Vianna, Lauro C.

Previous studies have indicated that acute slow‐paced breathing (6 breaths/min) significantly decreases arterial blood pressure (BP) in both normotensive and hypertensive individuals. More recent work suggested that muscle sympathetic nerve activity (MSNA) was unchanged during acute slow‐paced breathing, however, the pattern of MSNA firing was altered in that more periods of MSNA quiescence were present. Herein, we examined the ability of MSNA to modulate BP on a beat‐to‐beat basis. We hypothesized that the magnitude of the increase in BP following a burst of MSNA is attenuated during acute slow‐paced breathing contributing to the reduction in BP previously reported. BP (brachial catheter) and MSNA (peroneal microneurography) were continuously measured in 10 young (27 ± 1 years), healthy men during 10 minutes of normal spontaneous breathing (baseline) or paced breathing at six breaths per minute. Signal averaging was used to characterize changes in mean BP for 15 cardiac cycles following heartbeats associated with and without MSNA bursts. MSNA bursts were segregated as either single bursts (directly bordered by ≥1 heartbeat lacking MSNA) or multiple bursts (any burst positioned directly adjacent to another MSNA burst). Respiratory rate was 12 ± 1.1 breaths/min during baseline and 6 ± 0.7 breaths/min during slow‐paced breathing. Compared to baseline, mean BP was decreased during slow‐paced breathing (88±2 vs. 85±2 mmHg, P<0.05), whereas MSNA burst frequency was unchanged (13±2.2 vs. 13±2.4 bursts/min, P>0.05). At baseline, mean BP significantly increased following MSNA bursts (+3.2± 0.2 mmHg, P<0.05) and this increase was augmented during slow‐paced breathing (+5.0± 0.5 mmHg, P<0.05). There was a greater increase in mean BP following single as well as multiple MSNA bursts during slow‐paced breathing. In contrast, cardiac cycles without MSNA bursts (non‐bursts) were associated with a significant decrease in mean BP (−0.7± 0.1 mmHg, P<0.05), which was also augmented during slow‐paced breathing (−1.3± 0.3 mmHg, P<0.05). In summary, the pressor and depressor responses coupled to spontaneous oscillations in MSNA are augmented during slow paced breathing with a net effect of reduced BP. Thus, changes in MSNA firing pattern, without changes in burst frequency, can modulate resting BP. Support or Funding Information Funding: NIH HL‐083947 (MJJ)

The sympathetic nervous system is critical for the beat-to-beat regulation of arterial blood pressure (BP). Although studies have examined age- and sex-related effects on BP control, findings are inconsistent and limited data are available in postmenopausal women. In addition, the majority of studies have focused on time-averaged responses without consideration for potential beat-to-beat alterations. Thus we examined whether the ability of muscle sympathetic nerve activity (MSNA) to modulate BP on a beat-to-beat basis is affected by age or sex. BP and MSNA were measured during supine rest in 40 young (20 men) and 40 older (20 men) healthy subjects. Beat-to-beat fluctuations in mean arterial pressure (MAP) were characterized for 15 cardiac cycles after each MSNA burst using signal averaging. The rise in MAP following an MSNA burst was similar between young men and women (+2.64 ± 0.3 vs. +2.57 ± 0.3 mmHg, respectively). However, the magnitude of the increase in MAP after an MSNA burst was reduced in older compared with young subjects ( P < 0.05). Moreover, the attenuation of the pressor response was greater in older women (+1.20 ± 0.1 mmHg) compared with older men (+1.72 ± 0.2 mmHg; P < 0.05). Interestingly, in all groups, MAP consistently decreased after cardiac cycles without MSNA bursts (nonbursts) with the magnitude of fall greatest in older men. In summary, healthy aging is associated with an attenuated beat-to-beat increase in BP after a spontaneous MSNA burst, and this attenuation is more pronounced in postmenopausal women. Furthermore, our nonburst findings highlight the importance of sympathetic vasoconstrictor activity to maintain beat-to-beat BP, particularly in older men.