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Heart: sinus block – arrest
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.
Presenting signs
- Severe sinus arrest may cause syncopal episodes Syncope, seizures Seizures or episodic weakness, especially when exercising or when excited.
- May be induced by drugs, especially sedatives such as xylazine and medetomidine (alpha 2-adrenergic agonists).
- Rarely, cases occur with both intermittent sinus arrest and episodes of tachydysrhythmia. This is called Sick Sinus Syndrome
.
Acute presentation
- Asystole → death.
- If the sinoatrial node ceases to produce an impulse, recovery from sinus arrest relies on pacemaker tissue elsewhere in the heart taking over impulse generation to rescue the situation. These are called escape complexes. If this mechanism also fails or is suppressed by drugs, asystole occurs → death.
Age predisposition
- Mortality: usually older dogs.
Special risks
- Anesthesia, sedation.
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
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed papers
- Recent references from PubMed and VetMedResource.
Other sources of information
- Ettinger S J, Feldman E C, Cote E (2017) Cardiac Arrhythmias. In: Textbook of Veterinary Internal Medicine. 8th edn. Chapter 248. Elsevier.
- Smith F K W Jr, Tilley L P, Oyama M, Sleeper M (2016) Electrocardiography (chapter 3); Treatment of cardiac arrhythmias (chapter 17). In: Manual of canine and feline cardiology. 5th edn, Saunders Elsevier.
- Bonagura J D & Twedt D C (2014) Bradyarrhythmia (chapter 170); Bradyarrhythmias and Cardiac Pacing (Web Chapter 59). In: Kirks Current Veterinary Therapy XV. Elsevier Saunders.