R-peak time in clinically healthy dogs with different thoracic conformations.

R-peak time (RPT) is an electrocardiographic parameter that represents the time taken for electrical activation to spread from the endocardium to the epicardium. In human medicine, right ventricular RPT is measured from lead V1 to lead V2, and left ventricular RPT from lead V5 to lead V6. The aim of the present study was to define RPT duration in a group of clinically healthy dogs with different thoracic conformations. Sixty clinically healthy dogs underwent a 12-lead electrocardiogram recorded using a previously described precordial system. The dogs were allocated into three morphologic groups. In the brachymorphic group, the median and 25th-75th percentiles for RPT in V1 were 10.5 ms (10-12 ms); V2, 18 ms (16.5-20 ms); V3, 19 ms (18-22 ms); V4, 20 ms (17-23.5 ms); V5, 21 ms (18.5-24 ms); and V6: 22 ms (18.5-25.5 ms). In the mesomorphic group, RPT in V1 was 16 ms (14-18 ms); V2, 22 ms (20-24 ms); V3, 23 ms (21-25 ms); V4, 23 ms (22-25 ms); V5, 25 ms (23-27 ms); and V6, 28 ms (25-30 ms). In the dolichomorphic group, RPT in V1 was 15 ms (13-17 ms); V2, 29 ms (26-32.5 ms); V3, 30 ms (27-33.5 ms); V4, 29.5 ms (26-35 ms); V5, 30 ms (28-34 ms); and V6, 31.5 ms (28-35 ms). RPT in V1 was significantly shorter than RPT in V2 to V6 in all morphotypes (P < 0.05). In all precordial leads, RPT was significantly different between morphotypes (P < 0.05). These results are in agreement with previous findings in humans and with the observation that V1 reads the right ventricle and V2 to V6 read the left ventricle. These preliminary data provide RPT ranges in clinically healthy dogs of different morphotypes.

Heart rhythm characterisation during unexplained transient loss of consciousness in dogs.

The identification of the heart rhythm during an episode of transient loss of consciousness (TLOC) is considered the reference standard method to elucidate the underlying aetiology. This study aimed to characterise heart rhythm in dogs during TLOC using Holter and external loop recorder monitoring. We retrospectively reviewed 24-h Holter monitoring and external loop recorder tracings from 8084 dogs. Heart rhythms from dogs that experienced TLOC during the recording was analysed to identify rhythm disturbances that occurred during episodes of TLOC. Electrocardiograms (ECGs) were subsequently categorised into Type 1 (ventricular arrest), Type 2 (sinus bradycardia), Type 3 (no/slight rhythm variations), and Type 4 (tachycardia). Transient LOC was documented in 92 dogs over 230 episodes of TLOC. Percentage of cases with ECGs compatible with each classification were as follows: 72.1%, Type 1; 6.1%, Type 2; 20.9%, Type 3; and 0.9%, Type 4. Cardiac rhythm during the TLOC could have been a consequence of a neurocardiogenic mechanism in 46.7% cases, while intrinsic rhythm disturbances of the sinus node or of the atrioventricular node were diagnosed in 31.5% cases. In two cases, tachycardia was the possible cause of the TLOC. ECG patterns in dogs presenting with multiple TLOC episodes were completely reproducible during each episode. TLOC in dogs was primarily caused by ventricular arrest. Most dogs with TLOC had electrocardiographic finding suggestive of a reflex or neurally-mediated syncope, but one third had an ECG more suggestive of a conduction disorder. Distinguishing these two entities could help inform diagnostic, therapeutic, and prognostic plans.

Isorhythmic atrioventricular dissociation in Labrador Retrievers.

BACKGROUND: Isorhythmic atrioventricular dissociation (IAVD) is a rhythm disturbance in which atria and ventricles are driven by independent pacemakers at equal or nearly equal rates. OBJECTIVES: To describe electrocardiographic and electrophysiologic features of IAVD in a group of 11 Labrador Retrievers and its possible correlation with focal junctional tachycardia (FJT). ANIMALS: Between December 2004 and October 2010, medical records of 11 Labrador Retrievers with surface electrocardiographic findings compatible with IAVD were retrospectively analyzed. METHODS: Twelve-lead surface electrocardiograms, thoracic radiographs, and echocardiographic findings of each dog and electrophysiologic mapping results of 3 dogs were retrospectively analyzed. RESULTS: In 10 of 11 dogs, the ECG pattern revealed the presence of IAVD with type I synchronization. In 5 of 10 dogs, IAVD with type I synchronization was interrupted by periods of junctional tachycardia with 1 : 1 ventriculo-atrial conduction. One of 11 dogs presented IAVD with type II synchronization. The ECG diagnosis of IAVD with type I and type II synchronization, and junctional rhythm with 1 : 1 ventriculo-atrial conduction was confirmed in 3 of 11 dogs with endocardial mapping in which the diagnosis of focal junctional tachycardia was made. CONCLUSION AND CLINICAL IMPORTANCE: IAVD with type I synchronization is more common than IAVD with type II synchronization in Labrador Retrievers, and a correlation between IAVD and FJT can be hypothesized.