Objective—To determine the effects of initial handling
and training on autonomic nervous functions in
Animals—63 healthy Thoroughbreds.
Procedure—All horses were trained to be handled
and initially ridden in September of the yearling year
and then trained until the following April by conventional
training regimens. To obtain the heart rate (HR),
electrocardiograms were recorded in the stable
before initial handling and training and following 7
months of training; variations in HR were then evaluated
from the power spectrum in terms of the low frequency
(LF; 0.01 to 0.07 Hz) power and high frequency
(HF; 0.07 to 0.6 Hz) power as indices of autonomic
nervous activity. To evaluate the fitness, the V200
(velocity at HR of 200 beat/min), which is reflective of
the aerobic capacity of the horse, was measured.
Results—Mean (± SE) resting HR decreased significantly
from 41.5 ± 0.8 to 38.7 ± 0.4 beat/min following
7 months of training. The LF power of horses
increased significantly from 1,037 ± 128 milliseconds2
in September of the yearling year to 2,944 ± 223 milliseconds2
in the following April. Similarly, the HF
power increased significantly from 326 ± 30 milliseconds2
to 576 ± 39 milliseconds2 at the corresponding
time points. The V200 increased significantly following
Conclusions and Clinical Relevance—Increases in
LF and HF powers indicate that parasympathetic nervous
activity increases in horses by 7 months of training.
The decrease in resting HR may be dependent on
the training-induced increase of parasympathetic nervous
activity in Thoroughbreds. (Am J Vet Res
OBJECTIVE To quantify fatigue-induced electromyographic changes in hind limb muscles in horses.
ANIMALS 8 Thoroughbreds.
PROCEDURES The left and right hind limb longissimus dorsi, tensor fasciae latae, gluteus medius, and biceps femoris muscles were instrumented for surface electromyography. Hoof strain gauges were attached to confirm stride cycle. Each horse was galloped on a treadmill (grade, 3%) at a constant speed (12.6 to 14.7 m/s) to achieve fatigue after approximately 360 seconds. Before and after this exercise, the horses were trotted at 3.5 m/s. At 30-second intervals during galloping an integrated electromyography (iEMG) value for a stride and the median frequency of muscle discharge (MF) in each limb were measured. The mean of stride frequency (SF), iEMG value, and MF of 5 consecutive strides at the start and end of galloping for the lead and trailing limbs were compared. For trotting, these variables were compared at 60 seconds before and after galloping.
RESULTS The mean ± SD value for SF decreased over time (2.14 ± 0.06 to 2.05 ± 0.07 stride/s). In both the lead and trailing limbs, fatigue decreased the iEMG values of the gluteus medius and biceps femoris muscles but not those of the longissimus dorsi and tensor fasciae latae muscles. The MF did not change for any muscle during galloping with fatigue. The SF, iEMG value, and MF did not change during trotting with fatigue.
CONCLUSIONS AND CLINICAL RELEVANCE Fatigue induced by high-speed galloping decreased the gluteus medius and biceps femoris muscles' iEMG values in Thoroughbreds. Fatigue of these less fatigue-resistant hind limb muscles would affect a horse's speed.
Objective—To determine the influence of transportation by road and air on heart rate (HR) and HR variability (HRV) in horses.
Animals—6 healthy horses.
Procedures—ECG recordings were obtained from horses before (quarantine with stall rest [Q]; 24 hours) and during a journey that included transportation by road (RT; 4.5 hours), waiting on the ground in an air stall (W; 5.5 hours), and transportation by air (AT; 11 hours); HR was determined, and HRV indices of autonomic nervous activity (low-frequency [LF; 0.01 to 0.07 Hz] and high-frequency [HF; 0.07 to 0.6 Hz] power) were calculated.
Results—Mean ± SD HRs during Q, RT, W, and AT were 38.9 ± 1.5 beats/min, 41.7 ± 5.6 beats/min, 41.5 ± 4.3 beats/min, and 48.8 ± 5.6 beats/min, respectively; HR during AT was significantly higher than HR during Q. The LF power was significantly higher during Q (3,454 ± 1,087 milliseconds2) and AT (3,101 ± 567 milliseconds2) than it was during RT (1,824 ± 432 milliseconds2) and W (2,072 ± 616 milliseconds2). During Q, RT, W, and AT, neither HF powers (range, 509 to 927 milliseconds2) nor LF:HF ratios (range, 4.1 to 6.2) differed significantly. The HR during RT was highly correlated with LF power (R2 = 0.979), and HR during AT was moderately correlated with the LF:HF ratio (R2 = 0.477).
Conclusions and Clinical Relevance—In horses, HR and HRV indices during RT and AT differed, suggesting that exposure to different stressors results in different autonomic nervous influences on HR.
