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QT Interval and Cardiac Restitution Ratio Complexity in Standardbred Racehorses From Rest to Maximal Effort: Insights Into Arrhythmia Risk.
Abstract: Sudden cardiac death is common in racehorses. Factors associated with the QT interval that could predispose to fatal cardiac arrhythmias are unknown. Cardiac restitution, expressed as a ratio of QT/TQ, has been used in humans to assess arrhythmia risk but has not been described in horses during maximal intensity exercise. Objective: Describe factors associated with the QT interval and cardiac restitution ratio (CRR) in clinically normal Standardbred racehorses under race-day conditions. Methods: Archival electrocardiograms from 42 Standardbred horses during live racing in Ontario. Methods: Observational study performing an automated cardiac restitution analysis. Cardiac cycles were obtained from rest, non-race exercise, non-race recovery, live racing, and post-race recovery periods. Multivariable linear regression analysis was performed with both QT interval and CRR as outcomes of interest. Results: Analysis of 3827 sequential pairs of cardiac cycles was performed. Exercise period and RR interval were highly associated with both QT interval and CRR. Other significant associations varied by exercise period and included: racing gait, sex, age, whether the horse received furosemide, and whether the horse experienced complex ventricular arrhythmias after racing. Interactions between gait and furosemide, and sex and gait were also significant. Conclusions: An automated cardiac restitution analysis is feasible in exercising racehorses. The QT-RR interval relationship is multifactorial, and there are numerous significant associations that must be considered to interpret changes in QT interval and CRR in horses.
© 2025 The Author(s). Journal of Veterinary Internal Medicine published by Wiley Periodicals LLC on behalf of American College of Veterinary Internal Medicine.
Publication Date: 2025-09-10 PubMed ID: 40926462PubMed Central: PMC12420912DOI: 10.1111/jvim.70207Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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Overview
- This study investigates how the QT interval and cardiac restitution ratio (CRR) behave in Standardbred racehorses from rest through maximal effort during races, aiming to identify factors related to arrhythmia risk that could explain sudden cardiac death in racehorses.
Background
- Sudden cardiac death is a serious concern in racehorses, but specific factors related to cardiac electrical activity that might increase this risk are not well known.
- The QT interval on an electrocardiogram (ECG) represents the time for ventricular depolarization and repolarization, a crucial aspect of heart rhythm.
- Cardiac restitution, quantified as the QT/TQ ratio (CRR), has been used in humans to assess the risk of arrhythmias but has not previously been studied in horses during maximal exercise.
Study Objectives
- To describe factors associated with the QT interval and cardiac restitution ratio in clinically normal Standardbred racehorses on race days.
- To explore how these factors change from rest, through non-race exercise and maximal racing, to post-race recovery.
Methods
- Data Source: Archival electrocardiograms from 42 Standardbred horses during live racing in Ontario.
- Study Design: Observational study performing automated analysis of cardiac restitution over multiple exercise phases.
- Recording Periods: Cardiac cycles were analyzed at rest, during non-race exercise, non-race recovery, live racing, and post-race recovery.
- Analysis:
- 3827 sequential pairs of cardiac cycles were reviewed.
- Automated algorithms calculated QT intervals and CRR for each pair.
- Multivariable linear regression was used to identify factors associated with QT interval and CRR.
Key Findings
- The QT interval and CRR were strongly influenced by:
- Exercise period (rest, exercise, racing, recovery).
- RR interval (the interval between successive heartbeats).
- Other important factors showed variable influence depending on the exercise phase, including:
- Racing gait (trot or pace).
- Sex of the horse.
- Age.
- Use of furosemide, a diuretic medication often used in racehorses.
- Occurrence of complex ventricular arrhythmias after racing.
- Significant interactions between variables were found, such as:
- Between gait and furosemide administration.
- Between sex and gait.
Conclusions
- Automated cardiac restitution analysis using ECG data is practical and effective in exercising racehorses.
- The relationship between the QT interval and RR interval is complex and influenced by multiple factors, including physiological state and horse-specific characteristics.
- Interpreting changes in QT interval and CRR in horses requires taking many variables into account, which may provide insight into arrhythmia risk and potentially help prevent sudden cardiac death.
Cite This Article
APA
Avison A, Goderre BG, Pyle WG, Physick-Sheard PW.
(2025).
QT Interval and Cardiac Restitution Ratio Complexity in Standardbred Racehorses From Rest to Maximal Effort: Insights Into Arrhythmia Risk.
J Vet Intern Med, 39(5), e70207.
https://doi.org/10.1111/jvim.70207 Publication
Researcher Affiliations
- Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada.
- Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada.
- IMPART Team Canada, Dalhousie Medicine, Dalhousie University, Saint John, New Brunswick, Canada.
- Women's Health Research Institute, BC Women's Hospital + Health Centre, Vancouver, British Columbia, Canada.
- Department of Population Medicine, Ontario Veterinary College, University of Guelph, Ontario, Canada.
MeSH Terms
- Animals
- Horses / physiology
- Male
- Female
- Arrhythmias, Cardiac / veterinary
- Arrhythmias, Cardiac / physiopathology
- Electrocardiography / veterinary
- Physical Conditioning, Animal / physiology
- Horse Diseases / physiopathology
- Horse Diseases / etiology
- Risk Factors
Grant Funding
- EG2023-09 / Equine Guelph
Conflict of Interest Statement
Authors declare no off‐label use of antimicrobials. The authors declare no conflicts of interest. The software used in the present study is intended to be published in the Biomedical Engineering literature and will be made available at no cost to researchers. There has not been, nor will there be financial gain from the software.
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