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The SA node is composed of P cells. Blood supply is from the SA nodal artery which is easily damaged. When damaged, ischemia results, fibrosis develops, and the node becomes dysfunctional. Initially the dysfunction may produce only mild symptoms, depending on the rate and duration of pauses. In more advanced disease, dizziness and possible syncopal episodes may be seen.
Three types of SA nodal dysfunction are covered here. They include abnormal overdrive suppression, sinus arrest, and sick sinus rhythm. With sick sinus syndrome the problem may be in the SA node or in the AV node.
In normal conduction, when an early beat occurs (such as a PAC), there is a pause in the rhythm. In healthy hearts, the conduction system “resets” itself and the rhythm resumes after the early beat. This is normal overdrive suppression.
When there is a defect in the conduction system, the heart is not able to reset and resume at the same rate after the early beat. The resulting rhythm is slower and represents abnormal overdrive suppression due to disease of the SA node.
Sinus block and sinus arrest may both be seen in the clinical setting. In sinus block, the impulse cannot get out of the SA node. After a pause, the rate will resume on time. In sinus arrest, the SA node is diseased and does not fire correctly. After a pause the rhythm is not able to resume on time and will be slower due to the dysfunction. The pause in both rhythms can be for any length of time. The longer the pause, the more symptomatic the patient will be.
(Rhythm resumes on time and the rate remains essentially unchanged.)
(SA node is unable to recover after the pause and resumes at a slower rate.)
Sick sinus syndrome is abnormal function of the SA node because of a wide range of abnormalities. Shifting of the pacing sites within the SA node accounts for the changes in heart rate and P wave configurations which will be seen.
Sick sinus syndrome may also be due to abnormal function of the AV node which is commonly seen in conjunction with disease of the SA node. In atrial fibrillation or flutter variable conduction through the AV node occurs and the heart rate can become very slow. It is estimated to occur in up to 60% of patients with atrial fibrillation or flutter.
Sick sinus syndrome is often known as “tachy-brady” syndrome. The slower the rate, the more symptomatic the patient will be. Generally, a pacemaker will be inserted. Anticoagulation should be considered if a very marked tachy-brady syndrome as clots may form during the bradycardic episodes and be released with the tachycardic ones.
(Sinus tachycardia to marked bradycardia with ischemia noted.)
(Atrial fibrillation with slow ventricular response, then rapid, then back to slow.)