Vol. 85, Issue 3, 1030-1036, September 1998
Blood-gas measurements adjusted for temperature at three sites
during incremental exercise in the horse
L. E.
Taylor,
D. S.
Kronfeld,
P. L.
Ferrante,
J. A.
Wilson, and
W.
Tiegs
Department of Animal and Poultry Sciences, Virginia Polytechnic
Institute and State University, Blacksburg, Virginia 24061-0306; and
Waltham Centre for Equine Nutrition and Care, D-2810 Verden, Germany
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ABSTRACT |
Rectal temperature
(Tre) is often used to adjust
measurements of blood gases, but these adjusted measurements may not
approximate temperatures during intense exercise at main sites of gas
exchange: muscle and lung. To evaluate differences in blood gases
between sites, temperatures (T) were measured with thermocouples in the rectum (re), in mixed venous blood (
), in
gluteal muscle (mu), and on the skin (sk) in seven Arabian horses as
they underwent an incremental exercise test on a treadmill. Blood
samples were drawn from the carotid artery and pulmonary artery (mixed
venous) 30 s before each increase in speed and during recovery. Blood gases and pH were measured at 37°C, and all variables were adjusted to Tre,
, and
Tmu. Adjusted variables during
exercise and recovery were significantly different from each other at
the three sites. Linear and polynomial equations described the time
course of venous temperature and
from
Tre and
Tsk during exercise and from
Tsk during recovery.
Interpretation of changes in muscle metabolism and gas exchanges based
on blood-gas measurements is improved if they are adjusted
appropriately to Tmu or
, which may be predicted from
Tsk in addition to
Tre during strenuous exercise and
from Tsk during recovery.
Arabian horses; muscle temperature
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INTRODUCTION |
BLOOD GASES and pH are often measured in studies of
exercise. Adjustment of blood-gas data for changes in temperature
during moderate and strenuous exercise is important for proper
interpretation. These measures are dependent on dissociation constants
that vary with temperature (4, 8), which rises during moderate and strenuous exercise. Rectal
(Tre), blood, and muscle
temperatures (Tmu) have been
used previously in horses for adjustment factors during exercise (2,
6-8, 14); Tre has
underestimated temperatures in working muscle and lung, which would
lead to an overestimation of pH and an underestimation of
PO2 and
PCO2. No reports have been found
concerning temperature changes during prolonged strenuous exercise. In
addition, no reports have been found in Arabian horses, a breed adapted
to endurance exercise that has certain characteristic responses (11,
17, 23, 25).
The present investigation had three objectives:
1) to determine simultaneously
Tmu, temperature in mixed venous
blood (), Tre, and temperature on the skin
surface (Tsk) during incremental exercise in conditioned Arabian horses,
2) to compare the differences in
Tre,
, and
Tmu with respect to mixed venous
and arterial H+ concentration
([H+]) and blood
gases, and 3) to derive regression
equations for prediction of Tmu
and from
Tre and
Tsk during exercise and from
Tsk during recovery.
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MATERIALS AND METHODS |
Experimental animals.
Seven Arabian horses [4-5 yr, 410 ± 41 (SE) kg body
wt] were conditioned for 20 wk on a treadmill (Mustang 2200, Kagra). The right carotid artery was surgically relocated to a
subcutaneous position in each horse 
6 mo before the study. Horses
were fed a cracked corn and oat mix and grass hay to meet requirements for moderate exercise in horses (13). The protocol and procedures were
approved by the university's animal care committee.
Experimental protocol.
Feed, but not water, was withheld overnight for 12 h before the
morning of the exercise test. The average ambient temperature of the
climate-controlled barn that housed the treadmill was 12°C; relative humidity was 40%. Horses were brought to the barn and allowed
1 h of acclimation before the study. A sterile 18-gauge catheter
(Angiocath, Becton-Dickinson) was introduced into the carotid artery
and kept patent with heparinized saline.
A sterile polyethylene catheter (PE-240, Intramedic) was introduced
into the pulmonary artery. Placement of the tip of the catheter was
determined by changes in blood pressure via water manometer. Subsequent
studies with the same group of horses confirmed via pressure transducer
that catheters had been placed in the pulmonary artery.
