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.