Definition

• Normal sinus impulse formation characterized by a phasic variation in sinus cycle length. An irregular R-R interval is present that has more than 10% variation in sinus cycle length (or variability of 0.12 seconds [dog], 0.10 seconds [cat], or more exists between successive P waves).
• Two basic forms exist-respiratory sinus arrhythmia (RSA): P-P interval cyclically shortens during inspiration due primarily to reflex inhibition of vagal tone and lengthens during expiration; non-respiratory sinus arrhythmia: phasic variation in P-P interval unrelated to the respiratory cycle.

Pathophysiology

Sinus node discharge rate depends on the two opposing influences of the autonomic nervous system. Vagal stimulation decreases spontaneous sinus nodal discharge rate and predominates over sympathetic stimulation. Negative intrathoracic pressure occurring with inspiration causes decreased pressure on the vagus nerves. Feedback from the cardioregulatory and vasomotor centers in the medulla produces cardiac acceleration by decreasing vagal restraint on the sinus node; the opposite occurs during exhalation. The genesis of sinus arrhythmia also depends on reflexes involving pulmonary stretch receptors (Hering-Breuer reflex), pressure-volume sensory receptors in the heart (Bainbridge-atrial stretch stimulates receptors in the atrial wall causing vagal inhibition and increase in heart rate, baroreceptor-receptors in the carotid sinus and aortic arch elicit inverse changes in heart rate with acute changes in arterial blood pressure), blood vessels, and chemical factors of the blood.

Systems Affected

Cardiovascular-generally no hemodynamic consequence, but marked SA may produce a long enough sinus pause to produce syncope if not accompanied by an escape rhythm.

Genetics

N/A

Incidence/Prevalence

Most frequent form of arrhythmia in the dog

Signalment

Signs

Causes

• Normal cyclic change in vagal tone associated with respiration in the dog; heart rate increases with inspiration and decreases with expiration.
• Underlying conditions that increase vagal tone-high intracranial pressure, gastrointestinal disease, respiratory disease, cerebral disorders, digitalis toxicity, organophosphates.
• Carotid sinus massage or ocular pressure (vagal maneuver) may accentuate.

Risk Factors

• Brachycephalic conformation
• Digoxin therapy
• Any disease that increases vagal tone

Differential Diagnosis

• Auscultation of SA is often confusing; ECG helps differentiate normal SA from true pathologic arrhythmia.
• Wandering sinus pacemaker frequently associated and a variant of sinus arrhythmia. Site of impulse formation shifts within the sinoatrial node or to an atrial focus or AV node, changing the configuration of the P wave.
• Important to differentiate normal SA from pathologic arrhythmias including atrial premature complexes, SSS, slow atrial fibrillation, and AV dissociation.

CBC/Biochemistry/Urinalysis

N/A

Other Laboratory Tests

Cats with chronic respiratory disease may be positive for feline leukemia or feline immunodeficiency virus.

Imaging

Radiographs, CT, MRI of head and neck to assess for abnormal anatomic conformation that might predispose to airway problems.

Diagnostic Procedures

• Pharyngoscopy/laryngoscopy if upper airway disease suspected.
• Atropine challenge test (administer atropine 0.04 mg/kg IM followed by ECG in 30 minutes or 0.04 mg/kg atropine IV followed by ECG in 10 minutes) if associated with sinus bradycardia and primary dysfunction of sinus node is suspected.

Pathologic Findings

See specific disease

Appropriate Health Care

Generally, specific treatment required only when associated with symptomatic sinus bradycardia; if not related to respiration, underlying cause is treated. If patient is suffering respiratory distress, appropriate inpatient management indicated until patient is stable.

Nursing Care

None unless associated with underlying disease (see also below).

Activity

Not restricted unless associated with specific disease (e.g., brachycephalic animals may need to limit exercise, especially in high ambient temperatures).

Diet

Caloric restriction for obese animals with airway compromise.

Client Education

None unless associated with specific disease.

Surgical Considerations

None unless associated with specific disease.

Drug(s) Of Choice

• Generally no therapy indicated; this is a normal rhythm.
• Infectious respiratory diseases require appropriate antibiotic therapy.
• If associated with symptomatic sinus bradycardia or sinus arrest or block, anticholinergics may be indicated-atropine (0.02–0.04 mg/kg IV, IM, SC) or glycopyrrolate 5–10 µg/kg IV, IM, SC).

Contraindications

Discontinue digoxin if toxicity is a problem.

Precautions

Avoid atropine in patients with respiratory disease; an adverse effect is drying of secretions.

Possible Interactions

N/A

Alternative Drug(s)

N/A

Patient Monitoring

Only if associated with specific disease.

Prevention/Avoidance

N/A

Possible Complications

N/A

Expected Course and Prognosis

N/A

Associated Conditions

• SSS
• Brachycephalic airway syndrome
• Asthma
• Chronic obstructive pulmonary disease

Age-Related Factors

Generally more pronounced in young adult

Zoonotic Potential

N/A

Pregnancy/Fertility/Breeding

Increased incidence of arrhythmias

Synonyms

• Non-respiratory SA = non-phasic SA; sinus irregularity.
• Respiratory SA = phasic SA.
• Ventriculophasic SA-form of non-phasic SA in which atrial cycles containing ventricular complexes are shorter than those in which they are absent. That is, the P-P interval that includes the QRS complex is shorter than the P-P interval without a QRS complex. This can be seen with second-degree AV block, complete AV block or in the presence of ventricular premature complexes with a full compensatory pause.

See Also

• Brachycephalic Airway Syndrome
• Sick Sinus Syndrome
• Sinus Arrest and Sinoatrial Block

Abbreviations

• AV = atrioventricular
• ECG = electrocardiogram
• RSA= respiratory sinus arrhythmia
• SA = sinus arrhythmia
• SSS = sick sinus syndrome