Arthur B. King DVM

The early recognition and proper management of metabolic problems in equine athletes participating in intense sports such as marathon driving, three-day-eventing, competitive trail and endurance riding presents a challenge for both veterinarians and competitors. Despite recent advances in the use of electrolytes and improved veterinary control at equestrian competitions, metabolic problems are not uncommon. The challenge of managing these problems under field conditions and often without diagnostic clinical pathology services, can be unique.

A number of metabolic problems have been recognised as being due to the fluid losses and changes in electrolyte concentrations that occur during prolonged and strenuous aerobic or anaerobic exercise. Underlying and often unrecognised pre-existing pathology may be a contributing factor to the development, severity and ultimate outcome of a particular problem in afflicted horses. The most commonly recognised problems associated with fluid and electrolyte changes during exercise are:

  • Dehydration
  • Synchronous Diaphragmatic Flutter (SDF)
    • thumps
  • Metabolic Myopathies
    • cramps, tying-up, azoturia
  • Laminitis - founder
  • Colic
  • Exhausted Horse Syndrome
  • Post-exhaustion Syndrome

An understanding of the pathophysiology of metabolic disorders is essential to their early recognition and proper management. A number of factors, often in combination, play a role in the development of metabolic problems. It is not unusual for two apparently similar horses under the influence of apparently similar factors to develop different metabolic problems. For example two horses competing under similar conditions at the same endurance ride may both show signs of dehydration and delayed recovery following the ride. However one horse may develop colic while the second horse develops laminitis.

The common factors contributing to metabolic disorders are:

  • heat production during exercise
  • lactic acid production
  • water loss in sweat
  • electrolyte losses in sweat
  • electrolyte recompartmentation
  • energy depletion
  • splenic contraction leading to hemoconcentration
  • metabolic alkalosis

Researchers have studied a number of parameters in horses at rest and during exercise. Normal ranges of values for these parameters have been determined. Some parameters have a wide range between normal resting value and acceptable value during maximal exercise whereas other parameters must be maintained within very narrow limits whether the horse is at rest or exercising. For example the heart rate at rest can be in a range as low as 32 to 24 beats per minute whereas during exercise the rate can increase by 8 to 10 or more times the resting rate. Blood pH on the other hand must be maintained within a narrow range whether the horse is exercising or resting under normal conditions. The veterinarian or competitor faced with management of a metabolic disorder must understand the normal physiological responses during exercise and during recovery following exercise.

When compared to a horse at rest, the horse during exercise manifests the following readily observable changes:

  • increased heart rate
  • increased respiratory rate
  • increased body temperature
  • increased sweating
  • increased bio-mechanical forces associated with speed, acceleration, deceleration, carrying or pulling weight, and jumping efforts

We know from research under laboratory conditions that other changes occur which cannot be as readily assessed under field conditions. For example:

  • oxygen consumption increases
  • carbon dioxide production increases
  • energy consumption increases
  • heat production increases
  • lactic acid production increases
  • cardiac output increases
  • stroke volume increases
  • PCV increases due to splenic contraction
  • blood flow patterns shunt with an increased flow to some organs e.g. muscle, skin, and with a decreased flow to other organs, e.g. gastro-intestinal tract, kidneys
  • blood and tissue pH changes vary - the development of acidosis or alkalosis depends upon the intensity and duration of the exercise and the extent of fluid and electrolyte changes

As the horse exercises the demand for energy increases. This demand is met when glucose is broken down with or without the use of oxygen. The production of energy from this source also results in heat being produced. This heat must be dissipated to prevent thermal overload. Endurance horses running at aerobic levels will rarely have rectal temperatures in excess of 40.5 C and usually are less than 39C. Three-day-event horses running anaerobically have had rectal temperatures as high as 43C. Temperatures in excess of 43C, even for short periods of a few minutes to an hour can be life threatening. Temperatures above 40.5C can be life threatening if maintained for longer than a few hours.

The need to dissipate heat causes the horse to sweat. The horse is cooled as the sweat evaporates. In hot, humid weather this is the primary method of cooling . In cooler weather heat can be dissipated through conduction and convection. Heat is also lost through the heating of the expired air, but this is far less significant than heat loss through the skin. The need for the horse to cool itself through sweating creates two significant demands on the horse's normal physiology. The redistribution of bloodflow to the subdermal capillary bed results in less blood being available for other organs. Secondly the loss of water and electrolytes in the sweat results in dehydration and significant changes to the acid-base balance. Horses exercising intensely in conditions where the ambient temperature is 22C and the relative humidity is 75 % can lose up to fifteen litres of sweat per hour. Delayed skin tent times are normally not evident until the horse has reached at least 3% dehydration. Skin tent times of 3 seconds may indicate a loss of about 10%. The hemoconcentration created by the loss of fluid greatly increases blood viscosity. This increased viscosity, the shunting of blood to the skin from other organs, and the acid-base changes associated with the loss of electrolytes combine to alter the normal perfusion of tissues. This decrease in tissue perfusion is a contributing factor in the development of laminitis, metabolic muscle myopathies, and colic. Normal thermo-regulation is also compromised since reduced tissue perfusion results in less heat being removed from tissues and reduced blood flow to the skin for cooling.

