Hematologic Responses to Exercise and Training

Symposium on Exercise Physiology

Hematologic Responses to Exercise and Training Reuben]. Rose, B.V.Se., Ph.D., F.R.C.V.S., M.A.C.V.Se., F.A.C.B.S.,* and ] ennifer R. Allen, B. V. Se. t

Because the examination of hematology has always been a relatively accessible diagnostic aid to the veterinarian in practice, it has been widely used in attempts to provide information about disease states, performance problems, and fitness in performance horses. The emphasis placed on the hemogram has been based on the relationship between erythrocyte numbers and oxygen transport. The reasoning has been that if a horse has a low erythrocyte count, then it will not be able to transport oxygen to the working muscle as efficiently as a horse with a high erythrocyte count. Although this may be true in extreme cases, it must be remembered that the erythrocyte is only one part of a complex chain of oxygen transport. Conditions such as partial obstructions of the upper respiratory tract and pulmonary disease can prevent sufficient oxygen from reaching the alveoli; decreases in blood pH or increases in temperature or concentration of 2,3 diphosphoglycerate (DPG) will shift the oxyhemoglobin dissociation to the right, thus lowering the degree of oxygen saturation, even though the total erythrocyte numbers may be unchanged. Finally, at the level of the working muscle, where the majority of blood flow is diverted during exercise, the degree of capillary development will affect the amount of oxygen that can be taken up by the tissues. Thus, it is obvious that care must be taken when interpreting the results of equine hematology, particularly in the analysis of something as nonspecific as reduced performance. The aim of this article is to review some of the hematologic responses to exercise and training. With this background, the results of hematology performed in the investigation of performance prob-

* Associate

Professor, Department of Veterinary Clinical Studies, Equine Exercise Physiology Laboratory, University of Sydney, Sydney, Australia t Resident, Department of Veterinary Medicine and Surgery, Washington State University College of Veterinary Medicine, Pulhnan, Washington

Veterinary Clinics of North America: Equine Practice-Vol. 1, No.3, December 1985

461

462

REUBEN

J.

ROSE AND JENNIFER R. ALLEN

lems or on a routine basis in horses in training can be interpreted with more precision.

METHODOLOGY Since the 1960s, the use of automated cell-counting techniques (for example, Coulter Counter, Coulter Electronics) has permitted the accurancy of total erythrocyte and leukocyte counts to be ilnproved. Stewart and Steel44 exalnined the repeatability of measurements of hemoglobin (Rb), hematocrit (PCV), erythrocyte count (RBC), and total leukocyte count (WBC) using 36 duplicate blood samples. They found that the precision of Ineasurement was ± 5 per cent; therefore, changes in excess of this would have to be found on repeated measurements from an individual horse before any significance could be attributed. Differential leukocyte counts have usually been perforlned manually, using a battlement technique. 36 The accuracy of this technique has been questioned by Allen, 1 who developed an automated method using electronic volume analysis for Ineasuring the total neutrophils, lymphocytes, and eosinophils. In addition, Allen found that the use of the normal blood smear technique produced a falsely low value for the percentage of monocytes, as these cells tended to aggregate to the edge of the smear. For this reason, he advocated the use of spinner slides 49 to prepare smears, ratl1er tl1an the standard techniques performed by hand. When assessing resting hematology, the technique of collection, attitude of the horse in relationsl1ip to feeding, and time of day are important variables that can influence results. For this reason, it is important that the standardization of collection techniques be achieved wherever possible. The simplest technique for collection of equine blood salnples is the use of evacuated bl90d collection tubes (for example, Vacutainer*). Althougl1 tl1is technique has been questioned by Archer4 as one in which the vaCUUln could damage the erytl1rocytes and give false values, this has not been found to be a practical probleln. 14 Storage of blood salnples overnight, prior to analysis, will cause a slight elevation of PCV and MCR, probably due to slight enlargement of the erythrocytes. 45 The demeanor of the horse during blood salnple collection can have an important affect on both the erythrocyte and leukocyte values. 5,13,16 Stewart and colleagues 45 classified horses as being either placid (stood still and remained placid), tilnid (stood stiff, but a forceful jugular pulse and elevated heart rate noted), apprehensive (horse pulled back during venipuncture), or excited (resisted venipuncture and moved about forcefully). When 32 Thoroughbred

* Becton-Dickinson,

Cockeysville, Maryland.

