Heart rate variability in the conscious dog signifi- cantly depends on duration of diastole

Heart Rate (HR) Variability (V) analysis provides information about cardiac regulation. Although HRV affects pump function on a beat-to-beat basis, no systematic study addresses the relative contribution of systole (S) and diastole (D). Thus, we analyzed the duration (T) by determining TS and TD in 2 conscious dogs instrumented with aortic flow probe. We selected 5 min epochs at rest (R) and 20 ug/kg.min dobutamine (Do) or 3 ug/kg.min medetomidine (M). V ofV of TS and TD are expressed as the T of beat n divided by the previous (n-1), and presented as mean, standard deviation (SD), with absolute values in brackets: TS/SD [ms] TD/SD [ms] HR/SD [bpm] R 1.000/0.024 1.083/0.444 1.048/0.326 [0.180/0.004] [0.536/0.174] [88.360/19.379] Do 1.000/0.027 1.002/0.082 1.002/0.056 [0.156/0.012] [0.335/0.074] [125.006/17.641] M 1.011/0.152 1.925/2.059 1.599/1.368 [0.203/0.020] [1.706/0.937] [44.816/29.319] Conclusions: V of TD is more pronounced than V in S (differences significant at p<0.05), suggesting that beat-to-beat variations in both preload (related to filling time) and afterload (due to exponential decrease of aortic pressure) are contributing factors and may have consequences in interpreting the regulation of ventricular function in terms of HRV.