Antiarrhythmics

How do antiarrhythmic drugs work?

The heart normally beats in a regular rhythm that is controlled by electrical impulses. These impulses originate in the sinus node, the heart's natural pacemaker, and spread to the atria and ventricles via the conduction system. In this way, the blood is pumped efficiently through the body.

In an arrhythmia, this rhythm is disturbed, either because the sinus node is not working properly, because additional impulses are generated or because the impulses are blocked. This can cause the heart to beat too quickly (tachycardia), too slowly (bradycardia) or irregularly (extrasystoles, atrial fibrillation).

Antiarrhythmic drugs are medications that intervene in the electrical processes of the heart and thus normalize or stabilize the rhythm. There are various classes of antiarrhythmic drugs, which are differentiated according to their mechanism of action and target group. The most important are

• Class I: Sodium channel blockers. They slow down excitation conduction in the heart and prolong the refractory period, i.e. the time during which the heart does not react to a new impulse. They are mainly used for ventricular arrhythmias. Examples are lidocaine, mexiletine and quinidine.
• Class II: beta blockers. They inhibit the effect of adrenaline and noradrenaline on the heart and thus reduce the heart rate and blood pressure. They are mainly used for supraventricular arrhythmias, i.e. those that occur in the atria. Examples are atenolol, metoprolol and propranolol.
• Class III: Potassium channel blockers. They prolong the action potential duration, i.e. the time during which the heart is excited, and thus prevent the occurrence of new impulses. They are mainly used for atrial fibrillation. Examples are amiodarone, sotalol and dronedarone.
• Class IV: calcium channel blockers. They slow down conduction in the AV node, which transmits the impulses from the atrium to the ventricle, and thus reduce the ventricular rate. They are mainly used for supraventricular tachycardia. Examples are verapamil and diltiazem.

When are antiarrhythmic drugs used?

The decision as to whether or not a dog with an arrhythmia needs to be treated depends on various factors, such as the type and severity of the arrhythmia, the presence of symptoms or concomitant diseases and the risk of complications. Not every arrhythmia needs to be treated, some are harmless or even physiological, i.e. normal for the dog.

The most common symptoms of arrhythmia are

• Weakness
• tiredness
• shortness of breath
• coughing
• Fainting
• Blue mucous membranes
• Heart murmur
• Irregular pulse

If your dog shows one or more of these symptoms, you should definitely take him to a vet who can make an accurate diagnosis. They will carry out a clinical examination and take an electrocardiogram (ECG), which records the electrical activity of the heart. Sometimes further examinations such as an X-ray, an ultrasound or a blood test are also necessary to find the cause of the arrhythmia.

Treatment with antiarrhythmic drugs is usually only recommended if the arrhythmia is symptomatic or increases the risk of serious complications. The aim is to normalize or at least stabilize the heart rhythm, alleviate the symptoms and improve the dog's quality and expectancy of life.

What are the side effects of antiarrhythmic drugs?

Like any medication, antiarrhythmic drugs can also have side effects, which can vary depending on the active ingredient, dosage and individual sensitivity of the dog. The most common are

• Gastrointestinal complaints such as nausea, vomiting, diarrhea or loss of appetite
• Nervous system disorders such as drowsiness, tremors, ataxia or seizures
• Cardiovascular disorders such as hypotension, bradycardia or heart block
• Skin reactions such as itching, rash or hair loss
• Liver or kidney damage
• Blood count changes such as anemia, thrombocytopenia or leukopenia

To minimize the risk of side effects, it is important that you follow your vet's instructions exactly and adhere to the prescribed dose and intake interval. You should also have your dog checked regularly to check the effectiveness and tolerability of the therapy. If you notice any side effects in your dog, you should take him to the vet immediately and not change or stop the medication on your own.