Pilot Study: Heart Rate and Heart Rate Variability Indices in Mules Evaluated by 24-Hour Electrocardiogram.

Overview

• This study evaluated heart rate (HR) and heart rate variability (HRV) in healthy mules over a 24-hour period using ambulatory electrocardiogram (aECG) recordings.
• The researchers compared mule cardiac parameters to those known for horses and donkeys to better understand physiological and autonomic differences and assessed the presence of arrhythmias in mules.

Background and Purpose

• Mules and donkeys represent the fifth largest equine population in the U.S., yet there is limited data on their basic cardiac physiology, specifically heart rate and arrhythmia prevalence in mules.
• Heart rate variability (HRV) analysis is a useful tool to gain insights into autonomic nervous system regulation and physiological differences among equine species.
• The study aimed to pilot the use of 24-hour electrocardiogram monitoring to establish baseline heart rate and variability indices in healthy mules.

Methodology

• Seven healthy, athletic, privately owned mules located in Northern California were enrolled in the study.
• Each mule underwent 24-hour ambulatory electrocardiogram (aECG) recording using Vision 5 Holter Analysis Software for data acquisition and review of cardiac rhythm.
• The software enabled detailed analysis of heart rate, heart rate variability, and identification of any cardiac arrhythmias over the monitoring period.

Key Findings

• The average heart rate across the mules was between 32 and 42 beats per minute (bpm), with a mean value of 36 ± 3 bpm over approximately 24 hours.
• Minimum recorded heart rates ranged from 16 to 24 bpm, and maximum heart rates ranged from 70 to 156 bpm, indicating variability depending on activity or autonomic state.
• Several types of arrhythmias were detected in individual mules, including:
  • Supraventricular complexes (abnormal beats originating above the ventricles)
  • Sinoatrial (SA) heart block (delays or interruptions in impulse generation from the sinoatrial node)
  • Second-degree atrioventricular (AV) block (intermittent failure of impulse conduction from the atria to ventricles)

Interpretation and Comparisons

• Heart rate in mules was found to be similar to that of horses but lower than that of donkeys, suggesting some physiological similarities to horses in regard to cardiac autonomic control.
• The observed arrhythmias were consistent with what is typically seen in horses, indicating these cardiac events may be normal variants rather than pathological findings in mules.
• Arrhythmias in mules appeared more frequent than those reported in donkeys, highlighting species differences in cardiac rhythm prevalence.

Implications and Future Directions

• The study pilot demonstrates the feasibility and importance of using 24-hour ECG monitoring to establish cardiac reference values specific to mules rather than relying solely on horse reference ranges.
• Results indicate a need for larger, more comprehensive studies involving a greater number of mules to:
  • Confirm these preliminary HR and HRV values
  • Determine if mule-specific reference intervals should be developed
  • Further explore the frequency, types, and clinical significance of arrhythmias observed in this species
• Improved understanding of mule cardiac physiology may aid in better diagnosis and management of cardiac conditions in these animals.