Objective—To determine whether evaluation of heart rate (HR) and HR variability (HRV) during prolonged road transportation in horses provides a sensitive index of autonomic stimulation.
Animals—Five 2-year-old Thoroughbreds.
Procedure—ECGs were recorded as horses were transported for 21 hours in a 9-horse van. Heart rate, high-frequency (HF) power, low-frequency (LF) power, and LF-to-HF ratio from Fourier spectral analyses of ECGs were calculated and compared with values recorded during a 24-hour period of stall rest preceding transportation.
Results—HR, HF power, and LF power had diurnal rhythms during stall rest but not during road transportation. Heart rate was higher and HF power and LF power lower during road transportation than stall rest, and HR, HF power, LF power, and LF-to-HF ratio all decreased with time during road transportation. Heart rate during stall rest was weakly and inversely associated with LF power, but during road transportation was strongly associated with LF power, HF power, and LF-to-HF ratio. Neither LF power nor HF power was correlated with LF-to-HF ratio during stall rest, but LF power was strongly and HF power weakly correlated with LF-to-HF ratio during road transportation. High-frequency power and LF power were significantly correlated with each other during stall rest and road transportation. Heart rate was significantly influenced by LF power and LF-to-HF ratio during stall rest (R2 = 0.40) and by HF power and LF-to-HF ratio during road transportation (R2 = 0.86).
Conclusions and Clinical Relevance—HR is influenced by different sympathovagal mechanisms during stall rest, compared with during road transportation; HRV may be a sensitive indicator of stress in transported horses.
OBJECTIVE To determine cardiorespiratory responses of Thoroughbreds to uphill and downhill locomotion on a treadmill at identical gradients.
ANIMALS 5 highly trained Thoroughbred geldings.
PROCEDURES Thoroughbreds were exercised for 2-minute intervals on a treadmill at 1.7, 3.5, 6.0, 8.0, and 10.0 m/s at a 4% incline, 0% incline (horizontal plane), and 4% decline in random order on different days. Stride frequency, stride length, and cardiopulmonary and O2-transport variables were measured and analyzed by means of repeated-measures ANOVA and Holm-Šidák pairwise comparisons.
RESULTS Horses completed all treadmill exercises with identical stride frequency and stride length. At identical uphill speeds, they had higher (vs horizontal) mass-specific O2 consumption (mean increase, 49%) and CO2 production (mean increase, 47%), cardiac output (mean increase, 21%), heart rate (mean increase, 11%), and Paco2 (mean increase, 1.7 mm Hg), and lower Pao2 (mean decrease, 5.8 mm Hg) and arterial O2 saturation (mean decrease, 1.0%); tidal volume was not higher. Downhill locomotion (vs horizontal) reduced mass-specific O2 consumption (mean decrease, 24%), CO2 production (mean decrease, 23%), and cardiac output (mean decrease, 9%). Absolute energy cost during uphill locomotion increased linearly with speed at approximately twice the rate at which it decreased during downhill locomotion.
CONCLUSIONS AND CLINICAL RELEVANCE Findings suggested that for Thoroughbreds, downhill locomotion resulted in a lower energy cost than did horizontal or uphill locomotion and that this cost changed with speed. Whether eccentric training induces skeletal muscle changes in horses similar to those in humans remains to be determined.
Objective—To determine the effects of immersion in
warm springwater (38° to 40°C) on autonomic nervous
activity in horses.
Animals—10 male Thoroughbreds.
Procedure—Electrocardiograms were recorded from
horses for 15 minutes during a warm springwater bath
after being recorded for 15 minutes during stall rest.
Variations in heart rate (HR) were evaluated from the
power spectrum in terms of low frequency (LF, 0.01 to
0.07 Hz) power and high frequency (HF, 0.07 to 0.6 Hz)
power as indices of autonomic nervous activity.
Results—Mean (±SE) HR during stall rest and immersion
in warm springwater was 31.1 ± 1.7 and 30.3 ±
1.0 beat/min, respectively. No significant difference
was found between the HR recorded during stall rest
and that recorded during immersion in warm springwater.
The HF power significantly increased from
1,361 ± 466 milliseconds2 during stall rest to 2,344 ±
720 milliseconds2 during immersion in warm springwater.
The LF power during stall rest and immersion
in warm springwater was 3,847 ± 663 and 5,120 ±
1,094 milliseconds2, respectively, and were not significantly
different from each other. Similarly, the LF:HF
ratio did not change during immersion in warm springwater.
The frequency of second-degree atrioventricular
block, which was observed in 2 horses, increased
during immersion in warm springwater, compared
with during stall rest.
