We measured muscle sympathetic nerve activity (MSNA, peroneal microneurography) in 5 healthy humans under conditions of matched tidal volume, breathing frequency, and end-tidal CO 2 , but varying respiratory motor output as follows: (1) passive positive pressure mechanical ventilation, (2) voluntary hyperventilation, (3) assisted mechanical ventilation that required the subject to generate -2.5 cm H 2 O to trigger each positive pressure breath, and (4) added inspiratory resistance. Spectral

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... yses showed marked respiratory periodicities in MSNA; however, the amplitude of the peak power was not changed with changing inspiratory effort. Time domain analyses showed that maximum 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 changes in intrathoracic pressure were without influence on the respiratory modulation of MSNA. In all conditions, within-breath changes in MSNA were inversely related to small changes in diastolic pressure (1 to 3 mm Hg), suggesting that respiratory rhythmicity in MSNA was secondary to loading/unloading of carotid sinus baroreceptors. Furthermore, at any given diastolic pressure, within-breath MSNA varied inversely with lung volume, demonstrating an additional influence of lung inflation feedback on sympathetic discharge. Our data provide evidence against a significant effect of respiratory motor output on the within-breath modulation of MSNA and suggest that feedback from baroreceptors and pulmonary stretch receptors are the dominant determinants of the respiratory modulation of MSNA in the intact human. (Circ Res. 1999;85:457-469.) Figure 4. Individual and mean (nϭ3) data for the within-breath changes in MSNA, R-R interval, and diastolic pressure (DP) in 3 subjects during voluntary hyperventilation (A) and added inspiratory resistance (B).