Figure 3.

In dogs with advanced myxomatous mitral valve disease, 9 variables representing the parasympathetic modulation of heart rate (HR) (including (A) HFn₂₄) were significantly lower in dogs with a history of syncope compared with dogs without a history of syncope. However, 1 variable, (B) LFn₂₄, suggested to represent both the parasympathetic and sympathetic modulation of HR, was significantly higher in dogs with a history of syncope. HFn₂₄, high frequency measured over a 24‐hour period; LFn₂₄, normalized low frequency measured over a 24‐hour period.