Review on Cardiovascular Systems

The cardiovascular system of the horse is designed to transport oxygen from the lungs to the body tissues via blood. “Blood is the main transporter through the body of essential water, oxygen, nutrients, chemical regulators and waste products. The blood is propelled along the blood vessels by the pumping action of the heart” (Hastie, 2012)

Blood vessels are composed of arteries, veins, and capillaries. Arteries are the biggest vessels; their purpose is to carry oxygenated blood away from the heart, the blood in these vessels is under high pressure. The walls of the arteries are composed of smooth muscle and elastic fibers, which contract and relax to be able to push blood through the cardiovascular system. The veins are responsible for transporting deoxygenated blood back towards the heart; the layers of the vein are much thinner and less elastic than those in the arteries. Veins contain valves that prevent blood from being able to flow backward, arteries do not have this function; the blood in the veins is transported at a much lower pressure than the arteries. Capillaries are the smallest of the blood vessels; their cells are one cell thick. Their function is to allow easy exchange of components such as nutrients, oxygen, carbon dioxide, and salts and to allow removal of waste from cells between body tissues and blood.

The equine heart is made up of two chambers that are made up of atria and ventricles. Atria are the chambers that collect the blood returning back to the heart and the ventricles are the chambers that pump blood from the heart around the body. The right atrium receives deoxygenated blood from the network of veins that feeds back to the heart; the atrium pushes the blood past the tricuspid valve into the right ventricle. From the right ventricle, blood is pushed through the pulmonary arteries to the lungs where the blood is oxygenated with the oxygen that is inhaled via respiration. The oxygenated blood is sent back to the left atrium through the pulmonary veins when it then supplies the left ventricle via the mitral valve. To complete the cycle, the left ventricle pumps the oxygenated blood past the aortic valve and throughout the entire body via the aorta.

The purpose of this study is to establish whether the horse’s heart rate is affected by the gait at which they are moving. Previous studies looking at heart rate variability have been used to assess stress response in horses with laminitis after short and long-term treatments; results showed that heart rate decreased after treatments (Gehrke et al., 2011). Another looked at heart rate variability in relation to stress and welfare; their results concluded, “HRV analysis in horses appears to be a sensitive measure of both physical and emotional stress responses” (von Borell et al., 2007). Studies have also been carried out on heart rate monitors to look at their accuracy in recording heart rate data. The results of (Evans et al., 1986) showed that all four heart rate monitors, (PEH 100, PU10, HR14 and HRM-7) used in their experiment showed significant correlations with the heart rate recordings taken by a simultaneous telemetry electrocardiography (ECG) that could suggest that they are accurate however there are still areas for errors in the recordings. More recent experiments using Polar Equine Monitors, used in this experiment, were deemed to be able to obtain accurate heart rate variability in horses in (Gehrke et al., 2011) the Polar Equine Monitors use human frequency ranges, which allows a method of being able to compare horse data to human data. (Ille et al., 2014) also confirms the results of (Gehrke et al., 2011) in that their findings concluded the Polar Equine Monitors to be as adequate as ECG readings.

The accuracy of these Polar Equine Monitors in these studies is good validity for using them in this study, as it will improve result accuracy. This study uses similar methods to (Ille et al., 2014) in that water and ultrasound gel were both used to increase the contact between the Polar Equine Monitor electrodes and the skin. Recordings were carried out with 2 horses at the same time for both studies and both studies lasted approximately for 60 minutes in each recording session. Data in both studies were transmitted at the end of the session to a laptop computer. (Gehrke et al., 2011) also states “gel should be used liberally to enhance signal transmission.” A study by (Harris et al., 2007) is very similar to this study, the study used a sample of horses that were lunged in a familiar environment during a 5 or 7m radius. The speed of each lap was recorded and the number of laps the horses completed and the overall distance measurement were used to determine the speed. They concluded that HR and speed were highly correlated and that the HR: speed relationship can be obtained by using an unridden lunge test. This study also requires the horses to be lunged in a familiar environment, the distance of the lunge line from the horse and the time it took to complete the lap were also used in this study to determine speed.

The overall aim of this study was to collect heart rate data from horses in three different gaits, to then compare this data to see whether there is a correlation between speed and heart rate and to compare the data of the horses exercised indoors and outdoors to see if there is a distinct difference in heart rate.