463

HEMATOLOGIC RESPONSES TO EXERCISE AND TRAINING

horses were classifi~d according to these categories, it was found that only the horses classed as excited had elevated erythrocyte and leukocyte counts, when compared to the placid group (Fig. 1). The fact that the timid and apprehensive groups did not have changes in their hemograms was probably due to the blood samples being collected within 30 seconds of entering the stable. Using 51Cr-Iabelled erythrocytes and scanning the splenic area with a scintillation detector, Persson and colleagues 25 found that erythrocytes were mobilized from the spleen 30 to 60 seconds after the intravenous injection of adrenaline. Thus, it would appear that, provided a blood sample for a hemogram can be obtained rapidly, tIle demeanor of the horse should not affect the result. However, Rose and Hodgson 33 found that an apprehensive group of endurance horses had PCVs that were significantly higher than those of a quiet group of horses studied over a 12-week training period (see Fig. 1). Therefore, it is important to note the attitude of the horse during blood collection so that the results can be interpreted correctly. The other major factors affecting the helllogram are the time of day that the blood sample is collected and the relationship to previous

70 5. EXCITEMENT

60

4.

~

-

0

DEMEANOUR

3.

50 FEEDING

> u a.

1.

2.

40

30 pre post

a

A

PTA E

rest pre- ~.~. race adrenaline

Figure 1. Resting packed cell volulne in horses.l, Data froln Kerr and Snow. 15 pre before feeding; post = 1 hour after feeding hay. 2, Data froln Rose and Hodgson. 33 Endurance horses: Q = quiet group; A = apprehensive group. 3, Data froln Stewart et al. 45 Thoroughbreds: P = placid; T = tilnid; A = apprehensive; E = excited. 4, Data froln Revington. 3o Thoroughbreds: Rest = in stable, day before race; pre-race = at racetrack, ilnn1ediately before race. 5, Data froln Irvine. 13 pre = before adrenaline; post = after adrenaline. =

464

REUBEN

J.

ROSE AND JENNIFER R. ALLEN

exercise. When Thoroughbred horses remained resting in their stables, Allen and Powel13 found no significant changes in hemograms from the same horses, in blood samples collected either at 8:30 AM or 4:00 PM, apart from a slightly higher total lymphocyte count in the afternoon. However, when horses were exercised following the 8:30 AM blood samples, the 4:00 PM hemograms showed a higher total leukocyte count and increased neutrophil:lymphocyte (N :L) ratio. This was probably due to a change in leukocyte population following exercise and response to increased plasma cortisol. 35 Also important when examining resting hematology is the time of blood sample collection in relation to feeding. Kerr and Snow15 found that feeding of hay produced a 15.8 per cent increase in resting PCV within 1 hour, and this remained elevated for several hours (see Fig. 1).

