New Approaches to Quantifying Sympathetic Nerve Activity

Burke SL, Lambert E, Head GA.
Curr Hypertens Rep. 2011 Jun;13(3):249-57.

http://www.springerlink.com/content/515x5218g67h2l58/


        This review focus on some of the advances in quantifying symapthetic nerve activity (SNA). Sympathetic nerve activity (SNA) is given as a value that relates to both the burst frequency and amplitude. More often than not it is rectified, integrated and averaged over time.
        Some of the indirect methods by which global SNA can be assessed are ganglionic blockade and spectral analysis. In order to measure regional SNA, norepinephrine isotope dilution method is widely used.The main advantage of this method is that it can be applied to various vascular beds to measure the regional SNA. Microneurography is being used for direct recording of multifiber SNA in humans.
        An important discussion in this review is whether single unit recording is a much better approach to quantify SNA. In this method the firing properties of individual muscle vasoconstrictor neurons are assessed. The properties of the firing sympathetic neurons are analyzed based on mean firing frequency, firing probability and the number of spikes a unit generates per cardiac interval. The reviewers believe that this method will be highly useful to determine the underlying pathology since many healthy individuals have high multiunit SNA but with low firing probabilities, rates and low incidence of multiple firing. 
        A number of methods have been used to compare integrated multifiber SNA between different groups, one such method is to measure all SNA as a percentage of baseline SNA. Another way is to show SNA as a percentage of maximum response to unloading the baroreceptors. In addition to the above methods, a combination of methods are also used, after measuring SNA as a percentage of baseline, it was compared in raw microvolts. In another study SNA was expressed as percentage changes from baseline and the frequency of bursts in baseline SNA was compared. 
        This review also discusses about the usage of telemetry for chronic recording of SNA. One of the pressing problems that remains to be solved in this method is the accounting for signal decay over time and comparing nerve activity between different animals, this makes calibration of nerve activity difficult.

Posted by Madhan