Safe and efficacious dosage of flecainide acetate for treating equine atrial fibrillation

To determine a safe and efficacious dose of flecainide acetate for treating equine atrial fibrillation (Af), the safe dosage level was determined by injecting 1, 2, or 3 mg/kg i.v. of 1% flecainide acetate solution at a rate of 0.2 mg/kg/min to five clinically healthy horses. Clinical signs and the ECG were monitored (HR, PR, QRS, and QT intervals) and blood was taken to measure the plasma flecainide concentration pre- and post-administration. No abnormal signs were observed in the 1- or 2-mg/kg groups, while agitation was observed in three of five horses in the 3-mg/kg group. The QRS, and QT intervals for the 3-mg/kg group increased significantly. The peak plasma flecainide concentrations were 1.316 +/- 358 (SD) ng/ml, 1,904 +/- 314 ng/ml, and 2,251 +/- 387 ng/ml for the 1-, 2-, and 3-mg/kg groups, respectively. To evaluate the efficacy of flecainide, Af was induced by right atrial pacing in six clinically healthy horses, and 1% flecainide acetate solution was then administered until they converted to sinus rhythm. All horses with induced Af converted. For the conversion, a total dose of 1.40 +/- 0.63 mg/kg flecainide was required, the duration of administration was 7.00 +/- 3.15 min and plasma flecainide concentration at conversion was 1,303 +/- 566 ng/ml. In conclusion, flecainide acetate is a safe and effective antiarrhythmic agent for equine Af, and the clinically effective dosage is 1 to 2 mg/kg.

Pathologic and electrocardiographic findings in sudden cardiac death in racehorses

Five racehorses in apparently normal condition succumbed to sudden cardiac death (SCD) during or shortly after intensive training exercise. Cardiopathologic examination was performed. In 1 of the 5 horses, the use of an electrocardiogram (ECG) recording taken continuously for 440 sec enabled us to analyze some of the arrhythmias in the terminal event of SCD. The ECG tracing exhibited the R-on-T phenomenon following a pair of ventricular premature contractions (VPCs). The phenomenon rapidly degenerated into ventricular fibrillation, which led to cardiac arrest. In all 5 horses cardiopathologic examination revealed the following lesions: (i) foci of myocardial fibrosis in the right atrium located close to the sinoatrial (SA) node, (ii) fibrotic and/or fibroplastic changes in the upper portion of the interventricular septum, including the atrioventricular (AV) conduction system, and (iii) arterio- and arteriolosclerosis of the SA and AV node vessels. Pathogenetically, the process by which the focal lesions of myocardial ischemia secondary to vascular sclerosis progressed into fibrosis and/or fibroplasia could play a major role in the genesis of arrhythmias. Presumably the fibrotic and/or fibroplastic changes in the area of the AV bundle and its bundle branches are closely related to the onset of fatal ventricular arrhythmias such as VPCs, deteriorating into ventricular fibrillation. SCD in training and racing Thoroughbred horses appears to be due to arrhythmia.

Application of V200 values for evaluation of training effects in the young thoroughbred under field conditions

The purpose of this investigation was to study, under field conditions, the usefulness of an incremental exercise test to measure V200 (running velocity [V] at heart rate HR[ of 200 beats/min) for evaluation of training effects in the young Thoroughbred. Five to 112 two-year-old Thoroughbreds were used over 4 years to investigate the correlation between HR and V, the influence of the track and rider's weight to V200, reproducibility of V200 and the changes of V200 as training progresses. The high correlation coefficient between HR and V, and V200 were obtained in horses that exercised smoothly during gait changes and had acceleration during the canter in relaxed and nonexcitable states. The reproducibility of V200 as statistically significant providing the conditions were constant. Furthermore, the regression lines between HR and V gradually declined in parallel as training progressed. The average V200 in 2-year-old Thoroughbreds significantly increased from 623 +/- 55 m/min to 691 +/- 64 m/min after 5 months of training (P < 0.05). We concluded that V200 could easily be measured, that it was useful in evaluating treatment effect and that practical application under field conditions was possible.

Intrinsic atrioventricular conductive function in horses with a second degree atrioventricular block

The atrioventricular (AV) conductive functions were investigated before and after the pharmacological autonomic nervous blockade (PAB) through the electrophysiological study on six horses (AV block group) in which the dropped beats occurred more frequently (over 200 beats/24 hr) and on five horses (control group) in which the dropped beats occurred sporadically (under 200 beats/24 hr) or in which the arrhythmias were not recognized at all on the long-term electrocardiogram. As a result, AV conductivity, conduction time and refractoriness in the AV block group were lower, longer and higher, respectively, than those in the control group even after PAB. These results indicate the intrinsic AV conductive functions in the AV block group were withdrawing than in the control group.

Electrocardiographic observation on spontaneously occurring arrhythmias in chickens

In order to clarify the spontaneous occurrence of arrhythmias in chickens, electrocardiograms using standard bipolar limb leads were recorded for 461 chickens of 7 breeds, including 3 pure breeds: Rhode Island Red (RIR, n = 125), Japanese Game (JG, n = 101), and White Leghorn (WL, n = 52); and 4 crossbreeds of RIR: the first filial generation (F1) obtained by mating RIR with JG (RIR x JG, n = 44), the backcross obtained by mating the Fl of RIR x JG with the parental JG ((RIR x JG) x JG, n = 10), the F1 obtained by mating RIR with WL (RIR x WL, n = 73), and the F1 obtained by mating RIR with the commercial hybrid chicken, Dekalb Amber Link (DAL)(RIR x DAL, n = 56). Nine types of arrhythmias were observed in 107 chickens (23.2%, 107/461): 66 had sinus arrhythmia, 19 had atrial premature contraction, 9 had ventricular preexcitation syndrome, 7 had ventricular premature contraction, 2 had second-degree atrioventricular block. 1 had atrial fibrillation, I had aberrant ventricular conduction. I had intraventricular conduction disturbance, and 1 had ventricular electrical alternans. Except for sinus arrhythmia, the incidence of other arrhythmias in RIR was significantly high, compared with that in pure breeds other than RIR (p < 0.001) and that in crossbreeds of RIR (p < 0.01). Arrhythmias other than ventricular premature contraction and sinus arrhythmia will be the first to be described as spontaneously occurring arrhythmias in chicken. Although the reason for both the highest incidence of sinus arrhythmia in WL and the predominance of other arrhythmias in RIR were obscure, the present results suggest the possibility of using chickens, especially RIR which shows a high incidence of arrhythmias, as a relevant animal for studying arrhythmias.