Kidneys, adrenal glands, lungs and special regions in the brain all work together in an amazingly complex system to maintain the internal chemistry within acceptable limits. There are many causes of acid-base disturbance, but in the exercising endurance horse, acid-base changes generally indicate that an exercising horse is working somewhat beyond his immediate capacity. Depending on the type and extent of changes, it may mean that the horse needs to slow down, cool off, or may be indicative of major metabolic changes. Significant changes in a resting horse that has not undergone recent exercise may indicate a disease process.
pH – Most people are somewhat familiar with the concept that pH is an indication of a solution’s acidity. Lower pH indicates a more acidic solution; the higher the pH, the less acidic (and therefore more alkaline or “basic”). The normal pH range of blood is between 7.32 – 7.44(12). Most systems in the body only operate efficiently within this narrow pH range. If blood pH is either too low or too high, the horse’s condition is referred to as “acidosis” or “alkalosis”, respectively. Under endurance conditions, low pH (acidosis) is a good indication that the horse has recently been exercising beyond his aerobic capacity, and lactic acid of anaerobic muscle metabolism is accumulating faster than the body can recycle it. The lower the pH, the further the horse has been pushed beyond his limits, and the longer it will take for him to recover. As with all other blood parameters, it’s important to look at the total picture—if the horse has recently raced into the finish line, pH levels may be temporarily somewhat decreased due to the release of lactic acid from hard-working muscles. Or it may indicate nothing more than muscles that have not yet warmed up and fully shifted into aerobic metabolism. However, a low blood pH observed along with other acid-base indicators, elevated muscle enzymes, muscle stiffness and other clinical signs help the veterinarian identify ongoing disease processes, such as tying-up or exhausted horse syndrome.
Blood pH level higher than normal often indicates that a horse is overheated and is panting to help with excess heat dissipation. During rapid breathing or “hyperventilating”, the body will lose significant amounts of carbon dioxide, which in turn raises blood pH (more alkalotic or basic). Increases in pH have other effects in the body, such as decreasing the availability of physiologically active calcium.
The normal TC02 (total carbon dioxide) concentration is 28 mEq/liter, and also contributes to the ‘big picture’ of acid-base status. TC02 levels of 20-27 mEq/liter indicate a mild acidosis as described above; TC02 of less than 20 mEq/liter indicate severe, possibly life-threatening, acidosis (12).
HC03 refers to bicarbonate, a buffer released by the kidneys to help prevent changes in the acid-base balance. A normal value is between 24-30 mEq/liter. Although the pathways within the body for regulating bicarbonate within the body are far too complex for these few pages, low levels during endurance exercise would contribute to a diagnosis of metabolic acidosis. Levels slightly above normal indicate mild alkalosis, and might be expected in horses exercising under hot conditions.
Base Excess (BE) is the mathematical sum of several of the above positively and negatively-charged ions that contribute to acid-base status. Changes in base excess evaluate “unmeasured anions”, usually lactic acid secondary to strenuous anaerobic
exercise, inflammation, dehydration or infection. A base excess of zero indicates no abnormal changes in acid-base status, A positive base excess in a horse generally indicates alkalosis secondary to heat and panting. A negative base excess increasingly indicates acidosis, usually secondary to strenuous, anaerobic exercise. Dehydration may be a contributing factor to a negative BE.
Of the nutrients, electrolytes, both cations and anions, are somewhat unique in their ability to affect performance. Not only do electrolytes have effects individually, because of their propensity to be ionized, they have the ability to interact with one another to create additional effects. Strictly defined, an electrolyte is a chemical compound that ionizes when dissolved or molten to produce an electrically conductive medium. Physiologically, electrolytes are required to regulate the electric charge across cell membranes and participate in a number of reactions necessary for life. Sodium, potassium, and chloride are the minerals most often thought of when the term electrolyte is used, although calcium, phosphorus, magnesium, and sulphur may also exist in ionized states in the body. Other compounds such as lactate, bicarbonate, and even proteins may act as electrolytes as well.
Blood pH – The pH of the blood is affected by acids and bases. An acid is any compound capable of donating a hydrogen ion to the solution and a base is any compound capable of accepting a hydrogen ion from the solution. The strength of an acid or base is measured by its ability to donate or accept hydrogen Ions at the pH of body fluids. Hydrochloric acid, for example, is a strong acid because it is completely dissociated into a hydrogen ion (H+) and a chloride ion (CI-) at a pH of 7.0. In other words, it is a strong hydrogen ion donor. Other strong acids normally found in body fluids include lactic acid, phosphoric acid, sulphuric acid and acetic acid. On the other hand, carbonic acid (H2C03) is a weak acid because it tends to stay in that form at norma! body pH and not dissociate as easily into a hydrogen ion (H+) and bicarbonate ion (HC03 -). An ion with a positive charge is referred to as a cation and an ion with a negative charge is referred to as an anion. The sum of all the charges theoretically must be zero. In other words, for each cation in the body, there must be an associated anion. They may not be in the same place, i.e. the cation in the plasma and the anion inside of a cell, but the body must be electrically “neutral”.