Hemoconcentration also occurs as a result of splenic contraction. This further increases blood viscosity adding to the decreased tissue perfusion throughout the body.

The effects of sweating are basic to an understanding of how electrolyte losses lead to the acid-base changes which cause the metabolic problems of high performance horses. The primary anions lost in sweat are Na+, K+, Ca++, and Mg++. Of these sodium and potassium are more important since they are present in higher concentrations than the others. The primary cation is Cl-. Sweat is hypertonic relative to plasma. This is of significance since water is lost from the plasma at a lower rate than electrolytes as the horse sweats. The horse's cerebellar sensors of thirst fail to recognize the loss of water because the loss of water has not resulted in an increased concentration of sodium in the plasma. This results in dehydrated horses with little or no desire to drink.

Another effect of the electrolyte loss in sweat is metabolic alkalosis. The loss of chloride in sweat has a physiologic effect similar to the loss of chloride in vomiting cats and dogs and in neonates with diarrhea. The metabolic alkalosis results in binding of ionized calcium to plasma proteins, since protein binding is a factor of blood pH. The decrease in ionised calcium leads to an increase in neural irritability. This change of neural irritability in the phrenic nerve leads to it being stimulated by the polarisation-depolarisation cycle of the cardiac muscle. This results in the diaphragm contracting in response to the heart contractions, hence a diaphragmatic flutter, synchronous with the heart beat. This is commonly referred to as thumps since the flank muscles often respond with a hiccup-like "thump".

The electrolyte losses in sweat also result in deficits at the cellular level. This in turn has an effect on normal cell function. This effect is often noticed in the muscular system with an increased risk of fasciculations, or tying up, muscle cramps and other myopathies. The cardiac muscle can also be affected by these electrolyte imbalances. Arrhythmias associated with exercise and sweating are not an infrequent finding at veterinary checks during competition.

The smooth muscle in the gastro-intestinal system is also influenced by the electrolyte changes and decreased tissue perfusion. Reduced gut sounds are a common finding in endurance horses during competition. Colic is a very common finding in horses suffering from metabolic problems following or during endurance rides. When dehydration is marked, the risk of impaction colic is greater.

The lack of tissue perfusion leads to:

  • lack of oxygenation
  • accumulation of metabolic by-products such as carbon dioxide and heat
  • alterations in pH

This results in impaired tissue and organ function especially in muscle, gut, hoof laminae, kidney, liver, and circulatory system.

Energy depletion becomes a significant factor in prolonged endurance events, especially if the ambient conditions are cold and wet. All body tissues require energy for normal function, not only those in the muscular system. Many of the signs of general fatigue are due to the combination of reduced tissue perfusion and decreased energy levels.

The chart below was prepared to assist riders and trainers in understanding the clinical evaluation of equine athletes under field conditions with a minimum of equipment. These parameters are routinely used at endurance rides to assess horses at rest prior to competition, at veterinary check points during competition and during the recovery period following competition. Values for the parameters are listed in the three columns to the right of the column of parameters. The first column lists values or findings that would be acceptable at rest, during exercise, or during recovery. Horses with findings in this range prior to or during competition are fit to compete. If values fall in this range following competition or recovery, they indicate that the horse has not suffered undue stress or fatigue as a result of exercise. The values and findings listed in the second column are those found in horses showing signs of stress and fatigue as the result of exercise. Horses with several parameters in this area, especially if no improvement or recovery is evident with a period of 30 minutes rest, should be withdrawn from competition. These horses should also be monitored closely for evidence that they are eating, drinking, urinating, defecating, and gradually returning to resting values for the listed parameters. Horses that fail to show progressive recovery in the 30 minutes to one hour after exercise, or that develop worsening symptoms, require treatment. Horses with values in the red or danger zone require treatment unless they show marked evidence of recovery with rest alone. The decision on when to initiate, continue, or discontinue treatment is based on the clinical signs, their severity, and whether there is evidence that the horse is recovering.

