The heart is an amazing organ. Made of solid muscle, your dog’s heart will contract at least once every second, from the moment it develops in the foetus, day in, day out, until their days are up. Never missing a beat. That is, unless they suffer from a condition called ‘heart block’.
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What makes a heart beat?
To understand heart block, it is helpful to grasp the basics of the electrical conduction system which makes the heart beat. Unlike skeletal muscles, which contract because signals from our brain tell them too, the heart contracts independently of the brain. Electrical signals are produced in a small part of the right atrium called the sinoatrial (SA) node. From here, the signals pass through the atria to the atrioventricular (AV) node which lies in between the atria and ventricles, and then spread through the ventricles, causing them to contract.
These electrical signals are rhythmically produced, and travel along this same, set route each time. This results in a beautifully synchronised, well controlled heart beat. The atria contract together to pump blood into the larger ventricles, which then contract together to pump blood around the body, before it returns to the atria to repeat the journey. The heart relies on these precisely coordinated electrical signals to contract and pump blood effectively around the body.
What is heart block?
Heart block in dogs is a condition that results from a disruption, or blockage, to the electrical conduction system of the heart, which interrupts the normal rhythm and beat. This interruption can happen because of a benign process, with no significance to your dog’s health. But it can also be a result of severe cardiac disease and be very dangerous. The block can happen at different points along the conduction pathway, resulting in three different types of heart block with increasing significance; first degree, second degree and third degree block.
Types of heart block
1) First-degree heart block:
First degree heart block occurs when there is a delay in the conduction of electrical impulses from the atria to the ventricles. The atria contract first as normal. But the signal to make the ventricles contract comes late i.e. all electrical impulses eventually reach the ventricles, just slower than normal. In most cases, first-degree heart block does not cause noticeable symptoms. And it may be detected incidentally during a routine vet check.
2) Second-degree heart block:
Second-degree heart block is the occasional failure of electrical impulses to pass from the atria to the ventricles. (i.e. the signal is not just delayed as with first-degree block, but it completely fails to pass to the ventricles). Here, the atria contract as normal, but the ventricles do not contract for that beat. The heart will beat normally some of the time, with these abnormal blocked beats only happening intermittently.
To confuse things, there are different types of second-degree heart block – Mobitz Type I and Mobitz Type II. No treatment is needed for Type I block, and it carries a good prognosis. However, the more serious Type II block is generally caused by underlying cardiac disease and may progress to complete heart block.
3) Third-degree (complete) heart block:
Third-degree heart block is the most severe form. And it is always a result of underlying heart disease such as fibrosis or cancer of the heart. It is a complete failure of the electrical impulses to pass from the SA node down to the ventricles. The ventricles therefore do not contract in synchrony with the atria. Instead, they get their electrical signal to contract from areas of the heart other than the SA node (called subsidiary pacemakers). In this type of heart block, the atria and ventricles are not contracting in synchrony and beat independently of each other, leading to a very irregular heartbeat and inefficient pumping of blood around the body.
Symptoms of heart block
The severity of symptoms will vary depending on the type of heart block present as each type affects the heart’s ability to pump blood differently. First-degree block will often have no symptoms, whilst third-degree block can have profound symptoms and be life-threatening.
Typical symptoms will include:
- Lethargy
- Weakness
- Exercise intolerance
- Fainting or collapse
- Irregular heartbeat (detected by a vet)
- Difficulty breathing
If you recognise any of these signs in your dog, even if they are short lived and happening irregularly, it’s important to seek veterinary attention as a prompt diagnosis and treatment can improve the prognosis.
Diagnosis
Heart block may be picked up either if an irregular heartbeat is picked up incidentally during a routine vet visit, or because you have noticed worrying symptoms in your dog at home which prompted a vet check.
Your dog may be otherwise very well, but if the vet hears an irregular heartbeat on examination, it cannot be ignored as it is hard to differentiate between the different types of heart block just by listening. To make a firm diagnosis, your vet may recommend blood tests for general health parameters as well as cardiac markers, and a heart scan (echocardiogram). Ultimately, an ECG trace is needed to diagnose heart block.
Because of the intermittent nature of some of the blocks, it may be recommended that your dog wears a 24-hour ECG holter monitor so a long period of time can be analysed in the hope of capturing the abnormal rhythm. Each different type of heart block will have specific changes on the ECG trace which will need to be interpreted by a vet – or, often, a heart specialist – to give a firm diagnosis.
Prognosis and treatment
As with symptoms, the prognosis for dogs with heart block depends on the type and severity of the block.
Generally, the prognosis for first-degree and second-degree Type I heart blocks is good. Many dogs with these more benign blocks live normal, healthy lives without requiring specific treatment.
The prognosis for second-degree Type II heart block is more variable, depending on the specifics of the conduction abnormalities. The more serious the block, the more guarded the prognosis; they may require close monitoring, oral medications to treat the underlying heart disease, or if severe, then even surgical implantation of a pacemaker.
The prognosis for third-degree block is guarded to poor. Dogs with complete heart block are at risk of cardiac arrest and sudden death. They may require ongoing medical management, but implantation of a pacemaker to regulate the heart’s rhythm is the only reliable treatment.
Conclusion
In conclusion, while serious heart block in dogs is not common, it is crucial as a pet owner to recognize the symptoms and seek veterinary advice promptly if you are worried. Most dogs with an irregular beat will have a mild form of heart block, which won’t require any intervention. However, ignoring clinical signs can have catastrophic implications for your dog. By being vigilant about changes in your dog’s health, exercise ability and energy levels, and seeking timely advice from your vet, you will give him the best chance for an early diagnosis and treatment, and ensure the best possible prognosis for him.
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