THE NORMAL RESTING HEMOGRAM Although a number of breed differences have been reported for the resting hemogram (Table 1), most breeds and classes of performance horses will fit within tIle normal range found for the young Thoroughbred 3,45 as presented in Table 2. However, as horses increase in age, the MCV will become elevated. In general, Thoroughbreds have resting erythrocyte indices that are higher than those of Standardbred trotters and pacers or endurance horses. Although most performance horses will have hemograms that fall within the ranges quoted in Table 2, Stewart and Steel44 reported that Thoroughbred horses with hemograms falling more than one standard deviation below the mean did not win races on metropolitan racecourses. This observation followed an earlier report by Steel and Whitlock42 that the best racehorses had hemograms that fell inside narrow limits within the normal range for the breed. Similar conclusions, postulating a red cell "threshold:>:> associated with optimal performance, were reached by a number of other research workers. 8,18,41 A more complex study undertaken by Blackmore 7 examined the relationship between performance and a variety of hematologic and biochemical measurements. Retrospective analysis showed that there was a positive correlation between RBC, plasma sodium, and performance. Individual horses with higher RBC and plasma sodium values tended to have a higher Timeform rating (a score based on previous racing form). However, at least six estimates of an individual horse:> s RBC and sodium were necessary to establish its normal values. There was no correlation between a single observation ofRBC and plasma sodium and the horse:>s Timeform rating. Great care is needed when attempting to use the resting hemogram in any predictive fashion, however. Owing to the variable contribution of splenic erythrocytes to the circulating pool, the resting values for PCV, RBC, and Hb can be quite labile. Persson 21 ,22,23 examined the resting hemoglobin values in three Standardbred horses, with daily blood sam-

Table 1.

=t

Normal Hematologic Values (Mean or Mean ± SD) Reported for Adult Horses at Rest

tr1

3:

BREED AND TRAINING STATE

(X

RBC 106/MM)

HB (GM/100 ML)

PCV (0/0)

WBC (x 103/MM)

~

o t"'" o o

n

Thoroughbreds Macleod and Ponder 17 2 and 3 year olds More than 3 years old

~

10.8 11.6

tr1

14.1 15.4

CJ)

~

o z CJ)

Irvine 13 2 years old, not in training More than 3 years old, not in training 2 years old, in training More than 3 years old, in training

tr1

CJ)

8.1 7.4 6.7

12.5 13.4 11. 7 11.4

~

o 43.3 39.0 36.0

t-rj

:x tr1 ::0

o CJ)

tr1

Archer and Miller6 In fraining

9.5 ± 1.1

14.7 ± 0.9

41.2 ± 6.6

8.4 ± 2.2

Steel and Whitlock42 In training Sykes 46 2 years old, in training 2 years old, in training 2 years old, in training 3 years old, in training 3 years old, in training 3 years old, in training More than 4 years old, More than 4 years old, More than 4 years old,

less than 1 rno 3-6 rno more than 6 rno less than 1 rno 3-6 rno more than 6 rno in training less than 1 rno in training 3-6 rno in training more than 6 rno

6.8

13.9

43.0

9.7 ± 1.3

13.4 ± 1.9

41. 7 ± 4.5

10.2 11.0 11.1 10.5 11.0 11.0 10.6 10.9 10.9

13.6 15.3 15.5 14.5 15.7 15.6 14.8 15.1 15.2

40.4 45.8 45.7 42.9 46.7 46.2 44.0 44.8 45.1

o

....,

Brenon8 In training

> Z

~

z zo 10.4

....

~ ~

~ ~ ~

Continued

Table 1.

RBC BREED AND TRAINING STATE

(x 106/MM)

Tasker 47 In training Stewart, Clarkson, and Steel43 In training Allen and Archer2 and Archer4 2 years old 3 years old 4 years old More than 4 years old

10.3 ± 1.5

9.9 9.7 9.3 8.8

± ± ± ±

1.0 1.1 1.0 1.1

WBC (x 103/MM)

HB (GM/100 ML)

PCV (%)

