Role of respiratory motor output in within-breath modulation of muscle sympathetic nerve activity in humans

We measured muscle sympathetic nerve activity (MSNA, peroneal microneurogra phy) in 5 healthy humans under conditions of matched tidal volume, breathin g frequency, and end tidal CO,, but varying respiratory motor output as fol lows: (1) passive positive pressure mechanical ventilation, (2) voluntary h yperventilation, (3) assisted mechanical ventilation that required the subj ect to generate -2.5 cm H2O to trigger each positive pressure breath, and ( 4) added inspiratory resistance. Spectral analyses showed marked respirator y periodicities in MSNA; however, the amplitude of the peak power was not c hanged with changing inspiratory effort. Time domain analyses showed that m aximum MSNA always occurred at end expiration (25% to 30% of total activity ) and minimum activity at end inspiration (2% to 3% of total activity), and the amplitude of the variation was not different among conditions despite marked changes in respiratory motor output. Furthermore, qualitative change s in intrathoracic pressure were without influence on the respiratory modul ation of MSNA. in all conditions, within-breath changes in MSNA were invers ely related to small changes in diastolic pressure (1 to 3 mm Hg), suggesti ng that respiratory rhythmicity in MSNA was secondary to loading/unloading of carotid sinus baroreceptors. Furthermore, at any given diastolic pressur e, within-breath MSNA varied inversely with lung volume, demonstrating an a dditional influence of lung inflation feedback on sympathetic discharge. Ou r data provide evidence against a significant effect of respiratory motor o utput on the within-breath modulation of MSNA and suggest that feedback fro m baroreceptors and pulmonary stretch receptors are the dominant determinan ts of the respiratory modulation of MSNA in the intact human.