Analysis Of Muscle Sympathetic Nerve Activity Research Articles

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.

A Case for a Standardized Method for the Normalization of Muscle Sympathetic Nerve Activity (MSNA) Amplitude

MSNA has no agreed upon standard for analysis. For example, mean burst strength (MBS), total activity (TA) and total MSNA (TMSNA) rely on amplitude measurements assigned arbitrary units which are highly dependent on the method of normalization. We investigated the hypothesis that computer algorithms used for automatic normalization of MSNA amplitudes differ from those obtained with manual normalization leading to erroneous interpretations. MSNA from healthy human subjects (n=8, 2 female) was obtained by microneurography of the peroneal nerve while resting quietly in a 30o semi‐recumbent position. Data was analyzed by 3 different normalization methods (Min‐Max Auto (MM), Mean Auto (MA), and Rectified Manual (RM)). MSNA is quantified as burst frequency (BF), burst incidence (BI), MBS, TA, and TMSNA. There were no differences in BF or BI between the three analysis methods. However, significant differences were detected between MM and RM in MBS, TA and TMSNA (p=0.002, p=0.009, and p=0.004). Rankings according to activity were not different between MM and RM but were different in MA compared to either MM or RM. There is an overwhelming need to standardize the analysis of MSNA. The current study points out that the variables BF and BI can be compared across studies. However, our data identify that the RM normalization is the only method to quantify MBS, TA and TMSNA for cross‐study comparisons.

Pulse-synchronous sympathetic burst power as a new index of sympathoexcitation in patients with heart failure.

The upper limit of incidence of muscle sympathetic neural bursts can lead to underestimation of sympathetic activity in patients with severe heart failure. This study aimed to evaluate the pulse-synchronous burst power of muscle sympathetic nerve activity (MSNA) as a more specific indicator that could discriminate sympathetic activity in patients with heart failure. In 54 patients with heart failure, the pulse-synchronous burst power at the mean heart rate was quantified by spectral analysis of MSNA. Thirteen patients received a central sympatholytic agent (guanfacine) for 5 days to validate the feasibility of this new index. Both burst incidence and plasma norepinephrine level showed no significant difference between patients in New York Heart Association functional class III (94 +/- 6 per 100 heartbeats and 477 +/- 219 pg/ml, respectively) and class II (79 +/- 14 per 100 heartbeats and 424 +/- 268 pg/ml, respectively). In contrast, the burst power was useful for discriminating patients in class III from those in class II (61 +/- 8% vs. 39 +/- 10%; P < 0.05). Inhibition of sympathetic nerve activity by guanfacine was more sensitively reflected by the change of burst power (-36 +/- 25%) than by that of burst incidence (-12 +/- 14%; P < 0.001). The sympathetic burst power reflects both burst frequency and amplitude independently of the absolute values and provides a sensitive new index for interindividual comparisons of sympathetic activity in patients with heart failure.

Spectral analysis of heart rate, arterial pressure, and muscle sympathetic nerve activity in normal humans.

We investigated the frequency components of fluctuations in heart rate, arterial pressure, respiration, and muscle sympathetic nerve activity (MSNA) in 11 healthy women using an autoregressive model and examined the relation among variables using Akaike's relative power contribution analysis with multivariate autoregressive model fitting. Power spectral analysis of MSNA revealed two peaks, with low-frequency (LF) and high-frequency (HF) components. The LF component of MSNA was a major determinant of the LF component of arterial pressure and R-R interval variability (0.70 +/- 0.07 and 0.18 +/- 0.05, respectively). The effect of the LF component of MSNA on arterial pressure showed no change in response to propranolol but was diminished (0.35 +/- 0.08) by phentolamine (P < 0.02). The effect of the LF component of MSNA on R-R interval was not altered by pharmacological sympathetic nerve blockade. The HF component of MSNA did not influence other variables but was influenced by R-R interval, arterial pressure, and respiration. These findings indicate that the LF component of MSNA reflects autonomic oscillations, whereas the HF component is passive and influenced by other cardiovascular variables.

A new method of quantifying human muscle sympathetic nerve activity for frequency domain analysis

We present a new method for quantitative analysis of muscle sympathetic nerve activity (MSNA), expressed as muscle sympathetic burst area (MSBA). This technique is likely to be useful for inter-individual comparisons and for frequency domain analysis of MSNA. After standardization of MSNA so that the burst of highest amplitude in the integrated MSNA trace was 1000 units, MSNA was assessed by measuring the area of each burst in the integrated MSNA trace, with baroreflex latency of about 1.3 sec while triggered by consecutive R-waves of the ECG. We examined the relationship between MSBA and burst rate (burst number/min) or plasma norepinephrine levels at rest in 50 healthy subjects, aged 23–82 years. MSBA showed positive correlations with burst rate ( r = 0.91, n = 50) and plasma levels of norepinephrine ( r = 0.64, n = 22). During head-up tilting in 6 subjects, MSBA showed linear correlations with sine values of tilt angles and with plasma norepinephrine levels. These results suggest that MSBA is a useful index of MSNA for evaluating both intra-individual and inter-individual variations of MSNA.