• Cause: multiple, including hypervagotonia, exposure to toxins, inflammation, iatrogenic (medications), myocardial infarction, electrolyte abnormalities and neoplasia.
• Signs: usually incidental finding and concomitant or secondary to another disease process (eg urethral obstruction, intrathoracic neoplasia, etc) or sudden onset during anesthesia or post-anti-arrhythmic medication initiation.
• Diagnosis: electrocardiogram with a QRS following every P wave and P for every QRS, but the PR interval is prolonged (>0.08 s).
• Treatment: treat underlying cause if known. Otherwise, none is required. Monitor over time as this may progress to 2^nd or 3^rd degree AV block.
• Prognosis: usually excellent (unless the underlying cause carries a poorer prognosis).

• Incidental finding or caused iatrogenically.

• Presented for concomitant condition or seen during anesthesia.

• Not applicable.

• There are no reports in rabbits; however, based on other species, degenerative and neoplastic causes (including hypervagotonia from intra-thoracic neoplasia) tend to be seen in older animals.
• Inflammatory causes causing myocarditis can be seen in younger animals (based on information in other species).
• No age predisposition for other causes.

• None known.
• In one study of 59 cases of pet rabbits with cardiovascular disease, four (21.1%; 95% CI, 6.1-45.6%) had atrioventricular block, but the type/degree is not described.

• Electrocardiography Cardiovascular: electrocardiography.
• Echocardiography Echocardiography.
• Biochemistry Blood biochemistry: overview and hematology Hematology, urinalysis Urinalysis: overview.
• Thoracic radiography Radiography: thorax.
• Abdominal ultrasonography.
• Neurological examination.

• It may be exacerbated by medications, eg anesthetics/antiarrhythmics, or procedures that increase vagal tone, eg intubation, ocular manipulation.
• Alpha-2 agonists and antiarrhythmics may exacerbate the block.
• Free-roaming rabbits exposed to plants containing cardiac glycosides, eg foxgloves Poisoning overview, are potentially at an increased risk (if ingested).

• Degenerative.
• Neoplastic: neoplasia affecting the AV node or extra-cardiac tumors, increasing vagal tone.
• Inflammatory; myocarditis.
• Toxin: cardiac glycosides such as digitalis (Foxglove) Foxglove (Digitalis pupurea), convalatoxin (Lily of the valley) Lily of the valley (Convallaria majalis), Oleandrin (oleander).
• Normal: not reported in rabbits, but in other species where the individual is athletic 1^st degree AV block may be a normal variant due to increased vagal tone.
• Electrolyte disorders, hyperkalemia Hyperkalemia, and hypercalcemia Hypercalcemia have been implicated in some literature sources, but this remains controversial.
• Iatrogenic: secondary to ß-blockers, Ca²⁺ channel blockers, digoxin, or alpha-2 adrenergic antagonists.
• Myocardial infarct: likely transient or short-term occlusion of terminal branches of the left and right coronary arteries causing transient hypoxia of the AV node and bundle of HIS. Non-transients are thought to result in more severe forms of AV block.

General

• Athleticism in other species (unconfirmed in rabbits).

• The block may be secondary to structural causes (lesion within the tissue affecting the condition) or, more likely, a functional defect (a physiological change affecting conduction).
• 1^st degree AV block is a conduction delay rather than a true block characterized by prolongation of the PR interval.
• PR interval (ECG) represents all events that happen from the formation of the depolarization impulse in the SA node to the activation of the ventricles; these include:
  • Atrial myocardium depolarization.
  • Impulse propagation within the AV node, the bundle of HIS and the proximal bundle branches.
• Block can occur in 4 different regions. Normal QRS complexes are seen in 3 of these:
  1. Infra-atrial block (the block is above the AV node and within the atrial tissue).
  2. Intra-nodal (block is within the AV node itself).
  3. Infra-Hissian (block is in the bundle of Hiss).
• Prolonged QRS and bundle branch blocks with prolonged PR intervals may be seen in infra-Hissian blocks (where the block is below the bundle of HIS and where the left and right bundle branches separate).
• Physiological 1^st degree AV blocks may self-resolve if the initiating cause is treated.

Refereed papers

• Recent references from PubMed and VetMedResource.
• Ozawa S, Guzman D S, Keel K & Gunther-Harrington C (2021) Clinical and pathological findings in rabbits with cardiovascular disease: 59 cases (2001–2018). J Am Vet Med Assoc 259 (7), 764-776 PubMed.
• Holmqvist F & Daubert J P (2013) First-degree AV block – An entirely benign finding or a potentially curable cause of cardiac disease? Ann Noninvasive Electrocardiol 18 (3), 215-224 PubMed.
• Lord B, Boswood A & Petrie A (2010) Electrocardiography of the normal domestic pet rabbit. Vet Rec 167 (25), 961-965 PubMed.
• Pariaut R (2009) Cardiovascular physiology and diseases of the rabbit. Vet Clin North Am Exot Anim Pract 12 (1), 135-144 PubMed.
• James T N (1967) Anatomy of the cardiac conduction system in the rabbit. Circ Res 20 (6), 638-648 PubMed.

Other sources of information

• Orcutt C L & Malakoff R (2020) Cardiovascular Disease. In: Ferrets Rabbits Rodents. pp 250-257.
• Willis R (2018) Bradyarrhythmias and Conduction Disturbances. In: Guide to Canine and Feline Electrocardiography. Eds: Willis R, Oliveira P & A Mavropoulou. Wiley Blackwell, UK. pp 79-107 WileyOnline.