Sinus Arrhythmia in Cats
The sinoatrial node (SA Node, or SAN), also called the sinus node, is the initiator of electrical impulses within the heart, triggering the heart to beat, or contract, by firing off electrical surges (also called the heart's pacemaker). The sinus discharge rate depends on two opposing influences of the nervous system: stimulation from the vagus nerves (nerves that originate in the lower brain stem [medulla] and send signals to the autonomic organs of the body) decreases the spontaneous discharge rate and predominates over sympathetic stimulation (mediating the neuronal and hormonal stress response commonly known as the fight-or-flight response). During inhalation, feedback from the respiratory and cardiac centers produce cardiac acceleration by decreasing restraints on the vagus nerves; the opposite occurs during exhalation.
Arrhythmia is caused by an abnormal variation in the cycling of impulses that regulate the heart's beating action, resulting in an irregular rhythm. The heart may beat too fast, too slow, or it may skip beats. An irregular heart beat is the primary symptom of arrhythmia.
Sinus arrhythmia also depends on reflexes involving stretch receptors in the lung, pressure-volume sensory receptors in the heart, blood vessels, and chemical factors of the blood. There is generally no consequence in blood flow, but marked sinus arrhythmia may produce a long enough pause in the heart's beating action to produce loss of consciousness if not accompanied by an escape rhythm.
Arrhythmia is relatively uncommon in cats and is not always a cause for concern. An occasional irregular heart beat may not cause any other health problems. However, because an irregular beat may be an early symptom of a more serious underlying condition, it is wise to have it checked out by your veterinarian. Some breeds appear to be more predisposed to sinus arrhythmia than others, most notably brachycephalic breeds like Persians and Himalayans.
Symptoms and Types
- Primary symptom is a heartbeat that is too fast, too slow, or that skips a beat, which is also referred to as an irregular heartbeat
- Physical weakness may develop if pauses between beats are excessively long; loss of consciousness can also occur, however, these side effects are uncommon
- In general, symptoms are more common in nonrespiratory than in respiratory form
- Normal cyclic change in the vagus nerves associated with respiration; heart rate increases with inspiration and decreases with expiration
- Underlying conditions that increase vagal tone: high intracranial (within the skull) pressure, gastrointestinal disease, respiratory disease, cerebral disorders, digitalis toxicity, congestive heart failure
- Brachycephalic conformation
- Digoxin therapy (digitalis)
- Taurine deficiency – rare, since most cat foods now include this ingredient; cats that are fed home-made meals may be deficient in taurine
- Any disease that affects the vagus nerves
Your veterinarian will perform a thorough physical exam on your cat, taking into account the background history of symptoms and possible incidents that might have led to this condition. You will need to give a thorough history of your cat's health and onset of symptoms. The history you provide may give your veterinarian clues as to whether there are any underlying disorders, or if there are other organs that are being affected by this disorder. This information may help to make the diagnosis that much easier to conclude.
An electrocardiogram (ECG, or EKG) recording can be used to examine 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). X-rays of the head and neck may be used to assess for abnormal anatomic conformation that might predispose your cat to airway problems. If upper airway disease is suspected, your veterinarian can use a minimally invasive technique called a pharyngoscopy or laryngoscopy, by which a tubular device with a camera attached is inserted into the respiratory tract (pharynx and larynx, respectively) in order to visually examine the space.