The heart is an amazing organ- every second of every hour of every day of every year of our (and our pets’) lives it must continually keep pumping blood around our bodies. If it misses even just a few beats it has a dramatic effect on us. Imagine lifting a small dumbbell every second, for 24 hours a day for the rest of your life, and if you stop for more than a minute you will die. The heart truly is an outstanding evolutionary achievement.
And yet most of us know so little about it. We’re dimly aware there are two parallel circulatory systems to the lungs and body, and that each beat is triggered by a “pacemaker”, but beyond that we expect that little miracle to keep on working, without us even being aware of it most of the time.
We can divide up problems we see in the heart into a number of broad failure categories:
- electrical impulse generation and conduction
- heart muscle
- heart valves
- heart anatomy
Electrical problems commonly come about as a result of the usual pacemaker going awry – instead of generating a signal every half second or so it pulses more or less frequently, or in a chaotic rhythm. Sometimes a secondary pacemaker can start to generate a second competing signal leading to very confused heart muscles! Occasionally the circuitry of the heart can fail and the signal does not propagate correctly through it.
Heart muscle problems are the major cause of human heart attacks. Atherosclerosis – a thick fatty scarring layer – builds up in arteries and when a clot forms or gets stuck, it prevents oxygenated blood reaching the muscles, so they stop, and bits die off. Myocardial (muscle / heart) ischaema (=starvation of oxygen) is why we human have heart attacks. Our pets luckily don’t tend to suffer from atherosclerosis, so “heart attacks” as we know them in humans very rarely occur in animals. However, clots can and do occasionally cause myocardial ischaemia. Heart muscle problems in animals broadly divide into two camps- over and under development. Over development, or hypertrophic cardiomyopathy, occurs when the heart muscle becomes inflated like a body builder’s muscle- usually due to having been made to work too hard; cats with an overactive thyroid commonly have this problem. Like a body builder, the muscle becomes less flexible, and hence less able to pump blood. Under development, or dilatative cardiomyopathy, is a problem where the heart muscle becomes weak and floppy; it’s generally a genetic problems, and most commonly seen in breeds like the Dobermann.
All mechanisms can wear out, and heart valves are no exception. Some breeds such as Cavalier King Charles are prone to early genetically driven valvular degeneration, or diseases such as septicaemia can lead to bacterial valvular endocarditis- infection on the heart valve causing it to break down and become leaky. When the valve stops working properly, blood sloshes back through the valve under pressure, rather than forward to where it was supposed to go. Not only do organs then become starved of oxygenated blood, but the back pressure this causes leads to ballooning of heart chambers, and overpressure in tissues leading to oedema, or swelling. In people this is classically around the ankles- in pets we see a build up of fluid in the abdomen for right sided failure. Left sided heart failure leads to overpressure in the lungs and so we see fluid build up in the lungs causing breathlessness, coughing and exercise intolerance.
Heart anatomical problems such as hole in the heart or a narrow aorta (aortic stenosis e.g. Boxers) lead to all kinds of problems- mixing of oxygenated and deoxygenated blood, backflow of blood, excessive pressure required to pump blood forward etc.
The heart is marvellous at compensating for these problems- it pumps faster or harder, retains fluid to maintain blood pressure and alters a whole host of hormones and electrical activity to maintain that all important blood flow. When it stops being able to cope, we call that “decompensation”, and early treatment can put this moment off for as long as possible.
What can we do to diagnose heart problems?
Abnormal sounds in the heart (murmurs), problems in heart rate and rhythm can be picked up with a stethoscope, but a good clinical examination to look for signs of heart failue such as weak pulses or a congested chest are essential. Blood tests for heart damage enzymes (proBNP and Troponin) help but can pick up false positives and miss true positives. A heart scan by an experienced operator is a great way of assessing blood flow, heart muscle health and cardiac anatomy and is essential in most cases. An electrocardiogram (ECG) can detect problems with electrical conduction, and plain xrays or even 3d xrays (CT scan) can help us to understand problems.
Ultimately treatment needs to be aimed at solving existing problems e.g. diuretics to reduce fluid in coughing, and to reduce the speed at which the problem gets worse e.g. pimobendan for dogs with valvular heart disease. Rarely a drug such as digoxin can be used to correct a problem such as an abnormal heart rhythm. The recent EPIC trial for congestive heart failure suggests early treatment in asympotmatic dogs can increase the number of quality life years they have ahead of them
A careful approach is needed to work out the exact problem with the exact blend of diagnostic tools before reaching a treatment decision- heart disease is complicated!