Sub Aortic Stenosis
Dilated Cardio-Myopathy
Heart

HEART

Interpreting Heart Test Certificates

Dr. Jo Dukes-McEwan BVMS, MVM, PhD, DVC, Dip.ECVIM-CA(Cardiology), MRCVS

RCVS & European recognised Specialist in Veterinary Cardiology

Senior Lecturer in Veterinary Cardiology

Small Animal Teaching Hospital, University of Liverpool, Leahurst, Chester High Road, Neston, Wirral CH64 7TE

J.Dukes-McEwan@liv.ac.uk

 

This review is to encourage and support members of the Newfoundland Club to continue to screen Newfs for potentially serious heart disease prior to use in breeding. The two major heart diseases of concern in Newfoundlands are subaortic stenosis(SAS) and dilated cardiomyopathy (DCM). However, screening will also identify other heart diseases such as patent ductus arteriosus (PDA). Heart disease can be congenital or acquired, the latter appearing later on in life, with the young animal being totally normal.

Congenital heart disease is a structural heart defect the pup is born with. Sometimes, it can progress over time as the pup grows (particularly SAS). Congenital heart diseases will almost always have an audible heart murmur if they are severe. Therefore, routine veterinary checks of the pup prior to or after sale, and at vaccinations, should identify the most serious conditions. Veterinary surgeons may refer affected puppies to veterinary cardiologists confirm the diagnosis and how severe the defect is, and whether any surgical correction or medical management is possible.

Aortic stenosis leads to narrowing of the aortic valve, separating the heart from the circulation. Normally, this is due to a band beneath the valve, so this is sub-aortic stenosis. For conditions such as SAS, the grade (loudness) of the heart murmur correlates with the severity of the disease. So there is no problem for the primary vet or a cardiologist to identify severely affected dogs. The heart murmur grade correlates with the speed (velocity) of blood passing through the valve; this is measured with Doppler. The more narrowed the valve is, the faster blood has to go to pass through this obstruction to supply the circulation.

There is a problem with making the diagnosis of mild aortic stenosis. Very mild aortic stenosis can lead to slight turbulence of flow across the aortic valve, and minor increases in blood velocity. A soft heart murmur may be detected, or there may be no audible murmur (since the massive conformation of the Newfoundland, the dense fur and tendency to pant may mask any murmur present). Other factors can also give increased velocity of aortic flow measured by Doppler. If the dog is very stressed, the adrenaline causes increased force of contraction of the heart, and more blood passes by a normal aortic valve. This leads to a “grey area” – some Newfs with an increased velocity across the valve may merely be stressed, or may have mild aortic stenosis. Furthermore, the aortic flow varies beat to beat as well, so an average velocity has to be taken. It is also clear that an accurate velocity measurement has to be taken; if the Doppler angle is not parallel to flow passing the valve, the cardiologist will seriously under-estimate the flow velocity. It is easier in some dogs to get parallel to flow, and getting parallel to flow in every dog is a learning curve for cardiologists – which is why ideally, only cardiologists who have the additional training and accreditation in echo-Doppler should be involved in screening. An experienced cardiologist, in deciding whether a Newf is normal or has aortic stenosis will take into account the following:

  • An audible heart murmur suggesting aortic stenosis
  • Degree of stress and excitement of the dog during the study
  • The aortic velocity (average of at least five good quality aortic flow spectra).
    • From studies in UK Newfoundlands, and published in the cardiology literature from the USA, over 95% of Newfoundlands have aortic velocity less than 1.7 m/s. This is therefore normal.
    • Even with excitement and stress, it is very unlikely in a Newf that adrenaline will result in aortic velocity of over 2.0 m/s. This would therefore be abnormal, particularly if there are other features of aortic stenosis (detailed below).
    • Newfs with velocities of between 1.7 m/s and 2.0 m/s may have mild aortic stenosis, or may be normal (equivocal). In deciding which is true, the other factors are taken into account
  • Imaged abnormalities of the aortic valve or the region beneath the valve which are consistent with aortic stenosis.
  • A leaky aortic valve (aortic regurgitation) suggests the valve is abnormal
  • The Doppler technique can include “walking” the sample volume where velocity is recorded to cross the valve. If the velocity suddenly “steps-up” from normal to high (step-up of over 0.4 m/s is abnormal), as it indicates that the valve is narrowed.

This is clearly a difficult area. As with many tests with biological variation, results are not always “black and white”, but there will be a grey area. Owners and breeders often wish to be retested. This is fine. Studies investigation repeatability of Doppler measurements show <10% variation in Doppler velocity measurements between two studies on the same dog, even if different cardiologists are doing the scan, provided they have a similar level of experience and training. Cardiologists are more likely to make an error of underestimating aortic velocity, if they fail to get aligned with flow. It is not possible for them to over-estimate the velocity.

