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Keywords:

  • horse;
  • cardiac arrhythmias;
  • dressage;
  • exercise;
  • heart rate

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

Reasons for performing study: Exercise-associated arrhythmias are important differentials when evaluating poor performance sport horses. However, most studies of arrhythmias have been conducted in racehorses and therefore there is a paucity of knowledge concerning the prevalence and significance of exercise-associated arrhythmias in riding horses.

Objectives: The objective of this study was to examine the prevalence of arrhythmias, particularly supraventricular premature complex (SVPCs) and ventricular premature complex (VPCs), associated with exercise in normal performing dressage horses.

Methods: In total, 21 normal performing dressage horses, aged 5–16 years (mean 9 years), were examined clinically and echocardiographically to detect the prevalence of valvular regurgitation. Electrocardiographic (ECG) examinations were performed during rest and during a standardised dressage exercise programme, as well as during recovery period 1 h after exercise. All ECG recordings were analysed for presence and frequency of arrhythmias and arrhythmias were correlated with HR. Mean HR for walk, trot, canter and for specific dressage movements, as well as maximum HR was calculated. Fisher's exact test was used to test for associations between arrhythmias, valvular regurgitation, age and gender.

Results: SVPCs occurred rarely during rest, but occurred during exercise in 6 (28.6%), and in 13 (61.9%) horses during recovery period. Most horses had ≤3 SVPCs. Ventricular premature complex were seen in one horse, which developed twice during exercise. Mild valvular regurgitation was seen in 11 (52%) of the horses. No significant associations between arrhythmia and age, gender or valvular regurgitation were observed.

Conclusions: Reference values for normal performing dressage horses are presented for the first time, demonstrating that arrhythmias occur infrequently during exercise and recovery. Maximum HR in dressage horses is low compared to both showjumping and racehorses. Studies of dressage horses with moderate to severe valvular regurgitation at rest or poor performance are needed to further elucidate the significance of cardiac arrhythmias.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

Cardiac disease is a known cause of poor performance and, in severe cases, sudden death in horses (Kiryu et al. 1999; Martin et al. 2000). When a horse presents with a history of poor performance, cardiac disease is an important differential diagnosis. In a large retrospective study of 348 racehorses with poor performance, 9.5% had clinically significant cardiac arrhythmias and 29.3% had murmurs that required further examination (Martin et al. 2000). In the racing industry, cardiac disease leading to fatal arrhythmias is known to be a cause of sudden death occurring on the racetrack or during training (Gelberg et al. 1985; Brown et al. 1988; Kiryu et al. 1999). A recent study has found cardiovascular problems to be the cause of 7.9% of deaths in middle-aged and older riding horses (Stevens et al. 2009). Cardiac function and pathology in racehorses has been studied extensively, however we have very little knowledge of normal cardiac function in riding horses, other than conclusions extrapolated from studies of racehorses. With a greater knowledge of cardiac function in normal dressage horses, it will become easier to evaluate the significance of cardiological findings in these horses when examined for poor performance.

The objective of this study was to describe the prevalence of exercise-associated arrhythmias and heart rate in healthy dressage horses during different gaits and dressage exercises.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

Horses

Twenty-seven horses from 3 different dressage yards in Denmark were included in the study. They were all competition horses and had been in full training for at least 4 weeks prior to the study and were performing normally at the time of the examination. Five (18.5%) of these horses were excluded from the study due to unsatisfactory electrocardiographic (ECG) recordings. Three horses had ECGs of unacceptable quality and 2 had destroyed the ECG equipment in the stable during the recovery period. Another horse had to be excluded because it was sold during the study period. Hence, 21 dressage horses completed the study. The horses were aged 5–16 years, mean 9 years. There were 16 geldings (76.2%), 4 mares (19%) and one stallion (4.8%). All were Warmbloods; 15 Danish Warmbloods, 2 Dutch Warmbloods (KWPN), one Hannoverian, one Oldenburger, one Trakehner and one Holstein Thoroughbred cross. Ten of the horses were performing dressage at Elementary level, 7 at Medium level and 4 at Prix St Georges level. All horses presented for the study received a thorough clinical examination including temperature, auscultation of heart and lungs, an ECG examination, as well as a standardised echocardiographic examination. We included horses with a normal clinical examination and for auscultation of the heart we accepted horses with murmurs <IV/VI and no pathological arrhythmias during auscultation or resting ECG.

