Cardiac Autonomic Measures during Baseline Resting Conditions in Autism Spectrum Disorders: Two Meta-Analyses

Background: Cardiac autonomic markers such as heart rate (HR) and heart rate variability (HRV) are increasingly being related to behavioral characteristics of autism spectrum disorder (ASD). Theories such as the neurovisceral integration model (Thayer & Lane, 2000) and the polyvagel theory (Porges, 2001, 2007) also relate cardiac autonomic functioning to appropriate social and environmentally adjusted behavior. Two recent reviews (Benevides & Lane, 2015; Klusek, Roberts & Losh, 2015) have provided qualitative descriptions of research findings so far regarding baseline levels of cardiac autonomic functioning as well as cardiac autonomic reactivity to different kinds of tasks in individuals with ASD. In general, they report mixed results regarding baseline functioning and some evidence for altered cardiac autonomic reactivity to tasks. Unfortunately quantitative analyses, such as meta-analyses, are lacking.

Objectives: The aim of the present review was to improve current knowledge on cardiac autonomic functioning in baseline conditions among individuals (all ages) with ASD by means of meta-analyses.

Methods: Two meta-analyses were conducted to investigate whether individuals with ASD differ from typically developing individuals regarding their baseline levels of heart rate (meta-analysis 1) and heart rate variability (meta-analysis 2). For this purpose, systematic searches were conducted among the databases of PsychINFO, Web of Science and PubMed for the period 1990 - august 2015.

Results: Regarding HR, ten studies could be included with a total sample size of 339 ASD participants and 291 control participants. For HRV, 15 studies were included with a total sample size of 537 ASD participants and 410 control participants. Results of the meta-analyses revealed that individuals with ASD have higher heart rate (g = 0.68) and lower HRV (g = -0.46) in baseline conditions than typically developing individuals without ASD, irrespective of age and intellectual functioning. While the heart rate findings seem very robust, the heterogeneity with respect to the studies focusing on HRV was large. This implies that the HRV findings need to be interpreted cautiously.

Conclusions: The present results of heightened HR and lowered HRV in baseline conditions among individuals with ASD provide a theoretical basis for the lack of adaptation and unresponsiveness to environmental challenges frequently encountered in individuals with ASD, for example in social situations. This theoretical basis relies on the described assumptions of the neurovisceral integration model and polyvagel theory that cardiac autonomic functioning is related to appropriate social and environmentally adjusted behavior.