Discussion
India is known to have several unique breeds of equines since many
centuries. Many of the native equine animals had gone either extinct or
got mixed up with other breeds losing breed identity. More than 2500
year old Tamil Language Texts called “Sangam” Corpuses had documented
about the ancient equines of Tamil Nadu, which is one of the ancient and
still surviving civilization of the World. Kathiawari is one of the
unique equine breeds of India and has home tracts in the Kathiawar
peninsula in the modern day Gujarat, India. This horse has a historical
role as a war horse, serving until the end of the First World War for
the Indian army and also used as a cavalry mount. The Kathiawari horse
breed has experienced a decline in its population over the years. The
decline in the population of Kathiawari horses underscores the
importance of immediate need for conservation measures to safeguard this
unique Indian horse breed for future generations. This study is unique
because it represents acquisition and assessment of the exercise
physiology related values in routinely heavy working Kathiawari horses
and not in the laboratory or post exercise on a treadmill.
Kathiawari horses are well known runners and cover long distances of
running under desert climatic conditions of India and have phenomenal
endurances. Their ECG studies were helpful and there was a significant
decrease in the duration and increase in the amplitude of the P wave
after the exercise compared to before exercise. Similar findings in
other horse breeds were also reported by Piccione et al . (2003).
An increase in the P-wave amplitude on an electrocardiogram (ECG) could
indeed be related to an increase in sympathetic tone. When sympathetic
tone is increased, it can lead to an increase in atrial contractility
and, consequently, an increase in the amplitude of the P-wave on an ECG.
It’s important to note that an increase in P-wave amplitude can also be
associated with other factors, such as atrial enlargement or conditions
that affect the conduction of electrical impulses within the atria.
A highly significant increase (p<0.01) in heart rate was
observed 5min after exercise when compared with before exercise. An
increased heart rate in the present study is in concurrence with Hassanet al. (2015). The elevated heart rate following exercise may be
attributed to the activation of the sympathetic nervous system and the
subsequent rise in catecholamine levels. P wave changed their bifid
positive form to single positive form which is due to the fusion of the
P1 and P2 components of P wave, because of increased heart rate. Similar
findings were also reported by Mathapati & Saini (2019) in
Thoroughbred horses during exercise. Morphology of the P wave was
influenced by alterations in the heart rate which is also observed by
Schade et al . (2014).
Highly significant reduction of PQ, QT and RR interval after exercise
was observed in the present study. PQ interval measures the time
required for the electrical impulse to travel from the sinoatrial node
to the ventricular muscle (Mirvis and Goldberger, 2004). In normal
physiological conditions, PQ interval depends primarily on the velocity
of the electrical impulse conduction through the atrial muscle. The
duration of conduction through the atrioventricular junction is
subjected to modulation by the autonomic nervous system. Mirvis and
Goldberger (2004) found that sympathetic activation accelerates this
conduction velocity, leading to a shorter PQ interval after exercise in
horses which was in accordance with our study.
T wave morphology is the most variable component in ECG of the horse.
Inversion of polarity was the most common electrocardiographic
abnormality (Verheyen et al., 2010a ) especially related
to stress and strenuous training.
QT interval represents the duration of depolarization and repolarization
of the ventricular myocardium (Mirvis and Goldberger, 2004). Similar
findings were also found by Pasławska et al. (2012) after
exercise testing PQ, QT and R-R intervals were shorter than the resting
conditions.
An irregular ST segment could suggest myocardial hypoxia or variations
in potassium levels and blood pH (Babusci & López, 2006). ST segment
irregularities exceeding 0.3 mV may suggest conditions such as shock,
endotoxemia, abdominal pain, or electrolyte disorders (Diniz et
al. , 2011, Dumont et al., 2011).
There were no echocardiographic studies conducted in Kathiawari horses.
In the present study of Kathiawari horses, among all echocardiographic
parameters, LVID during diastole and systole, LVPWd, EDV, ESV, SV, Ao
and LA were significantly increased after exercise. The present study
was in accordance with Buhl and Ersboll (2012) reported in Standardbred
horses. Left ventricular internal diameter in diastole (LVIDd), left
ventricular mass (LVM), and mean wall thickness (MWT) were increased
after physical training. These findings support the concept of athlete’s
heart, wherein training induces cardiac hypertrophy. A similar finding
of cardiac hypertrophy was also observed in our study.
Training-related increases in left ventricular (LV) dimensions have been
consistently observed in various horse breeds, including Thoroughbred
horses (Young, 1999), Standardbreds (Buhl et al., 2005; Buhl and
Ersboll 2012), and elite Arabian horses (Sleeper et al., 2014).
Additionally, Trachsel et al. (2016) reported that training
serves as a potent stimulus for cardiac hypertrophy.
FS% and EF% were increased significantly after exercise in our study.
Young et al. (2005) found that there was a positive association
between left ventricular ejection fraction and left ventricular mass
with race rating in older flat racehorses running over sprint and longer
distances. Gunther-Harrington et al. (2018) also found that
increased FS% after exercise. It may be secondary to sympathetic
activation and its positive inotropic effect. Increased heart rate may
also contribute to increase in systolic function as a result of Bowditch
effect.
In the present study, the LV internal diameter and systolic function had
the strongest relationship with exercise performance. Pluim et
al. (2000) found that increases in both wall thickness and left
ventricular internal diameter signify the presence of eccentric left
ventricular hypertrophy in endurance-trained athletes.
While cardiac biomarkers are important tools, so far no studies were
conducted to evaluate the cardiac biomarkers in Kathiawari horses. In
our study there was a highly significant increase in cTnI, cTnT, CK-MB
and NT-proBNP 3 hours after exercise in Kathiawari horses. Similar
findings were reported in other breeds by Holbrook et
al. (2006) and Pourmohammad et al . (2020) suggested that
exercise induced a certain level of myocardial stress in Thoroughbred
horses. Nostell and Häggström (2008) also suggested that horses
typically exhibit low cardiac troponin I (cTnI) values at rest. However,
intense, short-term exercise leads to a mild increase in cTnI
concentrations, which can persist for up to 10-14 hours. Durandoet al. (2006) found that higher than normal values of cTnI even
after 12 and 24 h after exercise. Walton (2014) reported that cTnI
values of clinically healthy horses are less than 0.2 ng/mL and those
with greater than 0.3 ng/mL are considered to be abnormal. Shieldset al. (2018) suggested that the increased cTnT concentration
after high intensity short term exercise may not have been solely due to
cardiac muscle release but also possibly as a result of impaired renal
function, exercise induced dehydration or skeletal muscle damage.
Exercise can lead to various physiological changes, including the
release of catecholamines, which may induce transient myocardial
ischemia. Additionally, increased heart rates and blood volume can cause
mechanical stretching of cardiac myocytes, dehydration and acid-base
imbalances may also contribute to cell damage (Weippert et al., 2016). These factors collectively result in short-lived elevations in
circulating cardiac troponin (cTn) concentrations.
CK-MB is a less sensitive cardiac biomarker than cardiac troponin for
evaluation myocardial injury (Sim et al., 2008). Serum CK-MB
activity was a less specific cardiac biomarker in horses but was
included in the study to support the cTnI concentration results (Flethøjet al., 2016). BNP is expressed in the ventricular myocytes and
in response to pressure or volume overload and mechanical stresses to
ventricle walls, contribute to increase in BNP level after exercise
(Alter et al., 2007 and Sadeghian Chaleshtori et al., 2022). These findings were in accordance with our study which showed
elevation of CK-MB and NT-proBNP after exercise.