The normal pH of venous blood is 7,4. If the pH is higher than 7.4 then a state of alkalosis is said to exist and if the pH is lower than 7.4, a state of acidosis is said to exist. Certainly, there are varying degrees of each of these conditions and for a given individual, “normal” might be slightly above or below 7.4.
SODIUM, POTASSIUM, CHLORIDE, CALCIUM & MAGNESIUM
Electrolytes are a critical element in cellular metabolism, muscle contraction, nerve transmission and enzyme reactions. Imbalances or deficits lead to impaired athletic performance at best and life-threatening metabolic disruption or death at worst. It is important to realize that the body has no mechanism for storing “extra reserves” of electrolytes. Therefore, while electrolytes are closely regulated by the body, much is lost in the sweat, urine and feces during exercise and hence is an important parameter in monitoring a horse’s ongoing status.
Sodium, Chloride, & Potassium – The electrolyte ions lost to the greatest extent in sweat production, although diarrhea, kidney dysfunction and other pathologies can also be a cause of electrolyte imbalance. Sodium is a primary ion in the body involved in virtually every metabolic process from glucose transport to neural transmission. The body does not store reserves of these electrolytes in tissue (as is the case with some minerals such as calcium), therefore losses during exercise which are not replaced through supplementation or other dietary intake will result in a progressive depletion. Assuming baseline level was within normal ranges, measurements of high serum levels of sodium or chloride during an endurance ride or race usually only reflect recent intake before the kidneys have filtered out and disposed of excess ions in the urine. Low levels indicate depletion and are often a predisposing factor, along with dehydration, in fatigue, muscle cramps, colic, synchronous diaphragmatic flutter (“thumps”), diarrhea and other symptoms of exhausted horse syndrome. Even seemingly normal or high-normal levels may in reality be lower, but appear higher due to concentration secondary to dehydration as measured by total protein and albumin levels. Therefore, levels at the lower end of the normal range should be evaluated relative to concurrent dehydration.
Potassium- High serum levels of potassium during an endurance ride are generally not a concern. These increases often reflect nothing more serious than a delay between blood collection (when potassium is actively sequestered inside cells) and sample measurement (after potassium has had time to “leak” from inside the cells out into the plasma or serum). Decreased levels may be indicative of depletion, changes in acid-base status, fecal losses or renal disease.
Calcium- (total and ionized) – one of the most highly regulated ions in the body, and essential for muscle contraction. Normally, adequate serum levels of ionized calcium (the physiologically active form) will be maintained by mobilizing reserve stores in bone. However, new supplies may not be able to keep up with sweat or urine losses, especially during prolonged exercise under hot conditions, resulting in a progressive depletion of available serum calcium. The availability of ionized calcium can also be affected by changes in acid-base status (i.e., after a long sprint or during hot weather when the horse ‘pants’ excessively to cool himself). It should be noted that low serum ionized calcium is not an indication that the horse is lacking in total calcium in bone stores, simply that the body may be unable to mobilize calcium from bone into the bloodstream quickly enough. As depletion
of ionized calcium, as well as for other electrolytes, progresses, the muscle ceils lose their ability to contract and relax, resulting in thumps, muscle cramps and poor gut motility. Therefore, as with other electrolytes, progressively decreasing levels of ionized calcium throughout a ride may provide hints to explain muscle fatigue, metabolic failure or poor recoveries, as well as avenues of management by which performance may be improved. High serum levels of total calcium are unusual, but if measured in conjunction with abnormal levels of other electrolytes and phosphorus, could possibly be indicative of kidney disease.
Magnesium – Lost only in trace amounts in the sweat during exercise, magnesium is a relatively minor factor in evaluating electrolyte and mineral status in endurance horses. Deficiencies may contribute to development of cramps, thumps and tying up. We believe that magnesium also plays a role In regulating temperament and excitability. Like calcium, blood levels of magnesium do not reflect total body stores. A dietary deficiency is unlikely. Most deficiencies are likely to be secondary to interference of calcium in high-calcium rations (such as those high in alfalfa).
ONE – the destruction of red cells and other tissues from hard work usually greater than the horse is conditioned for.
TWO – reduced drainage and elimination of toxins through the gallbladder/bile duct from liver to gut.
The other obvious problem could, of course, be a combination of both.
