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The Pride Project
How To Speak CBC in One Easy Lesson
Susan Garlinghouse, MS; Barney Fleming, DVM
You signed up for the project, rode the miles and held the horse at vet
checks while blood was drawn. A few weeks later, you received a confusing
packet of mysterious numbers that might just as well be Superbowl statistics
as blood chemistry results. How are you supposed to translate this intouseable information?
The basic purpose of measuring blood biochemistry is to assess the current
status of the body---how well different substrates are being produced and
utilized, whether by-products are being efficiently eliminated, whether the
different organ systems are doing their job at the right time and at the
correct rate. Blood panels will also often give indicators that something
isn't quite right long before outward signs appear. Does the horse have an
infection brewing somewhere? Dehydrated? Are the muscles or kidneys having
trouble keeping up with the workload? Is the liver damaged?
While this guide should in no way replace the services of a veterinarian, or
provides a complete explanation of each parameter, it may help the owner in
understanding why each are measured and what abnormal results might suggest.
MUSCLE ENZYMES - CPK, LDH, SGOT/AST
Levels of specific enzymes help indicate the presence of muscle injury or
disease, its severity and progression. Measured blood levels, along with
the observation of other clinical signs, such as lameness, pain or dark
urine, help tell the veterinarian 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 (a ride last weekend, an
unplanned midnight gallop through camp, even recent vaccinations) may have
contributed to chemistry results. High---even very high---enzyme levels
after a long ride 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 panel to measure subsequent
increases or decreases is often a good idea.
CPK - refers to creatine kinase (also called CK), 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 IU/liter) without signs of clinical muscle damage(1). Distance
and intensity of exercise are significant factors, so that one horse who
completes slow 100 miles may have higher CK levels than an equally fit horse
who finishes a brisk 50, with neither suffering clinical 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. Elevated levels in a resting horse that has not exercised
intensively for several days, however, may indicate disease such as
infection, electrolyte imbalances or chronic rhabdomyolysis(2).
AST/SGOT - refers to aspartate aminotransferase (the SGOT refers to an
earlier term for the same enzyme), 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 as a result of prolonged
exercise without necessarily indicating damage(1,3). AST levels rise more
slowly, and remain 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(2,4).
LDH - refers to lactate dehydrogenase, yet another enzyme released by
both cardiac and skeletal muscle cells during stress. Although LDH levels
are used to diagnose cardiac disease, elevated levels without other
characteristic signs of heart trouble are almost certainly due to itsrelease from skeletal muscle(2). As with the other muscle enzymes,
increased levels may only indicate that the horse has undergone an intense
and prolonged bout of exercise, without necessarily 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 immediate and past clinical history. Likewise, clinical signs
similar to tying-up without concurrent increases in enzyme levels may be
indicative of other diseases such as laminitis or colic.
LIVER FUNCTION - GGTP, BILIRUBIN, ALKALINE PHOSPHATASE
GGTP - 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. As seen in the muscle enzyme
discussion, some enzyme levels may increase for variety of reasons, but GGTP
is indicative of liver function only---not cardiac or skeletal muscle
activity. Therefore, increases above the normal range of 3-30 IU/liter
during or after an endurance ride, possibly concurrent with increases in
bilirubin and alkaline phosphatase, are not due simply to prolonged
exercise, but may indicate liver disease such as an obstruction of the bile
duct(5). If your lab report indicates significantly increased levels, a
follow-up examination by your regular veterinarian is strongly
recommended.
Bilirubin - 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(6),
increased bilirubin levels must be considered along with other factors such
as GGTP and alkaline phosphatase.
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(6). 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).
When high levels of GGTP and/or bilirubin accompany serum increases of AP, afollow-up examination for liver dysfunction is very highly recommended.
ELECTROLYTES - SODIUM, POTASSIUM, CHLORIDE, CALCIUM
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 its worst. While electrolytes are closely
regulated by the body, many are lost in the sweat or otherwise depleted
during exercise and hence are an important parameter in monitoring a horse's
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. Measurements of high serum levels of sodium or chloride almost
always 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 muscle cramps,
colic, synchronous diaphragmatic flutter ("thumps"), diarrhea and other
symptoms of exhausted horse syndrome(8). High serum levels of potassium
generally not a concern and 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).