Copper-constantan thermocouples (Physitemp Instruments) were used to
measure temperatures. One thermocouple was passed through the tubing
into the pulmonary artery for measurement of (model IT-18EXLNG).
Tmu was measured 8 cm deep in the
left middle gluteal muscle (model MT 23/8),
Tre was measured 10 cm deep in the
rectum (model ESO-1), and Tsk was
measured on the dried surface of the skin over the right gluteal muscle
(model SST-1). Thermocouples were calibrated initially in a water bath, starting with ice water and heating to 45°C using a mercury
thermometer marked to 0.1°C (ERTCO Precision model 1033, Baxter).
Subsequent calibration was tested in a water bath at 37°C during
each experiment. Heart rates were recorded before measurement of
Tmu, just before each change in
speed, with a commercial digital heart monitor (Polar Pacer, Polar
CIC).
Exercise test protocol.
Exercise consisted of an incremental test, with moderate increases in
speed at each step to elicit steady changes in temperature. Horses
walked for 4 min at 1.5 m/s at 0% slope, the slope was then raised to
6% (3.6°), and the speed of the treadmill was increased every 4 min by 0.5 m/s. Total time of the exercise portion of the test was 52 min, with horses reaching a top speed of 7.5 m/s. Horses then completed
a 16-min walking recovery period at 0% slope.
Sampling protocol.
Resting arterial and mixed venous blood samples and resting
temperatures were recorded simultaneously before exercise. Samples were
drawn every 4 min, 30 s before the treadmill was stopped for 5 s to
record Tmu.
Tre,
, and
Tsk were measured during blood sampling times. After measurement of
Tmu, the treadmill speed was
increased and had returned to the desired speed within 10 s. Samples
were also taken every 4 min for 16 min during the walking recovery
period.
The arterial and mixed venous samples (2 ml) were drawn into plastic
heparinized syringes (300 U lithium heparin; Sigma Chemical) and stored
in an ice-water bath until analyzed for pH,
PCO2, and
PO2 at 37°C (Stat Profile 1, Nova
Biomedical) within 30 min. All measurements were adjusted to
Tre,
, and
Tmu with the following equations
(12)
![= pH + [−0.0147 + 0.0065(7.400 − pH)(T − 37)]](/content/vol85/issue3/fulltext/1030/img004.gif)
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(1)
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(2)
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(3)
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where
y = e3.88 × ln(PO2).
Changes in the dissociation constant of carbonic acid in plasma with
varying pH and temperature were also taken into account (20). The
[H+] was calculated
from pH values.
Data are summarized as means ± SE and examined by ANOVA for
repeated measures (19). Dunnett's
t-test was used for comparison of
preexercise means with means during exercise and recovery. P < 0.01 was considered significant.
Regression equations were based on mean values for the seven horses at
each step and were obtained with a curve-fitting program (22).
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RESULTS |
Temperatures and heart rate.
Tsk was lower than
,
Tre, and
Tmu at all times, and
Tmu was higher than
Tre and
Tsk at all times (Fig.
1).
was lower than
Tre during rest and recovery but
higher during exercise. Temperature increased at all four sites during
exercise (Fig. 1), with Tmu higher
than the others for the walking sample (4 min). Temperatures were
different from each other at all four sites by 20 min, and all four
temperatures had increased above resting values by 20 min (3.5 m/s,
medium trot). Increases from rest to the end of exercise were 33.5 ± 0.06 to 37.9 ± 0.1, 37.8 ± 0.05 to 40.4 ± 0.1, 37.4 ± 0.05 to 41.0 ± 0.1°C, and 37.8 ± 0.05 to 42.0 ± 0.1°C for Tsk,
Tre,
, and
Tmu, respectively.
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Fig. 1.
Temperature measured in rectum, blood, and muscle and on skin in 7 horses during incremental exercise and recovery. Values are means ± SE.
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Tre continued to rise for 5 min
during the walking recovery period, but
Tsk,
, and
Tmu decreased immediately,
Tmu at a relatively slower rate.
All four temperatures remained significantly different from each other
throughout the recovery period. Only
returned to the preexercise
value by the completion of the 16-min recovery period.
