Endurance riding is a strenuous equestrian discipline; hence, much effort is spent on monitoring horses to ensure that their welfare is not compromised. The metabolic consequences of an endurance ride have been identified in several studies1–9 of exercise-induced changes in hematologic and biochemical variables in horses. Some of these studies1,5,7 also involved examination of the association between heart rate and biochemical variables and found an increased heart rate to be indicative of blood volume depletion and skeletal muscle fatigue. Clinical variables such as heart rate and cardiac recovery index are therefore commonly used to assess the metabolic status of the horses and their capability to continue the ride.10,11 However, whereas previous studies primarily focused on heart rate during the ride, to the authors' knowledge no investigations have been conducted into changes in heart rate during the recovery period.
A study12 involving human endurance athletes revealed an increase in heart rate and decrease in HRV subsequent to a long-distance run. Endurance horses are likewise anecdotally believed to have an increase in heart rate for hours after an endurance ride, which could suggest an altered sympathetic-parasympathetic balance resulting in a decrease in HRV and increase in susceptibility to arrhythmias. Frequent occurrence of supraventricular and ventricular premature beats has been identified in trotting horses during the immediate postrace recovery phase and is most likely related to vagal reactivation during deceleration of the heart rate.13,14 Nevertheless, despite the known systemic effects of endurance riding, no studies have been conducted to assess HRV or frequency of arrhythmias in endurance horses during recovery after an endurance ride.
Studies15,16 involving human endurance athletes have revealed increases in circulating concentrations and activities of cardiac biomarkers following exercise, leading to the hypothesis that prolonged exercise in humans affects the myocardium. A correlation has even been identified between magnitude of cardiac biomarker release and heart rate and HRV during recovery.12 Increases in plasma cTnI concentration have also been reported for horses after endurance rides, but in the associated study17 an association with heart rate was not investigated. The objective of the study reported here was to investigate changes in heart rate, HRV, and arrhythmia frequency in horses during the initial 12 hours of recovery after completing a 120- or 160-km endurance ride. A second objective was to evaluate changes from preride values in serum cTnI concentration and CK-MB activity after the ride and their potential correlation with heart rate.
Funded by the University of Copenhagen.
None of the authors have a financial or personal relationship with other individuals or organizations that could inappropriately influence or bias the content of this report.
The authors thank Dr. Rebecca Langhorn for assistance with analysis of cardiac biomarkers, Dr. Søren Saxmose Nielsen for assistance with statistical analysis, and Tina Roust and Camilla Malec for handling of blood samples.
Creatine kinase isoenzyme MB
Cardiac troponin I
Maximum mean heart rate calculated over a
5-minute sequence within a 30-minute period Mean heart rate calculated over a 30-minute period
Minimum mean heart rate calculated over a 5-minute sequence within a 30-minute period
Heart rate variability
SD of normal R-R intervals calculated over a 30-minute period
Supraventricular premature complex
Ventricular premature complex
Televet 100, Engel Engineering Services GmbH, Heusenstamm, Germany.
Animal Polster, Snøgg AS, Kristiansand, Norway.
PROC MEANS, SAS Enterprise Guide, version 6.1, SAS Institute Inc, Cary, NC.
BD Vacutainer, Becton, Dickinson & Co, Franklin Lakes, NJ.
CryoPure, Sarstedt AG & Co, Nümbrecht, Germany.
ADVIA 2120 hematology system, Siemens Healthcare Diagnostics Inc, Tarrytown, NY.
ADVIA 1800 chemistry system, Siemens Healthcare Diagnostics Inc, Tarrytown, NY.
ACL TOP coagulation analyzer, Instrumentation Laboratory, Bedford, Mass.
ADVIA Centaur CP TnI-ultra, Siemens Healthcare Diagnostics Inc, Tarrytown, NY.
ADVIA Centaur CP CK-MB, Siemens Healthcare Diagnostics Inc, Tarrytown, NY.
PHASE haptoglobin assay, Tridelta Development Ltd, Maynooth, Kildare, Ireland.
PROC MIXED, SAS Enterprise Guide, version 6.1, SAS Institute Inc, Cary, NC.
1. Rose RJ, Purdue RA, Hensley W. Plasma biochemistry alterations in horses during an endurance ride. Equine Vet J 1977; 9:122–126.
2. Rose RJ, Hodgson DR, Sampson D, et al. Changes in plasma biochemistry in horses competing in a 160 km endurance ride. Aust Vet J 1983; 60:101–105.
