QT instability, an indicator of augmented arrhythmogenesis, increases with the progression of myxomatous mitral valve disease in dogs

Objectives: To investigate QT instability in dogs with myxomatous mitral valve disease (MMVD) and to determine if this is associated with arrhythmogenesis.

Animals: One hundred sixty-seven MMVD dogs that met the study criteria were included.

Methods: Echocardiographic and electrocardiographic data were gathered. Fifty consecutive QT intervals were recorded for each dog. Both heart rate-corrected and uncorrected QT intervals were used to calculate average QT (QTa), QT variance (QTv), total instability (TI), short-term instability (STI), and long-term instability (LTI). Sensitivity and specificity of QTa, QTv, TI, STI, and LTI in identifying arrhythmias and cardiac remodeling were calculated. Patient follow-ups were obtained for analyses of disease progression and survival.

Results: An increase related to progression was documented for all the studied indices. QTa and STI best identified dilated hearts and arrhythmias, respectively. Dogs with QTa >272 ms and STI >8 ms were 15% more likely to develop ventricular arrhythmias (likelihood ratios of 2.31 [P = 0.0008] and 2.09 [P = 0.0049], respectively). A QTa >258 ms discriminated American College of Veterinary Internal Medicine stage B1 from stages B2/C disease with a sensitivity of 63% and specificity of 61%. Dogs in American College of Veterinary Internal Medicine stage C of MMVD have higher STI and 3.34 times increased risk of developing arrhythmias when values more than 8 ms are reached. All indices except LTI and QTv showed prognostic value, with increases relating to all-cause mortality.

Conclusion: Analyses of QT intervals demonstrated changes in STI, LTI, and TI. Increased QT prolongation and instability are significantly related to mortality and may be useful in determining prognosis of MMVD patients.