Heart rate was 33 ± 0.7 beats/min at rest and increased linearly
with exercise time to 173 ± 2.3 beats/min at the last step (52 min,
7.5 m/s) of the exercise test and returned to 80 ± 4.3 beats/min at
16 min of recovery.
[H+].
Changes in arterial and mixed venous
[H+] are summarized in
Fig. 2. The three adjusted arterial
[H+] values were
between 38.9 and 38.0 nM at rest and decreased during exercise,
reaching the lowest value at 30 min (5.0 m/s) before returning to near
resting values at the end of exercise. By 28 min the changes from rest
in temperatures at the measured sites (Fig. 1) resulted in a difference
between adjusted arterial
[H+] values at the
three sites. Arterial values were lowest for
[H+] adjusted to
Tre, followed by
[H+] adjusted to
, with
[H+] adjusted to
Tmu being highest. The rapid
decline in resulted in
[H+] adjusted to
being the lowest during
recovery, with [H+]
adjusted to Tmu remaining the
highest.
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Fig. 2.
H+ concentration
([H+]) in arterial
(A) and mixed venous
(B) blood in 7 horses at a standard
temperature of 37°C and adjusted to rectal, blood, and muscle
temperatures during incremental exercise and recovery. Values are means ± SE.
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[H+] adjusted to
values were between 41.7 and
40.7 nM at rest (Fig. 2); they reached a low point at 32 min and then
increased steadily until the end of exercise. Estimates of mixed venous [H+] exhibited a
pattern similar to the values of arterial
[H+] when adjusted for
Tre,
, and
Tmu.
PCO2.
Changes in arterial
(PaCO2) and mixed venous
(
)
PCO2 are summarized in Fig.
3. The adjusted
PaCO2 values were
41-42 Torr at rest. The increased Tmu resulted in a difference in
PaCO2 and
adjusted to
Tmu at the first sampling time.
PaCO2 at the three sites declined
steadily during exercise, with differences between the three adjusted
values by 20 min of exercise. None of the values had returned to
preexercise levels by the end of recovery. The rapid decline in
during recovery resulted in
PaCO2 adjusted to
being the lowest;
PaCO2 adjusted to
Tmu was the highest.
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Fig. 3.
PCO2 in arterial
(PaCO2,
A) and mixed venous
(,
B) blood in 7 horses at a
standard temperature of 37°C and adjusted to rectal, blood, and
muscle temperatures during incremental exercise and recovery. Values
are means ± SE.
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The adjusted values were
47-48 Torr at rest (Fig. 3) and were increased by 10 Torr during exercise. The values were decreased by 20 Torr during the first 4 min of recovery and did not
return to preexercise values by the end of recovery.
adjusted to
Tmu was highest throughout
exercise, followed by adjusted for and
Tre.
HCO
3 concentration.
Changes in HCO3 concentration
([HCO3]) are summarized in
Fig. 4. The adjusted arterial [HCO3] values were
25.5-26.0 mM at rest, increased at the walk, and then decreased
steadily during exercise. Arterial [HCO3] adjusted to
Tmu was always the highest value,
followed by [HCO3]
adjusted to and Tre, respectively. All values
increased during recovery and approached preexercise levels by 16 min
of recovery.
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Fig. 4.
HCO3 concentration in arterial
(A) and mixed venous
(B) blood in 7 horses at a standard
temperature of 37°C and adjusted to rectal, blood, and muscle
temperatures during incremental exercise and recovery. Values are means ± SE.
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Adjusted mixed venous [HOC3]
was 27.5-28 mM at rest and remained higher than arterial
[HCO3] at all times, reaching
a peak value of 33 mM at 16 min (3 m/s) before returning to preexercise
levels by the end of exercise (Fig. 4). Adjusted values were different
at all three sites after 20 min of exercise. Mixed venous
[HCO3] adjusted to
Tmu was higher than the other
values at the first sampling, and all values increased during recovery
but failed to reach preexercise levels.
PO2.
Changes in arterial (PaO2) and mixed
venous (
)
PO2 are summarized in Fig.