When faced with treating a horse for a metabolic problem during or following a competition, it is important to consider the events that contributed to the development of the problem. Simply put, they will be loss of sweat and loss of energy. The loss of sweat may be minimal or considerable, but irregardless there will be significant losses after only one to two hours of competition. An animal with some signs of poor recovery, signs of colic, or other disorder, but with clinically normal findings for hydration, will often benefit from administration of either oral or intravenous fluid therapy.

Fluids can be given orally at the rate of 4 to 6 litres every 30 minutes until the horse appears to be recovered to normal. Oral fluids should be isotonic mixtures containing 40 g sodium chloride and 30 g potassium chloride per 10 litres, with 250ml to 500ml of 50% glucose added if energy depletion is a factor. If the risk of colic is too great or the horse fails to show any signs of recovery, or worsens within 30 minutes following administration of oral fluids, intravenous fluid therapy should be initiated. Fluids suitable for intravenous use include Ringer's, with or without 5% glucose, or normal saline, preferably with up to 3 g added potassium chloride and with or without the addition of up to 5% glucose. Avoid solutions containing lactate or bicarbonate. Horses with SDF (thumps) will usually respond to the chloride therapy since the chloride reverses the alkalosis, thereby releasing the protein bound calcium as ionized calcium. If the SDF is severe, 250 to 500 ml of calcium borogluconate solution as would be used for hypocalcemia in cattle, may be indicated.

Intravenous fluids should be given at the rate of up to 20 litres in the first hour, reducing to 10 to 15 litres per hour after that, until the horse has recovered. Volumes of 40 to 60 litres or more may be required to replace deficits and allow for maintenance during the administration period. Catheters should be at least of 14 gauge. Administer the fluids under pressure or by pump and with one catheter in each vein if necessary to achieve the high flow rates. Ensure that the intravenous tubing is of sufficient inside diameter to allow for these high flow rates. Horses are not at risk for pulmonary edema. Many of the pharmaceuticals commonly indicated for the treatment of the metabolic problems which may be encountered e.g. acepromazine, analgesics, NSAIDs, diuretics, etc., are contraindicated in dehydrated horses with electrolyte imbalances. The rapid administration of high volumes of balanced electrolyte solutions will reverse the dehydration and electrolyte deficits, thereby allowing for the safer administration of any pharmaceutical agents that may be indicated for treatment of the secondary conditions. It is not uncommon for the clinical signs to subside with the rehydration and improved tissue perfusion alone, with no need for any other pharmaceuticals. Horses on intravenous fluids will not likely drink but they will frequently eat. Therefore if the horse is not drinking but all other signs are normal the intravenous therapy can be discontinued.

A veterinarian would normally have equipment, materials, and supplies available for the first aid treatment and management of traumatic injuries at an equestrian competition. The list below suggests what is needed for management of the metabolic problems that may arise.

  • halters, leads, and twitch
  • stethoscope
  • thermometer
  • refractometer
  • pails
  • water for sponging and drinking
  • towels and ice
  • stomach tube, funnel, and pump
  • oral electrolytes
  • mineral oil and other laxatives
  • syringes and needles
  • 14 g catheters and IV tubes (large bore)
  • local anaesthetic
  • suture pack
  • intravenous fluids (one treatment may require 40 to 60 litres or more) - Ringer's or saline
  • potassium chloride solution
  • 50% dextrose solution
  • calcium borogluconate solution
  • NSAID's
  • sedatives
  • analgesics
  • diuretics

In summary, the treatment of metabolic disorders under field conditions at equestrian events requires a basic understanding of the effects of the fluid, electrolyte and energy losses that contribute to the development of the disorders. Early recognition and treatment with oral or intravenous fluids, even in the absence of clinical dehydration, in the quantities outlined above, will often alleviate the disorders without any other pharmaceuticals. Furthermore the pharmaceuticals that may be indicated for treatment of these disorders are often contraindicated in dehydrated horses.

The author wishes to acknowledge the invaluable assistance of Gayle Ecker, Equine Research Centre, Guelph, Ontario, Canada in the preparation of this article.

Ecker G: University of Guelph, 1994 Msc. Thesis
Ecker G, Lindinger M: Fluids and Electrolytes: In Short Supply? The Equine Athlete 7:3,4 1994
Ecker G, Lindinger M, Equine Vet Journal (accepted for publication)
Jones Wm E: Equine Sports Medicine 1989
Persson SGB, Lindholm A, Jeffcott LB: Equine Exercise Physiology 3, 1991
Sloet van Oldruitenborgh-Oosterbaan MM: The Treatment of the Exhausted Horse Under Field Conditions. Equine Practice 16:8 27- 33, 1994

2002 Arthur B. King DVM

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