Echocardiography

The echocardiographic examination was performed using 2D, M-mode and colour flow Doppler imaging techniques using an ultrasound machine (Vividi)1 with a 1.5 MHz phased array probe with harmonic imaging. The examination was performed by one operator (R.B.) using a standardised protocol described by Buhl et al. (2004) including measurement of heart size and detection of valvular regurgitation. For tricuspid and mitral valvular regurgitation the area of the regurgitation jet in comparison with the approximated size of the atrium was used to categorise the size of the regurgitation. A very small jet occupying <10% of the area of the atrium was classified as mild regurgitation (Buhl et al. 2004). For aortic regurgitation maximal diameter of the regurgitation just below the aortic valve was measured. The diameter of the jet in comparison with the diameter of the left ventricular outflow tract (LVOT) was used to categorise the size of aortic regurgitation (Willems et al. 1997). We defined mild regurgitation as the maximal diameter of the regurgitant jet relative to LVOT being less than one third. A moderate aortic regurgitation was defined as the relation between the maximal diameter of the regurgitant jet relative to LVOT being one third to one half. The operator was blinded to the electrocardiographic results obtained in the horses.

Electrocardiograms

The ECG was recorded using a telemetric ECG unit (KRUTECH Televet)2, with electrodes placed in a modified base-apex conformation. The red (negative electrode) and black (earth) electrodes were placed on the left proximal scapular region and the green (positive electrode) and yellow (positive/negative electrode) electrodes just to the left of the ventral midline, approximately 10 cm caudal to the girth. Adhesive foam tape3 was used as an extra safety to secure the electrodes.

The ECG unit was fixed to the saddle and electrode cable was connected to the ECG unit. ECG examinations consisted of 3 phases; rest, exercise and recovery. A timer was started once the recording was started. Resting ECG was recorded before the horse was ridden, for a minimum of 2 min. Following this, the horse was taken to the riding arena. All events occurring during ridden work and the corresponding times were noted, so that any arrhythmias found upon later analysis could be correlated with the work being performed. The riders were asked to ride approximately the same programme, consisting of a warm-up in walk and trot, 10 min of loosening work, 10 min of collected work and finally, a dressage programme corresponding to the horse's dressage performance level.

The horses at Elementary level performed walk, trot and canter with tempo changes, counter-canter and single flying changes and shoulder-in in walk and canter.

The horses at Medium level performed as the Elementary horses, but also demonstrated travers in trot and canter, serial flying changes and 2 were able to do pirouettes in canter.

The horses at Advanced level (Grand Prix and Prix St Georges) performed as above, but further demonstrated pirouettes in canter, as well as passage and piaffe.

In total, each horse was exercised for approximately 45 min. The ECG recording was continued for 1 h post exercise, in order to detect any arrhythmias during recovery period.

The ECG recordings were subjected to a rhythm analysis by the integrated software program2. The software is able to recognise R-waves and thereby calculate R-R intervals and HRs. The maximum accepted deviation in R-R interval was set to 20% during rest and recovery phase and at 10% during exercise according to a previous study (Nørgaard et al. 2008).

All R-R interval deviations were controlled manually by the author and thereafter classified based on the following arrhythmia definitions:

  • • 
    Sinus pause: >20% (during rest) or 10% (during exercise) increase of distance between 2 R waves (Garcia and Miller 2004; Ulfberg and Clark 2006).
  • • 
    Second degree AV Block: P wave not associated with any following QRS complex and double length of R-R interval (Bonagura and Miller 1985).
  • • 
    Supraventricular premature complex (SVPC): R-R interval decreased >20% (during rest) or 10% (during exercise) in distance from the previous R-R interval and no change in configuration of the QRS-complex (Nørgaard et al. 2008).
  • • 
    Supraventricular tachycardia (SVT): a series of >3 consecutive SVPCs.
  • • 
    Ventricular premature complex (VPC): R-R interval decreased >20% (during rest) or 10% (during exercise) in distance from the previous R-R interval and configuration of QRS complex was of obviously higher amplitude, longer duration and abnormal morphology than the previous sinus QRS complex (Reimer et al. 1992; Nørgaard et al. 2008).