14.5 ± 1.1

40.4 ± 3.8

15.7 ± 1.8

40.4 ± 4.6

14.6 15.1 15.0 14.6

39.9 41.4 40.8 39.8

± ± ± ±

1.4 1.5 1.7 1.6

± ± ± ±

4.0 4.2 4.7 4.7

~

t!j

c: t:::O t!j

Stewart and Steel 44 In training

9.5 ± 1.3

15.0 ± 2.0

40.0 ± 5.5

Schalm et al. 44

9.6 ± 1.1

15.2 ± 1.4

43.6 ± 3.9

Z ~

9.8 ± 1.4

~

oen t!j

Stewart, Riddle, and Sahnon 45 In training

;>

9.1 ± 1.0

14.2 ± 1.4

40.3 ± 4.0

8.4 ± 1.2

z

u ~

t!j

Allen and PowelP Before training After 5 months of training

9.2 ± 0.8 10.2 ± 1.2

13.6 ± 1.0 15.2 ± 1.7

37.0 ± 2.0 41.0 ± 4.0

9.8 ± 1.3 9.6 ± 1.1

z z

~

t!j

::0

~

> t"'" t"'"

t!j

Z

Revington 3O Racing

=t 9.6 ± 0.9

15.1 ± 1.0

41.9 ± 3.1

8.9 ± 1.3

~

> ~

Standardbreds Steel and Whitlock42

~

8.7 ± 1.4

12.4 ± 1.9

39.4 ± 4.4

14.9 ± 1.5

38.7 ± 3.7

9.8

0 ~ 0 CJ

()

Tasker47

~ ~

rJ)

Schalm et al. 36

8.3 ± 0.7

13.7 ± 0.9

39.3 ± 2.5

7.9 ± 1.0

8.4 ± 1.2

13.8 ± 2.1

39.3 ± 5.0

9.5 ± 2.3

13.9 ± 2.2

37.6 ± 5.0

13.8 ± 1.7

40.0 ± 5.0

13.2 ± 1.6

37.1 ± 4.7

~

Arabian Schalm et al. 36

~

0 Z rJ) rJ)

Quarter Horse Tasker47

~

0 trj

>< ~

::0

0 rJ)

Schalm et al. 36

9.1 ± 1.4

9.7 ± 1.3

'0

"Equitation and Polo Horses" Tasker47

~

~

Endurance Horses Carlson 9

7.3

Carlson et al. 10 Rose 31

~

> Z

34.7

7.7

35.9 ± 2.6

7.5 ± 1.2

7.9 ± 0.5

13.0 ± 1.1

37.0 ± 2.6

8.8 ± 1.9

7.5

11.5

35.0

8.5

Z Z CJ

Cold-Blooded Breeds Schalm et al. 36

~ ~

.....:t

468

REUBEN

J.

ROSE AND JENNIFER R. ALLEN

Values for the Normal Resting Hemogram in the Adult Thoroughbred Horse

Table 2. VALUE

NORMAL RANGE

MEAN

7.0-11.0 11.0-17.0 32.0-48.0 42.0-47.0 33.0-38.0 14.0-17.0 6.0-11.0 2.5-6.5 2.0-5.5 0.2-0.8 0.1-0.4

9.0 14.0 40.0 44.0 35.0 15.5 8.5 4.5 3.5 0.5 0.2

RBC (x 106/mln 3) Hb (gIn/100 Inl) PCV(%) MCV (cu) MCHC (gm/100 Inl) IvlCH (pg) WBC (x 103hnm 3) Neutrophils Lymphocytes Monocytes Eosinophils

pIes collected for 7 days. He reported up to a 30 per cent variation in the resting Hb values and warned that the resting Hb values provided no useful indication of the total body Hb (Fig. 2). However, duplicate blood salnples collected after exercise or adrenaline administration showed excellent correlations of O. 98 and 0.99, respectively, indicating tIlat good repeatability was possible after mobilization of the splenic erythrocyte pool. In studies of Thoroughbred racehorses, both Laufenstein-Duffy 16 and Revington 30 found no significant correlation between the pev prior to racing and subsequent racing performance. Although this is undoubtedly true, it is worthwhile noting that on repeated hemograms there is little variation in individual results when blood salnples are taken under standard conditions. 4,7 Thus, the norlnal range for tIle individual horse is Inuch narrower than that for tIle group.

