sinus Node Dysfunction

Single Chamber AAI Pacemaker Still the Treatment of Choice?

Dual Chamber Pacemaker Sick Sinus Syndrome
AV Node (Blue Dot). The
gateway between the top
chambers and bottom
chambers.

This pacemaker will certainly treat the bradycardia associated with sinus node dysfunction, however it is now accepted that a certain percentage of people with sinus node disease will go on to develop disease of the AV node. This is the gateway for electrical signals passing from the top of the heart (atria) to the bottom of the heart (ventricles). Disease of this node can lead to heart block and the treatment of heart block requires a dual chamber pacemaker. It is for this reason that patients with snus node dysfunction are often fitted with a dual chamber pacemaker preempting this eventuality. 

This can minimise the amount of surgeries a patient undergoes and prevent them ever experiencing symptoms from a development in their disease process.

For those patients that have had dual chamber pacemakers fitted they are commonly set AAI=DDD (a particular mode) because the AV Node may be still working well. I have fully explained this pacing mode in this post, but essentially the pacemaker will perform like a single chamber AAI pacemaker until the AV Node fails (heart block) in which case the pacemaker mode will change and the device will perform as a dual chamber pacemaker (DDD).

If you are interested to know more about AAI = DDD Pacing mode you can read these two posts.

Rate Response

Another ‘problem’ with sinus node dysfunction is that the disease to the sinus node means that a person can become chronotropically incompetent. This long terminology refers to the ability for the heart rate to increase when a person requires a faster heart rate – when they are exercising or rushing about the house. If the heart rate does not increase to match the demands of the body then a person may feel excessively short of breath or dizzy when exerting themselves.

As a result patients with sinus node dysfunction often require the rate response setting to be activated in their pacemaker. Rate response uses a few different methods to ascertain when the person is active and will actively increase their heart rate. One example of rate response is the accelerometer used in Medtronic devices, others include closed loop stimulation and minute ventilation.

VVI Pacemaker? No.

It should also be mentioned in this post that a VVI pacemaker could in theory be used to prevent the heart rate from going too slowly in sick sinus syndrome. This is NOT the treatment of choice though for two main reasons. Firstly it promotes ventricular pacing (we try to avoid this where possible), but also there is no synchronising of the atrial contraction and ventricular contraction. This can cause pacemaker syndrome and in time potentially reduce the heart’s ability to pump.

If you are interested in a full introduction to pacemakers then my book Pacemakers Made Easy is available electronically on Ibooks or in hard copy.
Thanks for reading,
Carl

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