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Arrhythmias in Thoroughbreds During and After Treadmill and Racetrack Exercise
Journal of Equine Veterinary Science 42 (2016) 19–24
Contents lists available at ScienceDirect
Journal of Equine Veterinary Science journal homepage: www.j-evs.com
Original Research
Arrhythmias in Thoroughbreds During and After Treadmill and Racetrack Exercise Cristobal Navas de Solis*,1, Christina M. Green, Raymond. H. Sides, Warwick. M. Bayly Department of Veterinary Clinical Sciences and College of Veterinary Medicine, Washington State University, Pullman, WA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 16 December 2015 Accepted 16 March 2016 Available online 4 April 2016
The objectives of the study were to develop information regarding the frequency of recurrence of exercising arrhythmias and the relationship of arrhythmia development to exercise intensity and type of exercise in Thoroughbred horses. Electrocardiograms (ECGs) were recorded on nine Thoroughbreds during maximal or submaximal exercise on a racetrack (Ra) and treadmill (Tm). The frequency of arrhythmias on a Ra and Tm was compared, and their relationship to exercise intensity (expressed as HR/HRmax [%]) was evaluated. Sixty-five workouts were analyzed: 46 workouts were on a Tm and 19 on a Ra; median number of workouts/horse was four, and the range was 2–14. Exercising arrhythmias were detected in 4/9 horses (12/65 workouts), and there were postexercise arrhythmias in 7/9 horses (19/65 workouts). Arrhythmias were detected at some point in 8/9 horses. For 7/9 horses, the same rhythm result was obtained during exercise in repeated recordings. For 7/9 horses, the postexercise rhythm was variable: postexercise arrhythmias were present in median: 21%; range: 0%–75% of workouts. The presence of arrhythmias was positively related to exercise intensity (P ¼ .01; odds ratio ¼ 1.2) and all occurred during workouts at 94% of HR/HRmax (%). Arrhythmias during exercise were more frequent on the Ra than on the Tm (P ¼ .009). A single ECG did not always display all the arrhythmias detected over several exercise tests. The presence/absence of exercising arrhythmias was more consistent than postexercise arrhythmias. Arrhythmias were more likely to be detected at maximal or near-maximal intensities and during gallops on the Ra. A larger population needs to be studied before more definitive conclusions are drawn. Ó 2016 Elsevier Inc. All rights reserved.
Keywords: Horse Exercising arrhythmias Electrocardiogram Arrhythmogenesis Sudden cardiac death
1. Introduction Healthy and poorly performing horses can display arrhythmias during exercise and immediately afterward. The frequency of arrhythmias described in the literature varies with the discipline and the characteristics of the study population (healthy vs. poorly performing horses) [1–8]. Arrhythmias during or immediately after exercise are a
* Corresponding author at: Cristobal Navas de Solis, Texas Veterinary Medical Center, 4475 TAMU, College Station, TX 77843-4475. E-mail address: [email protected] (C. Navas de Solis). 1 Cristobal Navas de Solis present address is Texas Veterinary Medical Center, Texas A&M University, 4475 TAMU, College Station, TX, 77,8434475. 0737-0806/$ – see front matter Ó 2016 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2016.03.018
potential cause of sudden cardiac death [9–11]. However, in many cases, these arrhythmias are inconsequential [1,2,4,6]. The risk that the detection of arrhythmias implies or the limits between acceptable and nonacceptable rhythms are incompletely understood. The frequency of arrhythmia detection varies between studies. Up to 96% and 76% of exercising electrocardiograms (ECGs) in normal showjumpers [1] and dressage [2] horses, respectively, can be expected to have supraventricular or ventricular premature depolarizations (SVPDs or VPDs) if the warm-up period is included. The occurrence of arrhythmias is less frequent if only peak exercise is considered, and overall, ventricular arrhythmias are less frequent than supraventricular arrhythmias [2–6]. The immediate postexercise period is a vulnerable time for
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arrhythmogenesis, with arrhythmia development reported in a large percentage of horses likely due to the rapid return of parasympathetic tone [6,7]. To be able to interpret the clinical significance of exercising arrhythmias and to preserve the health of horses and safety of their riders, studies in differing populations of performance horses and about the reproducibility of exercising ECGs are needed and have recently been recommended by a panel of experts [12]. The present study was designed to gain insight into the frequency of recurrence of arrhythmias during and after exercise and the relationship of arrhythmia development to exercise intensity and type of exercise.