Statistics

From the results of the clinical, ECG and echocardiographic examinations, prevalence of arrhythmias occurring during rest, exercise and recovery, as well as prevalence of valvular regurgitation was calculated. To evaluate associations between variables, Fisher's exact test was chosen, due to its suitability to smaller sample sizes. The following variables were tested for association: Age group: 1 (≤6 years of age), 2 (7–10 years of age), 3 (≥11 years of age); Gender: M (geldings and stallion), F (mares); valvular regurgitation (+/-); SVPC (+/-); VPC (+/-); sinus pause (+/-). All statistical calculations were performed using Minitab4 statistical software and statistical inferences were made on the 0.05 level.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

Clinical findings

Rectal temperature, respiratory rate and pulmonary auscultation were within normal reference values. Cardiac auscultations were unremarkable with resting HR within normal reference values, cardiac rhythm was normal and no murmurs were auscultated except in one horse, which had a grade III/VI holodiastolic murmur with a PMI over the aortic valve area. Since murmurs of grade III/VI or less were acceptable according to the inclusion criteria of the study, this horse was not excluded.

Echocardiography

The echocardiographic examinations revealed that 11 (52.4%) of the horses had valvular regurgitation (mitral: n = 1, tricuspid: n = 5, aortic: n = 5), they were all classified as mild except one aortic regurgitation that was classified as moderate. This was the same horse that had a murmur diagnosed during the clinical examination. No significant enlargement of the size of left ventricle or abnormal contractility of the left ventricle was observed.

Heart rate

Mean heart rate (HR) during walk, trot, canter and for specific dressage movements found in this study are presented in Table 1. Overall, the maximum HR reached during the dressage exercise programme ranged from 125–193 beats/min, with a mean of 140 beats/min. Maximum HR occurred when performing dressage movements such as serial flying changes, pirouettes during canter, or passage or piaffe. In 4 horses, the maximum HRs were generated when the horses were frightened or being disobedient (e.g. bucking). Although only the 4 horses at Advanced level were able to perform passage and piaffe, and 6 were capable of pirouettes and travers in canter, these horses performed the movements several times and therefore there were sufficient recordings to calculate mean HR for these movements.

Table 1. Mean ± s.d. heart rate (beats/min) for the horses in the present study during walk, trot, canter and for selected dressage manoeuvres
 Heart rate
  1. n, number of horses that demonstrated the different movements during the ridden ECG recording.

Walk (n = 21)80 ± 9
Trot (n = 21)110 ± 7
Canter (n = 21)128 ± 5
Serial flying changes (n = 8)124 ± 16
Travers, canter (n = 6)126 ± 10
Pirouette, canter (n = 6)131 ± 15
Passage (n = 4)131 ± 11
Piaffe (n = 4)132 ± 13

ECG

The arrhythmias detected in this study were second degree AV block, SVPCs, VPCs, SVT and sinus pauses (Fig 1). Their prevalence and distribution for the individual horses are shown in Table 2.

image

Figure 1. ECG appearance of some of the arrhythmias (marked by arrows) found in the present study. (a) Second degree AV block, (b) Sinus pause, (c) SVPC during exercise (HR 139 beats/min) and (d) VPC during exercise (HR 102 beats/min). Paper speed 50 mm/s, gain 20 mm/mV.

Download figure to PowerPoint

Table 2. Total number of SVPCs, VPCs and SVT seen during rest, exercise and recovery. If double or triple SVPCs were found, this is specified in brackets
HorseSexAge (years)BreedDressage levelSVPC (rest)SVPC (exercise)SVPC (recovery)VPC (exercise)SVT (recovery)
  1. G, gelding; M, mare; S, stallion; DV, Danish Warmblood; O, Oldenbourger; KWPN, Dutch Warmblood; Trak, Trakehner; HxTb, Holstein Thoroughbred cross; Hann, Hannoverian; E, Elementary; M, Medium; PSG, Prix St Georges; GP, Grand Prix.

1M16DVE00100
2G6DVM00100
3G6DVM00200
4G8DVM00000
5G6DVE00300
6M9OE00100
7G9DVE109 (3 double + 1 triple)01
8G9DVM00200
9G6DVE003 (1 single + 1 double)00
10G9DVM00000
11M8DVM002 (1 double)00
12G6DVE01000
13G11DVPSG00200
14G11KWPNGP00000
15S16TrakPSG08 (4 single + 2 double)2 (1 double)00
16G13HxTbPSG00000
17G5DVE03200
19G5DVE03 (1 single + 1 double)100
20G8HannE00000
25G13DVM0102 (single)0
26M9KWPNE01000

Four of the 21 horses in the study had second degree AV block. In all 4 horses the arrhythmia disappeared during exercise and returned during the recovery period, once HR had returned to resting levels.

