Sensitivity to mental effort and test–retest reliability of heart rate variability measures in healthy seniors

Abstract

Objectives

To determine (1) whether heart rate variability (HRV) was a sensitive and reliable measure in mental effort tasks carried out by healthy seniors and (2) whether non-linear approaches to HRV analysis, in addition to traditional time and frequency domain approaches were useful to study such effects.

Methods

Forty healthy seniors performed two visual working memory tasks requiring different levels of mental effort, while ECG was recorded. They underwent the same tasks and recordings 2 weeks later. Traditional and 13 non-linear indices of HRV including Poincaré, entropy and detrended fluctuation analysis (DFA) were determined.

Results

Time domain, especially mean R-R interval (RRI), frequency domain and, among non-linear parameters – Poincaré and DFA were the most reliable indices. Mean RRI, time domain and Poincaré were also the most sensitive to different mental effort task loads and had the largest effect size.

Conclusions

Overall, linear measures were the most sensitive and reliable indices to mental effort. In non-linear measures, Poincaré was the most reliable and sensitive, suggesting possible usefulness as an independent marker in cognitive function tasks in healthy seniors.

Significance

A large number of HRV parameters was both reliable as well as sensitive indices of mental effort, although the simple linear methods were the most sensitive.

Introduction

Mental effort, a varying capacity for cognitive processing closely related to arousal and attention, has been studied using physiological measures (Kahneman, 1973, Kramer and Weber, 2000, Oken et al., 2006). One such measurement is heart rate variability (HRV), the analysis of beat-to-beat intervals. HRV is sensitive to several clinical conditions, especially those associated with cardiac disease or autonomic neuropathy (Task Force of the European Society of Cardiology, 1996, Hennessy et al., 2001, Pope et al., 2001, Sandercock et al., 2005, Paul-Labrador et al., 2006). Additionally, HRV has been used to assess interventions that might impact sympathetic–parasympathetic balance in conditions not associated with cardiac disease or autonomic neuropathy (Fu et al., 2006, Paul-Labrador et al., 2006).

Previous research reported that HRV is sensitive to changes in mental effort (Mulder and Mulder, 1981, Aasman et al., 1987, Jorna, 1992, Veltman and Gaillard, 1993). With an increase in mental effort, there is a decrease in power around 0.10 Hz, in what they referred to as the mid-frequency band, 0.07–0.14 Hz (Jorna, 1992). This is now commonly known as the low frequency band, 0.04–0.15 Hz (Task Force of the European Society of Cardiology, 1996). The underlying mechanism may be due to increased sympathetic activation or the subjects’ pattern of breathing (Althaus et al., 1998). In other studies, apart from a strong inverse relationship between mental effort and HRV power, mean RR interval appears to be the most sensitive measure (Capa et al., 2008, De Rivecourt et al., 2008, Henelius et al., 2009, Weippert et al., 2009).

While HRV has useful applications both clinically and as an autonomic measure in physiological research- the inter-subject variability of HRV measures is rather high (Gerritsen et al., 2003, Pinna et al., 2007). Thus, it is difficult to generalize the results from any one given study to others with subjects of different demographic characteristics. Additionally, the within-subject reliability of HRV measures remains unclear. The degree of reproducibility of HRV measurements from short-term recordings in healthy people is inconsistent and can at best be considered moderate (Pinna et al., 2007, Sandercock et al., 2005), except under highly controlled resting conditions (Melanson, 2000). Studies of reliability of HRV parameters during mental effort tasks is lacking in the literature.

Of late, non-linear methods of HRV analysis have gained prominence, with the promise of unraveling the inherent complexity in heart rate rhythms and an aim to model the complex interplay of influencing factors (Schumacher, 2004, González and Pereda, 2004). These new methods seem to contribute to the prognosis and treatment of different diseases (Huikuri et al., 2009). While most studies using non-linear techniques have been carried out on long-term time series, like a 24-h ambulatory Holter monitor (Bigger et al., 1996, Huikuri et al., 2000, Maestri et al., 2007a, Maestri et al., 2007b), it is of immense interest and benefit to see if these results are reproducible in short-term HRV, which is critical for studying cardiovascular autonomic changes. Shorter time series analysis techniques are desirable because many human psychophysiologic studies under controlled lab conditions are limited in how long they can be optimally maintained. While several methods suitable to assess short-term non-linear properties have been proposed (Guzzetti et al., 2005, Balocchi et al., 2006), the reliability of these indices remains unclear. Studies carried out on patients with heart failure (Maestri et al., 2006, Maestri et al., 2007a, Maestri et al., 2007a) and short term records in control subjects (Maestri et al., 2007b) show a large variability in these indices Moreover, the reliability and usefulness of these indices as markers in the study of mental effort tasks has not been established. Hence, in the current study, we investigated the sensitivity and reliability of HRV to mental effort tasks in healthy seniors, using time domain, frequency domain and non-linear approaches to HRV analysis.