2. Materials and Methods Electrocardiograms were recorded prospectively on nine Thoroughbreds during maximal or submaximal exercise on a racetrack (Ra) and treadmill (Tm). The workouts were part of normal exercise routines of a research group of horses to maintain fitness or part of their preparation for other exercise physiology studies. Only workouts for which the maximal heart rate exceeded 185/ min and for which the recording quality was good were considered. Recording quality was considered good when the R-R interval could be followed throughout the recording. The sampling was of convenience, and there was no randomization. During the study period (August 7, 2013–October 10, 2013), horses exercised other times apart from the times when the ECGs were recorded. The number of resting days between exercise tests was variable (median [range], 2 [1–29] days). There were three mares and six geldings of age (median [range]), 11 [4–18] years that were fit to run at maximal speed. All horses had normal general physical examinations including cardiac auscultation. Horses were ridden on a dirt, 0.8 km Ra, and/or exercised on a high-speed Tm. All horses were participating in other exercise physiology studies, and their maximal heart rates (HRmax) had been previously determined. The intensity of the exercise was expressed as HR/HRmax (%) where HR was the maximal heart rate recorded during each exercise test. A modified base-apex lead was used to allow the horse to be ridden with a saddle or exercised on the Tm with a fitted surcingle [1,12–14]. Adhesive ECG electrodes were glued beneath each other near the left side of the withers in front of the saddle, and two electrodes were placed similarly, just to the left of ventral midline, behind the girth or surcingle. An ECG Televet unit (Engel Engineering Service GmbH, Heusenstamm, Germany) was connected to the electrodes and fixed with adhesive tape to the front of the saddle or surcingle as previously described [1]. Electrocardiogram recordings were stored on a Secure Digital card and reviewed using Televet 100 software (Engel Engineering Service GmbH, Heusenstamm, Germany). For the purpose of this study, sinus arrhythmia or block, second degree AV block, or sinus pauses were ignored. Only arrhythmias during exercise tests (excluding warm-up) and during fast heart rate deceleration, while horses were walking and until the heart rate was 100/min, were considered.
Rhythm disturbances were classified as SVPDs and VPDs [15]. Supraventricular premature depolarizations were defined as complexes for which the R-R interval decreased >10% in duration from the previous R-R interval and for which there was no change in configuration of the QRS complex. Ventricular premature depolarizations were defined as complexes for which the R-R interval decreased >10% in duration from the previous R-R interval and for which configuration of the QRS complex was obviously abnormal when compared to the previous sinus QRS complex. The relationship of the presence of arrhythmias with the type of exercise test (Ra or Tm) and their relationship to exercise intensity (expressed as HR/HRmax [%]) was evaluated by logistic regression analysis. The model corrected for the effect of the individual animal by including horse into the model as a repeated effect. The method used was generalized estimating equations as described by Liang and Zeger, and commercial statistical software (SAS 9.2. SAS Institute Inc, 100 SAS Campus Drive, Cary, NC 27513-2414) was used. The binary outcome was presence (or absence) of arrhythmia. Type of exercise test (Ra or Tm) and HR/HRmax (%) was used as explanatory variables (fixed effects). The analysis was performed separately for arrhythmias during exercise and after exercise.
3. Results Electrocardiograms from 65 workouts were analyzed (Figs. 1–3). Forty-six workouts were on a Tm and 19 on a Ra; median number of workouts/horse was four, and the range of workouts/horse was [2–14]. Recordings of seven exercise tests were not included in the analysis: two due to HR being under 185/min, two due to a poor quality tracing, and three due to problems with data recording. The (mean standard deviation) HR/HRmax (%) was 96 4% (range 87%–100%), and the HR was 215 13/min. There was no difference in HR (P ¼ .06) or HR/HRmax (%) (P ¼ .29) between exercise bouts performed on the TM and the Ra (Table 1). Exercising arrhythmias were detected in 4/9 horses (12/65 workouts), and there were postexercise arrhythmias in 7/9 horses (19/65 workouts). Overall, arrhythmias were detected at some point of the study in 8/9 horses (25/65 workouts). Tables 1 and 2 summarize rhythm results. The rhythm result during exercise was repeated in all exercise tests in 7/9 horses (Table 2). For 2/9 horses, the rhythm during exercise varied (Table 2). The rhythm result after exercise was repeated in all exercise tests in 2/9 horses. For 7/9 horses, the postexercise rhythm result was variable (Table 2). Presence of arrhythmias during exercise (P ¼ .01; odds ratio ¼ 1.2; confidence interval ¼ 1.1–1.3), but not after exercise, was associated with HR/HRmax (%) and all occurred during workouts at 94% of HR/HRmax (%). Exercising on the Ra was associated with a higher frequency (P ¼ .009) of arrhythmias during exercise (Table 1) but not after exercise.
C. Navas de Solis et al. / Journal of Equine Veterinary Science 42 (2016) 19–24
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Table 1 Summary of exercising and postexercising arrhythmias in exercise tests on a treadmill and a racetrack. Group
Racetrack Treadmill Total
n
HR; HR/HRmax (%)
209 9; 95 14 217 14; 96 4 215 13; 96 4
19 46 65
Arrhythmia Exercise
Arrhythmia Postexercise
Proportion
SVPDs/Test; Proportion
VPDS/Test; Proportion
Proportion
SVPDs/Test; Proportion
VPDs/Test; Proportion
5/19 Testa 7/46 Testsa 12/65 Test
0 (0–8); 5/19 Tests 0 (0–11); 7/46 Tests 0 (0–11); 11/65 Tests
0 (0–1); 1/19 Tests 0; 0/46 Tests 0 (0–1); 1/65 Tests
5/19 15/46 Tests 20/65
0 (0–2); 3/19 Tests 0 (0–3); 6/46 Tests 0 (0–3); 9/65 Tests
0 (0–2); 2/19 Tests 0 (0–5); 8/46 Tests 0 (0–5); 10/65 Tests
Abbreviations: HR, maximal heart rate during the exercise test; HR/HRmax, HR during the exercise tests/maximal heart rate for the horse 100; n, number of exercise tests; proportion, number of exercise tests that presented the indicated arrhythmia/n; SVPD, supraventricular premature depolarization; VPD, ventricular premature depolarization. a Denotes statistically significant difference (P ¼ .009). SVPDs and VPDs are reported as median (range).
