by Debbie “The Rat Lady” Ducommun
In the late 1990s I started realizing that congestive heart failure is very common in rats, and especially older rats. The most common form is hypertrophic cardiomyopathy (thickened ventricle walls which reduce the size of the left ventricle), but dilated cardiomyopathy (an enlarged heart) can also occur. Rats with mycoplasma infections seem to be quite prone to heart failure and I think many rats have died from heart failure or a combination of heart failure and lung disease rather than just lung disease as is usually assumed.
I do as many necropsies of rats as I can, and between 1998 and 2003 I took measurements of the hearts of 150 rats. Externally I measured the length and width of the heart, and internally I measured the thickness of the left ventricle walls and the width of the left ventricle. I also measured the length of the rat when stretched out from nose tip to anus.
Average external length of heart = 21 mm (most between 20-23 mm)
Average external width of heart = 13 mm (most between 13-15 mm)
Average thickness of external wall of left ventricle = 3 mm (most between 2.5-3 mm)
Average width of left ventricle = 3.85 mm (most between 3-4 mm)
Average body length = 223 mm
An external wall of the left ventricle thicker than 3 mm tend to be associated with hypertrophy and a wall thinner than this with dilation. An external wall thicker than the width of the ventricle itself also seems to be associated with hypertrophy. A left ventricle 6 mm or wider seems to be associated with dilated cardiomyopathy.
The worst case of hypertrophic cardiomyopathy in my sample:
external length = 22 mm
external width = 13 mm
thickness of external wall of left ventricle = 4 mm
width of left ventricle = 1 mm
length of body = 191 mm long
The smallest heart in the sample:
external length = 14 mm
external width = 9 mm
thickness of external wall of left ventricle = 5 mm
width of left ventricle = 3 mm
length of body = 255 mm
This heart belonged to a 10-month-old male who died under anesthesia for an abscess. This heart was one of the worst examples of hypertrophy in the sample
The largest heart in the sample:
external length = 30 mm
external width = 15 mm
thickness of external wall of left ventricle = 3 mm
width of left ventricle = 9 mm
length of body = 255 mm
This was a male rat at least 26 months old who died from congestive heart failure.
During a necropsy I often find the right atrium to be dramatically enlarged while the heart is still in place in the chest. Once the heart is removed, the atrium collapses to a more normal size. Sometimes the right ventricle also appears enlarged externally. Rarely the left atrium is enlarged.
Of the 54 rats I owned and necropsied while collecting heart measurements, 10 (18%) had dilated hearts and 13 (24%) had hypertrophic hearts. These 23 rats underwent treatment for congestive heart failure. The average age of onset of treatment was 24 months and treatment for these older rats tended to be very successful. Rats who had an onset of symptoms before 18 months of age did not have a good prognosis.
There were also 9 of my rats (15%) who were successfully treated for heart failure whose heart measurements fell within the average range. That means a total of 57% of these rats had some type of heart abnormality.
For more information on Congestive Heart Failure in rats, including a link to a page with photos of hearts, see the article Respiratory & Heart Disease.
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