(Part 1 – Which Branch?)

I have witnessed over 800 implantable cardioverter defibrillator (ICD) shocks during DFT testing and ER visits.  In one memorable ER visit, I watched helplessly as a patient got shocked every 35 seconds until a strong dose of Lopressor was administered.  These shocks were appropriate. But many are not. So ICDs need to be capable of reducing inappropriate shocks.

Patient Care

Shocks are violent.  Most patients describe a shock as like getting kicked in the chest by a mule.  One patient was so dismayed by a shock, the device was explanted.  Several others lived in daily fear.

Each year 260,000 ICD’s are implanted worldwide (not including the US).  Studies show 10%-25% of ICD patients will receive at least one inappropriate shock during their lifetime.  

These occur following atrial arrhythmias with a rapid ventricular response (RVR) or sensing errors and can be painful, reduce a patient’s quality of life, cause psychiatric disturbances, and be potentially arrhythmogenic.

Not exactly the quality of patient care we strive to deliver.  But it does beg a question.

Is There a Predictor for Inappropriate Shocks?

Well, several studies have shown a major predictor of inappropriate shocks is the presence of A Fib or A Flutter.  A retrospective study of 148 patients detailed in the Journal of Korean Science, found that 67.7% of inappropriate shocks were due to A Fib or A Flutter with a high ventricular response.  This was followed by other SVT’s at 23.5% and abnormal sensing at 8.8%.[1]

Inappropriate Shocks, Percentage

So clearly there is a need for an ICD to be capable of distinguishing between AF/AFL with RVR and VT/VF.  Otherwise, each inappropriate shock places our patient on the path towards a lower quality of life.In the presence of AF/AFL with RVR, Abbott ICD’s have several discriminators that activate which allows the device to know whether or not to shock.

The Secrets Behind SVT Discriminators

But before we present the first discriminator, let’s cover some underlying SVT secrets.  First, sensitivity and specificity (say that quickly ten times).

Sensitivity is the ability of an ICD to detect VT.  Specificity is the ability of the ICD to differentiate VT from SVT.  An ICD requires a high level of each to behave appropriately.  

Fortunately, Abbott ICD’s have a sensitivity of >90% and a specificity of >90%.  That means that more than 90% of the time, an Abbott ICD can correctly diagnose VT and distinguish VT from an SVT.  That’s awesome.

Next, all SVT discriminators are activated after VT is detected and before a rhythm diagnosis is made.  So, if the VT zone is set to 171 bpm (350ms) SVT discriminators awaken once the first   V-V interval is recorded that is faster than 171 bpm.  

SVT discriminators do not operate in the VF zone.  If the patient’s heart rate is fast enough to initially appear in the VF zone, therapy will begin after VF is diagnosed and confirmed.

Finally, we can program the upper limit for which SVT discriminators will operate.  If the device is programed with a VT 1, VT 2, and VF zone, an upper cutoff rate can be determined.  Above this rate, the patient will not receive discriminator analysis.  

For example, the device can be programed to use discriminators for the entire VT 1 and VT 2 zones or the entire VT 1 zone and only a percentage of the VT 2 zone.  The latter is illustrated below.  

Programmed Discriminators for VT Zones

In this example, the SVT discriminators will operate within the entire VT 1 zone and roughly the lower half of the VT 2 zone.  They will not operate in the upper half of the VT 2 zone nor the VF zone.

Introducing the First SVT Discriminator

Abbott ICD’s use three discriminators to differentiate AF/AFL with RVR from VT.  But first, we must classify the rhythm in the correct branch.  Below is the dual chamber ICD discrimination logic chart.  Today, we will discuss the first discriminator that is used…rate branch.

Reducing Inappropriate ICD Shocks Rate Branch

Rate branch evaluates the relationship between the A and V.  Obviously, proper sensing must be present for this algorithm to work effectively.  Inaccurate sensing can be caused by atrial lead dislodgements or farfield R wave oversensing (FFRW) oversensing, and if present, must be resolved.

In this evaluation there are three possible outcomes…

  • A>V = The AF/AFL branch
  • A=V = The sinus rate branch, and
  • A<V = The VT/VF branch

But how does the device place the AF/AFL with RVR patient onto the correct branch?After the first VT interval is detected, the device looks at the P-P and R-R intervals within a window whose size is determined by the number of VT detection intervals.  

After the first VT interval is detected, the device looks at the P-P and R-R intervals within a window whose size is determined by the number of VT detection intervals.  

VT Detection Criteria

Let’s assume the device is programed as above. Since the number of programed detection intervals for the VT 2 zone (14) is less than the number of detection intervals for the VT 1 zone (16), the size of the window is 14.  

If this was programed with only two zones, VT 1 and VF, the size of the window would be 16.  Remember, no discriminators are used in the VF zone so its number of detection intervals is irrelevant.

Back to our original assumption, the device calculates the median atrial rate and median ventricular rate over 14 cardiac cycles.  Then the device compares those two median intervals. 

If the median atrial and ventricular rates are within 30ms of each other, the rate branch algorithm determines A=V and places that rhythm in the sinus tach branch.  

If the median atrial rate is 31ms or slower than the median ventricular rate, rate branch places that rhythm in the VT/VF branch and, after confirmation, therapy is delivered.If the median atrial rate is 31ms or faster than the median ventricular rate, then rate branch classifies that rhythm as A>V and places it into the AF/AFL branch. Below is a table summary.

Rate Branch Summary for Reducing Inappropriate ICD Shocks

ICD Discriminator Logic Chart

So, below is our updated ICD discriminator logic chart.

Reducing Inappropriate ICD Shocks Completed Logic Chart

Once in the AF/AFL branch, there are several additional algorithms which will further determine if the rhythm is truly AF/AFL with RVR or VT.

But that’s a lesson for another day.  Next up… “looks do matter”.

[1] Jeong Hoon Yang, Kyeongmin Byeon, Hye Ran Yim, etc., Journal of Korean Medical Science, Predictors and Clinical Impact of Inappropriate Implantable Cardioverter Defibrillator Shocks in Korean Patients. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3369447/