4. Discussion The postexercise rhythm results were not repeated during consecutive workouts in many cases in this study. Exercising arrhythmias were more consistent than postexercise arrhythmias. In humans, arrhythmias detected in several exercise tests are considered more relevant as the repeatability of serious rhythm disturbances is high (76%) in human athletes [16]. However, in individuals with certain channelopathies, several exercise tests may be necessary to detect arrhythmias that evidence an increased risk for death [17]. The causes for sudden cardiac death are very different in horses when compared to humans, and channelopathies are currently not recognized in horses. Conclusions about how many exercising ECGs are needed in horses to determine if exercising arrhythmias are the cause of poor performance or increase the risk for collapse or sudden cardiac death cannot be drawn from the data reported here due to the small data set and the uncertain clinical relevance of the arrhythmias displayed by these horses. Studies in larger populations and in horses with cardiovascular disease are needed to resolve this matter. The fact that arrhythmias are more likely to be detected at higher exercise intensities and that are more likely to happen during the postexercise period has been reported previously [4,6]. The present study is the first to quantify the relationship with exercise intensity. This finding highlights the importance of performing exercise tests that are at an intensity equal or that slightly exceeds the horse’s customary activities [10].
Arrhythmias were more frequent when horses exercised on a Ra than on a Tm. The exercise intensity was not different between Ra and Tm exercise tests so perhaps, other variables such as the presence of a rider, psychological factors associated with the environment, and so forth can affect arrhythmogenesis in horses. This suggests that when evaluating arrhythmias during exercise, not only intensity, but also other circumstances of the exercise tests should be taken into account. Horses in the present study did not present cardiac abnormalities, and horses with cardiovascular disease may behave differently. The clinical significance of certain exercising and postexercising arrhythmias is a matter of debate in equine and human athletes [12,18,19]. Similarly to previous reports, arrhythmias during and immediately after exercise were frequent [1–6], and a large proportion of horses (6/ 9) would have been classified as having “clinically important” arrhythmias using previously described criteria [5]. The presence of VPDs during exercise was very rare (1 single VPD in 65 recordings) in the group of horses reported here as it was in previous studies [1–6]. The matter of which arrhythmias are clinically relevant cannot be resolved with the methods used in this study. It is important to realize that although some arrhythmias are common and innocuous, not all arrhythmias are innocuous and exercising and postexercising rhythm alterations are a potential cause of sudden death or collapse in horses [9–11]. Sudden cardiac death jeopardizes horses’ health, the safety of riders, and the public perception of welfare in equestrian sports. Additional studies are
Table 2 Summary of exercising and postexercising arrhythmias for each horse. Horse
n (TM/Ra)
1 2 3 4 5 6 7
2 2 2 4 4 11 12
(1/1) (2/0) (0/2) (1/3) (4/0) (9/2) (8/4)
8 9
14 (11/3) 14 (10/4)
Arrhythmia Exercise
Arrhythmia Postexercise
SVPDs/Test; Proportion
VPDs/Test; Proportion
SVPDs/Test; Proportion
VPDs/Test; Proportion
2–5; 2/2 3–11; 2/2 0 0 0 0–2; 1/11 2, 0–8; 7/12
0–1; 1/2 0 0 0 0 0 0
0–1; 1/2 0 0 0 0 0 0, 0–2; 3/14
0 0–1; 1/2 0 0–5; 1/4 0–3; 3/4 0 0, 0–4; 2/14
0 0
0 0
0, 0–1; 2/14 0, 0–2; 3/14
0, 0–2; 3/14 0
Description Rhythm Abnormality
Single PDs Single PDs Normal rhythm Run of 4 VPDs postexercise and 1 single VPD Couplets and triplet of R on T Single PDs Couplets of VPDs (2), couplet and triplet of SVPDs. Else single PDs Couplets (2) of R on T. Else single PDs Single PDs
Abbreviations: n, number of exercise tests; PD, premature depolarization; proportion, number of tests with arrhythmias/n; Ra, racetrack; SVPD, supraventricular premature depolarization; TM, treadmill; VPDs, ventricular premature depolarization.
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Fig. 1. Normal exercising electrocardiogram of an 11-year-old Thoroughbred gelding exercising on a racetrack under saddle. The dotted lines mark the R waves, and instantaneous heart rates are depicted at the bottom.
needed to determine which type and frequency of arrhythmias affect performance and safety, to quantify risks, and to determine which circumstances increase the likelihood of arrhythmia development.
