Learning Objective #1: Describe patterns of heart rate variability in cardiac patients with ICD devices

Learning Objective #2: Explain the reasons for measuring heart rate variability during both rest and ambulation

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Heart rate variability (HRV) has not been well-characterized in seriously ill populations such as cardiac patients receiving an implantable cardioverter defibrillator (ICD). HRV changes in response to physical activity may indicate illness severity, since HRV reflects the dynamic effects of neurocardiac modulation. This study examined relationships between resting and walking HRV over time in patients at one-day (n = 34), 3-months (n = 18) and 6-months (n = 21) post-ICD insertion. Subjects were initial ICD recipients, predominantly Caucasian (76%) males (76%) with an ejection fraction < 30% (78%) and mean age 56 ± 11 years. Subjects underwent two 10-minute Holter ECG recordings: first while resting supine then walking. RR interval data were analyzed in the time domain via standard deviation (SDNN) and in the nonlinear domain via recurrence quantification analysis (RQA). RQA produces seven variables signifying aspects of nonlinear dynamics; the variable “trend” denotes dynamic stationarity. The percent change (%change) between resting and walking values was calculated for each variable, then categorized into 3 groups: none, positive and negative change. Nonparametric statistics showed SDNN %change increased significantly over time (p = 0.04) despite low median rest / walk SDNN values: 27/ 26 ms at baseline, 28 / 37 ms at 3-months and 26 / 33 ms at 6-months. The categorized %change values for heart rate and RQA variable “trend” exhibited a significant negative correlation at baseline and 6 months (r = -0.53, p = 0.001 and r = -0.53, p = 0.01, respectively) with 69% of heart rate exhibiting no change and 59% of “trend” exhibiting negative change with activity. While further analysis is needed to explain the SDNN behavior pattern over time, the results suggest that the neurocardiac dynamics governing HRV adversely shift towards a stationary state during physical activity in these ICD patients.