Regular intense exercise is known to be able to induce cardiac hypertrophy in dogs engaged in different physical activities, such as that of search and rescue [1
] or sporting activities such as agility or mushing. Numerous studies have been carried out on Alaskan sled dogs, demonstrating the cardiac remodeling that these dogs present over time due to such activity [3
In recent years, there has been an increase in the participation of canine athletes in other types of mushing, taking place on dry land rather than snow. One of these, bikejöring, has significantly increased in popularity in Central European countries, with optimal weather conditions being present there for this sport. In this discipline, cyclists are assisted by one or two dogs which pull the bicycle along and there are organized races which may be sprints (up to eight kilometers) or ones of medium distance (approximately 30 km).
In these competitions, the Greyster is a popular breed and yet this is not one of those typically studied in the literature, which has primarily focused on sled dogs running on snow. These dogs were originally bred in Norway in the 1980s, from a series of crossings between the German Shorthaired Pointer and the Greyhound, for the original purpose of participation in sledging races. As with other such breeds, they have been adapted and selected for their occupation through improved endurance, cardiac function, blood flow and cognitive performance [5
Greysters have been shown to be highly competitive mushing dogs, demonstrating great athletic performance. There are no data in the scientific literature regarding exercise-induced hemodynamic changes in Greyster dogs, but a high degree of training would be expected to lead to altered echocardiographic values in these dogs in comparison with the usual values for sedentary dogs. These echocardiographic changes could make these dogs appear to have cardiomyopathies, which could carry a risk in competition. Given that, in the pre-competition examination, the veterinarians attending the races do not have enough time to individually evaluate each of the participating dogs, we propose the performance of a cardiovascular clinical study in the field, imitating the real conditions in order to study cardiovascular changes in physical training.
The objective of this comparative study was therefore to perform a clinical evaluation of different cardiovascular parameters before and after physical training of 22 Greyster dogs, using non-invasive methods.
2. Materials and Methods
Healthy adult Greyster dogs (>18 months of age) who regularly take part in bikejöring (minimum 3 h/week) were included in this study. One dog initially included had to be excluded due to a recent injury, so 22 dogs were ultimately enrolled on the full study. All of them were of the same breed and engaged in the same sporting activity. The study was conducted on the same day for all dogs, during the final phase of the sports season. The dogs belonged to a total of twelve owners, with two of them owning 13 of the dogs, more than half of the sample. All the dogs were from the same geographical area, with the same orography and similar training systems.
The physical training consisted of completing an identical route of 4.37 km. Each dog wore a sports harness and was attached to the owner’s bicycle (or “musher”) by a shooting line. The equipment is approved by the International Federation of Sleddog Sports for this sport.
All records were taken without sedation or restrictions. The owner was always with the dog and the animals were allowed a period of time (less than 1 min) to adapt to the procedures for taking blood pressure, heart rate and echocardiography to minimize the effect of excitation.
Post-exercise measurements were made individually, over three consecutive days with temperatures ranging from 0 °C to 5 °C. Average speed and maximum speed during exercise session were recorded for each pair (dog-musher). A maximum speed of 42.6 km/h was reached in the 4.37 km race.
2.2. Blood Pressure Measurement
For the measurement of blood pressure, an Omron® RS6 automatic blood pressure monitor was used, whose length covers 80% of the perimeter of the thoracic limb of these dogs.
For the pre-exercise measurement (T0), we waited three minutes after fitting the monitor before taking the measurement. The sphygmomanometer was placed on the left forelimb, near the elbow joint, on the superficial brachial artery. The post-exercise blood pressure measurement (T1) was performed immediately after the exercise was completed.
2.3. Electrocardiographic Recordings
One of the veterinarians devoted herself exclusively to performing the electrocardiographic recordings, before (T0) and immediately after exercise (T1). ECGs were recorded and analyzed with an SRM® CardioScout three-channel electrocardiograph connected via Bluetooth with the associated SRM® software. The electrocardiogram was performed on each dog on the right lateral decubitus and the electrodes were attached to the skin folds using suitable forceps, and the areas of the skin in contact with the electrodes were impregnated with an alcohol solution. The electrodes were fixed on the palmar surface of the right and left thoracic limbs at the olecranon and on the internal surface of the groin. A six-lead baseline ECG recording at 25 mm·sg−1 and with a calibration of 0.1 mV·mm−1 was obtained. The rhythm and heart rate at rest, the electrical axis, the amplitude and duration of the P, Q and R waves and the QRS complex were analyzed, as well as the duration of the PR and QT intervals. Checks for possible conduction disturbances were made.