Horses of this research group were not race fit and were classified as normal based on the absence of signs of cardiac disease during physical examination and the lack of a history of cardiac disease or poor performance. Echocardiograms,
Fig. 2. Exercising electrocardiogram of a 13-year-old Thoroughbred gelding exercising on a treadmill. The vertical red lines mark the presence of supraventricular premature depolarizations. The dotted lines mark the R waves, and instantaneous heart rates are depicted at the bottom.
C. Navas de Solis et al. / Journal of Equine Veterinary Science 42 (2016) 19–24
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Fig. 3. Postexercise electrocardiogram of a 15-year-old Thoroughbred gelding. The first three vertical red lines mark the presence of ventricular premature depolarizations. The following two vertical lines are normal complexes. The dotted lines mark the R waves, and instantaneous heart rates are depicted at the bottom.
24-hour continuous ECGs, and measurements of cardiac biomarkers would have been an appropriate means of further characterizing this group and confirming or refuting the clinical impression that these horses had normal cardiovascular systems. Likewise, measuring electrolytes would have been interesting as electrolyte disturbances are a commonly mentioned reason for arrhythmia development. The study had several limitations such as the differences in the exercise tests performed between horses, the different intervals between tests, the size of the group, or the recording of a different amount of tests/horse. These limitations introduce variables that make drawing firm conclusions difficult. The absence of blinded reading of the ECGs is another limitation of the study. The interpretation of ECGs during exercise in horses presents some unresolved difficulties for which controversies and gaps in knowledge exist. A 10% variation in the R-R interval was chosen to define premature beats in this study based on previous literature [1,2,7]. This number is somewhat arbitrary and provides a conservative approach to the detection of arrhythmias. Some clinicians may use a shorter change in the R-R interval to determine prematurity. Further studies are needed to define the normal variation of the R-R interval and the relationship between prematurity and ectopy. In addition, the distinction of ventricular and supraventricular arrhythmias is not always clear during exercise tests. We followed the most commonly cited criteria for this classification [1–4,7]. These criteria have limitations, and future studies should better define the limits between SVPDs, VPDs, and aberrant ventricular conduction.
5. Conclusions We concluded that a single exercise test did not always display all the arrhythmias detected over several workouts and that the exercising rhythm was more consistent than the postexercise rhythm. Arrhythmias are more likely to be detected at maximal or near-maximal intensities and during exercise tests on a Ra when compared to a Tm. A larger population needs to be studied before firm conclusions are drawn. Acknowledgments The authors gratefully acknowledge Dr Kathy Seino for her assistance in data collection. There are no ethical considerations or competing interests to disclose. The study was funded internally by the Department of Veterinary Clinical Sciences of Washington State University that had no involvement in the study apart from authors being part of this department. The procedures were covered under the Animal Care and used protocol of the Exercise Physiology group at Washington State University. References [1] Buhl R, Meldgaard C, Barbesgaard L. Cardiac arrhythmias in clinically healthy showjumping horses. Equine Vet J Suppl 2010;38: 196–201. [2] Barbesgaard L, Buhl R, Meldgaard C. Prevalence of exerciseassociated arrhythmias in normal performing dressage horses. Equine Vet J Suppl 2010;38:202–7. [3] Jose-Cunilleras E, Young LE, Newton JR, Marlin DJ. Cardiac arrhythmias during and after treadmill exercise in poorly performing thoroughbred racehorses. Equine Vet J Suppl 2006;36:163–70.
