Autonomic modulation of sinus and atrioventricular nodes in premature low-birth-weight infants

Respiratory vagal activity is expressed by heart rate variability (HRV) at approximately 1 month of age in premature low-birth-weight infants (PLBWI). However, the autonomic inputs into the sinus node (SAN) and atrioventricular node (AVN) in PLBWI are unclear. We evaluated the variability in PP and PR intervals at day zero (day 0) and 1 month (1 month) after birth in 16 PLBWI (gestation 32.3 ± 1.3 weeks, birth weight 1.578 ± 257 g). The polygraph was recorded during sleep on day 0 and at 1 month. PP and PR intervals and the number of respiratory cycles were measured, and frequency analysis was performed by auto-correlation fast Fourier transforms. Power spectral density (PSD: ms²) was calculated for the low frequency domain (LF: 0.036-0.146 Hz), high frequency domain (HF: 0.146-0.390 Hz), total frequency (TF: 0.036-2.000 Hz), and respiratory sinus arrhythmia (RSA: frequency bandwidth of 0.3 Hz with peak respiratory frequency as median), and the PSD ratio in the PP and PR intervals (LF/HF, RSA/TF) were compared. Compared with day 0, a decrease in the LF/HF ratio and an increase in the RSA/TF ratio in PP intervals were observed at 1 month, consistent with expression of respiratory vagal activity. For PR intervals, on the other hand, the LF/HF ratio increased, indicative of accentuated sympathetic activity. However, the respiratory vagal input was weak, and the RSA/TF ratio remained unchanged. These observations suggest that, in PLBWI at 1 month, AVN conduction was not predominatly influenced by respiratory-related vagal activity, but was controlled by autonomic regulation, independent of the SAN.