The effect of thermal stimulation on the heart-rate variability in neonates

Thermoregulation in humans can be divided into three broad mechanisms of control, namely: shivering, sweating and vasomotor activity. Previous investigations suggested the presence of an autonomic rhythm, originating in the central nervous system, possibly related to thermal vasomotor control and directly affecting heart rate by reflex changes in cardiac sympathetic and parasympathetic activity. The objective of the present work was to study the maturation process of the thermoregulatory system in newborns. We used peripheral thermal entrainment and focused on the reflections of vasomotor control in the heart-rate (HR) power spectrum (PS). The study included three groups of neonates at three different ages: 10 premature infants, 6 full-terms and 7 older infants (4 to 6 months). Thermal stimulation was achieved by placing a hot and cold surface on the subject's right palm alternately at three different rates: replacing the touching surface every 4 s (0.25 Hz), 7 s (0.14 Hz) and 10 s (0.1 Hz). 'Double period' stimulation was defined as the total duration of each period of hot and cold stimulation at the three rates, namely 8 s (0.125 Hz), 14 s (0.07 Hz), 20 s (0.05 Hz). The ECG of every infant was measured and recorded during the various stages of the experiment. The HR power spectrum from 0.02 Hz up to 2.00 Hz was considered, focusing on narrow ranges around the thermal stimulation frequencies. We found that in most subjects, clear peaks arise in the HR PS at the thermal entrainment frequency and its corresponding half frequency ('double period'). In premature infants, the reaction is best in response to the longest (10 s) stimulus (9 out of 10 prematures reacted positively), in group B (full-term infants) the reaction responds best to the 7-s stimulus (6 out of 6 reacted) and in older infants the reaction is slightly better at the 4-s stimulus. Since sympathetic control is slower, this ability to entrain the control system at increasing frequencies, might be related to the gradual maturation of parasympathetic control after birth. The different reaction of the three groups may help to understand the maturation process of the thermoregulation system.