Measuring Cardiac Autonomic Nervous System (ANS) Activity in Toddlers - Resting and Developmental Challenges

Summary

We describe the methods for continuous monitoring of the autonomic nervous system under resting and challenge conditions with 18 month old children. Results revealed that this protocol can produce meaningful physiological responses in both branches of the autonomic nervous system and elicit significant individual variability in patterns of responses.

Abstract

The autonomic nervous system (ANS) consists of two branches, the parasympathetic and sympathetic nervous systems, and controls the function of internal organs (e.g., heart rate, respiration, digestion) and responds to everyday and adverse experiences ¹. ANS measures in children have been found to be related to behavior problems, emotion regulation, and health ^2-7. Therefore, understanding the factors that affect ANS development during early childhood is important. Both branches of the ANS affect young children’s cardiovascular responses to stimuli and have been measured noninvasively, via external monitoring equipment, using valid and reliable measures of physiological change ^8-11. However, there are few studies of very young children with simultaneous measures of the parasympathetic and sympathetic nervous systems, which limits understanding of the integrated functioning of the two systems. In addition, the majority of existing studies of young children report on infants’ resting ANS measures or their reactivity to commonly used mother-child interaction paradigms, and less is known about ANS reactivity to other challenging conditions. We present a study design and standardized protocol for a non-invasive and rapid assessment of cardiac autonomic control in 18 month old children. We describe methods for continuous monitoring of the parasympathetic and sympathetic branches of the ANS under resting and challenge conditions during a home or laboratory visit and provide descriptive findings from our sample of 140 ethnically diverse toddlers using validated equipment and scoring software. Results revealed that this protocol can produce a range of physiological responses to both resting and developmentally challenging conditions, as indicated by changes in heart rate and indices of parasympathetic and sympathetic activity. Individuals demonstrated variability in resting levels, responses to challenges, and challenge reactivity, which provides additional evidence that this protocol is useful for the examination of ANS individual differences for toddlers.

Introduction

Individual differences in children’s autonomic nervous system (ANS) reactivity play an important role in the development and maintenance of physical and mental health problems ^12-16. A growing body of evidence reveals that individual differences in ANS measures at rest and reactivity are predictive of variations in children’s internalizing and externalizing psychopathology ^4,9,17-19 and physical health ^7,20. In addition to these direct or main effects of ANS functioning, an accumulating body of studies have found that children’s reactivity can interact with contextual risk factors, such as adverse events or marital conflict, to moderate those effects on children’s wellbeing and health (see reference ²¹ for a review). Despite this growing evidence that children’s autonomic reactivity plays a role in health across the life course, and the accompanying interest in those processes, there is need for additional research examining the development of ANS functioning in children under three years of age.

The ANS consists of two branches, the parasympathetic and sympathetic nervous systems. It regulates the function of internal organs (e.g., heart rate, respiration, digestion, and sexual arousal), largely through unconscious mechanisms, in response to everyday and adverse experiences ¹. The parasympathetic nervous system (PNS) is the ‘rest and restorative’ system that maintains a low resting heart rate and restorative state when sleeping or relaxing The sympathetic nervous system (SNS) is the ‘fight or flight’ system that responds to emergencies or threatening situations by accelerating one’s heart rate ²³. Respiratory sinus arrhythmia (RSA) is a reliable index of the PNS influence on cardiac functioning, and pre-ejection period (PEP) is a reliable index of the SNS influence on cardiac functioning. Both RSA and PEP have been found to be valid measures through experimental pharmacological blockade in samples of adults ^24,25. More recently, this work has been extended into child and adolescent samples, further establishing RSA and PEP as valid and reliable measures of ANS activity throughout development ^8,26,27. Reliable impedance cardiography measurements (PEP) using band and spot electrodes have been used for considerable time in adult samples ²⁸. More recently, parallel measurement and analytic techniques have also been shown to be reliable and valid in samples of children ^11,29,30, although collection of PEP measures in young children is rare.

There is an extant large body of literature on PNS functioning in infants and young children, including vagal tone and respiratory sinus arrhythmia measures. There are fewer studies of the SNS using PEP, and very few simultaneously examining both the parasympathetic and sympathetic nervous systems, which is critical for building understanding of their highly integrated functioning. Of the few studies of young children’s PEP, most find that their protocols do not produce significant group level changes in PEP in response to challenges ^8,31. Studies have demonstrated that there is stability in resting PNS measures starting in infancy and they have relations with temperament, behavior, and health,^{13,16,26,32,33} but there is a limited understanding of SNS responding and its stability over time, relations to development, and the factors that shape that its developmental trajectory. PNS responses have been attributed primarily to social engagement ¹⁶ and SNS responses proposed to reflect “fight or flight responses” ¹ as well as reward sensitivity (see reference ³⁴ for a review). In young children, it is challenging to simulate specific challenges in a laboratory and to engage them due to their short attention span. There are also measurement challenges for toddlers who may experience discomfort with the seven spot electrodes needed to measure PNS and SNS simultaneously. In addition, within the fields of developmental science, there few standards about how to elicit “stress reactivity” sufficiently while also respecting children’s developmental needs and the ethics in working with children. This article presents information on the administration and scoring of a standardized protocol comprised of resting and challenging conditions designed to elicit parasympathetic and sympathetic nervous system responses from 18 month old children.

