Postnatal development of baroreflex sensitivity in infancy

Baroreflex sensitivity (BRS) using spontaneous sequence analysis in the time domain is not fully applicable in infancy, as the time delay for heart period to change (heart period delay, HPD) after an arterial pressure change is unknown. We estimated and compared HPD and BRS in the frequency (BRSsp, HPDsp) and time domains (BRSseq, HPDseq) from systolic blood pressure (SBP) and heart period fluctuations. Continuous SBP, using photoplethysmography, and heart period measurements were performed on 30 term infants at 2-4 weeks, 2-3 months and 5-6 months postnatal age. Cross-spectral analysis between SBP and heart period fluctuations was used to estimate BRSsp and HPDsp. Spontaneous sequence analysis was used to estimate BRS using a fixed beat delay of 1-12 beats (BRSseq) or a variable delay identified by a novel method accounting for epoch-epoch variability in HPD (BRSseqvar). HPDsp averaged 3.4 s (similar to 7 beats); BRSsp averaged 11.4 ms mmHg-1. BRSseq and BRSseqvar were consistently lower than BRSsp (P < 0.05), but the three BRS estimates were strongly correlated using a HPD of similar to 5-6 beats. BRSseqvar resulted in the average estimate (8.9 ms mmHg-1) closest to BRSsp and overall had the strongest correlation with BRSsp (R2 = 0.61; P < 0.001). All three BRS estimates increased progressively with postnatal age, with BRSsp averaging 6.4, 10.5 and 16.0 ms mmHg-1 at 2-4 weeks, 2-3 months and 5-6 months, respectively (P < 0.05). Accounting for the HPD of infancy provides estimates of BRS in the time domain that closely parallel spectral estimates, and provides a novel analytical tool to assess normal development and dysfunction of the baroreflex in infants.