Heart rate and respirator rate response to exercise in horses (Proceedings)

The coordinated actions of the cardiovascular and respiratory systems result in the transport of oxygen and energy products (glucose, fatty acids) to the muscle fibers, where they are used for aerobic energy production, and the removal of waste products (lactate, carbon dioxide, water) from them. During exercise, oxygen delivery is improved by increases in the volume of air breathed, the amount of blood pumped by the heart, and the oxygen carrying capacity of the blood, together with a selective redistribution of the blood flow from the viscera to the working muscles. Many of these exercise-related adaptations are initiated by epinephrine release from the adrenal glands.

Cardiac Response to Exercise

VO2 Max: The athletic capacity of horses is attributable to a large number of physiologic and anatomic adaptations that allow an extremely high maximal rate of O2 consumption (VO2 max). VO2 max is the maximal volume of oxygen the horse's body can use per minute. VO2 max of a horse can be 200 ml/kg/min vs. 85 ml/kg/min in an athletic man. Various adaptations including large lung size, high cardiac output and stroke volume, high hemoglobin concentration and the capacity for splenic contraction increases the horse's oxygen carrying ability of blood by 50%. Training in horses usually increases VO2 max by 10-25%. In addition, horses have a very efficient thermoregulatory system which allows heat generated during exercise to be lost by evaporation from skin surface through the production of large volumes of sweat, evaporation from respiratory tract and from convective loss from skin and mucous membranes. Furthermore horses have developed an efficiency of anatomy and gait, whereby muscle mass makes up a large proportion (45-55%) of bodyweight and muscular work is halved by energy storage in elastic structures. The structure of muscle itself has various adaptations including a high mitochondrial content for aerobic energy production and large stores of energy substrates

Heart Rate Monitoring Methods:

Heart rate measuring technology is rapidly progressing in horses and is being used more and more by trainers in their training programs. Several methods have been available for many years.

     1. Feel the digital or mandibular pulse. A simple, low cost method, however, it is difficult to consistently count heart rates above resting values, and it is impractical during exercise.

     2. Stethoscope – more consistent than the finger pulse method. However, it is difficult to accurately determine the beats per minute at heart rates > 100 bpm and is impractical to use during exercise.

     3. Electronic monitoring - Most reliable method during and while recovering from exercise. More recently they have become more affordable.

Heart Rate: The normal resting heart rate of a mature horse is between 30 to 40 beats per minute. This rate is difficult to obtain in some situations, as certain horses become excited by external stimuli, which elevates the resting heart rate. Although resting heart rate in humans can decrease dramatically as a result of physical conditioning, the resting heart rate of horses does not appear to change appreciably with fitness. Athletic horses often have low resting HRs. This indicates large stroke volume capacity. During exercise the rise in HR is the major contributor to the increase in cardiac output and it is responsible for 53% of the increase in oxygen consumption. At exercise onset, HR increases rapidly from approximately 30 beats/min to approximately 110 beats/min via parasympathetic withdrawal, with the consequence that at low running speeds heart rate may elicit an early over shoot. At faster speeds further elevations are achieved less rapidly and are driven by the sympathetic nervous system and circulating catecholamines.

The anaerobic threshold in horses is around 150 to 170 beats per minute. Heart rates below this threshold indicate that a large percentage of exercise is being performed aerobically. When exercise intensity or duration increase, the requirements of the cardiovascular system increase, which in turn results in an elevated heart rate. Heart rates above the anaerobic threshold characterize rates of metabolism that exceed the abilities of the oxygen dependant pathways supplying energy. Heart rates of 170 beats per minute or greater characterize a large percentage of metabolism occurring anaerobically or without oxygen. Anaerobic metabolism is supported primarily with glucose and glycogen as the fuel source. The threshold heart rate, like resting heart rate, does not change dramatically with physical conditioning in horses. It is likely that the threshold HR for anaerobic exercise may be influenced by genetics.