3. Rose RJ, Arnold KS, Church S, et al. Plasma and sweat electrolyte concentrations in the horse during long distance exercise. Equine Vet J 1980; 12:19–22.
5. Sloet van Oldruitenborgh-Oosterbaan MM, Wensing T, Barneveld A, et al. Heart rate, blood biochemistry and performance of horses competing in a 100 km endurance ride. Vet Rec 1991; 128:175–179.
7. Schott HC II, Marlin DJ, Geor RJ, et al. Changes in selected physiological and laboratory measurements in elite horses competing in a 160 km endurance ride. Equine Vet J Suppl 2006; (36):37–42.
8. Cywińska A, Szarska E, Górecka R, et al. Acute phase protein concentrations after limited distance and long distance endurance rides in horses. Res Vet Sci 2012; 93:1402–1406.
9. Boucher JH, Ferguson EW, Wilhelmsen CL, et al. Erythrocyte alterations endurance exercise in horses. J Appl Physiol 1981; 51:131–134.
10. Fielding CL, Meier CA, Balch OK, et al. Risk factors for the elimination of endurance horses from competition. J Am Vet Med Assoc 2011; 239:493–498.
11. Robert C, Benamou-Smith A, Leclerc J-L. Use of the recovery check in long-distance endurance rides. Equine Vet J Suppl 2002;(34):106–111.
12. Aagaard P, Sahlén A, Bergfeldt L, et al. Heart rate and its variability in response to running-associations with troponin. Med Sci Sports Exerc 2014; 46:1624–1630.
13. Buhl R, Petersen EE, Lindholm M, et al. Cardiac arrhythmias in Standardbreds during and after racing—possible association between heart size, valvular regurgitations, and arrhythmias. J Equine Vet Sci 2013; 33:590–596.
14. Physick-Sheard PW, McGurrin MKJ. Ventricular arrhythmias during race recovery in Standardbred racehorses and associations with autonomic activity. J Vet Intern Med 2010; 24:1158–1166.
15. Neumayr G, Gaenzer H, Pfister R, et al. Plasma levels of cardiac troponin I after prolonged strenuous endurance exercise. Am J Cardiol 2001; 87:369–371.
16. Shave R, Baggish A, George K, et al. Exercise-induced cardiac troponin elevation: evidence, mechanisms, and implications. J Am Coll Cardiol 2010; 56:169–176.
17. Holbrook TC, Birks EK, Sleeper MM, et al. Endurance exercise is associated with increased plasma cardiac troponin I in horses. Equine Vet J Suppl 2006;(36):27–31.
18. Buhl R, Meldgaard C, Barbesgaard L. Cardiac arrhythmias in clinically healthy showjumping horses. Equine Vet J Suppl 2010;(38):196–201.
19. Verheyen T, Decloedt A, De Clercq D, et al. Oesophageal electrocardiography in healthy horses. Equine Vet J 2012; 44:640–645.
20. Verheyen T, Decloedt A, De Clercq D, et al. Electrocardiography in horses—part 2: how to read the equine ECG. Vlaams Diergeneeskd Tijdschr 2010; 79:337–344.
21. Langhorn R, Willesen JL, Tarnow I, et al. Evaluation of a highsensitivity assay for measurement of canine and feline serum cardiac troponin I. Vet Clin Pathol 2013; 42:490–498.
22. Hautala A, Tulppo MP, Makikallio TH, et al. Changes in cardiac autonomic regulation after prolonged maximal exercise. Clin Physiol 2001; 21:238–245.
23. Amory H, Votion DM, Fraipont A, et al. Altered systolic left ventricular function in horses completing a long distance endurance race. Equine Vet J Suppl 2010;(38):216–219.
24. Barbesgaard L, Buhl R, Meldgaard C. Prevalence of exerciseassociated arrhythmias in normal performing dressage horses. Equine Vet J Suppl 2010;(38):202–207.
26. Rishniw M, Simpson KW. Cloning and sequencing of equine cardiac troponin I and confirmation of its usefulness as a target analyte for commercial troponin I analyzers. J Vet Diagn Invest 2005; 17:582–584.
27. Serra M, Papakonstantinou S, Adamcova M, et al. Veterinary and toxicological applications for the detection of cardiac injury using cardiac troponin. Vet J 2010; 185:50–57.
28. Argiroudis SA, Kent JE, Blackmore DJ. Observations on the isoenzymes of creatine kinase in equine serum and tissues. Equine Vet J 1982; 14:317–321.