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

Abstract

Research on effort and motivation commonly measures 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 sympathetic outcome, but assessing PEP requires identifying subtle points on cardiac waveforms. The present research examined the value of the RZ interval (RZ), which has recently been proposed as a measure 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. 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. 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.

Do depressive symptoms “blunt” effort? An analysis of cardiac engagement and withdrawal for an increasingly difficult task

“…The pattern, shown in Table 1, essentially paralleled the pattern for PEP. As an aside, the model R 2 was slightly higher for RZ (11.9%) than for PEP (10.6%), which replicates the larger effect sizes found for RZ found in recent experiments using incentives (Silvia et al, 2016) and other effort paradigms (Silvia, Beaty et al, 2014; Silvia, Nusbaum et al, 2014). …”

“…Also known as the initial systolic time interval (ISTI; Meijer, Boesveldt, Elbertse, & Berendse, 2008), RZ is increasingly popular as a measure of contractility. Several effort studies have found that it works at least as well as PEP (Silvia, Beaty, Nusbaum, Eddington, & Kwapil, 2014; Silvia, Nusbaum et al, 2014; Silvia et al, 2016), possibly because it uses the salient R and Z points instead of the subtle Q and B points (van Lien, Schutte, Meijer, & de Geus, 2013). PEP and RZ were assessed by ensemble averaging (Kelsey et al, 1998) the ECG and dZ/dt waveforms within each 60 second period.…”

“…In some tasks—known as self-paced, piece-rate, or unfixed tasks (Wright, 2008)—the amount of reward is proportional to a person’s effort, such as a cognitive task that pays 5 cents for each correct response and allows people to work as quickly as they wish (e.g., Silvia, McHone, Mironovová, Harper, & Sperry, 2016). For such tasks, the intensity of effort is a function of the value of the incentives (Wright, Killebrew, & Pimpalapure, 2002).…”

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

“…See the online supplemental materials for information on the RZ interval, a similar cardiac outcome(Silvia, McHone, et al, 2018).…”