A transthoracic echocardiographic study was also performed before (T0) and after (T1) exercise, using a Sonosite® EDGE II portable device. The following parameters were evaluated in TM-mode in the right parasternal window on the transverse axis: the interventricular septum thickness at end-diastole (IVSd) and end-systole (IVSs), the diameter of the left internal ventricle in diastole (LVIDd) and systole (LVIDs), the left ventricle free wall in diastole (PWd-posterior wall thickness at end-diastole) and systole (PWs-posterior wall thickness at end-systole) and the separation of the septum from point E (EPSS).
The device automatically calculated the shortening fraction (LVFS) and the ejection fraction (LVFE-left ventricle ejection fraction). The diameter of the left atrium (LA) and the aorta (Ao) was calculated in mode B. The continuous doppler mode was selected for the assessment of the left ventricular outflow (LVOT). Finally, color Doppler enabled a visual assessment to be performed of the presence of abnormal flows in systoles in the mitral and aortic valve area. All echocardiographic studies were performed with the dogs in the right lateral decubitus position.
2.5. Data Analysis
A descriptive statistical analysis was carried out, with means and standard deviations being recorded and inferential statistical tests, such as independent t-tests, paired t-tests and Pearson’s correlation coefficient, were performed, using the IBM SPSS Statistics 24 software. First, the relationship between sex and all the blood pressure, heart rate and echocardiographic variables was tested, before and after exercise, in order to determine if the sample could be studied as a single group. Paired t-tests were carried out to determine if there were statistical differences between the pre- and post-exercise blood pressure, heart rate and echocardiographic variables. Finally, Pearson’s correlation coefficient was calculated for specific variables.
The objective of this study was to investigate variations in blood pressure, electrocardiographic and echocardiographic variables before and after exercise in 22 Greyster dogs under real conditions.
Before the full battery of pre and post-exercise analyses were performed, independent t-tests were carried out to determine if sex differences existed in the sample, but none were found and so the entire sample was analyzed as a single group.
Regarding the blood pressure variables, a significant correlation between heart rate and systolic pressure was found. Systolic blood pressure increases during progressive exercise as part of the necessary increase in cardiac output and oxygen consumption, while diastolic blood pressure remains relatively stable. Aerobic exercise in these trained and healthy dogs therefore produces a considerable increase in pulse pressure [6
]. This results in a decrease in general peripheral resistance, which guarantees greater blood supply and better oxygen supply for the tissues doing the work, mainly the muscles, enabling them to function more economically and effectively [7
Regarding the pre- and post-exercise differences, as expected, higher post-exercise values were found in SBP, MBP and SBPD. This is due to hemodynamic adaptation, affecting blood pressure and peripheral resistance. With exercise, systolic pressure increases with cardiac output, without diastolic changes, probably due to the drop in peripheral vascular resistance due to the activation of cellular chemoreceptors. Heart rate also increases due to reduced vagal tone and increased sympathetic tone, resulting in elevated systolic blood pressure [7
Second, regarding the electrocardiographic variables, we found a single significant correlation between the QT interval and the heart rate after exercise. The changes in QT interval could be due to many factors: HR, heart size, autonomic tone, serum calcium and potassium concentrations, and the administration of drugs could influence the duration of cardiac action potentials. HR was a negative determinant of QT-interval duration [8
]. The presence, therefore, of wider QT intervals before exertion is consistent with this correlation and can be explained by the effect of sympathetic hyperactivity.
In the case of the pre- and post-exercise amplitude values of the electrocardiographic waves, the R wave was found to be higher after exercise. The R wave represents the depolarization of both ventricular free walls from the subendocardial endings of the Purkinje fibers to the epicardium and the terminal depolarization of the bases of both ventricles and of the ventricular septa [9
] whereas, in other canine sports activities, such as agility, the R wave voltage is lower [8
] in sled dogs [10
], but in Greyhounds [11
] it is higher, suggesting larger external ventricular dimensions for sled dogs.
Regarding the Q wave, the Greyster dogs of this study did not present significant differences between males and females, although a somewhat higher pre-exercise voltage was observed in females. After exertion, however, a somewhat higher Q wave was observed in the males, although without significant differences. These appreciable amplitude increases in electrocardiography in Greyster dogs point to the need for echocardiographic studies to determine whether ventricular hypertrophy exists in these dogs taking part in bikejöring.