Persson, 1975 o ,

22 18

I

.. . . . • • ••

:J 16 ~

~14 ~12

10

. . ,

,

,,1·

,I'

'

'

18

.. .....

16

'

•

• ••

14

.',;

12

.-

20

,.Z. ..~ •

' ~.

10

12

14

16

18 20

~

18

"

16

6'

14

12

14

16

18

20

#

o~'

"

12 10

'.

~

t"

10

n:

,0

12

14

16

18 20 22

Hb (gm/dL) selected horses Unselected

horses

r= 0·36

(not excited during sample collection)

r: 0·70

After exercise

(0)

After adrenaline

c.)

o r-0·98 • r-0·99

Figure 2. Relationship between duplicate detenninations of resting helnoglobin concentration in horses. 21

469

HEMATOLOGIC RESPONSES TO EXERCISE AND TRAINING

EFFECTS OF EXERCISE ON THE HEMOGRAM Erythrocyte Indices Depending on tl1e intensity of the exercise, there will be variable effects on the hemograln. Most forms of exercise result in mobilization of splenic erythrocytes and, therefore, increase the oxygen transport capacity. The extent of the potential increase in circulating red celllnass is quite impressive, for Persson and Lydin 26 estilnated that the spleen could store up to half the total erythrocyte volume. The release of splenic erythrocytes into the systemic circulation is under the influence of catecholamines. The extent of the increase in PCV is a function of exercise intensity, because there is a linear relationship between PCV and speed20 ,31 up to a Inaxilnum PCV of approximately 60 to 65 per cent (Fig. 3). This autotransfusion of erythrocytes during exercise boosts the horse's aerobic capacity and is thought to be the main reason for the horse's maxilnal oxygen uptake (Vo 21nax) being much higher than that of other species. 48 In a study of horses before and after splenectomy, Persson and Lydin 26 found a decrease in work capacity after splenectolny and an increase in heart rate at submaximal work loads. Further studies by Persson and Bergsten 27 revealed that tl1e cardiac output during Inoderate treadmill exercise was decreased, mainly owing to a decreased stroke volume. These authors proposed that the equine spleen could be regarded as a car-

Rest Endurance exercise Treadmill exercise Trotting or pacing

3-DE cross country Galloping

30

35

40

45

50

55

Post-exercise PCV Figure 3.

60

65

<0;0)

Packed cell volulne following various fonns of exercise in horses.

470

REUBEN

J.

ROSE AND JENNIFER R. ALLEN

diovascular reserve that would be able to maintain ventricular filling at the high heart rates found during exercise. Persson 20 has found a significant correlation between the circulating red cell mass or total hemoglobin and the track performance (expressed as time per kIn) in Standardbred horses. The total Hb or red cell volume is deterlnined by lnobilizing the splenic erythrocytes, using either adrenaline or maximal exercise. A blood sample is collected to measure tIle helnatocrit and a technique such as Evan's blue dye dilution is used to lneasure plasma volulne. The blood volume can be measured froln these two deternlinations. The extent of the blood volulne increase with exercise is variable 24 and depends on age; it is less in young horses and increases progressively up to 5 years of age. Although there are obviously substantial benefits in oxygen transport capacity and cardiac output from the splenic erythrocyte reserve, the resultant increase in blood viscosity can have adverse effects on regional perfusion. This lnust be taken into consideration and is ilnportant in horses competing in endurance rides where large fluid losses in sweat can lead to hemoconcentration, in addition to the normal splenic mobilization of erythrocytes. The resulting increase in blood viscosity will limit the horse's endurance capacity and can lead to exhaustion and play a role in exhaustion-related disorders. 10 Even during maximal exercise, SOlne of the increase in PCV is due to temporary fluid shift out of the vascular compartment. However, it is interesting to note that values for plaslna viscosity are lower in the ThorougIlbred tIlan in other breeds. 3 The Leukocytes Significant differences in the response of the leukocytes to exercise are found when comparing sublnaximal endurance witIl lnaximal high-intensity exercise (Fig. 4). Following endurance exercise, there is a leukocytosis due to neutropIlilia and lylnphopenia. 10,31,38 This leukocyte response is associated with elevations in plaslna cortisol values,32 whereas the extent of the neutrophilia and IYlnphopenia is significantly correlated with speed. When a group of fast and slow horses cOlnpeting in a 160-km endurance ride were compared, it was found that the fast horses had a significantly greater degree of neutrophilia and lymphopenia tIlan the slow group,31 although the total leukocyte counts were not significantly different. Carlson and colleagues 10 compared a group of norlnal endurance horses with those eliminated due to exhaustion midway through a 160-km endurance ride. They found that the exhausted group had a greater degree of neutrophilia with a pronounced left shift, there being a mean of 8 per cent band form neutrophils. This suggests that the greater the degree of stress, the more extensive is the change in leukocyte population. As the speed increases or signs of exhaustion appear, the elevated cortisol values result in lnobilization of neutrophils and a decrease in circulating lymphocytes. Examination of the N:L ratio,

