Respiratory modulation of sympathetic activity in neonatal swine

Spontaneous efferent sympathetic activity (cervical sympathetic and splanchnic nerves) was recorded simultaneously with efferent phrenic discharge, with aortic pressure, and with the electrocardiogram in less than 1- to 51-day-old neonatal swine anesthetized with Saffan (alfaxalone). Power spectral analyses of sympathetic discharge revealed a frequency range of 4-36 Hz, with peaks distributed in four regions of the spectrum (4-6 Hz, 8-15 Hz, 16-22 Hz, and 28-36 Hz). Inspiratory modulation was readily apparent since sympathetic power spectral energy was greatest during the time of phrenic activity. Alterations in pulmonary afferent inputs (e.g., lung inflation tests and/or vagotomy) altered both phrenic and sympathetic activity. Use of the respiratory-cardiac modulation ratio indicated that respiratory modulation of sympathetic activity was greater than cardiac modulation. Complex afferent inputs, i.e., fictive Valsalva maneuver, indicated an immaturity of integration within the cardiovascular regulatory system. The adult patterns of blood pressure changes and heart rate responses were delayed until approximately 1 mo of age. Calculations of Valsalva ratios (ratio of maximal R-R interval to minimal R-R interval) indicated delayed onset of heart rate responses, reflecting the maturation of cardiovascular reflexes during the neonatal period.