Methods of assessment of the post-exercise cardiac autonomic recovery: A methodological review

Highlights

• Post-exercise autonomic analysis is a tool for the assessment of cardiac health.

• We reviewed the most commonly methods for post-exercise autonomic recovery analysis.

• Assessment of post-exercise autonomic function using the provided methods is encouraged.

• The reliability and clinical properties of these methods must be better addressed.

Abstract

The analysis of post-exercise cardiac autonomic recovery is a practical clinical tool for the assessment of cardiovascular health. A reduced heart rate recovery – an indicator of autonomic dysfunction – has been found in a broad range of cardiovascular diseases and has been associated with increased risks of both cardiac and all-cause mortality. For this reason, over the last several years, non-invasive methods for the assessment of cardiac autonomic recovery after exercise – either based on heart rate recovery or heart rate variability indices – have been proposed. However, for the proper implementation of such methods in daily clinical practice, the discussion of their clinical validity, physiologic meaning, mathematical formulation and reproducibility should be better addressed. Therefore, the aim of this methodological review is to present some of the most employed methods of post-exercise cardiac autonomic recovery in the literature and comprehensively discuss their strengths and weaknesses.

Introduction

Exercise is a valuable tool in screening for cardiovascular diseases [1]. In an intensity-dependent manner, the exercise stimulus leads to an autonomic adjustment evidenced by a reduced parasympathetic and an increased sympathetic outflow to the heart [2], [3]. The autonomic stress imposed by exercise continues during the post-exercise period and gradually diminishes with parasympathetic reactivation and sympathetic withdrawal [4]. This autonomic recovery is accompanied by a large decrease in heart rate (HR) [5]. It has been shown that the slower the recovery of cardiac autonomic activity and HR after exercise, the higher is the cardiovascular risk [6], [7], [8], [9].

A Pubmed search using the keywords “heart rate”, “recovery” and “exercise” (only with English language) has returned nearly three thousand titles. However, despite the early studies on HR recovery (HRR) being dated from the 1960s, a quantitative analysis of studies published per year reveals a marked increase in the number of titles issued in the last three decades. This overwhelming growth in the HRR related literature can be attributed to a series of studies elucidating the physiologic mechanisms underpinning HRR as well as its association with health and prognosis [4], [5], [8], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Milestones in the association between HRR and cardiovascular health were the studies conducted by Savin et al. [17], Arai et al. [4], Perini et al. [16] and Imai et al. [5] which demonstrated that HRR is determined by parasympathetic and sympathetic activities at the sinoatrial node. Further advances in the early 2000s included the demonstration of the clinical utility of HRR as a prognostic tool [10], [11], [15], [18], [19], specifically that HRR is an independent predictor of overall and cardiovascular mortality in both asymptomatic individuals [11] and in patients with cardiovascular disease [15], [18]. Since then, studies from other groups have confirmed the prognostic value of HRR [8], [13], [14], [20], [21], [22]. Assessment of HR variability (HRV) during recovery is a complementary method to assess the autonomic recovery after exercise [23], [24], but its prognostic significance has not been determined.

As the analysis of cardiac autonomic recovery after exercise is considered a valuable prognostic tool, several methods of assessment of cardiac autonomic recovery using HR- or HRV-derived indices have been used [5], [12], [16], [23], [25], [26], [27], [28]. While the variety of methods and indices bring a wide perspective in terms of post-exercise cardiac autonomic assessment, the clinical validity, physiologic meaning, mathematical formulation, and reproducibility of these approaches must be comprehensively addressed. In this regard, the aim of this methodological review is to present and critically discuss the pros and cons of the most used indices of assessment of post-exercise cardiac autonomic recovery.