Sick Sinus Syndrome (SSS) is a condition where the sinus node (normal pacemaker of the heart) is diseased and not responding normally to the body’s needs.
This disease is often a result of fibrosis/scarring of the sinus node or surrounding tissue, which prevents the signals from forming or being conducted.
Affected patients can range in severity from mild (no clinical signs) to severe (collapse, lethargy, weakness – especially with excitement). Often, the sinus node stops suddenly and a long pause occurs, followed by a ventricular escape beat.
This condition carries the risk of sudden death.
Medications may be attempted to increase the sinus rate; however, these are often ineffective or only effective in the short-term. A permanent pacemaker is usually required to ensure the heart rate does not drop below the minimum programmed rate.
3rd Degree Atrioventricular (AV) Block is a condition where the signals for the heart rate from the sinus node are not received or allowed through the AV Node.
This is usually a result of scarring/fibrosis of the AV Node where the conductive tissue is no longer allowing the signals to travel through this region.
This results in a rhythm called a ventricular escape rhythm that is enough to keep the patient alive, but is very slow (typically 30-40 beats per minute). Affected patients often collapse and have extreme exercise intolerance because their heart rates are not appropriate for higher activity levels.
This condition is typically incurable; however, this can be treated with a pacemaker, which ensures the heart rate never drops below the minimum programmed value.
Until a pacemaker can be implanted, medications can be attempted to increase the ventricular rate slightly.
These medications are typically effective for only a short time, if at all.
2nd Degree Atrioventricular (AV) Block is a condition where the AV Node allows only some sinus-initiated beats to travel through the node into the ventricle, while others are blocked.
This can range in severity from mild (occasional blocked beats) to severe (High-Grade 2nd Degree AV Block).
The mild forms are often a result of higher vagal (parasympathetic) tone and rarely if ever cause clinical signs.
Commonly, this is associated with intracranial (neurologic), gastrointestinal, respiratory, bladder, or ocular disease.
An atropine response test will differentiate vagal from primary nodal causes. Mildly affected patients should be monitored for any evidence of progression.
The severe forms (High-Grade 2nd Degree AV Block) can result in collapse, weakness, exercise intolerance, and lethargy.
This condition is almost always due to a fibrosis/scarring of the AV Node that prevents signals from conducting through appropriately.
In severe cases, a permanent pacemaker is required to ensure the heart rate does not drop below the minimum programmed heart rate.
Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) is a condition that most frequently occurs in boxer dogs.
This condition consists of the right ventricle being infiltrated with a fatty or fibrofatty tissue that destroys the normal myocardium.
This leads to the development of severe ventricular arrhythmias (VPCs and Ventricular Tachycardia) that can be life-threatening.
While there is no cure for this disease, antiarrhythmic therapy is utilized to attempt to reduce the risk of sudden death.
It is typically impossible to eliminate all arrhythmias and sudden death remains a possibility regardless of therapy.
24 hour Holter Monitor examinations are the best possible way to evaluate the arrhythmia frequency and grade.
With this information, we can further direct antiarrhythmic therapy.
Ventricular Premature Complexes or Ventricular Tachycardia is an electrical disturbance, where the ventricular myocardium depolarizes at incorrect times.
These may be due to numerous etiologies, which include; cardiac disease, pericardial effusion, metabolic disease and electrolyte disturbances, tickborne disease, fever, pain, anemia, altered autonomic tone, trauma, sepsis, DIC, gastric dilatation with/without volvulus, splenic disease, hepatic disease, gastrointestinal disease, and pheochromocytoma.
The primary risk with this condition is an R-on-T phenomenon that can cause
ventricular fibrillation and sudden death. Antiarrhythmic therapy is utilized in certain cases to reduce the frequency of these abnormal beats.
Addressing the underlying condition is the best possible strategy; however, this still may not eliminate all abnormal activity.
Follow up electrocardiograms and 24 hour Holter Monitor examinations are necessary for patients with this arrhythmia to continue to optimize medical therapy.
Atrial premature complexes are often the result of atrial dilation (left or right); however, additional considerations would be mechanical irritation (perihilar lymph node enlargement, atrial/auricular mass, heart base tumor, pericardial disease, etc.), infectious/tick-borne disease, metabolic disturbances, or potentially altered autonomic tone (sympathetic/parasympathetic imbalance).
This rhythm is fortunately not considered fatal in itself.
If atrial dilation is the underlying etiology, your pet may go into atrial fibrillation in the future. Antiarrhythmic therapy may be indicated based on the frequency and grade of atrial arrhythmias noted on the ECG.
Atrial fibrillation is a condition where the atrium (top chambers of the heart) are no longer allowing organized electrical activity.
Rather, there is a random oscillation of electrical waves that do not allow the atrium to contract as they should.
The Atrioventricular Node (AV Node) responds to this haphazard electrical activity in a very irregular and rapid pattern, causing a very irregular, rapid heart (pulse or ventricular response) rate.
This condition is typically the result of severely dilated chambers.
This condition can rarely be corrected; however, the ventricular response rate can be lowered to a reasonable level with medications, frequently the combination of Digoxin and Diltiazem.
Continued rechecks and monitoring for adequate heart rate control are essential to ensure these patients remain at an optimal heart rate and do not develop congestive heart failure (fluid in the lungs).