The maximal HR (HRmax) of a horse is in the range of 210-280 beats/min, which represents a 7 fold increase over resting values. Each horse has its own HRmax, which is reached at a particular exercise intensity. Once a horse reaches its HRmax a further increase in speed is still possible, but it does not elevate the HR any more. A horse with a high HRmax is favored as an athlete because it can perform more work at a specific heart rate (ie a horse with a lower HRmax works relatively harder at a given HR). Conditioning does not alter a horse's resting HR or the HRmax. After conditioning the horse reaches its HR max at a higher workload, and travels faster/works harder at a given HR. In humans HRmax decreases with age and an age related decline in HRmax has recently been described in horses such that as a horse ages it performs the same workload at a higher HR than in its youth. Maximal heart rates should not be used as a major part of conditioning programs; rather, they should be monitored as a danger zone suggesting that fatigue may occur quickly. The speed or velocity a horse can achieve or sustain at a submaximal heart rate of 140, 170, 200 beats/min (i.e. V140, V170, V200) provides information about stroke volume and cardiovascular capacity and can be used as an indicator of fitness and racing potential i.e. as a horse gets fitter the velocity at which it travels at a heart rate of 170bpm should increase.

Stroke Volume: (SV)

Amount of blood pumped during each systole. At rest 450kg horse SV ~ 900ml (2-2.5ml/kg), During exercise 450kg horse SV ~ 1200 ml (increases by 33%). Typically SV increases sharply at exercise onset up to around 40% VO2max. This is as a result of increased blood volume, venous return, and filling pressures according to Frank-Starling's mechanism. Maximal stroke volume may not coincide with maximal HR as during very high heart rates; diastole is insufficient, resulting in inadequate ventricular filling. SV will increase to a point with training.

Cardiac Output:

CO = HR X SV. The cardiac output is the amount of blood pumped by the heart each minute and is the most important means of increasing muscle oxygen delivery during exercise and thus is the principle determinant of VO2 max. Increases in CO are due mostly from an increase in HR and to a smaller degree by an increase in SV. During submaximal exercise CO increases linearly with running speed.

At rest 450kg horse CO ~ 30-45 l/min. During exercise 450kg horse CO increases to a maximum of about 240 l/min. In very fit TBs the CO has been measured up to 350L/min. Marked adjustments in capacitance of blood vessels are needed to accommodate the large increase in CO and to distribute blood appropriately. At rest only about 15% of the circulating blood is delivered to the muscles, but this increases to as much as 85% during strenuous exercise.

Blood Pressure:

BP depends on HR, blood volume, force of ventricular contraction and resistance to blood flow from blood vessels. Systemic circulation is normally under high pressure due to the force of the left ventricular contraction and the high resistance of vessels. The systemic arterial pressure is about 155/110mmHg at rest, rising to 250/120mmHg during strenuous exercise. There is no change in the maximal arterial pressure with training.

Heart size and scoring:

Heart size is very important in horses performing exhaustive sports such as endurance, eventing. After training, ventricular mass and volume is increased. The amplitude and direction of the ECG provides some information about the size of the heart chambers and the pattern of electrical conduction. It is particularly useful in the diagnosis of arrhythmias and conduction disturbances and can be used to detect abnormalities that may interfere with the horse's athletic ability. ECG has been used by some researchers as a measurement of heart size. The "heart score" is measured as the average of duration in ms of the QRS complexes in leads I, II and III. Generally there is a positive correlation between heart size and racing performance, and some research indicates that there may be a correlation between heart score and racetrack earnings. However many cardiologists are skeptical of the accuracy of determining heart size from the duration of the QRS interval.