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[4] Ryan N, Marr CM, McGladdery AJ. Survey of cardiac arrhythmias during submaximal and maximal exercise in Thoroughbred racehorses. Equine Vet J 2005;37:265–8. [5] Martin Jr BB, Reef VB, Parente EJ, Sage AD. Causes of poor performance of horses during training, racing or showing: 348 cases (1992-1996). J Am Vet Med Assoc 2000;216:554–8. [6] Physick-Sheard PW, McGurrin MK. Ventricular arrhythmias during race recovery in Standardbred racehorses and associations with autonomic activity. J Vet Intern Med 2010;24:1158–66. [7] Buhl R, Petersen EE, Lindholm M, Bak LR, Nostell K. Cardiac arrhythmias in Standardbreds during and after racingdpossible association between heart size, valvular regurgitations and arrhythmias. J Equine Vet Sci 2013;33:590–6. [8] Uhlendorf F, Gehlen H, Stadler P. Comparison of different electrocardiographic techniques for the detection of arrhythmias in horses. Tieraerztliche Praxis 2013;41:305–14. [9] Kiryu K, Machida N, Kashida Y, Yoshihara T, Amada A, Yamamoto T. Pathologic and electrocardiographic findings in sudden cardiac death in racehorses. J Vet Med Sci 1999;61:921–8. [10] Freel KM, Morrison LR, Thompson H, Else RW. Arrhythmogenic right ventricular cardiomyopathy as a cause of unexpected cardiac death in two horses. Vet Rec 2010;166:718–21. [11] Lyle CH, Uzal FA, McGorum BC, Aida H, Blissitt KJ, Case JT, Charles JT, Gardner I, Horadagoda N, Kusano K, Lam K, Pack JD, Parkin TD, Slocombe RF, Stewart BD, Boden LA. Sudden death in racing
[12]
[13]
[14]
[15] [16]
[17] [18] [19]
Thoroughbred horses: an international multicentre study of post mortem findings. Equine Vet J 2011;43:324–31. Reef VB, Bonagura J, Buhl R, McGurrin K, Schwarzwald C, van Loon G, Young LE. Recommendations for equine athletes with cardiovascular abnormalities. J Vet Intern Med 2014;28: 749–61. Young LE. Recent advances in diagnosing cardiac abnormalities with an ECG during exercise. A review. Proc Am Ass Equine Practnrs 2007;53:99–103. Norgaard LM, Johnsen EB, Restrepo J, Buhl R. Evaluation of R-R intervals from equine ECG recordings during submaximal and maximal exercise intensity: is there standard criterion for arrhythmia assessment? BEVA Congress 2008:138–9. Gatti L, Holmes JR. ECG recording at rest and during exercise in the horse. Equine Vet Edu 1990;2:28–30. Robinson B, Xie L, Temple J, Octavio J, Srayyih M, Thacker D, Kharouf R, Davies R, Gidding SS. Predicting utility of exercise tests based on history/holter in patients with premature ventricular contractions. Pediatr Cardiol 2015;36:214–8. Beckerman J, Wu T, Jones S, Froelicher VF. Exercise test-induced arrhythmias. Prog Cardiovasc Dis 2005;47:285–305. Walker J, Calkins H, Nazarian S. Evaluation of cardiac arrhythmia among athletes. Am J Med 2010;123:1075–81. Physick-Sheard PW. Seek and ye shall find: cardiac arrhythmias in the horse. Equine Vet J 2013;45:270–2.
Contents lists available at ScienceDirect
Journal of Equine Veterinary Science journal homepage: www.j-evs.com
Original Research
Arrhythmias in Thoroughbreds During and After Treadmill and Racetrack Exercise Cristobal Navas de Solis*,1, Christina M. Green, Raymond. H. Sides, Warwick. M. Bayly Department of Veterinary Clinical Sciences and College of Veterinary Medicine, Washington State University, Pullman, WA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 16 December 2015 Accepted 16 March 2016 Available online 4 April 2016
The objectives of the study were to develop information regarding the frequency of recurrence of exercising arrhythmias and the relationship of arrhythmia development to exercise intensity and type of exercise in Thoroughbred horses. Electrocardiograms (ECGs) were recorded on nine Thoroughbreds during maximal or submaximal exercise on a racetrack (Ra) and treadmill (Tm). The frequency of arrhythmias on a Ra and Tm was compared, and their relationship to exercise intensity (expressed as HR/HRmax [%]) was evaluated. Sixty-five workouts were analyzed: 46 workouts were on a Tm and 19 on a Ra; median number of workouts/horse was four, and the range was 2–14. Exercising arrhythmias were detected in 4/9 horses (12/65 workouts), and there were postexercise arrhythmias in 7/9 horses (19/65 workouts). Arrhythmias were detected at some point in 8/9 horses. For 7/9 horses, the same rhythm result was obtained during exercise in repeated recordings. For 7/9 horses, the postexercise rhythm was variable: postexercise arrhythmias were present in median: 21%; range: 0%–75% of workouts. The presence of arrhythmias was positively related to exercise intensity (P ¼ .01; odds ratio ¼ 1.2) and all occurred during workouts at 94% of HR/HRmax (%). Arrhythmias during exercise were more frequent on the Ra than on the Tm (P ¼ .009). A single ECG did not always display all the arrhythmias detected over several exercise tests. The presence/absence of exercising arrhythmias was more consistent than postexercise arrhythmias. Arrhythmias were more likely to be detected at maximal or near-maximal intensities and during gallops on the Ra. A larger population needs to be studied before more definitive conclusions are drawn. Ó 2016 Elsevier Inc. All rights reserved.