Calcium - 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, supply 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. It should be noted that low serum calcium is not an indication
that the horse is lacking in calcium in bone stores, simply that the body
may be unable to mobilize calcium from bone into the bloodstream quickly
enough(7). As depletion of calcium, as well as for other electrolytes,
progresses, the muscle cells lose their ability to contract and relax,
resulting in thumps, muscle cramps and poor gut motility(8,9). Therefore,
as with other electrolytes, progressively decreasing levels of calcium
throughout a ride may provide hints to explain muscle fatigue or poor
recoveries, as well as avenues of management by which performance may be
improved. High serum levels of calcium are unusual, but if measured in
conjunction with abnormal levels of other electrolytes and phosphorus, could
possibly be indicative of kidney disease.
KIDNEY FUNCTION AND HYDRATION STATUS - BUN, CREATININE, PHOSPHORUS, TOTALPROTEIN, ALBUMIN AND HEMATOCRIT
BUN, creatinine - BUN refers to blood urea nitrogen, a nitrogenous waste
by-product of protein metabolism, which is filtered out and excreted almost
exclusively by the kidneys. Creatinine is a normal by-product of muscle
metabolism that is also cleared exclusively by the kidneys. Both of these
substances are normally present in measurable quantities in the blood, and
increased levels, usually in relationship to each other, are used to assess
kidney function. However, it is important not to see increased levels and
automatically assume kidney damage. Increased urea alone may simply be in
response to a recent meal high in protein, such as a rich alfalfa hay, and
the body's attempts to excrete excess nitrogen(9). Increased creatinine
levels may indicate the onset of exertional rhabdomyolysis (tying-up), but
may also occur during dehydration. The two substances are often related to
each other by dividing BUN levels by the creatinine level. A ratio of
approximately 10:1 is normal, while ratios of 20-30:1 are often seen in
horses after a high-protein meal. A normal ratio, increased levels of both
BUN and creatinine, and increased levels of albumin and total protein would
generally indicate dehydration. Increased BUN/creatinine, with normal
protein levels might indicate impaired kidney function(11).
Phosphorus - Unlike sodium, chloride, potassium and calcium,
phosphorus is not lost through sweating and thus does not require
supplementation during a ride. Like calcium, serum phosphorus levels are a
poor indicator of the total phosphorus content of the body(7). High serum
phosphorus is often used to help diagnose renal (kidney) failure in mature
horses. In young horses, high serum phosphorus is nothing more than an
indication of rapid bone growth.
Total protein, albumin - Measuring the amount of protein in the serum
provides an index of hydration status, as well as indices of infection,
inflammation, increased protein loss or decreased protein production.
Increased protein levels in athletic horses are usually due to dehydration,
although can also be due to increased levels of globulins (antibodies to
fight infection). Low protein levels are unusual, but if detected, should
be further investigated to rule out possible sites of protein loss, such as
via the kidneys or gastrointestinal tract due to inflammation or
parasites(11).
Hematocrit - a measurement of the relative amount of red blood cells
present in a blood sample. After blood is drawn, a small tube is filled and
centrifuged to separate the heavier blood cells from the lighter white blood
cells and the even lighter fluid (plasma or serum) portion. A higher than
normal reading generally indicates dehydration (same number of cells in less
plasma volume). A low reading may indicate anemia, though not
invariably(4).
ACID-BASE STATUS - pH, TCO2, HCO3
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. The lower the pH, the more acidic a
solution is; the higher the pH, the less acidic (and therefore more alkaline
or "basic"). The normal pH range of blood is between 7.32 - 7.4412. 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 acids of muscle
metabolism are accumulating faster than the body can dispose of them. The
lower the pH, the further the horse has been pushed beyond his limits. 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 an endurance 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). The normal TCO2 (total carbon dioxide) concentration is 28
mEq/liter, and also contributes to the 'big picture' of acid-base status.
TCO2 levels of 20-27 mEq/liter indicate a mild acidosis as described
above; TCO2 of less than 20 mEq/liter indicate severe, possibly
life-threatening, acidosis(12).
HCO3 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(12).
OXYGEN TRANSPORT SYSTEMS - pO2, pCO2, sO2, Hb
During strenuous exercise, the amount of oxygen inspired is not nearly as
important as the amount that actually reaches the tissues. Various forces
and barriers have an effect on this delivery system, including infections or
obstructions that compromise respiratory function; dehydration, which
thickens the blood and forces the heart to work harder to circulate it; or
anemia, which results in fewer red blood cells to actually transport oxygen
and carbon dioxide.
pO2 and pCO2 represent the amount of dissolved oxygen and carbon
dioxide circulating in the bloodstream. "Normal" levels of each vary
depending on the fitness of the individual horse, but levels of
approximately 39 mmHg and 47 mmHg for oxygen and carbon dioxide,
respectively, would be considered normal for average, healthy horses.
