HCM Screening Form, what it says and what it means

By Misha Peersmans

What is where

The feline heart is a miniature copy of the human heart, both organs work in the exact same way. The heart contains 4 separate chambers, 2 on the left and 2 on the right side. The two top chambers are the right and the left atrium, the two lower chambers are the right and left ventricle.

The blood that flows towards the heart, after having given oxygen to the various parts of the body, enters the heart in the right atrium. From the right atrium, the blood is pumped to the right ventricle, where it is pumped along to the lungs, to collect fresh oxygen.
From the lungs, the blood comes back into the heart, this time at the left side. The blood enters the left atrium, then it is moved to the left ventricle and is then pumped into the aorta, to deliver the oxygen to the body parts.

The lower chambers of the heart, the ventricles, are the strongest pumping spaces of the heart. They need to pump the blood through the lungs and the entire body, whereas the atrium only needs to pump the blood into the ventricles.

To prevent the blood from flowing back into the atrium when the ventricle contracts, there are valves between these chambers. The valve between the right atrium and ventricle is called the tricuspid valve. The valve on the left side of the heart is called the mitral valve.

The valves could be pushed back into the atriums when the ventricles contract, because there is high pressure build up in the ventricles during these contractions. Luckily, this is prevented by small thread-like bands, called chordae tendineae. The chordae tendineae are a sort of anchor ropes between the two valves and the papillary muscles, which are located in the heart walls.

The terminology used in the form

  • Auscultation:
    • Normal: The heart should go "lub-dub lub-dub lub-dub"
    • Gallop: We speak of a galloping heart if the heart goes "lub-lub-dub-dub lub-lub-dub-dub lub-lub-dub-dub"
  • Murmur:
    • Grading: when a heart murmur is heard, it is graded according to severity
    • Dynamic: Not a regular sound, depending on the frequency and function (not always in every heartbeat)
    • Static: Regular sound, in relation to the heart's function, normally caused by a defect of the heart valves or the heart muscle (is present in every heartbeat)
  • Timing:
    • Systolic: is when the heart is at its maximum contraction
    • Diastolic: when the heart is maximum relaxed
  • Location: where the murmur is heard

First a short explanation of the 2-D and M-Mode:

  • 2D means two-dimensional; this way the heart is shown as if you cut a slice out of it
  • M-mode is as if you look at a LINE through the heart which shows the changes as the heart contracts and relaxes; it will show how the walls get thicker and then thinner, then thicker again, etc.
    And it shows also the "space" in the heart, the ventricle. When you look at the screen when the program is in M-Mode, you will see all sorts of funny waves going up and down. When this picture is frozen, the measurements can there be taken. In the M-mode view it is easy to see when there is diastole and systole (see the picture below for example).

The abbreviations:

First the measurements in diastole (when the heart is maximum relaxed)

  • IVSd: IntraVentricular Septum in Diastole
    Meaning the measurement of the septum between the ventricles (the septum is the wall that separates the left ventricle from the right ventricle)
  • LVIDd: Left Ventricle Inner Diameter in Diastole
    Meaning how large the actual ventricle - the space - is. This is first measured in diastole, and later in systole (and usually in the M-Mode)
  • LVFWd: Left Ventricle Free Wall in Diastole
    The free wall is the wall of the ventricle that is on the outer part of the heart

Now the measurements in systole (when the heart is at its maximum contraction)

  • IVSs: IntraVentricular Septum in systole
    Meaning the measurement of the septum between the ventricles (the septum is the wall that separates the left ventricle from the right ventricle)
  • LVIDs: Left Ventricle Inner Diameter in systole
    Meaning how large the actual ventricle - the space - is. This is first measured in diastole, and later in systole (and usually in the M-Mode)
  • LVFWs: Left Ventricle Free Wall in systole
    The free wall is the wall of the ventricle that is on the outside of the heart
  • SF: Shortening Fraction (expressed in %)
    This is the calculation of how much the heart contracts. It is calculated from the difference between the LVIDd and LVIDs (which are already measured), divided by LVIDd. This will give a percentage that shows how many percent smaller does the ventricle get in systole compared to when it is in diastole.
    Note: the contractions are not that powerful in a sedated cat.
  • Ao: aorta
    This is a measurement of the aorta, i.e. the diameter of the aorta. The blood goes from the ventricle out though the aorta and supplies the body with oxygenated blood.
  • LA: left atrium
    The diameter is measured.
    Ao and LA are always done in 2-D Mode.
  • LA/Ao: meaning LA divided by Ao
    This measurement gives the subjective LA size (i.e. the comparison between the LA size subjectively to the size of the heart as a whole).

