Pharmacokinetic/Pharmacodynamic Analysis of Quinidine-Induced Dominant Frequency Reduction in Thoroughbred Horses With Atrial Fibrillation.

Overview

• This study investigated how the drug quinidine affects the heart rhythm of Thoroughbred horses with atrial fibrillation (AF), specifically focusing on how drug levels in the blood impact the dominant frequency (DF) of atrial activation measured by electrocardiograms.
• The research developed a pharmacokinetic/pharmacodynamic (PK/PD) model to describe the relationship between quinidine plasma concentrations and changes in DF, aiming to identify the plasma concentration required for successful conversion to normal heart rhythm (sinus rhythm).

Background and Importance

• Atrial Fibrillation (AF) in Horses: AF is a common cardiac arrhythmia in horses characterized by rapid and irregular atrial activation that compromises cardiac function.
• Use of Quinidine: Quinidine sulfate is a standard pharmacological treatment to restore normal heart rhythm (sinus rhythm) in horses affected by AF.
• Challenge: Despite widespread use, the exact plasma concentration of quinidine needed to effectively convert AF back to sinus rhythm was previously unclear.
• Dominant Frequency (DF): DF, derived from electrocardiograms, represents the atrial activation rate during AF and is a measurable indicator of the arrhythmia’s severity and treatment response.

Study Design and Methods

• Subjects: Ten Thoroughbred horses naturally experiencing AF were included.
• Treatment: Each horse received quinidine sulfate administered nasogastrically (through a tube to the stomach).
• Measurements:
  • Plasma concentrations of quinidine were regularly measured during the treatment period.
  • Dominant frequency values were calculated from digitized atrial fibrillatory ECG segments to monitor atrial activity.
• Analysis: A pharmacokinetic/pharmacodynamic (PK/PD) model called the sigmoid inhibitory Emax model was used to describe the relationship between quinidine levels and DF reduction.

Key Findings and Model Parameters

• Baseline Measurements: The average (baseline) dominant frequency before treatment was approximately 6.27 Hz.
• Effect of Quinidine: Maximum observed dominant frequency reduction was about 3.63 Hz, resulting in a theoretical minimum DF around 2.64 Hz.
• Pharmacodynamic Parameters:
  • EC50 (the quinidine concentration producing half-maximal effect) was 0.78 μg/mL.
  • Hill coefficient (describes the steepness of the concentration-effect curve) was 1.81, indicating a steep response to changes in quinidine concentration.
• Clinical Outcome: Nine of the ten treated horses successfully converted to sinus rhythm.
• Frequency Change Before Conversion: DF decreased from a median of 6.4 Hz to 3.0 Hz just prior to conversion.

Model Simulations and Clinical Implications

• Pharmacodynamic Plateau: Increasing quinidine plasma concentration from 4 to 5 μg/mL led to only a minimal additional DF reduction (0.06 Hz), suggesting increasing the dose above a certain point yields diminishing returns.
• Target Plasma Concentration: The model predicted that a quinidine concentration of approximately 2.6 μg/mL corresponds to a DF of 3.0 Hz, which was near the frequency observed immediately before conversion, implying this may be an optimal target concentration to aim for in treatment.
• Clinical Utility: These findings can guide dosing strategies by providing a quantified target plasma concentration to maximize the probability of restoring normal rhythm while avoiding unnecessarily high doses.

Conclusion

• The study successfully established a quantitative PK/PD relationship between quinidine plasma levels and reduction in atrial fibrillation dominant frequency in Thoroughbred horses.
• It identified a target plasma concentration that could be used to optimize quinidine dosing for effective and safe pharmacological cardioversion of AF in clinical practice.
• These insights improve understanding of quinidine treatment dynamics and support evidence-based dosing regimens to improve outcomes in horses with AF.