Conclusions and Clinical Relevance—Increases in
HF power indicates that the parasympathetic nervous
activity in horses increases during immersion in warm
springwater. Thus, immersion in warm springwater
may provide a means of relaxation for horses. ( Am J
Vet Res 2003;64:1482–1485)
Objective—To determine whether withholding of food affects autonomic nervous system balance by analysis of heart rate (HR), HR variability (HRV), and frequency of second-degree atrioventricular block in horses.
Animals—5 healthy Thoroughbreds.
Procedures—For two 24-hour periods in a crossover study, food was withheld from horses or horses were maintained on their regular feeding schedule (control conditions) in their stalls and Holter monitor ECG recordings were obtained. The ECGs were analyzed by use of fast-Fourier transformation, and power spectrum densities were calculated for low-frequency (0.01 to 0.07 Hz) and high-frequency (0.07 to 0.6 Hz) variations in HR. Serum cortisol and plasma ACTH, norepinephrine, and glucose concentrations were measured at predetermined time points.
Results—Withholding of food resulted in significantly lower HR and more frequent second-degree atrioventricular block (the frequency of which was inversely related to the HR), compared with findings for control conditions. Circadian rhythms were similar during food-withholding and control conditions; peak HR was detected from 7:00 pm to 8:00 pm, and the lowest HR was detected in the early morning. During food-withholding conditions, the low-frequency and high-frequency components of HRV were significantly higher, and the low-frequency-to-high-frequency ratio was lower than during control conditions. Serum cortisol concentration was higher and plasma glucose concentration was lower at 6:00 pm in horses when food was withheld, compared with findings during control conditions.
Conclusions and Clinical Relevance—Indices of HRV seemed to be sensitive to changes in autonomic nervous activity and may be useful as clinical indices of the neuroendocrine response to stressors in horses.
OBJECTIVE To determine whether racehorses undergoing regular exercise at 2 intensities or stall rest during a period of reduced training (detraining) would differentially maintain their cardiopulmonary and oxygen-transport capacities.
ANIMALS 27 Thoroughbreds.
PROCEDURES Horses trained on a treadmill for 18 weeks underwent a period of detraining for 12 weeks according to 1 of 3 protocols: cantering at 70% of maximal rate of oxygen consumption (
o2max) for 3 min/d for 5 d/wk (canter group); walking for 1 h/d for 5 d/wk (walk group); or stall rest (stall group). Standardized treadmill exercise protocols (during which cardiopulmonary and oxygen-transport variables were measured) were performed before and after detraining.
o2max, maximal cardiac output, and maximal cardiac stroke volume of all groups decreased after 12 weeks of detraining with no differences among groups. After detraining, arterial-mixed-venous oxygen concentration difference did not decrease in any group, and maximal heart rate decreased in the walk and stall groups. Run time to exhaustion and speeds eliciting
o2max and maximal heart rate and at which plasma lactate concentration reached 4mM did not change in the canter group but decreased in the walk and stall groups.
CONCLUSIONS AND CLINICAL RELEVANCE Horses following the cantering detraining protocol maintained higher values of several performance variables compared with horses following the walking or stall rest protocols. These results suggested that it may be possible to identify a minimal threshold exercise intensity or protocol during detraining that would promote maintenance of important performance-related variables and minimize reductions in oxygen-transport capacity in horses.
Objective—To evaluate the effects of dorsal versus lateral recumbency on the cardiopulmonary system during isoflurane anesthesia in red-tailed hawks (Buteo jamaicensis).
Animals—6 adult 1.1- to 1.6-kg red-tailed hawks.
Procedures—A randomized, crossover study was used to evaluate changes in respiratory rate, tidal volume, minute ventilation, heart rate, mean arterial and indirect blood pressures, and end-tidal Pco2 measured every 5 minutes plus Paco2 and Pao2 and arterial pH measured every 15 minutes throughout a 75-minute study period.
Results—Respiratory rate was higher, tidal volume lower, and minute ventilation not different in lateral versus dorsal recumbency. Position did not affect heart rate, mean arterial blood pressure, or indirect blood pressure, although heart rate decreased during the anesthetic period. Birds hypoventilated in both positions and Paco2 differed with time and position × time interaction. The Petco2 position × time interaction was significant and Petco2 was a mean of 7 Torr higher than Paco2. The Paco2 in dorsal recumbency was a mean of 32 Torr higher than in lateral recumbency. Birds in both positions developed respiratory acidosis.
Conclusions and Clinical Relevance—Differences in tidal volume with similar minute ventilation suggested red-tailed hawks in dorsal recumbency might have lower dead space ventilation. Despite similar minute ventilation in both positions, birds in dorsal recumbency hypoventilated more yet maintained higher Pao2, suggesting parabronchial ventilatory or pulmonary blood flow distribution changes with position. The results refute the hypothesis that dorsal recumbency compromises ventilation and O2 transport more than lateral recumbency in red-tailed hawks.