Oxygen levels higher than this might be one indication of a horse who is
aerobically very fit. Low levels of pO2 might indicate some barrier
preventing adequate movement of oxygen from the lungs into the
bloodstream---for example, respiratory infection, partial paralysis of the
larynx (often seen in thoroughbred racehorses) or even horses bred for
'teacup' muzzles and accompanying small nostrils. Decreased oxygen levels
would be perfectly normal at high altitudes, where lower atmospheric
pressure is available to help drive oxygen across respiratory membranes. As
intensity of exercise increases, circulating oxygen tends to decrease, while
CO2 tends to increase. pO2 levels between 30 - 16 mmHg, and pCO2 levels of
50 -96 mmHg, respectively, as speed increased from a slow trot to a fast
gallop would not be abnormal13. Observing relative levels before, during
and after a ride gives a good indication of how aerobically stressed the
horse was at this intensity of exercise.
Hb refers to hemoglobin, a component of red blood cells which actively
binds and transports oxygen from the lungs to the peripheral tissues.
Normal levels in a healthy horse are between 10 - 18 g/dl4. Low hemoglobin
levels, along with a low hematocrit, might indicate anemia, a decrease in
the number of circulating red blood cells.
sO2 levels represent a measurement of how many of the available
hemoglobin molecules are currently involved in transporting oxygen. For
example, an sO2 level of 78 would indicate that 78% of the available
hemoglobin is being utilized to transport oxygen and that the horse is still
exercising at less than his maximum aerobic capacity. Horses with low
hemoglobin levels could usually be expected to have higher sO2 levels during
exercise (and therefore a reduced aerobic capacity), simply because a larger
proportion of available hemoglobin are being utilized to transport oxygen.
IMMUNE FUNCTION - WBC, POLYS, BANDS, LYMPHS, MONOS, EOS
Collectively known as white blood cells or leukocytes, measurement of these
parameters evaluate the presence of infection, inflammation and ongoing
systemic disease processes in the body.
WBC - a total count of all types of white cells (eosinophils,
basophils, neutrophils, etc), this count is made so that relative
proportions of its subunit cells can be calculated. It also provides a
general indication of normal balance between cell production in bone marrow
and tissue uptake.
Poly % - refers to the proportion of polychromatophilic (immature) red
blood cells present. Normal range for horses is between 30-65%. A high
proportion of polys indicate that there is an increased demand for red blood
cells that the bone marrow is unable to keep up with---therefore, the
release of cells into circulation prior to maturation in the bone marrow.
Causes for abnormal increases in red blood cells demand might include blood
loss or disease processes causing dysfunctional production or destruction ofcells.
Bands % - refers to the relative proportion of banded neutrophils.
Neutrophils in general are the predominant circulating white blood cell
whose function is to seek out, ingest and kill invading microorganisms, such
as bacteria. Mature neutrophils are referred to as "segmented", immature
neutrophils are referred to as "banded". A range of 0 - 1% of banded
neutrophils being present in the total white blood cell is considered
normal. Increased levels indicate acute inflammation that have stimulated
the bone marrow to release large numbers of neutrophils, including those not
yet mature.
Lymphs - refers to the relative proportion of lymphocytes, also known
as antibodies. Unlike neutrophils, which attack a broad spectrum of
invading microbes, lymphocyte differentiate into specialized cells that
attack and destroy very specific infecting antigens. Normal range for the
horse if between 25 - 70%. Low levels are often an indication of a "stress
leukogram", in which chronic stress results in suppression of the immune
system. This condition can also be the result of steroid use.
Monos - refers to monocytes, an immature stage of macrophage cells.
Like neutrophils, macrophages attack and engulf foreign bacteria, but are
also the "clean-up cells" which remove dead tissue wherever damage has
occurred, such as a healing wound site. The normal range for horses is
between 1 - 5% of the total WBC. Large numbers of circulating monocytes aregenerally an indication of an increased demand for macrophages, as might be
the case following injury.
Eos - refers to eosinophils, yet another type of leukocyte with
functions similar to those of neutrophils. Eosinophils have a role in the
inflammatory response, such as swelling, redness and pain following injury.
In addition, they have the major function of control of parasitic control,
in that they attack and damage parasites circulating in the bloodstream
(such as roundworms during their migratory phase). Normal ranges are
between 0 - 11% of the total WBC. Increased levels may be an indication of
infection by some species of parasites, or of inflammation in the body, such
as gastroenteritis.
