Nonrandom ventricular rhythm in horses with atrial fibrillation and its significance for patients.

The research paper focuses on the findings from a study of the behavior of RR intervals in horses experiencing atrial fibrillation, and how this behavior might be affected by certain medications.

Objective and Methodology

• The objective of this study was to analyze the RR interval sequences during spontaneous atrial fibrillation in horses – this is similar to previous studies performed on humans and dogs.
• The researchers used histograms and serial auto-correlograms, statistical tools useful for examining patterns in time series data, to perform their analysis.

Findings

• The researchers observed that unlike in humans and dogs, where ventricular rhythms remained random during episodes of atrial fibrillation, the ventricular rhythms in horses demonstrated noticeable patterning.
• The researchers found their histograms to be skewed with a median RR interval of about 1,000 ms in the horses. They also highlight the presence of long RR intervals in the horses, up to 5,000 ms in duration, a finding unmatched in human or dog studies.

Impact of Treatment

• The researchers then tested the impact of several drugs, like digitalis, quinidine, and atropine, on these RR intervals.
• Digitalis was found to increase the number of long RR intervals, i.e, increased the skewness while quinidine and atropine eliminated the longer RR intervals, thereby addressing the skewness.

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• In one particular horse, they even measured the intraarterial pressure. The fluctuations in pressure as RR intervals varied from over 3,000 to less than 500 ms were significant. They postulated that these changes were associated with baroreceptor responses potentially altering the electrophysiologic behavior of the atria and atrioventricular node, which led to the nonrandom patterns of ventricular rhythm observed.

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Implications

• The researchers postulate that the variation in the RR intervals and the resultant large fluctuations in intraarterial pressure may be due to baroreceptor responses, which can morph the electrophysiologic behavior of the atria and atrioventricular node.
• These changes lead to the nonrandom patterns observed in ventricular rhythm in horses.
• Because such very long RR intervals are not seen in humans or dogs during atrial fibrillation, the random ventricular rhythm in these groups is maintained, even when taking digitalis.