In order to provide a good supply of blood to the body, the heart needs to beat regularly, and at the right speed. If it beats too slowly (bradycardia) then not enough blood is pumped. If it beats too fast (tachycardia) then the heart does not have enough time to fill with blood before it contracts, and again not enough blood is pumped. In both cases this can lead to symptoms of weakness, exercise intolerance, collapse, fainting, and even death.
A fast heart rate can be a normal part of the fight, flight, fright response, but can be caused by a pathological (disease-related) arrhythmia. Similarly, a slow heart rate can be normal. Dogs and cats in the vet's surgery are usually at least a little agitated, and so the heart rate is quite high, but continuous electrocardiography (ECG or EKG) recordings called Holter monitors show that a sleeping dog can have a heart rate as low as 30 beats per minute, and the heart can actually stop for several seconds at a time, without causing problems. Again, however, if the slow heart rate is pathological, especially if it occurs at a time of high oxygen demand such as exercise or excitement, problems can occur.
Types of arrhythmia
Arrhythmias, or irregular heart beats, can be classified in several ways. One way is to classify them as fast (tachyarrhythmias) or slow (bradyarrhythmias). Tachyarrhythmias can be further divided into those coming from the atria, where the heart's natural pacemaker lives (atrial or supraventricular tachyarrhythmias) or from the ventricles, the main pumps of the heart.
Supraventricular tachycardias are often caused by diseases of the atria themselves, particularly diseases that stretch these chambers, such as cardiomyopathies or valvular disease in the dog or cat. In one example, atrial fibrillation, the atria stop beating all together and just appear to vibrate (fibrillate).
Ventricular tachyarrhythmias can occur because of diseases affecting the ventricles, but also serious non-cardiac illnesses such as tumours of the spleen and gastric torsion can provoke these abnormal rhythms. Ventricular tachyarrhythmias can consist of intermittent early beats, prolonged runs of abnormal beats or a continuous fast rhythm originating from the ventricle. Ventricular tachycardia has the potential to degenerate into ventricular flutter and fibrillation, where the heart effectively stops pumping, and death will rapidly ensue if not corrected.
Bradyarrhythmias include sinus bradycardia, which usually involves slowing of the heart for non-cardiac reasons such as hypothyroidism, sick sinus syndrome which is an abnormality of the heart's pacemaker, and heart block, in which signals from the heart's pacemaker do not reach the ventricles.
Treatment of arrhythmias
Treating an arrhythmia has the potential to make the arrhythmia worse rather than better, especially if the wrong anti-arrhythmic drug is used, but this can even happen in some cases where the correct treatment is used. In one human study in which treatment of ventricular arrhythmias was compared to placebo, the treated group had a mortality rate over 3 times the placebo group! (Pratt & Moye 1990). It is therefore essential that the correct diagnosis of an arrhythmia is made using a short ECG or a Holter monitor, and not just a diagnosis of arrhythmia by a stethoscope, and that the response to treatment is monitored.
Treatment of supraventricular arrhythmias may involve the use of drugs such as digoxin, or beta blockers such as propranolol or atenolol. Treatment of ventricular arrhythmias often involves injectable lidocaine as an emergency therapy, or longer term treatment with drugs such as sotalol or amiodarone.
Treatment of bradyarrhythmias can be difficult, and where the arrhythmia is severely affecting quality of life or is life-threatening, pacemaker implantation maybe required.
Conclusion
Arrhythmias in dogs and cats have a variety of causes, and can be benign or life threatening. Accurate diagnosis is essential prior to starting treatment, as treatment itself has the potential to be dangerous. However, in appropriate cases, dogs and cats treated for arrhythmias can have a dramatic improvement in quality of life, and benefit from a reduced risk of dying from their arrhythmia.
Source:
Pratt & Moye (1990) American Journal of Cardiology, 65, 20
Further reading:
Heart failure in dogs and cats
Treating heart failure in dogs and cats
Alex Gough - I am a qualified veterinary surgeon with 15 years experience, the last 9 of which have been working in referrals, seeing cases sent to be ...
