Correlation of PP and PR intervals in premature low birth weight infants.

We examined the developmental change by which autonomic neural activity associated respiration modulates spontaneous firing rate of sinus (SA) node and atrioventricular (AV) conduction in premature infants born with low birth weight (LBWI). The purpose of this study was to clarify whether variation of PR is correlated with that of PP or those are independent in LBWI with immature autonomic nervous system. We investigated, therefore, whether there are spontaneous functional differences in the innervation of SA and AV nodes. Further, we evaluated the maturation of autonomic nervous system progressing in the period, on the day of birth (Day 0) to approximately one month after the birth (Month 1). This study was performed in thirteen LBWI during deep sleep. EEG, EOG, ECG, respiratory waves were digitized on line, spontaneous firing cycle of SA node (PP), and AV nodal conduction time (PR) that were recorded on Day 0 and Month 1. Then, the data were analyzed as follows: 1) correlations among the means and standard deviations (SD) of PP, PR and RR, 2) variance evaluation of PP and PR intervals by Lorenz plot analysis method, 3) correlation analysis among PP, PR and RR intervals by linear regression method and 4) frequency analysis for PP and PR intervals by high-speed Fourier transform method (FFT) and determination of frequency density. The PP interval decreased as growing in the period. Contrary PR interval increased. In LBWI, the automatic nervous activities including parasympathetic nerve activity for spontaneous firing cycle of SA node and ventricular excitation cycle on Month 1 were higher than Day 0. It was assumed that the vagal nerve activity for the AV conduction was enhanced. However, there was no significant change in linear regression slope for the spontaneous firing cycle of SA node and the AV conduction time. Postnatal LF/HF changes for PP and PR obtained by frequency analysis, were opposite. Therefore, it was suggested that the maturity of autonomic nervous system progresses in the period, Day 0 to approximately Month 1, but the variations in PP and PR are independent each other.