Keywords: Horse Exercising arrhythmias Electrocardiogram Arrhythmogenesis Sudden cardiac death
1. Introduction Healthy and poorly performing horses can display arrhythmias during exercise and immediately afterward. The frequency of arrhythmias described in the literature varies with the discipline and the characteristics of the study population (healthy vs. poorly performing horses) [1–8]. Arrhythmias during or immediately after exercise are a
* Corresponding author at: Cristobal Navas de Solis, Texas Veterinary Medical Center, 4475 TAMU, College Station, TX 77843-4475. E-mail address: [email protected] (C. Navas de Solis). 1 Cristobal Navas de Solis present address is Texas Veterinary Medical Center, Texas A&M University, 4475 TAMU, College Station, TX, 77,8434475. 0737-0806/$ – see front matter Ó 2016 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2016.03.018
potential cause of sudden cardiac death [9–11]. However, in many cases, these arrhythmias are inconsequential [1,2,4,6]. The risk that the detection of arrhythmias implies or the limits between acceptable and nonacceptable rhythms are incompletely understood. The frequency of arrhythmia detection varies between studies. Up to 96% and 76% of exercising electrocardiograms (ECGs) in normal showjumpers [1] and dressage [2] horses, respectively, can be expected to have supraventricular or ventricular premature depolarizations (SVPDs or VPDs) if the warm-up period is included. The occurrence of arrhythmias is less frequent if only peak exercise is considered, and overall, ventricular arrhythmias are less frequent than supraventricular arrhythmias [2–6]. The immediate postexercise period is a vulnerable time for
20
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arrhythmogenesis, with arrhythmia development reported in a large percentage of horses likely due to the rapid return of parasympathetic tone [6,7]. To be able to interpret the clinical significance of exercising arrhythmias and to preserve the health of horses and safety of their riders, studies in differing populations of performance horses and about the reproducibility of exercising ECGs are needed and have recently been recommended by a panel of experts [12]. The present study was designed to gain insight into the frequency of recurrence of arrhythmias during and after exercise and the relationship of arrhythmia development to exercise intensity and type of exercise.
2. Materials and Methods Electrocardiograms were recorded prospectively on nine Thoroughbreds during maximal or submaximal exercise on a racetrack (Ra) and treadmill (Tm). The workouts were part of normal exercise routines of a research group of horses to maintain fitness or part of their preparation for other exercise physiology studies. Only workouts for which the maximal heart rate exceeded 185/ min and for which the recording quality was good were considered. Recording quality was considered good when the R-R interval could be followed throughout the recording. The sampling was of convenience, and there was no randomization. During the study period (August 7, 2013–October 10, 2013), horses exercised other times apart from the times when the ECGs were recorded. The number of resting days between exercise tests was variable (median [range], 2 [1–29] days). There were three mares and six geldings of age (median [range]), 11 [4–18] years that were fit to run at maximal speed. All horses had normal general physical examinations including cardiac auscultation. Horses were ridden on a dirt, 0.8 km Ra, and/or exercised on a high-speed Tm. All horses were participating in other exercise physiology studies, and their maximal heart rates (HRmax) had been previously determined. The intensity of the exercise was expressed as HR/HRmax (%) where HR was the maximal heart rate recorded during each exercise test. A modified base-apex lead was used to allow the horse to be ridden with a saddle or exercised on the Tm with a fitted surcingle [1,12–14]. Adhesive ECG electrodes were glued beneath each other near the left side of the withers in front of the saddle, and two electrodes were placed similarly, just to the left of ventral midline, behind the girth or surcingle. An ECG Televet unit (Engel Engineering Service GmbH, Heusenstamm, Germany) was connected to the electrodes and fixed with adhesive tape to the front of the saddle or surcingle as previously described [1]. Electrocardiogram recordings were stored on a Secure Digital card and reviewed using Televet 100 software (Engel Engineering Service GmbH, Heusenstamm, Germany). For the purpose of this study, sinus arrhythmia or block, second degree AV block, or sinus pauses were ignored. Only arrhythmias during exercise tests (excluding warm-up) and during fast heart rate deceleration, while horses were walking and until the heart rate was 100/min, were considered.
Rhythm disturbances were classified as SVPDs and VPDs [15]. Supraventricular premature depolarizations were defined as complexes for which the R-R interval decreased >10% in duration from the previous R-R interval and for which there was no change in configuration of the QRS complex. Ventricular premature depolarizations were defined as complexes for which the R-R interval decreased >10% in duration from the previous R-R interval and for which configuration of the QRS complex was obviously abnormal when compared to the previous sinus QRS complex. The relationship of the presence of arrhythmias with the type of exercise test (Ra or Tm) and their relationship to exercise intensity (expressed as HR/HRmax [%]) was evaluated by logistic regression analysis. The model corrected for the effect of the individual animal by including horse into the model as a repeated effect. The method used was generalized estimating equations as described by Liang and Zeger, and commercial statistical software (SAS 9.2. SAS Institute Inc, 100 SAS Campus Drive, Cary, NC 27513-2414) was used. The binary outcome was presence (or absence) of arrhythmia. Type of exercise test (Ra or Tm) and HR/HRmax (%) was used as explanatory variables (fixed effects). The analysis was performed separately for arrhythmias during exercise and after exercise.
3. Results Electrocardiograms from 65 workouts were analyzed (Figs. 1–3). Forty-six workouts were on a Tm and 19 on a Ra; median number of workouts/horse was four, and the range of workouts/horse was [2–14]. Recordings of seven exercise tests were not included in the analysis: two due to HR being under 185/min, two due to a poor quality tracing, and three due to problems with data recording. The (mean standard deviation) HR/HRmax (%) was 96 4% (range 87%–100%), and the HR was 215 13/min. There was no difference in HR (P ¼ .06) or HR/HRmax (%) (P ¼ .29) between exercise bouts performed on the TM and the Ra (Table 1). Exercising arrhythmias were detected in 4/9 horses (12/65 workouts), and there were postexercise arrhythmias in 7/9 horses (19/65 workouts). Overall, arrhythmias were detected at some point of the study in 8/9 horses (25/65 workouts). Tables 1 and 2 summarize rhythm results. The rhythm result during exercise was repeated in all exercise tests in 7/9 horses (Table 2). For 2/9 horses, the rhythm during exercise varied (Table 2). The rhythm result after exercise was repeated in all exercise tests in 2/9 horses. For 7/9 horses, the postexercise rhythm result was variable (Table 2). Presence of arrhythmias during exercise (P ¼ .01; odds ratio ¼ 1.2; confidence interval ¼ 1.1–1.3), but not after exercise, was associated with HR/HRmax (%) and all occurred during workouts at 94% of HR/HRmax (%). Exercising on the Ra was associated with a higher frequency (P ¼ .009) of arrhythmias during exercise (Table 1) but not after exercise.
