Daniel W. White

Methods and considerations for the analysis and standardization of assessing muscle sympathetic nerve activity in humans

The technique of microneurography and the assessment of muscle sympathetic nerve activity (MSNA) are used in laboratories throughout the world. The variables used to describe MSNA, and the criteria by which these variables are quantified from the integrated neurogram, vary among studies and laboratories and, therefore, can become confusing to those starting to learn the technique. Therefore, the purpose of this educational review is to discuss guidelines and standards for the assessment of sympathetic nervous activity through the collection and analysis of MSNA. This review will reiterate common practices in the collection of MSNA, but will also introduce considerations for the evaluation and physiological inference using MSNA.

Caffeine delays autonomic recovery following acute exercise

Background Impaired autonomic recovery of heart rate (HR) following exercise is associated with an increased risk of sudden death. Caffeine, a potent stimulator of catecholamine release, has been shown to augment blood pressure (BP) and sympathetic nerve activity; however, whether caffeine alters autonomic function after a bout of exercise bout remains unclear. Methods In a randomized, crossover study, 18 healthy individuals (26 ± 1 years; 23.9 ± 0.8 kg·m−2) ingested caffeine (400 mg) or placebo pills, followed by a maximal treadmill test to exhaustion. Autonomic function and ventricular depolarization/repolarization were determined using heart rate variability (HRV) and corrected QT interval (QTc), respectively, at baseline, 5, 15, and 30 minutes post-exercise. Results Maximal HR (HRmax) was greater with caffeine (192 ± 2 vs. 190 ± 2 beat·min−1, p < 0.05). During recovery, HR, mean arterial pressure (MAP), and diastolic blood pressure (DBP) remained elevated with caffeine (p < 0.05). Natural log transformation of low-to-high frequency ratio (LnLF/LnHF) of HRV was increased compared with baseline at all time points in both trials (p < 0.05), with less of an increase during 5 and 15 minutes post-exercise in the caffeine trial (p < 0.05). QTc increased from baseline at all time points in both trials, with greater increases in the caffeine trial (p < 0.05). Conclusions Caffeine ingestion disrupts post-exercise autonomic recovery because of increased sympathetic nerve activity. The prolonged sympathetic recovery time could subsequently hinder baroreflex function during recovery and disrupt the stability of autonomic function, potentiating a pro-arrhythmogenic state in young adults.

Effect of upper body position on arterial stiffness: influence of hydrostatic pressure and autonomic function

Objective: To evaluate changes in arterial stiffness with positional change and whether the stiffness changes are due to hydrostatic pressure alone or if physiological changes in vasoconstriction of the conduit arteries play a role in the modulation of arterial stiffness. Methods: Thirty participants’ (male = 15, 24 ± 4 years) upper bodies were positioned at 0, 45, and 72° angles. Pulse wave velocity (PWV), cardio-ankle vascular index, carotid beta-stiffness index, carotid blood pressure (cBP), and carotid diameters were measured at each position. A gravitational height correction was determined using the vertical fluid column distance (mmHg) between the heart and carotid artery. Carotid beta-stiffness was calibrated using three methods: nonheight corrected cBP of each position, height corrected cBP of each position, and height corrected cBP of the supine position (theoretical model). Low frequency systolic blood pressure variability (LFSAP) was analyzed as a marker of sympathetic activity. Results: PWV and cardio-ankle vascular index increased with position (P < 0.05). Carotid beta-stiffness did not increase if not corrected for hydrostatic pressure. Arterial stiffness indices based on Method 2 were not different from Method 3 (P = 0.65). LFSAP increased in more upright positions (P < 0.05) but diastolic diameter relative to diastolic pressure did not (P > 0.05). Conclusion: Arterial stiffness increases with a more upright body position. Carotid beta-stiffness needs to be calibrated accounting for hydrostatic effects of gravity if measured in a seated position. It is unclear why PWV increased as this increase was independent of blood pressure. No difference between Methods 2 and 3 presumably indicates that the beta-stiffness increases are only pressure dependent, despite the increase in vascular sympathetic modulation.

N-Acetylcysteine reduces hyperacute intermittent hypoxia-induced sympathoexcitation in human subjects

What is the central question of this study? This study evaluated the following central question: does N‐acetylcysteine (N‐AC), an antioxidant that readily penetrates the blood–brain barrier, have the capability to reduce the increase in sympathetic nerve activity observed during hyperacute intermittent hypoxia? What is the main finding and its importance? We demonstrate that N‐AC decreases muscle sympathetic nerve activity in response to hyperacute intermittent hypoxia versus placebo control. This finding suggests that antioxidants, such as N‐AC, have therapeutic potential in obstructive sleep apnoea.

Healthy aging and carotid performance: strain measures and β-stiffness index

Arterial stiffness is related to the risk of cardiovascular disease (CVD) and increases with aging. Functional impairment of the arterial wall can occur before structural changes and can be detectable before CVD symptoms. The elastic properties of the carotid arterial wall during the cardiac cycle can be evaluated by standard 2-dimensional (2D) ultrasound longitudinal or circumferential imaging of vascular deformation (strain) using speckle tracking. The purpose of this study was to compare standard 2D ultrasound circumferential and longitudinal images of vascular tissue motion and strain using speckle tracking in young and older individuals. Participants underwent recording of 2D ultrasound circumferential and longitudinal images of the common carotid artery. Circumferential carotid strain (CS) and CS rate were obtained and analyzed via speckle tracking software. Following the strain analysis, the circumferential strain β-stiffness (C-β) was calculated. Conventional longitudinal β-stiffness (L-β) was calculated and non-invasive blood pressure measurements were obtained from carotid artery pressure measurements in a resting supine position using applanation tonometry. C-β was significantly higher than L-β, and the association with age was greater (r = .824 vs. r = .547). CS and CS rate were significantly higher in the young compared to the older group. L-β does not explain as much of the age-dependent differences in the carotid artery compared with C-β. This is possibly due to the inclusion of whole arterial wall motion and deformation observed in the CS image. The ability of C-β to accurately predict the future risk of CVD independent of age still needs further investigation.

Effects of Exercise on Blood Pressure and Autonomic Function and Other Hemodynamic Regulatory Factors

The autonomic nervous system is important for maintaining cardiovascular homeostasis in daily living. Improper autonomic function (i.e., the set of neurological control systems regulating and maintaining cardiovascular homeostasis, particularly heat rate and blood pressure) can lead to cardiovascular disease such as hypertension. This chapter discusses the role of the autonomic nervous system in hypertension and the effect of exercise on the autonomic nervous system in restoring cardiovascular health. Within, the reader will find a tutorial of the anatomy and physiology of the autonomic nervous system. Various common techniques for assessing autonomic function are also included in this discussion. The chapter finishes with a review of the literature pertaining to the acute (short-term or immediate) and chronic (i.e., long-term or training) effects of exercise on autonomic function.