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EP Procedures - Sinus Node Evaluation

Sinus Node Recovery Time (SNRT)

          The sinus node recovery time, (SNRT - pronounced snert), is designed to test the automaticity of the sinus node. As we age, normal wear and tear takes its toll on our bodies. Sometimes, the tissue within the sinus node becomes damaged and does not function at optimal levels. This process may lead to a decrease in signal production in times of stress. In essence, the sinus node gets tired and periodically takes a small break. Now we all need a break from time to time, but this is one instance where a break is not only counterproductive, it can be down right dangerous.

          To determine if the sinus node is prone to taking an extra break now and then, we apply stress and document the response. This is accomplished by pacing the atrium near the sinus node at a rate that is slightly higher than the intrinsic rhythm. Pacing is continued for at least 30 seconds and then stopped. The time for normal sinus activity to resume is measured and documented. This process is repeated at several different pacing rates. An example of our pacing protocol for SNRT is shown below. I have included imaginary numbers to indicate the patient's intrinsic rate along with recovery measurements that show normal results (less than 1500ms for SNRT / less than 500 for cSNRT). The corrected SNRT is determined by subtracting the patient's intrinsic rate from the recovery time.

Intrinsic Rate
(cycle length in ms)

Pacing Cycle Length
Duration
(seconds)
SNRT
(ms)
Corrected SNRT
(ms)
953
700
60
1143
190
947
600
60
1279
332
891
500
60
1698
807
903
400
60
2103
1200
886
300
60
1906
1020

          The table above shows normal results for the first two pacing cycles. Once the pacing rate was increased to 500ms, the response became abnormal. The delayed response was seen in the next two pacing trains also. This patient would be classified as having sinus node disease and may require a permanent pacemaker. If all five SNRT values had been less than 1500ms, the results would have been considered normal. If you have one or two values that measure a little above the 1500 range, the test may be considered borderline abnormal. Usually the results are pretty clear cut. Our patient above had a two second pause (2103ms) after the 400ms pacing train. This is strong indication of problems with the sinus node. A prolonged SNRT is shown in the image below.

This image shows the end of a right atrial pacing train of 500ms over one minute. The time until the next sinus beat is 2110ms or 2.1 seconds. To determine the corrected SNRT, we would have to know what the cycle length of the patient's intrinsic rhythm was prior to the pacing train.

 

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