With respect to the echocardiographic values obtained, no significant differences were detected between males and females, and so all the dogs were studied as a single group, as with the blood pressure and electrocardiographic variables. Echocardiography has traditionally been used in veterinary clinics to measure the size of the atrial and ventricular chambers and so to predict the risk of congestive heart failure [12
].There are thus numerous published studies that make reference to echocardiographic values and functional indices in canines [12
], although to date there are no representative reference values for the entire canine species. Therefore, we do not have a reference bibliography for cardiovascular response to exercise in Greyster dogs.
The pre- and post-exercise echocardiographic values presented significant differences in several cases. The dogs in our study presented lower post-exercise shortening fraction (LVFS) values. In horses, a lower shortening fraction immediately after exercise has been associated with a decrease in physical performance [25
]. This has not yet been described in canine athletes, and, therefore, there is a need to perform echocardiographic studies of these same dogs at rest and 30 min after exercise.
E-point-to-septal-separation (EPSS) is a valuable additional parameter for the diagnosis of dilated cardiomyopathy (CMD), enhancing the diagnostic capabilities of TM-mode and performing as well as the Sphericity Index [26
]. EPSS has been suggested, in a study on Doberman Pinschers, as the most sensitive and specific criterion for early CMD, without showing overlap between healthy dogs and those with advanced CMD, thus demonstrating good sensitivity and specificity [26
]. Calvert and Meurs proposed a cutoff value of > nine millimeters to diagnose CMD in this breed [27
In our study, EPSS is significantly higher after exertion (10.13 mm; SD 0.25) and is also accompanied by an increase in the MA/ma ratio after exercise. Those results are consistent with the previous study [27
] in that EPSS seems to be more sensitive than TM-mode (at least when using the published references for TM-mode). However, there is a discrepancy in the cutoff values for EPSS suggested above and this study, so studies with a larger sample need to be carried out in Greysters in order to demonstrate definitively that EPSS can be used objectively as a parameter of systolic function. However, it is reasonable to expect that—as in the human athlete [28
]—the heart of the Greyster is also characterized by a global remodeling of the cavities, with a conserved biventricular systolic function or in the lower limit than normal. In any case, the higher EPSS of the dogs in our study does not decisively point to dilated cardiomyopathy, as one would expect this to be accompanied by decreased aortic blood flow, hypokinesis with fractional shortening (FS) < 25% and possible presentation of mitral/tricuspid regurgitation, aortic thrombosis, LVIDd > 46 mm, LVIDs > 38 mm and axis major-axis minor ratio < 1.5. In the resting state, therefore, the dogs under study did not present other indicators compatible with congestive heart failure due to dilated cardiomyopathy.
In addition, unsurprisingly, cardiac output was significantly higher post-exercise. This is due to cardiovascular changes, which occur at both the cardiac and peripheral levels. The first modification that dynamic exercise training such as bikejöring produces is an increase in the beating volume and, therefore, also in the cardiac output at a given heart rate. This is accompanied by an increase in end-diastolic volume as measured by echocardiography, which may be associated with a decrease in end-systolic volume, accompanied by an increase in the thickness of the ventricular wall [29
The behavior of the aorta in sports dogs is currently unknown, but we found no significant differences for LVOT between males and females in 2D-mode, unlike those described in humans [29
]. In human medicine, the aorta of athletes has not been found to follow the same adaptation pattern as other cardiac structures [30
] and, therefore, if dilation is detected in a canine athlete, it could provide the clinical veterinarian with evidence of possible aortic or aortic valve disease. In the Greyster dogs in our study, no differences were found between males and females, and therefore, further studies with larger samples will be required to determine whether the aortic values in Greyster dogs change according to sex, sports activity and training.
Likewise, no significant differences were found in the LA/AO ratio before and after exercise. This ratio is useful to detect possible left atrial enlargement and predicts the risk of congestive heart failure [8
]. In our study, we did not find any indicators of an enlarged LA, which can be suggestive of myxomatous mitral valve degeneration. Although it is thought that 10% of healthy dogs may reach a LA/AO value of 1.6 [31
], none of the Greyster dogs reached this value, with mean pre- and post-exercise values being 1.10 and 1.11, respectively.