C. Navas de Solis et al. / Journal of Equine Veterinary Science 42 (2016) 19–24
21
Table 1 Summary of exercising and postexercising arrhythmias in exercise tests on a treadmill and a racetrack. Group
Racetrack Treadmill Total
n
HR; HR/HRmax (%)
209 9; 95 14 217 14; 96 4 215 13; 96 4
19 46 65
Arrhythmia Exercise
Arrhythmia Postexercise
Proportion
SVPDs/Test; Proportion
VPDS/Test; Proportion
Proportion
SVPDs/Test; Proportion
VPDs/Test; Proportion
5/19 Testa 7/46 Testsa 12/65 Test
0 (0–8); 5/19 Tests 0 (0–11); 7/46 Tests 0 (0–11); 11/65 Tests
0 (0–1); 1/19 Tests 0; 0/46 Tests 0 (0–1); 1/65 Tests
5/19 15/46 Tests 20/65
0 (0–2); 3/19 Tests 0 (0–3); 6/46 Tests 0 (0–3); 9/65 Tests
0 (0–2); 2/19 Tests 0 (0–5); 8/46 Tests 0 (0–5); 10/65 Tests
Abbreviations: HR, maximal heart rate during the exercise test; HR/HRmax, HR during the exercise tests/maximal heart rate for the horse 100; n, number of exercise tests; proportion, number of exercise tests that presented the indicated arrhythmia/n; SVPD, supraventricular premature depolarization; VPD, ventricular premature depolarization. a Denotes statistically significant difference (P ¼ .009). SVPDs and VPDs are reported as median (range).
4. Discussion The postexercise rhythm results were not repeated during consecutive workouts in many cases in this study. Exercising arrhythmias were more consistent than postexercise arrhythmias. In humans, arrhythmias detected in several exercise tests are considered more relevant as the repeatability of serious rhythm disturbances is high (76%) in human athletes [16]. However, in individuals with certain channelopathies, several exercise tests may be necessary to detect arrhythmias that evidence an increased risk for death [17]. The causes for sudden cardiac death are very different in horses when compared to humans, and channelopathies are currently not recognized in horses. Conclusions about how many exercising ECGs are needed in horses to determine if exercising arrhythmias are the cause of poor performance or increase the risk for collapse or sudden cardiac death cannot be drawn from the data reported here due to the small data set and the uncertain clinical relevance of the arrhythmias displayed by these horses. Studies in larger populations and in horses with cardiovascular disease are needed to resolve this matter. The fact that arrhythmias are more likely to be detected at higher exercise intensities and that are more likely to happen during the postexercise period has been reported previously [4,6]. The present study is the first to quantify the relationship with exercise intensity. This finding highlights the importance of performing exercise tests that are at an intensity equal or that slightly exceeds the horse’s customary activities [10].
Arrhythmias were more frequent when horses exercised on a Ra than on a Tm. The exercise intensity was not different between Ra and Tm exercise tests so perhaps, other variables such as the presence of a rider, psychological factors associated with the environment, and so forth can affect arrhythmogenesis in horses. This suggests that when evaluating arrhythmias during exercise, not only intensity, but also other circumstances of the exercise tests should be taken into account. Horses in the present study did not present cardiac abnormalities, and horses with cardiovascular disease may behave differently. The clinical significance of certain exercising and postexercising arrhythmias is a matter of debate in equine and human athletes [12,18,19]. Similarly to previous reports, arrhythmias during and immediately after exercise were frequent [1–6], and a large proportion of horses (6/ 9) would have been classified as having “clinically important” arrhythmias using previously described criteria [5]. The presence of VPDs during exercise was very rare (1 single VPD in 65 recordings) in the group of horses reported here as it was in previous studies [1–6]. The matter of which arrhythmias are clinically relevant cannot be resolved with the methods used in this study. It is important to realize that although some arrhythmias are common and innocuous, not all arrhythmias are innocuous and exercising and postexercising rhythm alterations are a potential cause of sudden death or collapse in horses [9–11]. Sudden cardiac death jeopardizes horses’ health, the safety of riders, and the public perception of welfare in equestrian sports. Additional studies are
Table 2 Summary of exercising and postexercising arrhythmias for each horse. Horse
n (TM/Ra)
1 2 3 4 5 6 7
2 2 2 4 4 11 12
(1/1) (2/0) (0/2) (1/3) (4/0) (9/2) (8/4)
8 9
14 (11/3) 14 (10/4)
Arrhythmia Exercise
Arrhythmia Postexercise
SVPDs/Test; Proportion
VPDs/Test; Proportion
SVPDs/Test; Proportion
VPDs/Test; Proportion
2–5; 2/2 3–11; 2/2 0 0 0 0–2; 1/11 2, 0–8; 7/12
0–1; 1/2 0 0 0 0 0 0
0–1; 1/2 0 0 0 0 0 0, 0–2; 3/14
0 0–1; 1/2 0 0–5; 1/4 0–3; 3/4 0 0, 0–4; 2/14
0 0
0 0
0, 0–1; 2/14 0, 0–2; 3/14
0, 0–2; 3/14 0
Description Rhythm Abnormality
Single PDs Single PDs Normal rhythm Run of 4 VPDs postexercise and 1 single VPD Couplets and triplet of R on T Single PDs Couplets of VPDs (2), couplet and triplet of SVPDs. Else single PDs Couplets (2) of R on T. Else single PDs Single PDs
Abbreviations: n, number of exercise tests; PD, premature depolarization; proportion, number of tests with arrhythmias/n; Ra, racetrack; SVPD, supraventricular premature depolarization; TM, treadmill; VPDs, ventricular premature depolarization.