5. Adjusted PaO2 was 90-94 Torr at rest, and
all values increased gradually during exercise; they were different
from each other at 20 min of exercise. However,
PaO2 adjusted to
Tmu was greater than the other
values at the first sampling during exercise (4 min, 1.5 m/s) when the
horse was still walking. Also, PaO2
adjusted to was lower than at
the other sites at rest, partly because resting blood temperature was
at its lowest level. During exercise,
PaO2 adjusted to
Tmu was always the highest,
followed by PaO2 adjusted to
and
Tre, respectively. After the end
of exercise, all three adjusted values peaked at the 4th min of
recovery and then declined slightly during the remainder of the
recovery period. None of the values had returned to preexercise levels
by the end of recovery.
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Fig. 5.
PO2 in arterial
(PaO2,
A) and mixed venous
(,
B) blood in 7 horses at a standard
temperature of 37°C and adjusted to rectal, blood, and muscle
temperatures during incremental exercise and recovery. Values are means ± SE.
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The adjusted values were
37-38 Torr at rest (Fig. 5) and remained lower than
PaO2 values at rest, exercise, and
recovery. All three values decreased to 20-25 Torr by 20 min and
remained low until the end of exercise. adjusted for
Tmu was different at the
first sampling, and all three values were different by 20 min of exercise. adjusted for
Tmu was highest, followed by
adjusted for
and
Tre, respectively. Only
adjusted for
had returned to the
preexercise level by 16 min of recovery.
Arterial and venous values and arteriovenous differences of
PO2,
PCO2, and
[H+] are compared as
measured at standard temperature (37.00°C) and adjusted to the mean
Tmu (41.96°C) at the last step
of the exercise test in Table 1. The
magnitude of the differences between the standard and adjusted values
increased with increasing Tmu. The arteriovenous differences also increased with the rise in
Tmu during exercise. Mean
arteriovenous PO2 was linearly related to mean Tmu during
incremental exercise
(r2 = 0.80, n = 15, P < 0.001) and recovery
(r2 = 0.97, n = 5, P = 0.002).
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Table 1.
Arteriovenous differences measured at standard temperature and
adjusted to the highest muscle temperature observed
during incremental exercise
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Prediction equations.
Mean Tmu and
values were related linearly
(r2 > 0.94) to
Tre and
Tsk during exercise over the range
37-42°C and to Tsk but not Tre during recovery. Quadratic
equations yielded better fits (r2 > 0.97) for
the same relationships (Figs. 6 and
7).
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Fig. 6.
Prediction of muscle (Tmu) and
blood () temperatures by
quadratic equation during incremental exercise from mean rectal
temperature (Tre;
Tmu = 32.7107Tre  0.399646T2re 627.358, r2 = 0.994;
= 18.5858Tre 0.221481T2re 348.412, r2 = 0.991;
A) and mean skin
temperature (Tsk;
Tmu = 259.524488 13.241223Tsk + 0.197739T2sk,
r2 = 0.991;
= 244.826453 12.288956Tsk + 0.182112T2sk,
r2 = 0.989;
B) in 7 horses.
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Fig. 7.
Prediction of Tmu and
by quadratic equation during
walking recovery from mean
Tsk
(Tmu = 47.782332Tsk 0.640598T2sk 849.415829, r2 = 0.974;
= 199.388706 9.787845Tsk + 0.147774T2sk,
r2 = 0.980) in 7 horses.
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DISCUSSION |
This study extends the range of increases in temperature during
exercise found previously in limited studies (2, 7, 8, 14, 21) and
allows for the first time the derivation of quantitative relationships
between temperatures at different sites, including rectum, blood,
muscle, and skin. In addition, inclusion of
Tsk in the experiment has allowed
derivation of quantitative relationships and the prediction of
Tmu and
from
Tsk during recovery. The
temperature differences between sites are reflected significantly in
adjusted parameters, especially
PaO2. Previous reports
of hypoxemia during intense exercise may have included errors in
PO2 measurement of as much as 11 Torr. The predictive equations derived in this study could be used to
avoid or reduce such errors in evaluation of acid-base balance and
blood-gas variables during exercise.
Critique of methods.
The collection of blood samples into iced plastic syringes with
subsequent analysis performed within 30 min may have slightly affected
the present results. The PaO2 of human
blood samples collected in plastic syringes and stored for 30 min in
ice water was overestimated by 1-3% (10). The solubility of
O2 increases with cooling, and
plastic syringes do not form an impenetrable barrier to ambient air.
