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Wednesday, August 31, 2011

Intrepreting Holter Reports for Boxer Dogs

After you've successfully hooked up the holter monitor on your Boxer, they've left it on for the full 24 hours, and you've sent the tape/recording in to be analyzed, you get to play the waiting game. Some services are very fast, consistently offering 24-hour turn-around times once they receive your recording. Others takes days or even weeks. If you're not good with waiting, or if you need the results sooner rather than later, you might want to check out customer satisfaction and turn-around times before you rent or purchase your holter.

When the results finally come in -- after that five-seconds waiting with your heart in your mouth for the PDF file to open -- the first thing you want to look at is the section titled "Ventricular Ectopy" (or something similar -- different companies might use slightly different language). This section is where the abnormal heartbeats (VPCs) associated with ARVC in Boxers will be listed. Within the Ventricular Ectopy section, you want to look at the number of singles (isolated abnormal beats), pairs (two abnormal beats in a row), and most especially runs (three or more abnormal beats in a row). (Some reports list singles, couplets (pairs), triplets (three in a row), and call runs four or more abnormal beats in a row.) Runs of VPCs are what can lead to syncope (fainting) and sudden death. You also want to note the total time monitored and analyzed -- sometimes the tape stops working or the electrodes become detached in the middle of a recording. The time should be close to 24 hours.

This is the holter report from our "Fort Knox" model -- you can't ask for a better result. (Whew!) (Click on any of the images to enlarge for detail.)

Sometimes you'll get a result that's not as great, but in a grey area. This Boxer had 116 single VPCs in a 24-hour period. (It so happens that it was the dog's first holter.)

With this kind of result, we look a little further to see what might be going on. (The good news here is that the VPCs were all singles; if you have to have VPCs, singles are the best kind to have.) The next page of the holter report gives an hourly breakdown of events:

As you can see, the majority of the abnormal beats occurred in the first few hours the dog was wearing the monitor. This could be coincidence, it could be artifact from rubbing/shifting/adjusting the monitor, or it could be that the monitor itself caused anxiety which lead to VPCs.

Currently, 116 VPCs would be considered a grey area -- however, at the time of the recording the dog was 18 months old. Consultation with a board-certified veterinary cardiologist led to a "wait and see" approach. The dog has been holtered annually since that time, and has only thrown single VPCs, ranging from 2 to 23. The cardiologist cleared the dog for breeding  based on the low numbers in the follow-up holters. (It should be noted that Dr. Meurs' current advice is to begin holtering at 3 years of age, so these 116 VPCs may never have been seen. The American Boxer Club still recommends holtering beginning at 1 year of age.)

All too often, the holter report comes back with a high number of VPCs. This dog was 8 years old at the time of the holter -- the average age of onset for ARVC is 6 years.

Obviously, this is a less-than-desirable report. This particular dog was never used for breeding, but often a dog will have clear or low holter reports throughout its breeding life and then something like this will show up. This dog only had two runs of VPCs, each with three abnormal beats -- not a horrible report, all things considered. Some Boxers have tens of thousands of VPCs and multiple runs of four, five, or more beats.

Looking at the detail, we can see that this dog had abnormal beats throughout the day and night; there appears to be no correlation between time of day and VPCs, which is also the conclusion Dr. Meurs reached in one of her studies. When faced with this kind of a holter report, the next step is consultation with a board-certified veterinary cardiologist. They will want to see the full report, including the sample strips:

Most Boxer owners are probably better off not looking at these pages unless they're standing next to a cardiologist -- they can look quite alarming!  This strip shows one of the runs of VPCs the dog had. The dog fainted twice while wearing the monitor -- one episode coincides with this run of VPCs, but the other was during a relatively calm moment of heart activity. (This particular dog was not placed on medication and has not fainted since; it is possible something other than ARVC was causing the syncope and/or arrhythmia. A follow-up holter will be done to see if there are any changes.)

Finally, sometimes you see some out of the ordinary results on a holter. Occasionally the area called "Supraventricular Ectopy" will have some numbers in it; these beats are also called APCs, or atrial premature contractions. In most cases these beats are inconsequential; however a large number of them, or several runs, might warrant consultation with a cardiologist. Then there are ventricular escape beats:

This holter report can cause some flip-flopping in a breeder's stomach! The zeroes across the board for Ventricular and Supraventricular Ectopy are great, but then the eye travels down to see 5,000 ventricular escape beats, and panic sets in. Even though nowhere in the literature about ARVC are ventricular escape beats mentioned, 5,000 is a lot of them!

Ventricular escape beats are basically internal pacemaker beats; the normal heart rhythm pauses a tiny bit too long, and so the ventricles "kick start" it back up again. These beats are apparently not uncommon in athletic dogs with slow heart rates. Three board-certified veterinary cardiologists have stated that they are nothing to worry about, however. If you do happen to come across them on your holter reports, don't panic, especially if all the other information is perfectly normal, as in our example above.

Of course, there are as many variations on holter reports as there are dogs being holtered. This information should give you some basic knowledge on interpreting your holter report, but if you're ever in doubt, consult a board-certified veterinary cardiologist.

Read past blogs for more information on holtering a Boxer dog, or on the "Fort Knox" holter hook-up method.


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  2. Thank you so very much for taking the time to put this together. It is the only place I have found such an easy to understand in depth explanation. It has really helped me understand the raw data reports.

  3. Thank you so very much for taking the time to put this together. It is the only place I have found such an easy to understand in depth explanation. It has really helped me understand the raw data reports.

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  5. What are the current VPC levels that are acceptable for breeding and what range is considered "grey"? Who sets these guidelines? Are they published?

    1. I would also love to know the answer to that question.

  6. VERY well written. Thank you so much :) I have several heart holters from my 2 year old poodles if you ever need them for research/explanation purposes.