Cardiovascular Response to Exercise:

Increases in anticipation of exercise. The more excitable the more the anticipatory rise in HR. With the onset of exercise there is a rapid elevation of the horse's HR. When steady submaximal work is performed the HR rate shows an initial overshoot before falling to a plateau after 2-3 minutes. The size of the overshoot and the steady HR depends on the horse's fitness and work intensity. When the horse accelerates to top speed over a steady distance, there is no initial overshoot and the HR does not reach steady state. In horses galloping at a steady speed on flat ground, there is a linear relationship between speed and HR at speeds in the range of 350-700 m/min (13-26mph), which are roughly equivalent to HR of 140-200 beats/min. As the horse approaches HRmax, the HR response curve tends to flatten. The graph of HR vs. speed shifts to the left when the horse is sick and to the right with increasing fitness. When high intensity exercise ceases, there is a rapid deceleration of the HR, with the greatest reduction occurring in the first minute post-exercise. The rate at which the HR declines depends on the intensity and duration of the work, the horse's fitness and the environmental conditions (heat humidity). Generally the fitter the horse, the faster the HR returns to normal after a standard amount of exercise.

Effects of Conditioning:

The cardiovascular system shows considerable adaptations in response to conditioning and the changes occur relatively rapidly in comparison to the slow rate of adaptation of the musculoskeletal system. As the horse gets fitter there are reductions in the HR and CO at a given level of exercise, although HRmax does not change. The horse's ability to consume oxygen increases due to improvements in the efficiency of the CV system in delivering oxygen to the skeletal muscles and in the ability of the muscles to extract oxygen from the blood. There is an increase in capillarization of the muscles which slows capillary transit time and enhances gas exchange by allowing a longer period of contact between the RBCs and the muscle fibers. Some of the exercise induced changes in the CV system vary with the nature of the regular work. Endurance conditioning raises the plasma volume by about 20% and the hemoglobin concentration by about 34%. High intensity sprint conditioning stimulates greater increases in the PCV, RBCs and Hb than endurance exercise. There are short term changes in the WBC profile following strenuous exercise, and regular conditioning is associated with alterations in the total and differential WBC count.

Respiratory Response to Exercise

During exercise the primary function of the respiratory system is gas exchange which involves supplying oxygen and removing carbon dioxide from the blood in the pulmonary capillaries. The respiratory system also plays an important role in thermoregulation and acid-base balance. Horses are unusual in that they are obliged to breathe through the nose, unlike other animal species that have the option of breathing either through the nose or the mouth.

Terminology

Respiratory Rate

Horses have a normal resting respiratory rate of 12-20 breaths per minute. During exercise the respiratory rate rises as high as 180 breaths per minute. At a walk and to a certain extent at a trot and pace, the horse selects an appropriate respiratory rate for the intensity of exercise. In the canter and gallop, however, the respiratory rate is usually coupled to the stride rate with a 1:1 ratio (locomotor: respiratory coupling).

Tidal Volume

Amount of air inhaled and exhaled at each breath. In a 450kg horse at rest the tidal volume is about 4-7 liters, rising to a maximum of about 10 liters during exercise. In resting breathing the dead space accounts for about 70% of the tidal volume and the alveolar volume is about 30%. With exercise there is a large increase in alveolar volume and a small increase in dead space.

Minute Volume

Amount of air passing in and out of the lungs per minute.MV = RR X TV

At rest ~ 100l/min. At maximal exercise MV = 1500l/min (due to 7X increase in RR and 2X increase in TV)

Upper Airwary:

The resistance to airflow in the airways affects the energy expended to drive respiration. The greatest resistance occurs at the narrowest parts of the respiratory tract, which are the nostrils and the larynx. During exercise the resistance to airflow is reduced by flaring the nostrils and dilating the larynx. Other methods of reducing upper airway resistance during exercise include vasoconstriction of the nasal mucosa which increases the diameter of the nasal passages and straightening of the horse's head and neck, which decreases air turbulence by aligning the pharynx, the larynx and the trachea.