Alkaline Phosphatase – An enzyme used to help identify liver dysfunction. Although AP levels alone do not define hepatic problems, when supported by other clinical signs, may help confirm a diagnosis, especially when bile duct obstruction is suspected. AP is also essential in bone remodeling, the process by which bone tissue is continually responding to the stresses of exercise. Increased serum levels are a general indication of osteoblast activity, the cells involved in building new bone tissue(7). Therefore, AP levels will normally be higher in young animals during rapid growth. A follow-up examination is highly recommended for mature animals with high levels of GGTP and/or bilirubin accompanying serum increases of AP.
This enzyme appears in large amounts in liver, spleen, bone, gut-lining and kidneys. In Blood Interpretations, its main use is as an indicator of bone activity, particularly in young horses as an indicator of growth spurts.
In older horses, it is used to indicate excessive bone turnover often associated with arthritis, pedal bruising, or spinal bone changes. In older horses normal levels are 70-100 while in two-year-olds the normal level is 130-170, depending on size. Three-year-olds read out between 120-150 and, again, size and sex need to be taken into account.
AP will also rise in tissue damage to the spleen, gut, lungs, liver and particularly the biliary system (gall bladder and bile duct from liver to gut), so levels need to be compared to other enzymes before full bone activity can be assessed.
Horses with gut problems often have elevated AP with low chloride and low globulin readings, while horses with chronic airway and throat diseases often have elevated AP with raised Monocytes. Both these conditions will have stressed White Cells.
Thus, AP is a reading to be used with others and/or to assess bone functions.
Relating to bone function, AP readings over 200 in two-year-olds are often associated with shin fractures, severe pedal bruising and excessive shin soreness, and frequently indicate a warning to “back off”.
In pacers, raised AP readings can indicate Sessamoiditis and hock problems.
AP is an excellent measure to follow treatment of a leg bone injury or change in training. It is one enzyme that seems to be universally measured in the same way and there is very little, if any, laboratory variation.
A breakdown product of hemoglobin, the molecule which transports oxygen in blood.
High levels may be an indication of liver dysfunction, or may related to hemolysis, the process by which red blood cells are destroyed. As many different processes may cause hemolysis, including toxicity, drugs, immune deficiency and infectious disease, increased bilirubin levels must be considered along with other factors such as GGTP and alkaline phosphatase.
The Bilirubin pigment, which is mostly from old red blood cells and nitrogen metabolism, is a by-product eliminated through the biliary system.
In itself, it does not do any damage but the elimination route is the same as many of the toxic body products, so when the Bilirubin is high so too are a large number of other toxins.
A normal Bilirubin in adult animals is below 25 units and is higher in the very young. It rises and falls with the severity of exercise and the speed with which it falls is a very good indicator of the recovery rate of a horse from work or racing.
Often following a hard race, a horse will have a Bilirubin of 80 but if this falls quickly to say 50 and 30 within the next two days, the horse is recovering well and usually ready for fast work.
If the rate of fall is 80-70-60-50, the horse is unlikely to eat well until the fourth day and will still require two more days to be ready for fast work again.
Horses that try hard often have very high Bilirubin readings and are affected for some time.
Great racehorse competitors are often 100 after a race and take from ten to fourteen days to fully recover before they should be worked hard again. This means racing “underdone”
which produced more toxins on race day and therefore creates an ongoing circle.
As a rule of thumb, a Bilirubin over 50 will stop most fillies and mares from eating well and over 80 will slow or stop colts and geldings.
This enzyme is found in several tissues but is most significant in liver and muscle in the horse.
If has a long half-life which means the number you get reflects muscle changes over several days and sometimes weeks. AST levels rise in early preparation work and when young horses start working and generally level out at about twelve to fourteen weeks.
Normal levels are between 300-360iu but are usually up after racing. Levels up to 500 normally indicate increased muscle turnover from building up or minor muscle damage from lameness … examples of sore gluteal muscles with a sore knee or fetlock are likely to cause some moderate rises.
Levels up to 50 000 are not uncommon in full-blown “tying-up” episodes, often evident in two-year-old colts or three-year-old fillies.
Remember though, if a “tying-up” episode is acute, the AST from that episode may be normal when tested close to the event and rise over several days often when the horse is returning to normal. So, in effect AST is an overall long-term assessment of muscle functions.
LOW readings of AST – These are readings below 300 and are associated with either under work or under effort on the part of the horse, relative to their current stage of fitness.
LDH- (Lactate Dehydrogenase) Another enzyme released by both cardiac and skeletal muscle cells during stress.
Although LDH levels are used to diagnose cardiac disease primarily, higher levels without signs of heart trouble are normally due to its release from skeletal muscles.
