Electrocardiographic features of right bundle branch block in Netherland dwarf rabbits

The surface electrocardiogram (ECG) represents a fundamental part of cardiac investigation and is commonly used to diagnose and monitor cardiac arrhythmias and conduction disturbances in many species, including rabbits. However, most of the rhythm disturbances that have been described in rabbits are induced by drugs or toxins in laboratories. In contrast, data on naturally acquired rhythm disturbances of pet rabbits are still limited to a few case reports (Martin et al. 1987, Lord et al. 2011). Recently, two standardised electrocardiographic assessments have been performed in healthy pet rabbits. The first study included 46 pet rabbits (Lord et al. 2010) and, in the other, 40 Netherland Dwarf rabbits were evaluated (Chapel et al. 2017). In both studies, the primary aim was to establish a well-tolerated recording technique and reference values. Interestingly, during the study from Chapel et al. (2017), four rabbits were excluded from analysis because of a peculiar ECG pattern consistent with right bundle branch block (RBBB). Since the ECG tracings of these subjects have not been described in detail so far, the aim of this report is to illustrate for the first time the ECG characteristics of spontaneous RBBB in client-owned rabbits. The three Netherland dwarf rabbits from the report by Chapel et al. (2017) were judged healthy based on anamnesis, physical examination, indirect assessment of arterial blood pressure and a restricted blood test profile. There was one neutered female and two males. Age and body weight ranged between 1.2 and 1.5 years and 0.8 and 1.1 kg, respectively. All rabbits underwent a standardised surface electrocardiogram in sternal recumbency. The animals were not sedated and were allowed time to acclimatise. The hair was clipped and contact gel was used to enhance conduction. Toothless alligator clip electrodes were used and were applied cranially over the triceps brachii muscle (lateral head) in both thoracic limbs and craniadistally over the biceps femoris muscle, according to the description of Chapel et al. (2017). Six-lead electrocardiograms (leads I, II, III, aVR, aVL and aVF) were recorded for 2 minutes in all rabbits at a paper speed of 25 or 50 mm/s and paper sensitivity of 20 mm/mV. Three representative consecutive beats were used to measure electrocardiographic variables, and the results were averaged for each variable. In each rabbit, electrocardiographic analysis revealed a pattern characterised by: (1) a regular rhythm with normal heart rate (215–260 beats per minute [bpm]; reference interval [RI] 190–320 bpm [Lord et al. 2011], 199–249 bpm [Chapel et al. 2017]); (2) P waves with normal duration (0.020–0.023 s; RI 0.020–0.030 s [Chapel et al. 2017]), amplitude (0.025–0.033 mV; RI 0.025–0.053 mV [Chapel et al. 2017]), and polarity (positive in leads I, II, III and aVF, negative in leads aVL and aVR); (3) fixed PR intervals with normal duration (0.056–0.060 s, RI 0.047–0.061 s [Chapel et al. 2017]) and (4) wide QRS complexes (0.049–0.050 s, RI 0.033–0.045 s [Chapel et al. 2017]) with a large S waves (0.200–0.281 mV) with right axis deviation (−90–[−138°], RI −42–44° [Chapel et al. 2017]) (rS configuration in leads I, II, III and aVF) (Fig. 1). These findings were consistent with sinus rhythm with complete RBBB (Tilley 1992, Miller et al. 1999). Additionally, since the RBBB pattern remained unchanged irrespective of cardiac cycle length over the 2 minutes of recording, a heart rate-dependent disturbance of conduction was ruled out, and a persistent RBBB was considered most likely (Tilley 1992; Miller et al. 1999). In the rabbit, the conduction system is organised in a pattern generally similar to that of other small mammals (Saitanov 1960, James 1967). Briefly, in the normal heart, Purkinje fibres allow a rapid and simultaneous depolarisation of ventricles (Miller et al. 1999). An exception to this rule is represented by bundle branch blocks, partial or complete interruptions of impulse conduction in one or more bundle branches. According to the number and the type of branches affected, a distinct electrocardiographic pattern is expected and a specific nomenclature should be used. With specific regard to the topic of this report, RBBB is a monofascicular block in the right bundle branch. As a result, the cardiac impulse enters the left bundle branch to activate the left ventricle, and then crosses the interventricular septum to activate the right ventricle late and abnormally (Tilley 1992). The resultant aberrantly conducted QRS complexes consequently show a duration above the normal limit and a right axis deviation; additionally, large and wide S waves can typically be observed in leads I, II, III and aVF in many species (e.g. dog, cat) (Tilley 1992, Miller et al. 1999). Since specific ECG criteria for RBBB have not been established for rabbits, our diagnosis was based on canine and feline criteria (Tilley 1992, Miller et al. 1999). Since transthoracic echocardiography was not included among diagnostic procedures performed in rabbits from this report, it remains unresolved why some subjects develop RBBB and what this conduction abnormality means for their health. Moreover, other occult cardiac disease cannot be excluded for that reason. In conclusion, this report provides a description of naturally acquired RBBB in apparently healthy client-owned rabbits. Although the population study was small and echocardiographic examination was not available, this report contributes to expand our knowledge on spontaneous abnormalities of cardiac rhythm in this species. It is important to point out that echocardiography should be recommended in similar cases to screen for underlying cardiac disease and further research is necessary. We declare we have no competing interests.