OBJECTIVE To compare the efficacy of quinidine and flecainide in treating naturally occurring, recent-onset atrial fibrillation (AF) in Thoroughbred racehorses.
DESIGN Retrospective case series.
ANIMALS 107 Thoroughbred racehorses.
PROCEDURES Medical records of racehorses with AF that were treated with quinidine or flecainide between 1987 and 2014 were reviewed. Signalment, history, treatments, complications, and outcome data were collected. Horses were allocated to 2 groups according to the initial treatment: initial treatment with quinidine (group 1) or initial treatment with flecainide (group 2). Horses in group 2 that did not convert to sinus rhythm with flecainide were then administered quinidine (group 3). Complications, total quinidine dose, and duration of treatment were compared. Rates of conversion for horses treated with quinidine versus flecainide were also compared.
RESULTS Overall rate of cardioversion was 91% (97/107). There was a significant difference in the rate of cardioversion for quinidine alone (91% [71/78]), compared with flecainide alone (41% [12/29]). In group 3, the conversion rate after the addition of quinidine treatment was 82% (14/17). Total quinidine dose and treatment duration did not differ significantly between groups 1 and 3.
CONCLUSIONS AND CLINICAL RELEVANCE Overall rate of cardioversion for Thoroughbred racehorses with AF was similar to that in previous reports. Flecainide treatment was less effective than quinidine treatment, but the frequency of complications did not differ between quinidine and flecainide. Further investigation is suggested to evaluate the efficacy of flecainide for cardioversion in athletic horses.
Objective—To determine the frequency of epistaxis
during or after racing among racehorses and identify
factors associated with development of epistaxis.
Sample Population—247,564 Thoroughbred and
4,045 Anglo-Arab race starts.
Procedure—Race start information (breed, age, sex,
racing distance, and race type) was obtained for
Thoroughbred and Anglo-Arab horses racing in Japan
Racing Association-sanctioned races between 1992
and 1997. All horses that raced were examined by a
veterinarian within 30 minutes of the conclusion of
the race; any horse that had blood at the nostrils was
examined with an endoscope. If blood was observed
in the trachea, epistaxis related to exercise-induced
pulmonary hemorrhage (EIPH) was diagnosed.
Results—Epistaxis related to EIPH was identified following
369 race starts (0.15%). Frequency of EIPHrelated
epistaxis was significantly associated with race
type, age, distance, and sex. Epistaxis was more common
following steeplechase races than following flat
races, in older horses than in horses that were 2 years
old, following races ≤ 1,600 m long than following
races between 1,601 and 2,000 m long, and in females
than in sexually intact males. For horses that had an
episode of epistaxis, the recurrence rate was 4.64%.
Conclusions and Clinical Relevance—Results suggested
that frequency of EIPH-related epistaxis in
racehorses is associated with the horse's age and
sex, the type of race, and the distance raced. The
higher frequency in shorter races suggests that higher
intensity exercise of shorter duration may increase
the probability of EIPH. (J Am Vet Med Assoc
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 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 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 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 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 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 determine prevalence of atrial fibrillation
(AF) immediately after racing among racehorses
that finished well behind the winners and examine
potential risk factors for AF in these horses.
Animals—39,302 racehorses representing 404,090
race starts in races sanctioned by the Japan Racing
Association between 1988 and 1997.
Procedure—Horses that finished ≥ 4 (turf races) or 5
(dirt races) seconds behind the winner or that did not
complete the race were examined for AF within 5
minutes after the race. Logistic regression and χ2
analyses were used to determine whether sex, age,
race distance, race surface, year, or development of
epistaxis was associated with development of AF.
Results—Estimated minimum frequency of AF was
0.03% (123 instances of AF following 404,090 race
starts), and estimated minimum prevalence of AF
among racehorses was 0.29% (115 horses with AF
among 39,302 racehorses). Estimated frequency of
AF among horses that finished slowly or did not finish
was 1.39% (120 instances of AF among 8,639 examinations),
and estimated prevalence of AF in horses
that finished slowly was 1.23% (92 instances of AF
among 7,500 horses) or 1.01% when only the first
time a horse finished slowly was considered (76
instances of AF among 7,500 horses). Atrial fibrillation
was paroxysmal in most horses. Among horses that
finished slowly, 4-year-old and older horses and horses
that raced on turf were more likely to develop AF.
Conclusions and Clinical Relevance—Results suggest
that the likelihood of AF among racehorses that
finish slowly is related to age and racing surface. (J
Am Vet Med Assoc 2003;223:84–88)
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.