GLUCOSE -
Blood glucose levels, and the manipulation thereof, is probably one of the
most controversial subjects in endurance horse management. While horses
exercising at typical endurance speeds rely primarily on the oxidation of
fatty acids for energy production, a certain amount of glucose is always
required for certain metabolic pathways and by certain vital organs. The
brain, for example, is unable to utilize any substrate other than
glucose. At the same time, the animal body is able to store relatively
small amounts in muscle and liver tissue, and its depletion during exercise
is a major factor in fatigue. Normal levels for a horse with a "full gas
tank" range between 69 - 122 mg/dl. As adrenalin also raises blood glucose
levels, levels measured in excited horses might normally be at the high end
of the normal range.
In other species, very high glucose levels would often indicate diabetes.
However, diabetes is extremely rare in horses, and very high levels of blood
glucose would generally indicate recent extreme dietary manipulations. Low
levels below the normal range may indicate several conditions---if measured
fairly soon after the above mentioned glucose "spike", the result may be an
"insulin rebound", wherein large amount of insulin are released to sequester
the excess glucose, resulting in dramatically lowered glucose levels. If
low glucose levels are measured during or after sustained, strenuous
exercise, it is more likely due to glycogen depletion, in which the body is
rapidly reaching the end of its available glucose stores.
REFERENCES
1. Kerr MG, Snow DH; Plasma enzyme activities in endurance horses. In Snow
DH, Persson SGB, Rose RJ (eds): Equine Exercise Physiology. Cambridge,
Granta Edition, 1983, pg 432.
2. Kobluk CN, Ames TR, Geor RJ (eds); Clinical evaluation of muscle and
muscular disorders. In The Horse, Diseases and Clinical Management. WB
Saunders, Philadelphia, 1995, pg 1314-1318.
3. Rose RJ, Hodgson DR, Sampson D, et al; Changes in plasma biochemistry in
horses competing in a 160-km endurance ride. Austr Vet J 1983; 60:101.
4. Aiello, SE (ed): Diagnostic procedures for the practice laboratory. In
Merck Veterinary Manual, Eighth edition. Merck & Company, New Jersey.
1998. Pg. 1196-1197; 1205.
5. Trent AM, Bailey JV; Bovine peritoneum: fibrinolytic activity and
adhesion formation. 1986. Am J Vet Res 47:653.
6. Boulton E; Liver and biliary system and pancreas. In Kobluk CN, AmesTR, Geor RJ (eds); The Horse, Diseases and Clinical Management. WB
Saunders, Philadelphia, 1995, pg 364-367; 1080-1081.
7. Dalin G, Jeffcott
LB; Biomechanics, Gait and Conformation. In Hodgson DR, Rose RJ (eds): The
Athletic Horse, Principles and Practice of Equine Sports Medicine. WB
Saunders, Philadelphia, 1994, pg.33.
8. McConaghy F; Thermoregulation. In Hodgson DR, Rose RJ (eds): The
Athletic Horse, Principles and Practice of Equine Sports Medicine. WB
Saunders, Philadelphia, 1994, pg. 181-195.
9. Pringle J: Pathophysiology and diagnosis of urinary disease. In Kobluk
CN, Ames TR, Geor RJ (eds); The Horse, Diseases and Clinical Management. WB
Saunders, Philadelphia, 1995, pg 578-587.
10. Ehnene SJ, Divers TJ, Gillette D, Reef VB; Obstructive nephrolithiasis
and uretolithiasis associated with chronic renal failure in horses: eight
cases (1981-1987). 1990. J Am Vet Med Asoc 197:249.
11. Rose RJ, Hodgson DR; Hematology and biochemistry. In Hodgson DR, Rose
RJ (eds): The Athletic Horse, Principles and Practice of Equine Sports
Medicine. WB Saunders, Philadelphia, 1994, pg.64-75.
12. Holbrook, TC, Eades SC: Principles of Drug and Fluid Therapy. In
Kobluk CN, Ames TR, Geor RJ (eds); The Horse, Diseases and Clinical
Management. WB Saunders, Philadelphia, 1995, pg . 35-40.
13. Taylor AE, Rehder K, Hyatt RE, et al: Clinical Respiratory Physiology.
WB Saunders, Philadelphia, 1989.
Copyright Susan Garlinghouse, 2000. All rights reserved.
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