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Tachycardia refers to an abnormally rapid heart rate. In dogs, the normal heart rate at rest can range from 70 to 120 beats per minute, depending on the dog’s size. Smaller breeds have a faster heart rate while larger breeds have a lower heart rate.
The heart is constituted by two atria and two ventricles, that contract in a synchronized way. Each beat (contraction) is initiated in the sinus node, which is located in the right atrium. The electric impulse is then conducted through the atria, leading to the contraction of the atria cardiac muscle, pumping the blood into the ventricles. The electrical impulse then reaches the atrioventricular node, located between the atria and the ventricles, which slows down the electrical signal to allow the ventricles to fill in with blood. When the electrical impulse reaches the ventricles, they contract, pumping the blood to the lungs and the rest of the body.
There are three kinds of tachycardia: sinus tachycardia, atrial tachycardia and ventriculartachycardia.
Sinus tachycardia refers to a heart rate that initiates in the sinus node (where normal impulses begin) but at a faster rate. This is generally a regular rhythm. This may be a normal physiologic rhythm that results from exercise or excitement, but it can also occur in conditions like stress, pain, shock, fever, anemia, hyperthyroidism, among others.
Atrial tachycardia refers to abnormal, rapid impulses that originate in the atria, but not from the sinus node. This is generally a regular rhythm. This condition suggests severe myocardial or conduction system disease.
Ventricular tachycardia refers to repetitive ventricular impulses that initiate in the heart’s lower chambers, the ventricles. The rapid heart rate does not allow the heart to fill before it contracts, so blood does not get properly pumped throughout the body.In this case the rhythm tends to be regular. This condition can be well tolerated or life-threatening.
Dogs with tachycardia may show no clinical signs, especially if there is no underlying cardiac disease and if the rate is not too rapid. Tachycardia symptoms may include:
In extreme cases this condition may cause unconsciousness or cardiac arrest.
The most commonly used test to detect tachycardia is the electrocardiogram (ECG). This test represents the cardiac muscle’s electrical depolarization and repolarization processes. These electrical impulses are recorded as waves. The ECG may provide us information on the cardiac rhythm and frequency and intra-cardiac conduction of the electric impulse. This test can also suggest myocardial disease, ischemia, pericardial disease, drug intoxication, among others. But the ECG alone is not enough to get an accurate diagnosis.
Holter is a test that allows a continuous evaluation of the heart’s electrical function during everyday activities, sleep and exercise. This test is often used to detect intermittent cardiac conditions.
Chest X-rays are also used to evaluate patients with cardiac disease. This allows us to have access to the size, position and conformation of the heart.A chest X-ray may also help to determine whether any congenital heart defects are present. It can also detect other conditions that may explain the signs and symptoms of the dog.
Blood tests can help to determine whether thyroid problems or other substances may be factors contributing to the patient’s tachycardia. Blood tests can also inform the doctor about the existence of anemia or kidney problems, which canlead to tachycardia complications. Serum electrolytes may also be tested to determine sodium and potassium levels.
The most important thing is to determine the underlying cause of the tachycardia so that this can be treated. The aim of the tachycardia treatment is to slow down an accelerated heart rate, prevent subsequent episodes of tachycardia and reduce risk complications.
One way of slowing an abnormally rapid heartbeat is by performing a vagal maneuver. This maneuver is performed by applying ocular pressure or carotid sinus pressure. This stimulates the vagal nerve which causes the heartbeat to slow down. When this maneuver doesn’t work, you may have to recur to medications. This must not be performed without medical supervision because different situations may require different approaches or different types of medication.
Anti-arrhythmic medication may be necessary to restore a normal heart rate. The most commonly used anti-arrhythmic drugs are amiodarone, atenolol, atropine sulphate, digoxin, diltiazem, epinephrine, esmolol, glycopyrrolate, isoproterenol, lidocaine and propanolol.
Electrical cardioversion is a procedure in which an electrical shock is delivered to the dog’s heartthrough paddles or patches on your dog’s chest. The current generated by those devices affects the electrical impulses in the heart and restores a normal rhythm. This method should be used in refractory cases, when maneuvers and medications aren’t effective.
Radiofrequency catheter ablation following electrophysiologic mapping of accessory pathway circuits is available in some referral centers. This can represent a cure for the arrhythmia without permanent implation devices. In this process, small areas in the heart that are triggering abnormal electrical impulses and causing atrial fibrillation are destroyed.
A pacemaker is a device surgically implanted under the dog’s skin. When the device detects an abnormal heartbeat, it emits an electrical pulse that helps the heart to restore a normal rhythm.
Open-heart surgery is also used in some cases to destroy extra electrical pathways.Surgery is usually used only when other treatment options don’t work.
Complications of tachycardia
The main complications of tachycardia include:
Tachycardia can be a serious condition and should not be taken lightly, so if you think your dog may have this condition take him to the veterinarian for proper counseling.