Beyond Clinical Signs: The Interplay Between Vagal Tone and Lethargic Behavior in Preterm Infants.

In this cross-sectional, observational study, the association between lethargy and vagal tone was analyzed in thirteen low-risk preterm infants. Infants underwent behavioral assessment and vagal tone measurement indexed by high-frequency heart-rate variability (HF-HRV). A lower vagal tone cluster was associated with greater lethargy (p = .05), reduced handling (p = .003), excitability (p = .006), higher skin stress indicators (p = .13), and central nervous system stress indicators (p = .10) than cluster with a higher vagal tone cluster. A lower vagal tone could indicate autonomic nervous system dysregulation or immaturity, leading to neurobehavioral challenges, such as lethargy and further neurodevelopmental difficulties.