Respiratory sinus arrhythmia: Modeling longitudinal change from 6 weeks to 2 years of age among low-income Mexican Americans.

Parasympathetically-mediated heart rate variability (HRV), commonly indexed via respiratory sinus arrhythmia (RSA), is theorized to support the physiological regulation of emotion; however, little is known about the trajectory of change in resting RSA across early development among high-risk populations for whom emotion regulation is crucial. This study characterized resting RSA change from 6 weeks to 2 years of age among 312 low-income Mexican American infants. RSA was assessed longitudinally at 6, 12, 18, 24, 52, 78, and 104 weeks of age. On average, resting RSA increased as infants aged, and this change accelerated over time. There was significant variance between infants in resting RSA at 6 weeks of age, and in the slope, and acceleration of resting RSA change. Intraclass correlation among infants' resting RSA measures was minimal, indicating that resting RSA may not be "trait-like" during infancy. Results characterize early RSA development among a high-risk sample, which can inform theoretical understanding of the development of emotional, and behavioral self-regulation in a high-risk population, as well as efforts to promote wellbeing across early childhood.