Respiratory Response to Exercise:

Rate and depth of breathing are controlled in part by chemoreceptors in the blood vessels which respond to changes in arterial oxygen tension, carbon dioxide tension and pH. A rise in respiratory rate is stimulated by a reduced oxygen tension, an increased CO2 tension or a reduced pH. At the onset of exercise, the respiratory rate and TV rise rapidly in accordance with the body's need for oxygen. In the canter and gallop and to a lesser extent the trot and pace the respiratory rate is coupled to stride rate and so the mechanics of locomotion override the chemical control of breathing. When exercise ceases, the respiratory rate decreases due to the cessation of the locomotor forces that drive respiration. Typically the horse takes a few deep breaths, and then the respiratory rate settles in the range of 60-100 bpm with the horse breathing deeply until the oxygen debt is repaid. Following repayment of the oxygen debt the respiratory response depends largely on the horse's body temperature. Panting is rapid shallow breathing, where air passes through the nasal passage but the tidal volume is small. It is therefore important to observe both the rate and depth of respiration, together with HR and rectal temperature to assess whether a horse is overheated after exercise.

Locomotor-Respiratory CouplingNG

In energetic terms the most economical breathing strategy minimizes the muscular effort of respiration. Horses accomplish this using a phenomenon known as locomotor-respiratory coupling (LRC) in which respiration is driven by the locomotor forces associated with weight bearing on the front limbs, the pressure of the abdominal organs against the diaphragm and changes in orientation of the body axis. These factors are gait dependant and as a result LRC is most effective in canter and gallop where there is a strict 1:1 ratio between the respiratory and locomotor cycles. In the trot and pace there is greater flexibility and horses will select a 1:2, 1:3 or 2:3 ratio between the respiratory rate and stride rate. In the canter and gallop one of the major contributors to LRC is the compression of the thorax between the two scapulae as the front legs bear weight. As both front legs bear weight simultaneously during canter or gallop this effect is maximized. The diaphragm is the principle muscle of inspiration. LRC augments this action through movements of the abdominal organs known as the visceral piston which have considerable inertia. When the front legs impacts the ground there is a moment of deceleration and when the leading front leg pushes off there is a moment of acceleration which moves the visceral piston. Thus as the forelimbs hit the ground expiration occurs and as the forelimbs push off into the suspension phase inspiration is stimulated. This effect is also helped by the rocking action of the horse's trunk during the canter and the movement of the head and neck up and down. In the trot and pace there less compression of the chest with forelimb loading and the body moves at a more consistent speed. In addition the body axis does not tend to pitch up and down, thus the horse has some choice in his breathing pattern.

Implications of LRC for Sport Horses

At a constant stride the visceral piston settles into the stride rhythm and the energy used in breathing is minimized. However, each time the stride rate changes it takes several strides before the visceral piston catches up with the new rhythm. During this time the energetic cost of breathing is increased. Therefore it is energetically efficient for a horse to change speed by adjusting its stride length, while maintaining the same rhythm. This is particularly relevant in which long periods of gallop are involved such as in eventing. In addition a horse with a longer stride length has a slower stride rate compared to a horse travelling at the same speed with a shorter stride length. This means that the longer striding horse can breathe more deeply, which favors its ability to maintain a speed over a longer distance. Short stride length will limit the time available for inspiration, effectively reducing the tidal volume.

Effects of Conditioning

Conditioning improves the function of muscles that hold the upper airways open during exercise, particularly the muscles of the nostrils, pharynx and larynx. Some horses that have a slight respiratory noise at the beginning of a conditioning program often will lose it as they get fitter. In the lower respiratory tract, conditioning has relatively little effect; the alveoli, pulmonary capillaries, bronchioles, bronchi and chest wall show minimal adaptation in response to regular exercise. In addition there is little change in the MV and TV with training. This is in contrast to the marked adaptive responses of the CV and muscular systems and suggests that the respiratory system may be the limiting factor to athletic ability in a fit horse. In fact studies have shown that as horses get fitter they become more hypoxic and hypercapnic with exercise than pretraining blood gases. Consequently the respiratory system is regarded as the weak link in the oxygen supply pathway in horses.