SVPCs

The total number of horses that had SVPCs during any time of ECG recording (rest, exercise and/or recovery) was 16 (76.2%). The total number of SVPCs per horse was <3 except for one horse that had a total of 8 SVPCs during the exercise (4 single and 2 double SVPCs) period and another that developed 9 during recovery period (3 double and one triple) (Table 2).

The SVPCs during peak exercise occurred at a mean HR of 133 (±17), while the horses were trotting or cantering and performing controlled manoeuvres such as pirouettes, travers and collection. The prevalence of SVPCs was highest during the recovery period as it occurred in 13 (61.9%) of the horses in the study at a mean HR of 43 (±7).

The horse that had a total of 9 SVPCs during the recovery period was the only horse that had a triple SVPC and also had an episode of SVT during the recovery period as well as an SVPC during rest. The horse was 9-years-old and performing at Elementary level. However, this horse had no arrhythmias of any kind during exercise and the echocardiographic examination revealed no abnormalities.

VPCs

Ventricular premature complexes were only seen during exercise, and only occurred in one horse (4.8%), as seen in Table 2. The first VPC occurred at a HR of 120, while the horses was performing a pirouette in canter and the second VPC occurred at a HR of 102 when the horse was performing flying changes.

Sinus pauses

Sinus pauses occurred in 12 (57.1%) of the horses during the recovery period while the HR was returning to resting levels at a mean HR of 50 (HR range 33–65 beats/min). Seven of the horses had a single sinus pause, whereas 2 horses had 2 single sinus pauses and 3 horses had 4 single sinus pauses.

Statistical analysis revealed no significant associations (P>0.05) between age (Groups 1, 2, 3); gender (M, F); valvular regurgitation (+/-) or the following arrhythmia SVPC (+/-), VPC (+/-) and sinus pause (+/-).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

Prevalence of arrhythmias during exercise in dressage horses is presented for the first time and the present study provides normal reference values of arrhythmias in this particular segment of the equine population. Further, mean HR during walk, trot and canter, as well as during specific dressage movements are presented for the first time.

The protocol of this study included meticulous record keeping of the events occurring during riding, which allowed us to correlate HR and arrhythmias to the events during exercise. Study of HR in dressage horses has been published (Geering et al. 2006), but unfortunately the study only evaluated mean HR throughout the entire exercise period at 3 different dressage levels (Elementary, Medium and Prix St Georges) and did not differentiate between the gaits or movements. The study reported mean HRs during dressage competition for Elementary level (103 ± 14 beats/min), Medium level (105 ± 11 beats/min) and Prix St Georges level (112 ± 12 beats/min) (Geering et al. 2006). Other HR studies have been performed on a treadmill during walk, trot and canter (Scheffer et al. 1995; Scheffer and Sloet van Oldruitenborgh-Oosterbaan 1996), but exercising on a treadmill can not be directly compared to dressage riding in an arena. Mean ± s.d. HR for walk, trot and canter from the 2 aforementioned studies, which shows a significantly higher HR for the horses cantering on the treadmill (150 ± 18 and 157 ± 19 beats/min) in comparison to HR in horses cantering in the present study (128 ± 5 beats/min). The reason for this difference could be that exercising on a treadmill is not natural for a horse and thus the higher HRs could be a result of stress. It is our experience that it is difficult to make horses canter at a slower pace on a treadmill compared to when a rider is controlling the horse. Another possible explanation could be that horses in the treadmill studies were riding school horses and hence probably less fit than the competition level dressage horses of the present study.

Mild valvular regurgitation was observed in 11 (52%) of the horses which is lower than reported in racehorses, where 80–90% of clinically normal racehorses have one or more sites of mild regurgitation detected by colour flow Doppler echocardiography (Buhl et al. 2005; Young et al. 2008) and to our knowledge no such studies of dressage horses have been performed before. We did not find any significant correlation between valvular regurgitation and arrhythmias but as the regurgitations observed in the present study were mild, except one classified as moderate and no cardiac enlargement was observed, this result was to be expected. Future exercise ECG studies of horses suffering from moderate or severe valvular regurgitation are needed to evaluate the effect of valvular pathology on cardiac electrocardiographic activity.

The sinus pauses discovered in this study occurred in approximately half of the horses and purely during the recovery period, while HR was returning to resting levels. Sinus pause has been thoroughly described in human literature (Garcia and Miller 2004; Ulfberg and Clark 2006) and bears some resemblance to SA block, but is shorter in duration. Therefore, it could be assumed that sinus pauses occur as a result of returning vagal tone creating intermittent, minor delays in the SA node conduction rate. It is unlikely that this arrhythmia has any pathological effects.

