Preliminary reference values for electrocardiography, echocardiography and myocardial morphometry in the European brown hare (Lepus europaeus)

The study aimed at defining reference values for electrocardiographic (ECG) and echocardiographic parameters as well as macroscopic dimensions of the heart and microscopic dimensions of cardiomyocytes in the European brown hare. The studies were conducted on 30 adult, clinically healthy hares of either sex caught in Poland. ECG and echocardiography were performed supravitally on anaesthetized hares. After euthanasia, gross and microscopic myocardial and cardiomyocyte dimensions were determined. Heart rate amounted to 140 ± 37.5 beats/min, the leading rhythm involved the sinus rhythm. P wave time was 26 ± 5 ms, PQ time was 80 ms, QRS time was 29 ± 3.5 ms, and ST was 97.5 ± 7 ms. Echocardiography determined a left ventricular wall end-diastolic diameter of 8.6 ± 2.0 mm and an intraventricular septum end-diastolic diameter of 5.75 ± 1.0 mm. The thickness of the interventricular septum corresponded to that of the free wall of the left ventricle, a finding consistent with physiological hypertrophy. Preliminary reference values were established for echocardiography. The findings were similar to those obtained at necropsy. The ECG and echocardiographic studies represent the first supravital examination of cardiac function in the hare. The obtained results illustrate adaptation of hare's myocardium to its mode of life. The cardiac findings resemble the athlete's heart syndrome described in humans. The findings may prove useful in further studies on the physiology of the cardio-vascular system in the hare.


Findings
Studies on the physiology of the European brown hare (Lepus europaeus) have focused on organ morphology, blood biochemical parameters, methods of blood sampling, coagulation parameters and cardiovascular disor-ders [1][2][3][4][5][6]. Physiological and morphological studies of the heart have not been performed, so the electrocardiographic (ECG) and echocardiographic variables remain unknown. Also, cardiomyocyte morphology remains to be reported.
This study aims at defining reference values related to ECG and echocardiography and to determine gross and microscopic dimensions of the heart in the European brown hare.
The studies were conducted on 30 adult (body weight (BW) 3.2 ± 0.54 kg), clinically healthy hares (10 males and 20 females) out of 96 hares caught in south-eastern Poland. The 30 hares were selected randomly among the 96 hares by selecting every third clinically normal hare. Two hares were omitted from the sampling population due to low age and low BW, respectively. Examination of cardiac morphometry was done in 42 hares, including the 30 hares mentioned above, 8 hares euthanatized due to injuries to extremities, which had developed during transport and 4 hares, which died during the transport. The hares were euthanatized by phenobarbitaladministered intracardially.
ECG was conducted on animals positioned on their right flank, using a three-channel Sheiler AT-1 apparatus at the pass of 50 mm/s. On extremities the electrodes were placed in line with the generally accepted standards for small animals (Fig. 1) [7]. Amplitudes and duration of P, Q, R, S, T waves, QRS complex, time distances of PQ, QT, ST were measured in the second lead. Duration of P wave was measured from the beginning of the rise to the end of the decrease in the record line. PQ (PR) interval was measured from the beginning of P wave to the beginning of QRS complex. QRS complex was measured from the beginning of Q wave to the end of S wave. QT interval was measured from the beginning of Q wave to the end of T wave. Q wave represents the first negative wave of QRS complex and in several species it is absent from ECG records. R wave represents the first positive wave of QRS complex, the descending arm of which below isoelectric line passes into the negative S wave (Fig. 2). The mean electrical axis (MEA) was calculated on the basis of algebraic sum of QRS complex amplitudes in leads I and III plotted on the coordinate system. The mean electrical axis represents a direction of the resultant electromotive force of the heart and can be applied for diagnosing myocardial hypertrophy or disturbed intraventricular conductance. For every measured ECG parameter its mean value and standard deviation (SD) were calculated and the values allowed for calculation of relevant reference norms (mean ± 2 SD) from 25 cycles. Data from all acral leads (I, II, III, aVR, aVL, aVF) were analyzed to detect disturbances in cardiac rhythm (Fig. 3).
The echocardiographic examination was performed using an Aloka 8000 apparatus equipped with a 7.5-10 Mzh head. Left ventricular end-systolic diameter, left ventricular end-diastolic diameter (LVEDd), left ventricular wall end-diastolic diameter (LWDd) and left ventricular wall end-systolic diameter in diastole as well as intraventricular septum end-diastolic diameter (IVSDd) and intraventricular septum end-systolic diameter were measured. The measurements were taken in parasternal projection in the short axis, from the right hand side, and the probe was Sites of electrode placement for electrocardiography in a European brown hare Post mortem examination of the cardiac morphometry included measurements of height and width of right and Table 1: Obtained values (mean and standard deviation (SD)) and the corresponding determined reference values (mean ± 2 × SD) for electrocardiographic parameters in European brown hares (N = 30) anesthetized with xylazine and ketamine in the parasternal projection in short axis. Echocardiographic image  Tables 1  and 2, respectively. Gross and microscopic dimensions of the heart are shown in Table 3. Similar gross and microscopic dimensions were obtained independently of the cause of death (spontaneous death versus euthanasia).

Reference value Mean and SD
The performed ECG and echocardiographic studies are the first supravital examinations of cardiac function in the hare. Even when anesthetic drugs were administered no disturbances were observed in cardiac rhythm or cardiac contractility. Anesthesia is needed to perform such studies in wild hares and xylazin-ketamine anesthesia provided a safe anesthesia [2].
The study demonstrated relatively thick ventricular walls and a relatively high ejection fraction thus reflecting the adaptation of hare's myocardium to their mode of life. The findings resemble the athlete's heart syndrome described in humans [9]. The pronounced and frequently repeated exertion leads to concentric hypertrophy of the myocardium without augmentation of cardiac cavities when the main inducing factor involves pressure load in the left ventricle [8]. Such cardiac transformation aims at securing increased stroke volume with preservation of the normal systolic function. The relative wall thickness in humans and in pigs amounts to 0.45 [9,10]. The relative wall thickness of 1.2 ± 0.54 found in this study points to cardiac hypertrophy in hares. No significant differences have been disclosed in thickness of interventricular septum and of free wall in the left ventricle, which indicates physiological hypertrophy [9]. The thickness of the inter- Table 2: Obtained values (mean and standard deviation (SD)) and the corresponding determined reference values (mean ± 2 × SD) for echocardiographic parameters in European brown hares (N = 30) anesthetized with xylazine and ketamine in the parasternal projection in short axis.