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Sinus Tachycardia in Dogs

Sinus tachycardia (ST) is clinically described as a sinus rhythm (heartbeat) with impulses that arise at a faster-than-normal rate: greater than 160 beats per minute (bpm) in standard sized dogs, 140 bpm in giant breeds, 180 bpm in toy breeds, and 220 bpm in puppies. Changes in heart rate usually involve a reciprocal action of the parasympathetic and sympathetic divisions of the autonomic nervous system.

Severe tachycardia can compromise cardiac output, as too rapid rates shorten the diastolic filling time, the point in which the chambers of the heart dilate and fill with blood - which occurs in the space between heart beats. Particularly in diseased hearts, the increased heart rate can fail to compensate for decreased volume, resulting in decreased cardiac output, decreased coronary blood flow and a concurrent increase in oxygen demands. This is the most common benign arrhythmia in dogs. It is also the most common rhythm disturbance in postoperative patients. 

Symptoms and Types

• Often no clinical signs because condition is a compensatory response to a variety of stresses
• If associated with primary cardiac disease, weakness, exercise intolerance, or loss of consciousness may be reported
• Pale mucous membranes if associated with anemia or congestive heart failure
• Fever may be present
• Signs of congestive heart failure, such as shortness of breath, cough, and pale mucous membranes may be present when ST is associated with primary cardiac disease

Causes

Physiologic

• Exercise
• Pain
• Restraint
• Excitement
• Anxiety, anger, fright

Pathologic

• Fever
• Congestive heart failure
• Chronic lung disease
• Shock
• Fluid in the chest
• Anemia
• Infection/sepsis
• Low oxygen levels/hypoxia
• Pulmonary blood clot
• Low blood pressure
• Reduced blood volume
• Dehydration
• Tumor

Risk Factors

• Thyroid medications
• Primary cardiac diseases
• Inflammation
• Pregnancy

Diagnosis

Because there are so many things that can cause this condition, it is difficult to diagnose and differentiate from other similar diseases. Your veterinarian will most likely use differential diagnosis. This process is guided by a deeper inspection of the apparent outward symptoms, ruling out each of the more common causes until the correct disorder is settled upon and can be treated appropriately.

Your veterinarian will perform a thorough physical exam on your dog, taking into account the background history of symptoms that you have provided and possible incidents that might have led to this condition. A complete blood profile will be conducted, including a chemical blood profile, a complete blood count, and a urinalysis,which may show infections of the blood or disorders of the organs (e.g., heart, kidneys).

Your doctor may also order chest X-rays to look for possible evidence of primary cardiac disease or tumors. An electrocardiogram (ECG, or EKG) is essential for evaluating the electrical currents in the heart muscles, and may reveal any abnormalities in cardiac electrical conduction (which underlies the heart’s ability to contract/beat), and may show structural cardiac diseases that are affecting the heart. Ultrasound and angiography are also very useful for evaluating adrenal masses. Your doctor may also conduct a thyroid scan to evaluate your dog for hyperthyroidism.

Treatment

Your veterinarian will develop a treatment plan for your dog once a diagnosis has been confirmed. If there is an underlying cause, that will be the primary focus of treatment.

Living and Management

The care of your dog following diagnosis will depend on the specific disease that is found to be causing the sinus tachycardia. Restricting your dog's activity so that its heart rate does not increase excessively may be called for, but only if your dog is being adversely affected by the increased heart rate.