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Fig. 1. Normal exercising electrocardiogram of an 11-year-old Thoroughbred gelding exercising on a racetrack under saddle. The dotted lines mark the R waves, and instantaneous heart rates are depicted at the bottom.
needed to determine which type and frequency of arrhythmias affect performance and safety, to quantify risks, and to determine which circumstances increase the likelihood of arrhythmia development.
Horses of this research group were not race fit and were classified as normal based on the absence of signs of cardiac disease during physical examination and the lack of a history of cardiac disease or poor performance. Echocardiograms,
Fig. 2. Exercising electrocardiogram of a 13-year-old Thoroughbred gelding exercising on a treadmill. The vertical red lines mark the presence of supraventricular premature depolarizations. The dotted lines mark the R waves, and instantaneous heart rates are depicted at the bottom.
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Fig. 3. Postexercise electrocardiogram of a 15-year-old Thoroughbred gelding. The first three vertical red lines mark the presence of ventricular premature depolarizations. The following two vertical lines are normal complexes. The dotted lines mark the R waves, and instantaneous heart rates are depicted at the bottom.
24-hour continuous ECGs, and measurements of cardiac biomarkers would have been an appropriate means of further characterizing this group and confirming or refuting the clinical impression that these horses had normal cardiovascular systems. Likewise, measuring electrolytes would have been interesting as electrolyte disturbances are a commonly mentioned reason for arrhythmia development. The study had several limitations such as the differences in the exercise tests performed between horses, the different intervals between tests, the size of the group, or the recording of a different amount of tests/horse. These limitations introduce variables that make drawing firm conclusions difficult. The absence of blinded reading of the ECGs is another limitation of the study. The interpretation of ECGs during exercise in horses presents some unresolved difficulties for which controversies and gaps in knowledge exist. A 10% variation in the R-R interval was chosen to define premature beats in this study based on previous literature [1,2,7]. This number is somewhat arbitrary and provides a conservative approach to the detection of arrhythmias. Some clinicians may use a shorter change in the R-R interval to determine prematurity. Further studies are needed to define the normal variation of the R-R interval and the relationship between prematurity and ectopy. In addition, the distinction of ventricular and supraventricular arrhythmias is not always clear during exercise tests. We followed the most commonly cited criteria for this classification [1–4,7]. These criteria have limitations, and future studies should better define the limits between SVPDs, VPDs, and aberrant ventricular conduction.
5. Conclusions We concluded that a single exercise test did not always display all the arrhythmias detected over several workouts and that the exercising rhythm was more consistent than the postexercise rhythm. Arrhythmias are more likely to be detected at maximal or near-maximal intensities and during exercise tests on a Ra when compared to a Tm. A larger population needs to be studied before firm conclusions are drawn. Acknowledgments The authors gratefully acknowledge Dr Kathy Seino for her assistance in data collection. There are no ethical considerations or competing interests to disclose. The study was funded internally by the Department of Veterinary Clinical Sciences of Washington State University that had no involvement in the study apart from authors being part of this department. The procedures were covered under the Animal Care and used protocol of the Exercise Physiology group at Washington State University. References [1] Buhl R, Meldgaard C, Barbesgaard L. Cardiac arrhythmias in clinically healthy showjumping horses. Equine Vet J Suppl 2010;38: 196–201. [2] Barbesgaard L, Buhl R, Meldgaard C. Prevalence of exerciseassociated arrhythmias in normal performing dressage horses. Equine Vet J Suppl 2010;38:202–7. [3] Jose-Cunilleras E, Young LE, Newton JR, Marlin DJ. Cardiac arrhythmias during and after treadmill exercise in poorly performing thoroughbred racehorses. Equine Vet J Suppl 2006;36:163–70.
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