Cardiac and behavioural trends in toddler pain distress responses across early development

In the second year of life, the brain undergoes rapid maturation, allowing young children to progress from relying mainly on their caregiver for comfort during distress to acquiring cognitive, motor, and emotional skills necessary for self-regulation of distress. This longitudinal, naturalistic observational study examined developmental patterns in pain-related distress responding during routine immunizations across the second year of life via cardiac and behavioural measures of pain-related distress. Respiratory Sinus Arrhythmia (RSA) and Face, Legs, Activity, Crying, and Consolability (FLACC) scores were collected at 4 epochs (Baseline, Needle, 1 min post-needle, and 2 min post-needle) during toddlers' 12-, 18-, and 24-month vaccination visits (N = 223). No significant developmental changes in RSA were found. FLACC was most variable at 12 months. FLACC responding to pain significantly diminished with age such that toddler's reactivity decreased, and regulatory rate exhibited a notable increase from 12 to 18 months. In 18- and 24-month-olds, pain behaviours returned to baseline levels by the third minute after vaccination, whereas in 12-month-olds, they remained elevated. These findings suggest that the second year of life marks a period of substantial development in distress regulation that mainly occurs between 12 and 18 months. With increasing age, toddlers' initial response to an acute pain stimulus decreases while their capacity for regulation increases.