Equine blood differs somewhat from human blood, but small increases in
PO2 and
PCO2 have been found after 1 h of
storage in iced plastic syringes (1).
Temperature variation.
The present data cover a wider range of temperatures during exercise
than any reported previously in horses (2, 5-8, 14, 21). The
pattern of responses, however, at different measurement sites has been
seen previously in different breeds of horses at various exercise
intensities (6, 8). Tmu was always
highest during exercise, followed by
,
Tre, and
Tsk, respectively.
Tre has a characteristic lag
period during early recovery as it continues to rise before gradually
decreasing. This study emphasizes that the use of
Tre for the adjustment of
blood-gas data after exercise may not be appropriate, even for
submaximal exercise in moderate environmental conditions.
and
Tsk declined rapidly during
recovery, with Tmu decreasing at a
slower rate. At lower exercise intensities (5-15 min, 1.5-3.0
m/s), and
Tre were not different, and this has been observed previously in horses (5). Little cooling of the blood
occurs across the lung; differences approximate 0.4°C between the
pulmonary and carotid arteries during exercise in horses (6).
Increasing the exercise intensity in an incremental fashion in the
present study resulted in a significant difference in temperature at
the different sites after 28 min (4.5 m/s). During this study, the
highest mean temperature of 42.02°C was recorded 8 cm deep in the
middle gluteal muscle at the end of exercise. The horses used in this
study had been in training for 5 mo, and training has been shown to
attenuate the rise in Tmu during
exercise (21, 26).
Consequences of temperature differences.
All the variables studied showed differences between measurement at
standard temperature (37°C) and each of the three adjusted values,
with the most dramatic differences found in the arterial measurements
adjusted to and
Tmu.
[H+].
The pattern of the
[H+] response to
incremental exercise has been observed previously in ponies and
Thoroughbred horses during moderate exercise (14, 15). In this study
the difference in Tmu at the first
sampling time resulted in adjusted arterial and mixed venous
[H+] values being
different from corresponding values at the other two sites. Near the
end of exercise, in arterial and mixed venous [H+], there was a
difference between
[H+] adjusted to
and
[H+] adjusted to
Tmu of 1.8 nM. During exercise
[H+] in the working
muscle was actually 1.1-1.8 nM higher than would have been
estimated with Tre and
, respectively. These
differences may be important for the interpretation of muscle
metabolism during exercise.
PCO2.
The use of Tre or
compared with
Tmu after exercise at 3.5 m/s
would have underestimated
PaCO2 and
during exercise and
recovery. The overall response of
PCO2 to exercise has been seen
previously (6, 24) in other breeds of horses exercising at similar
intensities for 8-15 min compared with 52 min in the present
study. The steady decrease in all three PaCO2 values throughout incremental
exercise has not been reported previously and demonstrates the Arabian
horse's ability to hyperventilate, hence avoiding an exercise-induced
hypercapnia and acidosis during this type of exercise. Increased
PaCO2 values have been observed during
short, high-intensity exercise in other breeds and lead to an arterial
acidosis (2, 8). The Arabian horses were able to maintain a high level
of alveolar ventilation in addition to avoiding acidosis. Whether the
Arabian horse can avoid this arterial hypercapnia at maximal-intensity
exercise has not been investigated.
increased slightly at the
onset of exercise but never changed >5 Torr in either direction during exercise. The lack of a large or steady increase in
CO2 in the mixed venous blood may
be due to the submaximal exercise intensity. The arteriovenous
difference averaged 6 Torr at rest, increased to 20 Torr
after 28 min of exercise, and was 25 Torr at the end of
exercise. The steady decline in PaCO2
helped maintain , offsetting
any tendency for it to increase, by presenting the working muscle with
arterial blood that had a relatively low
PCO2.
[HCO3].
Differences in adjusted plasma
[HCO3] between the three
sites were small but significant after 20 min of exercise. The decline
in all three adjusted [HCO3] values (Fig. 4) suggests the development of acidosis during exercise in
arterial and mixed venous blood. Increases in blood lactate during
exercise will decrease the strong ion difference and, combined with a
rise in the total concentration of plasma proteins, will contribute to
a decrease in [HCO3].