As with the other muscle enzymes like CPK and AST, increased levels may only indicate that the horse has undergone intense exercise, without indicating damage. Interpretation of enzyme results should include consideration of other clinical signs such as muscle pain or myoglobin in the urine, as well as the horse’s clinical history. Likewise, clinical signs similar to tying-up without increases in enzyme levels may signal other diseases such as iaminitis, colic or kidney disease.
GGTP – GGT- Refers to gamma glutamyltransferase, an enzyme involved with liver function. GGTP levels, along with bilirubin, are used to indicate the presence of liver damage or disease. Some enzyme levels may increase for variety of reasons, but GGTP is indicative of liver function only, increases of this enzyme above the normal range of 3-30
lU/liter during or after heavy equine exercise and with increases in bilirubin and alkaline phosphatase, may indicate liver disease such as an obstruction of the bile duct. If your lab report indicates significantly increased levels, follow-up tests by your regular veterinarian is highly recommended.
The enzyme is sensitive to mild liver damage and levels increase from 60-150 often after intake of drugs or some nutrient changes. A bleeding reduction supplement and summer weeds will both push GGT over 100 and if acute liver disease is present, GGT readings will rise from 1000 to 10 000 but these causes (which include biliary obstruction, cancer, liver fluke, bacterial/viral hepatitis) are those reflected in severe liver disease and are reasonably rare in racing-age horses.
Again, as with AST, a GGT reading below 20 will indicate an under active liver. There seems to be a condition called Flat Liver Syndrome where the liver activity is below the level required to allow maximum improvement in fitness. This is commonly seen in horses after long spells or after grazing for periods on poor-quality, high-weed pastures. It appears the liver does not “tone up” to the workload of training in the way the muscles, bones and respiratory system do. Horses with this problem respond well to high levels of B-Group Vitamins.
This is a very accurate measure of current muscle activity and so is a good indicator, for example, of what happened in the morning workout.
Levels are usually around 80-120 and can double without a problem after work. Higher levels, for example, 500-700 in non- or light-working horses can indicate cardiac muscle damage. CPK is mostly used when checking on “tying-up” episodes, where levels up to 100 000 are not uncommon and, as CPK rises and falls very quickly, it can be used to closely monitor such episodes and assess progress. There are about many causes of “typing-up” and the above enzyme can prove valuable in assessing and treating various ones.
CPK levels rise and fall through larger ranges earlier in a preparation and tighten into a narrower range when the horse is attaining good fitness levels for the racing required.
AST / SGOT- Refers to aspartate aminotransferase, an enzyme released by both skeletal and cardiac muscle, as well as the liver as the result of protein metabolism. As with CK, AST levels may rise significantly because of prolonged exercise without necessarily indicating damage. AST levels rise more slowly, and remain in the blood for a
longer period, than do CK levels.
Elevated ASTs in a horse with normal CK would suggest that the horse has undergone intense muscular stress sometime during the prior week. High AST and CK levels in a horse that has not recently exercised at an intense or prolonged level may indicate an ongoing disease process occurring in the muscles. High AST levels in a horse that has not exercised recently, without a concurrent increase in CK levels, may be indicative of liver disease.
CPK, LDH & SGOT / AST- Levels of specific enzymes help indicate the presence of muscle injury or disease, its severity and progression.
Measured blood levels of enzymes, along with the observation of other clinical signs, such as lameness, pain or dark urine, help tell you whether, when and to what extent muscle damage has occurred. It is extremely important to consider whether any increased enzyme levels were measured before, during or after exercise; as well as whether any other stressful events may have contributed to results. High enzyme levels after hard work are not necessarily the voice of doom in predicting muscle damage. In some cases, however, obtaining a consultation with your veterinarian and possibly a follow-up blood AST to measure increases or decreases is often a good idea. This is especially true if signs of muscle damage are present, such as muscle stiffness or pain, signs of colic or dark urine during or after exercise.
CPK- Refers to Creatine Kinase or Creatine Phosphokinase, a muscle enzyme produced during exercise.
While horses suffering from exertional rhabdomyolysis (tying up) will demonstrate increased levels, other studies have shown that prolonged endurance exercise can result in very high levels (> 30,000 lU/liter) without signs of clinical muscle damage. An elevated level during or following an endurance ride (or other stressful event) indicates the horse has had a long, hard day, but should not necessarily be interpreted as “muscle damage” without considering other clinical signs such as muscle pain, stiffness or dark-colored urine. Elevated levels in a resting horse that has not exercised intensively for several days, however, may indicate disease such as infection, dehydration, electrolyte imbalances or chronic rhabdomyolysis.
How to Read Blood in Race Horses | HorsePreRace | 
January 21, 2010 6:04 am | 
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