Cycle length variation assists in differentiation of atrial, ventricular, and sinus arrhythmias in horses at rest.

Overview

• This study analyzed the variability in heartbeat cycle lengths to improve the differentiation between atrial premature complexes (APCs), sinus arrhythmia (SA), and ventricular premature complexes (VPCs) in horses at rest.

Background and Objective

• Arrhythmia classification in horses, especially distinguishing between APCs and sinus arrhythmia (SA), is currently insufficient.
• This research aimed to investigate changes in RR intervals (time between ventricular beats) and PP intervals (time between atrial beats) among SA, APCs, and VPCs using resting electrocardiograms (ECGs).

Study Design and Methods

• Thirty resting ECGs from horses were reviewed, divided into three groups (10 with SA, 10 with APCs, and 10 with VPCs).
• For each arrhythmia type, sequences of 10 RR and 10 PP intervals corresponding to 20 events per ECG were recorded.
• Percent deviations were calculated in two ways:
  • Between sequential intervals (%ΔSEQPP for PP intervals, %ΔSEQRR for RR intervals)
  • Compared to baseline average intervals 1 to 3 (%ΔAVGPP, %ΔAVGRR)
• Statistical analyses included two-way ANOVA and receiver operator characteristic (ROC) curve analysis to compare percent deviations between arrhythmia types.

Key Findings

• Cycle length variation (changes in RR and PP intervals) effectively distinguished SA from APCs and VPCs, particularly by examining the degree of prematurity (how early a beat occurs) and the return cycle length (time for rhythm to return to baseline).
• Using %ΔAVGRR, %ΔSEQPP, and %ΔAVGPP metrics provided better differentiation between SA and APCs regarding the degree of prematurity than %ΔSEQRR metric.
• All four metrics (%ΔAVGRR, %ΔSEQPP, %ΔAVGPP, %ΔSEQRR) clearly differentiated return cycle lengths among the arrhythmias.
• %ΔSEQPP was particularly useful in demonstrating that both the degree of prematurity and the return cycle length of APCs differ from those of SA and VPCs.
• Assessments using %ΔAVGRR and %ΔAVGPP reduced confounding effects of the degree of prematurity on return cycle length variation.
• Variability in sinus cycle lengths (specifically PP intervals 7 to 10) was observed following both VPCs and SA events.

Significance of Including PP Interval Analysis

• Incorporating PP interval measurements alongside RR intervals added valuable discriminatory power for distinguishing APCs from SA.
• This suggests atrial-level timing (PP intervals) provides additional diagnostic information beyond ventricular-level timing (RR intervals) alone.

Limitations

• Classification bias was noted as a limitation in the study, meaning potential inaccuracies in how ECG arrhythmias were initially categorized might influence results.

Conclusions and Practical Implications

• Evaluating cycle length variation using both RR and PP intervals is a useful supplementary tool for the classification of common resting arrhythmias in horses: sinus arrhythmia (SA), atrial premature complexes (APCs), and ventricular premature complexes (VPCs).
• This approach may improve veterinary diagnostic accuracy for equine arrhythmias and guide appropriate clinical management.