Despite the concerns voiced, members and Newfoundland owners should be encouraged about the following:

  • Dogs with significant aortic stenosis will be identified. As well as advising that these dogs should not be bred, since we know SAS is inherited, these dogs can be carefully monitored and treated where appropriate.
  • The screening is working! – it is now quite unusual for cardiologists to detect loud heart murmurs in Newfoundlands at shows or to see severely affected pups.
  • If the dog is equivocal, then it can be bred if otherwise it shows good breeding potential. Ideally, it should be bred to a completely normal dog / bitch, and the progeny carefully checked prior to sale and when over 12 months old.
  • Heart testing is only one component of making the decision about whether you use a dog to breed. You need to ensure you look at the whole dog, including temperament, conformation and general health, as well as the results of other health screening.

Acquired heart disease

Acquired heart disease is common in dogs. Degenerative valvular disease (mitral valve disease) predominantly affects small breed dogs, leading to heart failure, such as the Cavalier King Charles spaniel. Ageing degenerative changes of the heart valves is seen in all breeds and crosses. It will result in a heart murmur (mitral regurgitation) in an elderly Newfoundland, but this rarely results in heart failure.

Dilated cardiomyopathy is more likely to affect the large and giant breeds, including Newfoundlands. We now have robust evidence that this is an inherited disease, transmitted as an autosomal dominant trait, so dogs and females may both be affected, and, with one affected parent, up to 50% of the progeny can be affected. The age of onset can vary enormously. Some Newfs may show signs of heart failure at 2 years old, others may be 12 years old. Typically, in the UK, the average age of onset of signs of heart failure is 7 – 8 years old. DCM means that the heart muscle progressively dilates and fails to pump normally. As pressure build up within the heart chambers, fluid may dam back into the lungs (causing coughing and breathlessness) or into the belly (ascites). Abnormal heart rhythms such as atrial fibrillation are common. In DCM, there is often no heart murmur, and heart testing may not identify the Newfs with early disease, unless they show an abnormal heart rhythm. Echo (cardiac ultrasound) will allow measurement of the heart chambers and contractility of the heart, so this will allow the diagnosis to be made.

Serial longitudinal studies in UK Newfoundlands in families where DCM have been reported, have shown that several years before the dog has clear echo evidence of DCM, they will have some echo abnormalities. These can be impaired pumping ability of the heart (depressed contractility) or dilated chambers (left ventricular enlargement). Some of these echo abnormalities mean the dog has DCM, but over time, these progress. We have developed a scoring system for DCM. A score of over +6 means that the dog has DCM, but scores of +1, +2 etc., mean that this dog should be monitored; it cannot yet be given the diagnosis of DCM. The reason we are cautious is that other conditions – such as hypothyroidism (underactive thyroid gland) - can also impair pumping ability of the heart.

Heart testing with use of Echo-Doppler will identify Newfoundlands with DCM. However, in a young dog, it is inconclusive – a two-year-old may show no evidence of DCM, when he is destined to develop DCM at 8 years old. However, serial screening (e.g. every 18 months) will begin to identify minor echo abnormalities, which eventually progress to DCM.

The problems with echo screening of Newfs for DCM are:

  • Results are inconclusive in a young dog.
  • Echos need to be repeated every 12 – 24 months to exclude the possibility of DCM in breeding individuals.
  • Very often, the Newf is at the end of its breeding career by the time DCM results in clinical signs such as breathlessness or coughing or atrial fibrillation. Screening will identify dogs at an earlier stage.
  • It is to the benefit of the individual dog to be identified before they go into heart failure – there are supplements and treatments, which may slow down this progression, even though we cannot cure the condition.
  • There are robust criteria, used in the scoring system, to come to the diagnosis of DCM. Therefore, many dogs, in the several years prior to manifesting the disease, equivocal echo abnormalities will be documented (reported on the heart test certificate).
  • If DCM does develop, then progeny and siblings should be serially screened (scans every 18 months, or more frequently once echo abnormalities or a score is recorded). But remember up to 50% of the litter will not be affected and will not be carriers if there is only one affected parent!

As there are these problems, what we urgently need is a genetic test for the disease. Despite a huge amount of work and funding from the British Heart Foundation and the Kennel Club Charitable Trust, we looked first for a genetic marker, then we looked at fifteen different candidate genes, known to cause DCM in humans or other animals. However, we have still not yet found the gene causing Newfoundland DCM. If we continue to collect echo and pedigree data and DNA from Newfoundlands who have normal echos at an old age, as well as dogs with DCM, we may get a genetic test sometime soon! As well as helping Newfoundlands, this may help other dog breeds or even humans! If your Newf is having a blood sample for any reason, request that your vet takes some additional blood for DNA, and the sample can be submitted to the DNA archive: For further information, see:

http://pcwww.liv.ac.uk/DNA_Archive_for_Companion_Animals/

For these reasons, I strongly urge members to continue supporting screening for heart disease and collating information which will continue to help your beautiful dogs.

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