ANS reactivity is typically conceptualized as the physiological response to a discrete external stimulus relative to a comparison or resting state, which varies across individual organisms ²². Examination of the early life etiology of mental and physical health problems has led scientists to be particularly interested in understanding children’s reactivity to situations that evoke adaptive responding or can be thought of as stressful challenges. The preponderance of literature refers to this phenomenon as “stress reactivity”, although encountering challenging stimuli has the potential to elicit responses across a broad range of domains. Thus, the protocol described below was designed to elicit ANS responses across multiple domains including two forms of rest (while listening to a soothing lullaby and while watching a calm, neutral video) and three developmentally-appropriate challenges for 18-month olds: anticipation/startle (jack-in-the-box), sensory (lemon juice), and socioemotional (listening to a sick infant cry). The protocol was adapted from our existing protocols for 12 month olds ⁸ and 3-5 year olds ¹¹, in order to make it developmentally-challenging, engaging and tolerable for toddlers (18-21 months).

Here we present the Developmental Challenges Protocol (DCP) and ANS data from the 18 month old study visit of the Stress, Eating and Early Development (SEED) Study of pregnant women and their offspring. Maternal participants with healthy pregnancies were recruited during 2013-14 for a study of gestational weight gain and prenatal stress and were overweight, low income, and racially and ethnically diverse. Parents’ informed consent for the study of her offspring was obtained just after birth and again prior to the start of the data collection for the study reported here, when the offspring were 18-21 months of age during 2014-2015.

Protocol

This study was approved by the Committee on Human Research of the University of California, San Francisco.   1. Pre-protocol Set-up (See Materials Spreadsheet for Complete List of Equipment) Set up the equipment by turning on the computer and connecting the ANS data acquisition unit to a power source. NOTE: Do not turn the acquisition unit on. Prior to beginning any data collection, set up the ANS data collection software. Choose settings to match Figu…

Representative Results

The SEED study enrolled 162 mother-child dyads (37% African American; 30% Latina; 16% white; 17% other or multiracial). For the 18 month visit, we completed the in-person assessment with 140 children (87% of the enrolled sample); 6 participants moved away, and the remaining participants were unreachable or unavailable for this visit. The refusal rate for ANS data collection component of the visit was 3/140 (2%) for mothers available for this visit. Three visits were conducted after 21 months (more than 3 months…

Discussion

This study revealed that, within a sample of 18 month-old children, standardized resting and challenge tasks designed to elicit responses from a range of domains (startle, sensory, social, resting) led to a range of ANS responses. The mean ANS responses found under these resting and challenge conditions were similar to ANS values reported by others who have measured ANS responsiveness with young children using similar tasks ^8,11,27,29. The mean levels of the ANS measures found in our sample are between those t…

Materials

ANS equipment
Laptop and power cord	Dell	60-0107-3.0	Mindware BioLab software requires a PC with 2 USB ports
ANS data collection software	Mindware	contact company	BioLab acquisition program
ANS data acquisition unit and cords	Mindware	50-3711-08	BioNex 8-slot Chassis
Snap leads	Mindware	93-0498-00	2 white, 2 red, 2 brown, 1 black
Spot electrodes	Kendall	31112496	130 model; 7 for each assessment + extras-to be sealed in a ziplock bag at all times)
ANS scoring/editing software	Mindware	contact company	HRV Analysis, version 3.1.0F and IMP Analysis, version 3.1.0I
Protocol equipment
tablet computer	Apple		iPad; for playing audio and video clips
portable speaker	Polariod	PBT510
Jack in the Box	Schylling	SMJB	Sock Monkey theme
Lemon juice	ReaLemon	98077	0.14oz shelf-stable packets
Pasture pipettes	Grainger	6FAV9	1 ml size, disposable
Portion cups	Dart Solo Con	250PC	2.5 oz, disposable, plastic
audio files of baby cry	Alkon lab		30 second clip recorded in a neonatal intensive care unit
audio file of baseline lullaby	Rockabye Baby Music		60 seconds of "Across the Universe" on "Rockabye Baby Lullaby Renditions of the Beatles" CD
audio file of final lullaby	Rockabye Baby Music		60 seconds of "Here Comes the Sun" on "Rockabye Baby Lullaby Renditions of the Beatles" CD
video file of neutral video			2 minutes of Baby Einstein video
Retractable measuring tape	Rollfix	TM04
Miscellaneous materials
Surge protector			to plug in all electronics
2-prong adapter for surge protector
Video camera	Sony		Handycam HDR-CX440
Video Encoder, power and input cords	Mindware	50-8614-01	optional; to sync video and ANS data files
Baby wipes and gauze			for cleaning and drying oily skin
Toys			to distract children during assessment
Visual aids			pictures of child with electrodes on, description of protocol to show participants
Script of protocol for research assistants
Charging cords for tablet and speaker
Doll and reinforcer			for demonstration if necessary