Supraventricular premature complexes and VPCs were seen during exercise in 6 (28.6%) and one (4.8%) horses, respectively. Previous studies in racehorses have observed different prevalence and in general SVPCs were seen more frequently than VPCs during and immediately after exercise (Ryan et al. 2005; Jose-Cunilleras et al. 2006; Lindholm et al. 2008), which is in agreement with the present results. It is, however, problematic to compare dressage horses and racehorses, since there are great differences in the athletic demands expected of racehorses vs. riding horses, which may influence the prevalence of arrhythmias. The mean maximal HR reached by dressage horses during exercise in this study was 140 beats/min. For showjumping horses maximum HR is higher, at approximately 180 beats/min (Art et al. 1990; Sloet van Oldruitenborgh-Oosterbaan et al. 2006), whereas maximum HR reached during racing and training in racehorses is up to 240 beats/min (Betros et al. 2002). Therefore it is clear that dressage horses perform at a much lower athletic level and, inevitably, their hearts are generally working under fewer physical demands.

The previously recommended cut-off values for significance of premature contractions are >2 isolated premature contractions during peak exercise or multiple (>5) pairs or paroxysms of premature contractions detected during the recovery period (Martin et al. 2000). The cut-off values are derived from a retrospective study of Thoroughbred and Standardbred racehorses presented with poor performance without knowing the prevalence of premature contractions in healthy horses (Martin et al. 2000). Two studies of exercise-associated arrhythmias in Thoroughbred racehorses (Ryan et al. 2005; Jose-Cunilleras et al. 2006) have brought the current cut-off values into question as high prevalence has been observed in clinically normal, well performing racehorses (Ryan et al. 2005).

If these cut-off values are employed, 4 of the horses (19.0%) in the present study would be categorised as having significant arrhythmias despite apparently being healthy and performing well. In addition, the cut-off values do not differentiate between SVPCs and VPCs, but simply regard them as one category. It would be more appropriate to define separate cut-off values for SVPCs and VPCs, since SVPCs, according to the present and other studies, are more prevalent in healthy horses than VPCs (Raekallio 1992; Scheffer and Sloet van Oldruitenborgh-Oosterbaan 1996; Lindholm et al. 2008) and hence should be evaluated independently. Also, VPCs are more likely to reflect severe cardiac pathology than SVPCs (Reimer et al. 1992), which further supports the need for separate evaluation.

Although we expected the horses to be clinically normal, our inclusion criteria might not have diagnosed mild abnormalities. In particular, the horse with 3 double and one triple SVPC as well as an episode of SVT during recovery might have reduced cardiac performance. The horse was only performing at elementary level and may simply not be in hard enough training for a limitation on its performance to be recognised.

It should also be mentioned that the resting ECGs recorded in this study may not be true resting ECGs, due to the fact that some horses have increased adrenergic tone simply from being taken out of their stables, being saddled, etc. However, this is unlikely to have had any significant effect on the results.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

In conclusion, reference values of electrocardiographic changes in healthy dressage horses during exercise and recovery have been described for the first time. Some of the horses developed SVPCs and VPCs during exercise, whereas only sinus pause and SVPCs were registered during recovery, which probably should be considered normal findings when examining healthy dressage horses. The arrhythmias occurred in low number per horse and no correlation between prevalence of arrhythmias and valvular regurgitation was demonstrated. Heart rate during walk, trot and canter as well as during specific manoeuvres was registered for the first time in dressage horses. Maximum HR in dressage horses is low in comparison to both showjumping and racehorses. Normal reference values are a prerequisite when evaluating dressage horses presented with reduced performance and future studies should include more horses and focus on evaluating horses with moderate or severe valvular regurgitation, cardiac enlargement or reduced cardiac performance.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

The authors would like to thank the trainers Astrid Gemal, Maj Tofte Olesen, Lotte Wagner and Torben Frandsen as well as their staff for fantastic support and help during the study.

Manufacturers' addresses

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References

1 GE Healthcare, Brøndby, Denmark.

2 Kruuse A/S, Langeskov, Denmark.

3 Snögg A/S, Kristiansand, Norway.

4 Minitab, State Collage Pennsylvania, USA.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. Conflicts of interest
  10. Manufacturers' addresses
  11. References
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