Introduction

• Sinus or sinoatrial block and sinus arrest are electrocardiographic (ECG) abnormalities, characterized by absence of P-QRS-T complexes.
• Cause: they indicate a condition in which no heart beat occurs, due to temporary failure of the sinoatrial node to produce an electrical impulse, or failure of conduction from the sinoatrial node.
• Signs: often none if short periods of no heart beat; weakness or fainting due to cerebral hypoxia with longer periods of no heart beat. 
• Diagnosis: sinoatrial block is usually diagnosed when the pause between complexes is exactly equal to twice the previous P-P interval. Sinus arrest is the usual name for a pause which is greater than twice the previous P-P interval. Cannot be discriminated in dogs because of sinus arrhythmia.
• Long periods of sinus arrest are usually terminated by a junctional or ventricular escape complex.
• Treatment: drugs to increase heart rate (eg atropine) or an artificial pacemaker if appropriate.
• Prognosis: generally reasonable unless progress; good with an artificial pacemaker.

Pathogenesis

Etiology

• Sinoatrial node disease may be due to atrial dilatation, infiltration or fibrosis.
• Also due to excessive vagal tone, drugs, electrolyte disturbances - particularly hyperkalemia Hyperkalemia.
• Exaggerated sinus arrhythmia in brachycephalic breeds → long sinus pauses, especially during sleep.

Predisposing factors

• Brachycephalic airway obstruction syndrome Brachycephalic airway obstruction syndrome.
• Coughing or vomiting, which temporarily increase vagal tone.
• Metabolic diseases such as hypoadrenocorticism Hypoadrenocorticism.

Pathophysiology

• Sinoatrial node fails to produce an impulse, usually due to depressed automaticity (high vagal tone).
• May be described as normal in some dogs (where no clinical signs seen), especially in brachycephalic breeds.

• Sinoatrial node fails to produce an impulse.
• After a pause of several seconds, episode is usually terminated by an escape complex and normal sinus rhythm is re-established.

Diagnosis

Treatment

Outcomes