However, this decrease may be exaggerated during short,
maximal-intensity exercise if
[HCO3] values are not
adjusted for temperature changes.
PO2.
Differences in the adjusted PaO2 at the
three sites during exercise and recovery were the most dramatic among
the measured variables. At 40 min of exercise, there was an 11-Torr
difference between PaO2 adjusted for
Tre and
PaO2 adjusted for
Tmu and an 8-Torr difference
between PaO2 adjusted for
and
PaO2 adjusted for
Tmu. These differences could lead
to substantial error when O2
extraction by the working muscle or alveolar diffusion in the lung is
interpreted. They are important when variables, such as O2 saturation, are
calculated from PaO2.
A small, transient increase in PaO2 at
the onset of exercise and a concomitant decrease in
during exercise have been
seen previously in ponies and Thoroughbred horses exercising at
moderate intensities (15, 18). However, the magnitude of the steady
increase in PaO2 throughout exercise (Fig. 5) has not been previously reported. This may represent a breed
characteristic: the Arabian horse is very well suited for long-distance
aerobic exercise, as demonstrated by the predominance of slow-twitch,
high-oxidative muscle fibers and by the high activity of aerobic
enzymes (16). Our unpublished data have revealed that the Arabian horse
can maintain a PaO2 between 90 and 100 Torr during exercise at heart rates >210 beats/min.
Adjustment of arterial and venous
PO2,
PCO2, and
[H+] to
Tmu during exercise reveals
substantial differences from values at 37°C (Table 1). The 36%
increase in arteriovenous PO2 would
support a corresponding increase in aerobic metabolism, and increased
Tmu per se during exercise has
been shown previously to alter muscle glycolysis and glycogenolysis in
humans (3). This increase in arteriovenous
PO2 would tend to slow any increase
in intracellular acidity due to lactate accumulation. However, the 25%
increase in arteriovenous PCO2
probably reflects a higher intracellular
PCO2, which would tend to increase
intracellular [H+].
Prediction equations.
The quadratic equations found in this study (Figs. 6 and 7) could most
likely be used to predict Tmu and
from
Tre and
Tsk in incremental tests of
similar design. The use of Tsk
during recovery as well as exercise would be convenient. The
Tsk under the saddle during
exercise and recovery has been used consistently during 50- to 100-mile
endurance competitions. Unpublished reports show that it correlates
closely (0.5-1.0°F) with
Tre during exercise under
normal-to-hot environmental conditions but may have limited use when
the relative humidity is low and the ambient temperature is <70°F
(P. Semmler, personal communication). One study suggests that changes
in Tsk may help provide a signal
via cutaneous thermoreceptors for changes in breathing patterns during
thermal stress (9).
Data comparable to our results have been found in five reports (2,
6-8, 14). Our equations had little predictive value for rest or
light and moderate exercise when differences between Tre and
Tmu or
were <1.0°C. During
hard work, however, good agreement (±0.2°C) was found in
constant-speed and incremental test protocols between actual
measurements of and predicted estimates of from
Tre or
Tsk by our equations in some
studies (2, 6, 14) but not others (7, 8). The use of these predictive
equations may be limited by environmental conditions, as well as the
breed and fitness level of the horses. Exercise intensity and duration
are modifying factors that may affect the magnitude and speed of the
increase in temperatures. The results suggest that the differences
between Tmu and
Tre or
Tsk and the differences between
and
Tre or
Tsk need to be >1.0°C for
these equations to be useful in predicting
Tmu and
. The changes in temperature
during exercise and recovery have sufficient impact on
[H+] and blood gases
to warrant prediction of Tmu and
if these values cannot be
measured directly.
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ACKNOWLEDGEMENTS |
The authors thank Louisa Gay and Mark White for technical support
and animal care.
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FOOTNOTES |
This study was supported in part by Paul Mellon (Upperville, VA), by
the John Lee Pratt Animal Nutrition Fellowship Program, and by The
Waltham Centre for Equine Nutrition and Care.
Address for reprint requests: L. E. Taylor, Dept. of Equine Science,
Otterbein College, Westerville, OH 43081-2006.
Received 13 November 1995; accepted in final form 1 May 1998.
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Abstract
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