471

HEMATOLOGIC RESPONSES TO EXERCISE AND TRAINING

ENDURANCE HORSES

THOROUGHBREDS Snow et ai, 1983

Rose, 1982 14

14

Race

12

Slow 1 Workout

«Slow»2

12 ~

..J

10

10

~

(;j--

~)( ()

al

3:

-

8

~

8

u

6

b

~

)(

6

EXha~sted

F

[\

...J

Carlson, 1976 «Fast»3

-

n

?0 ...

CD

3:

4

4 2

2

0

0 A

8

A

8

C

~ Neutrophils

D

Lymphocytes

o Other

~

a

b

Mean speed 1 2

3

c

d

a

b

c

d

A'

8'

660- 750 mlmin 233 mlmin

143 m/min

Figure 4. Changes in white count after exercise. 1, Data froln Snow et a1. 40 Thoroughbreds: A = before exercise; B = iInInediately after exercise; C = 3 hours after exercise. 2, Data froln Rose. 31 Endurance horses: a = before ride; b = iInInediatelyafter ride; c = 30 Ininutes after ride, d = 24 hours after ride. 3, Data from Carlson et al. lO A' = control (rest); B' = exhausted horses, after ride.

therefore, provides a Ineans of assessing tIle degree of stress in horses following exhaustive long-distance exercise. The leukocyte response to Inaxilnal exercise is quite different than the response to endurance exercise. Immediately after galloping exercise, there is a change in the N:L ratio, but little change in the total leukocyte count. A transient lymphocytosis with a consequent decrease in tIle N:L ratio has been reported by a nUlnber of research workers. 11,35,40 This is probably the result of the splenic release of stored lympllocytes. 40 The lymphocytosis of Inaxilnal exercise appears to last only a few hours, at which tinle there is an increase in the N:L ratio. This is due to an increase in plasma cortisol that occurs very soon after exercise. The temporal relationship between the increase in N:L ratio and cortisol appears to be very close, but following exercise, the N:L response seems to lag a little behind the increase in cortisol values. 35

EFFECTS OF TRAINING ON THE HEMOGRAM Because of the interest in a possible relationship between the state of training and the resting hemogram, a number of research workers have investigated the response of the hemograln to various types of training. 3,4,11,12,19,20,33,37,39,42,43,46 No significant changes in the