    These two (LA and Ao) are normally fairly equal in size; it is more common to be slightly over than slightly below though (so 1, 1.2 and 1.3 are fairly normal; when you get more than 1.4 there might be a reason to worry).
  • Subjective Left Atrial Size:
    There are no clear limits here (as far as I know), but an enlarged atrium could be a sign of HCM or RCM.
  • SAM: Systolic Anterior Motion of the Mitral Valve
    The mitral valve is supposed to close when the heart contracts, thus preventing blood to flow back into the left atrium. SAM means that the mitral valve does not close fully and this causes abnormal flows, turbulence and also leakages can occur. The most common reason why the mitral valve does not close properly is that it sticks to a local thickening of the septum. Not all of the blood can leave the left ventricle during the contractions and will leak back into the left atrium. This is why SAM also leads to an enlarged left atrium.
    At autopsies you can sometimes see a spot of scar tissue or something where the mitral valve used to hit back against the wall again - SAM! So SAM can sometimes be seen even at autopsy, in this way.
  • End systolic Cavity Obliteration:
    This means that in the systolic phase - when the heart is in contraction - there is no room left in the left heart chamber (ventricle) caused by thickened walls or enlarged papillary muscles. There is always - in normal hearts - some room left. The walls thicken on the inside of the heart, not on the outside.
  • Papillary muscles:
    Those are the muscles located inside the left and right ventricle, at the walls and attached to "threads" which are in the other end attached to the mitral valve. These threads show on the screen as white lines. Papillary muscles can be enlarged, but they can also be longer than average. This is easy to confuse with thickening but is entirely different. Some cats can have split papillary muscles or extra small ones. These anomalies do not seem to affect the cats at all, contrary to enlarged papillary muscles.

Assessment or grading

For the assessment whether a cat is normal, equivocal or has HCM, we have to look at different things.

One of those things, are the wall measurements: IVSd and LVFWd. These are the most talked about by breeders and some even tend to think that is THE thing. Normal wall size ranges from 3,0 to 5,0 mm, with some small variations.
Earlier studies gave the cut-off value between normal and equivocal at 6.0 mm. We have to take into account that the equipment was less sophisticated at that time. Later studies have given 5.5 mm as the cut-off but now, more vets seem to want to lower that even further. The values that the veterinaries who work with the health programme are now working with a cut-off value of 5.0 mm. So general or local thickening(s) above 5 mm or so should cause the assessment to be equivocal (if there are no other anomalies observed) or HCM.

SAM is also one of the important signs to take into account. Since Dr. Kittleson has stated that SAM can in some cats be the first sign of HCM, the vets grade this anomaly as 'equivocal', even when no other signs are present.

A heart murmur could also be a sign of HCM (60% of the heart murmurs are HCM positives, but a heart murmur can also have other causes). A galloping heartbeat could also be a sign of HCM but not when no other signs are present.

Another thing are the papillary muscles: If the papillary muscles are enlarged (and no other anomalies are observed), they cat will be assessed as equivocal. That is according to Dr Kittleson's criteria. Some cats just have slightly larger papillary muscles in their genetic make-up. But it could also be the first signs of HCM; so they are considered equivocal. We just don't know at that point what it is. If however a cat has BOTH enlarged papillary muscles and SAM, Dr Kittleson says HCM positive.

VERY IMPORTANT: Equivocal means that anomalies have been observed but at the time of the testing, it is not clear what those anomalies mean or will mean. Not every cat who is assessed as equivocal, will develop HCM. On the other hand, the possibility does exist.