Baroreflex modulation of muscle sympathetic nerve activity at rest does not differ between morning and afternoon
Sarah L Hissen1, Vaughan G Macefield2, Rachael Brown3
1School of Science and Health, University of Western Sydney Sydney, NSW, Australia.
Insights
Cardiovascular events are more common in the morning. This study found no significant difference in vascular sympathetic baroreflex sensitivity between morning and afternoon in healthy young adults, suggesting consistent assessment of sympathetic function is possible.
Area of Science:
- Physiology
- Cardiovascular Research
- Autonomic Nervous System
Background:
- Increased incidence of cardiovascular events in the morning hours is a known phenomenon.
- Previous research linked this to impaired cardiac baroreflex control of blood pressure.
- The diurnal variation in vascular sympathetic baroreflex function, regulating blood pressure via muscle sympathetic nerve activity (MSNA), remains unexplored.
Purpose of the Study:
- To investigate diurnal variations in vascular sympathetic baroreflex sensitivity (BRS).
- To compare vascular sympathetic BRS between morning and afternoon measurements in the same individuals.
Main Methods:
- 10 healthy young participants (mean age 22 ± 2.9 years) underwent measurements.
- Continuous monitoring of blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) during rest.
- Vascular sympathetic BRS quantified using MSNA burst incidence (BRSinc) and total MSNA (BRStotal) plotted against diastolic pressure.
Main Results:
- Significant vascular sympathetic BRSinc and BRStotal slopes were obtained in all participants at both times.
- No significant difference was observed in vascular sympathetic BRSinc between morning (-2.2 ± 0.6% bursts/mmHg) and afternoon (-2.5 ± 0.2% bursts/mmHg) sessions.
- Similarly, vascular sympathetic BRStotal showed no significant difference between morning (-3.0 ± 0.5 AU/beat/mmHg) and afternoon (-2.9 ± 0.4 AU/beat/mmHg) sessions.
Conclusions:
- Baroreflex modulation of MSNA at rest does not exhibit diurnal variation in healthy young individuals.
- These findings suggest that MSNA recordings are reliable for assessing sympathetic function regardless of the time of day.
- This supports the validity of using MSNA measurements at different times for consistent sympathetic function assessment.
Abstract:
The incidence of cardiovascular events is significantly higher in the morning than other times of day. This has previously been associated with poor blood pressure control via the cardiac baroreflex. However, it is not known whether diurnal variation exists in vascular sympathetic baroreflex function, in which blood pressure is regulated via muscle sympathetic nerve activity (MSNA). The aim of this study was to compare vascular sympathetic baroreflex sensitivity (BRS) in the same participants between the morning and afternoon. In 10 participants (mean age 22 ± 2.9 years), continuous measurements of blood pressure, heart rate and MSNA were made during 10 min of rest in the morning (between 0900 and 1000 h) and afternoon (between 1400 and 1500 h). Spontaneous vascular sympathetic BRS was quantified by plotting MSNA burst incidence against diastolic pressure (vascular sympathetic BRSinc), and by plotting total MSNA against diastolic pressure (vascular sympathetic BRStotal). Significant vascular sympathetic BRSinc and vascular sympathetic BRStotal slopes were obtained for 10 participants at both times of day. There was no significant difference in vascular sympathetic BRSinc between morning (-2.2 ± 0.6% bursts/mmHg) and afternoon (-2.5 ± 0.2% bursts/mmHg; P = 0.68) sessions. Similarly, vascular sympathetic BRStotal did not differ significantly between the morning (-3.0±0.5 AU/beat/mmHg) and afternoon (-2.9 ± 0.4 AU/beat/mmHg; P = 0.89). It is concluded that in healthy, young individuals baroreflex modulation of MSNA at rest does not differ between the morning and afternoon. The results indicate that recording MSNA at different times of the day is a valid means of assessing sympathetic function.
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