RZ Interval as an Impedance Cardiography Indicator of Effort-Related Cardiac Sympathetic Activity

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 ≥ .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 = .432 [.310, .554]) was roughly 15% larger than PEP's effect size (g = .376 [.256, .496]). RZ thus appears promising for future research on sympathetic aspects of effort-related cardiac activity.

Appetitive motivation in depressive anhedonia: Effects of piece-rate cash rewards on cardiac and behavioral outcomes.

“…For PEP, there were significant linear (b ϭ Ϫ1.08, SE ϭ .23, p Ͻ .001) and quadratic (b ϭ .31, SE ϭ .11, p ϭ .007) within-person main effects of period (R 2 ϭ 13.4%). The trend reflects an overall decline in PEP from the baseline to the 3 cents to the 15 cents condition, with a flattening between the 3 cents and 1 See the online supplemental materials for information on the RZ interval, a similar cardiac outcome (Silvia, McHone, et al, 2018). This document is copyrighted by the American Psychological Association or one of its allied publishers.…”

Effects of Transcranial Direct Current Stimulation on effort during a working-memory task

“…As hypothesized, participants showed a modification of ISTI in the anodal stimulation condition compared to the cathodal and sham stimulation conditions. This suggests changes in levels of effort 18 . Specifically, anodal stimulation of the DLPFC is associated with shorter ISTI, which is interpreted as the result of higher effort.…”

“…In the current study, we decided to opt for Initial Systolic Time Interval (ISTI) as a measure of effort because it is less prone to errors, relying on salient waveform points that are easily and reliably identifiable. Further, ISTI has been shown to covary strongly with PEP reactivity 18 . ISTI is thus a powerful marker of effort.…”

“…The link between initial systolic time intervals such as Pre-Ejection Period (PEP) or ISTI and effort has been largely demonstrated 2 , 9 , 14 , 18 , 19 . Most of the previous research in this area used PEP as an index of resources mobilization (effort), based on the work of Wright 9 , who integrated the active coping approach with the motivation intensity theory 1 .…”

Validation of a new impedance cardiography analysis algorithm for clinical classification of stress states

“…A popular method to mitigate the noise in the ICG signal is ensemble averaging (EA; Czarnek et al, 2021; Kibler, 2018; Kizakevich et al, 1976; Malcolm et al, 2016; Silvia et al, 2020; Sornmo & Laguna, 2005), which we refer to as the “conventional EA algorithm” (CEA). In our prior work, we have improved upon this approach with an open‐source automated ICG noise removal algorithm − three‐stage EA algorithm (TEA; Sheikh, Shah, Levantsevych, et al, 2020).…”