RESUMO
INTRODUCTION: Increased muscle sympathetic nerve activity (MSNA) indicates an adverse outcome in heart failure. Decreased baroreflex modulation of MSNA is a well-known feature of the disease. The determinability of cardiovagal baroreflex sensitivity (BRS) in heart failure is low, however, the determinability of sympathetic BRS is not known. METHODS: We have assessed the spontaneous, MSNA burst incidence-based baroreflex index (BRSsymp) in 33 stable heart failure patients and in 10 healthy controls using the traditional r ≥ .5 cutoff for acceptable individual diastolic pressure-burst incidence slopes, and also a more stringent r ≥ .7 cutoff. We have also assessed the influence of 6/min breathing. RESULTS: The determinability of BRSsymp in heart failure patients was 64% during spontaneous breathing with r ≥ .5 cutoff, and 39% using the r ≥ .7 cutoff. The determinability of these indices further decreased during 6/min breathing, dropping to 29% with the r ≥ .7 cutoff. In contrast, the determinability of the cardiovagal BRS indices increased significantly with 6/min breathing (from 24% to 66%; p < .001). Patients who still had determinable BRSsymp at the r ≥ .7 cutoff had a significantly lower baseline burst incidence than those with an undeterminable index (70 ± 14 vs. 89 ± 10 burst/100 cycles; p < .002). Neither the 6/min breathing, nor the r ≥ .7 cutoff limit influenced the high availability of BRSsymp in healthy subjects. CONCLUSION: The determinability of BRSsymp in heart failure patients is limited, especially with the 0.7 limit for correlation. Undeterminable BRSsymp in patients is associated with higher sympathetic activity. 6/min breathing improves the determinability of cardiovagal BRS indices, but not that of BRSsymp.