RZ Interval as an Impedance Cardiography Indicator of Effort-Related Cardiac Sympathetic Activity
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RZ Interval as an Impedance Cardiography Indicator of Effort‑Related Cardiac Sympathetic Activity Paul J. Silvia1 · Ashley N. McHone1 · Zuzana Mironovová1 · Kari M. Eddington1 · Kelly L. Harper2 · Sarah H. Sperry3 · Thomas R. Kwapil3 Accepted: 3 November 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Research on effort and motivation commonly assesses how the sympathetic branch of the autonomic nervous system affects the cardiovascular system. The cardiac pre-ejection period (PEP), assessed via impedance cardiography, is a common outcome, but assessing PEP requires identifying subtle points on cardiac waveforms. The present research examined the psychometric value of the RZ interval (RZ), which has recently been proposed as an indicator of sympathetic activity, for effort research. Also known as the initial systolic time interval (ISTI), RZ is the time (in ms) between the ECG R peak and the dZ/ dt Z peak. Unlike PEP, RZ involves salient waveform points that are easily and reliably identified. Data from two experiments evaluated the suitability of RZ for effort paradigms and compared it to a popular automated PEP method. In Studies 1 (n = 89) and 2 (n = 71), participants completed a standard appetitive task in which each correct response earned a small amount of cash. As expected, incentives significantly affected PEP and RZ in both experiments. PEP and RZ were highly correlated (all rs ≥ 0.89), and RZ consistently yielded a larger effect size than PEP. In Study 3, a quantitative synthesis of the experiments indicated that the effect size of RZ’s response to incentives (Hedges’s g = 0.432 [0.310, 0.554]) was roughly 15% larger than PEP’s effect size (g = 0.376 [0.256, 0.496]). RZ thus appears promising for future research on sympathetic aspects of effort-related cardiac activity. Keywords Effort · Pre-ejection period · RZ · Initial systolic time interval · Impedance cardiography · Motivation Influenced by Obrist’s (1981) active coping model, research on the psychophysiology of effort has traditionally emphasized autonomic outcomes that reflect the sympathetic branch’s beta-adrenergic impact on the heart (Gendolla et al. 2019). Early work consistently found that systolic blood pressure (SBP) was sensitive to factors important to effort, such as the difficulty of a task and the value of goals and incentives (Wright 1996), and SBP continues to be prominent (e.g., Barreto et al. 2015; Hess et al. 2016; Silvia 2012). Recent work has emphasized the cardiac pre-ejection * Paul J. Silvia [email protected] 1
Department of Psychology, University of North Carolina at Greensboro, P.O.Box 26170, Greensboro, NC 27402‑6170, USA
2
Behavioral Science Division, VA Boston Healthcare System, National Center for PTSD, Boston, USA
3
Department of Psychology, University of Illinois at Urbana-Champaign, Champaign, USA
period (PEP; Kelsey 2012), a systolic time interval typically assessed via impedance cardiography. PEP is the time in ms between the onset of depolarization (the ECG Q po
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