472

REUBEN J. ROSE AND JENNIFER R. ALLEN

total or differential leukocyte count have been found in response to high- or low-intensity training. 3 ,33,39 However, a number of studies have shown a significant increase in resting erythrocyte indices (RBC, PCV, Hb) in response to Thoroughbred training, although a study by Rose and Hodgson 33 in endurance horses showed that no significant increases in RBC, PCV, or Hb occurred during a 12-week training program (Fig. 5). These differences in response may be a reflection of the different intensities employed in the training of Thoroughbreds when compared with endurance horses. Thoroughbred racehorses are trained at higher exercise intensities than endurance horses, perhaps stimulating erythrocyte production due to a greater demand for oxygen carriage. It should be noted, however, that in the study of Clarkson 12 differences were found in the training responses of Thoroughbred horses related to the PCV values prior to commencement of training. Horses with initial PCV values greater than 40 per cent were found to have no significant change in RBC, PCV, or Hb during training, whereas those with PCV values of less than 40 per cent had a mean increase of 20 per cent (from 36.1 to 43.4) in their PCV These results once again demonstrate the variation THOROUGHBREDS

ENDURANCE

Sykes. 1966 e 2-year-olds

50

50[ ' .

~.

- - Rose & Hodgson. 1983

~ ~ 40

~ > u a..

HORSES

- - - - - Carlson, 1975

~ > u a..

45

~

.........

............. .

/

apprehensive group

35 40

2

3

4

5

6

30 ~0-~2---'4----J6'----8"----1.L..-0-1~2--1~4

7

Time in training (weeks)

Time in training (months)

STANDARDBREDS 19

TREADMILL Rose et ai, 1983. Weeks in training

~

g

e7

55

Persson, 1975

Cl

~

17

Cl

50

o 45

0'

~40 a..

35 30

13 0 (Rest)

5

15

(Exercise)

~~ 45 7~

(Recovery)

o

234567 Time in training

(months)

Minutes

Figure 5.

Changes in packed cell volulne with training.

HEMATOLOGIC RESPONSES TO EXERCISE AND TRAINING

473

that can be found in the resting hemogram due to factors such as the demeanor of the horse at the time of the blood collection. When an increase in resting PCY is found during training, it may not represent an increase in circulating red cell mass but rather an increase in the excitability of the horse as it becomes fitter. For these reasons, the routine monitoring of the hemogram during training is not a useful mechanism for determining fitness. However, the deviation of values from an individual horse>s normal range may indicate a problem. 4 Although the resting hemogram appears to show variable changes in response to training, the total hemoglobin and total cell volume do show significant increase (see Fig. 5).20,24 This improvement in the ability to transport oxygen can only be assessed by determination of post-exercise PCY or Hb and measurement of plasma volume. The assessment of post-exercise PCY or Hb alone does not provide sufficient accuracy for determination of the total erythrocyte mass owing to the individual variation in plasma volume. 24 When training is prolonged, the increase in red cell mass may be excessive, leading to red cell hypervolemia. 20,28,29 This results in reduced racing performance and appears to be due to overtraining. Persson and colleagues 29 have characterized these horses as having a higher heart rate than normal in response to a standardized exercise test and adrenocortical insufficiency, in addition to the red cell hypervolemia. Due to divergence from the expected relationship between oxygen consumption and total red cell volume, Persson and colleagues 29 postulated that a lower oxygen uptake in overtrained horses was due to inadequate utilization of oxygen by the working muscles. This is probably a result of increased blood viscosity and, therefore, reduced capillary perfusion. These observations have all been made on Standardbred trotters, and it is unclear whether the same overtraining pllenomenon occurs in Thoroughbred racehorses.

CONCLUSION It is apparent that great care must be taken in the interpretation of the resting hemogram. Due to factors such as the temperament of the horse and time of collection and feeding, considerable variation in the heluogram can be found. The splenic reservoir of erythrocytes is responsible for the remarkable aerobic capacity of the horse when related to luan. Therefore, determination of the total cell volume or total hemoglobin provides a good index to the horse's performance potential in Standardbred horses. However, such may not be the case for Thoroughbred racehorses, which exercise at greater intensities and are more dependent upon their anaerobic capacity for performance. Further research needs to be undertaken to examine the relationship between total red cell mass and performance in Thoroughbred horses.

474

REUBEN

J.

ROSE AND JENNIFER

R.

ALLEN

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