Baroreflex control of muscle sympathetic nerve activity: a nonpharmacological measure of baroreflex sensitivity

Abstract

Hart EC, Joyner MJ, Wallin BG, Karlsson T, Curry TB, Charkoudian N. Baroreflex control of muscle sympathetic nerve activity: a nonpharmacological measure of baroreflex sensitivity. Am J Physiol Heart Circ Physiol 298: H816 -H822, 2010. First published December 11, 2009; doi:10.1152/ajpheart.00924.2009.-The sensitivity of baroreflex control of sympathetic nerve activity (SNA) represents the responsiveness of SNA to changes in blood pressure. In a slightly different analysis, the baroreflex threshold measures the probability of whether a sympathetic burst will occur at a given diastolic blood pressure. We hypothesized that baroreflex threshold analysis could be used to estimate the sensitivity of the sympathetic baroreflex measured by the pharmacological modified Oxford test. We compared four measures of sympathetic baroreflex sensitivity in 25 young healthy participants: the "gold standard" modified Oxford analysis (nitroprusside and phenylephrine), nonbinned spontaneous baroreflex analysis, binned spontaneous baroreflex analysis, and threshold analysis. The latter three were performed during a quiet baseline period before pharmacological intervention. The modified Oxford baroreflex sensitivity was significantly related to the threshold slope (r ϭ 0.71, P Ͻ 0.05) but not to the binned (1 mmHg bins) and the nonbinned spontaneous baroreflex sensitivity (r ϭ 0.22 and 0.36, respectively, P Ͼ 0.05), which included burst area. The threshold analysis was also performed during the modified Oxford manipulation. Interestingly, we found that the threshold analysis results were not altered by the vasoactive drugs infused for the modified Oxford. We conclude that the noninvasive threshold analysis technique can be used as an indicator of muscle SNA baroreflex sensitivity as assessed by the modified Oxford technique. Furthermore, the modified Oxford method does not appear to alter the properties of the baroreflex. modified oxford method; baroreflex threshold; spontaneous baroreflex THE BAROREFLEX IS a key modulator of tonic sympathetic nerve activity (28), so that sympathetic bursts are synchronized with transient reductions in arterial pressure and are silenced during increased pressure (9). The baroreflex influence on sympathetic nerve activity can be assessed by measuring baroreflex sensitivity, defined as the responsiveness or gain in the reflex during transient changes in arterial pressure.The most well-accepted method to examine baroreflex sensitivity involves a drug-induced manipulation of blood pressure. This approach was first developed to evaluate the cardiac baroreflex by Smyth et al. (23) in Oxford, England, who infused angiotensin to generate acute transient increases in arterial pressure and then measured resulting reflex decreases in heart rate. Subsequently, investigators have used a modified approach (sequential boluses of nitroprusside and phenylephrine) to determine baroreflex sensitivity for muscle sympathetic nerve activity (MSNA). This so-called "modified Oxford technique" involves measurements of both t...

End-tidal carbon dioxide tension reflects arterial carbon dioxide tension in the heat-stressed human with and without simulated hemorrhage

“…This plot further reveals a close relationship between measures of PET CO 2 and Pa CO 2 , as demonstrated by a small measurement bias of Ϫ0.4 mmHg. A proportional bias exists if the slope of the relationship between the difference of two measurements and the mean of the two measurements is significantly different from zero [i.e., the difference between the two methods changes as the average values from the two methods becomes smaller or larger (6,14,16)]. In the current study, a proportional bias was not detected (r ϭ 0.01, P ϭ 0.94, slope Ϫ0.004).…”

“…Comparisons between Pa CO 2 and PETCO 2 measures, during the three thermal conditions, were also evaluated using the methods of differences as described by Bland-Altman (2). The Bland-Altman plot was subsequently analyzed by linear regression analysis to determine if the slope of the relationship was greater than zero, which would indicate that a proportional bias was present [i.e., difference between two methods changes as the average values from the two methods becomes smaller or larger (6,14,16)]. Furthermore, a fixed bias is present if the 95% confidence limit of the Bland-Altman plot does not include zero (14,16).…”

Muscle sympathetic nerve activity during cold stress and isometric exercise in healthy older adults

“…This technique is highly correlated with sympathetic baroreflex sensitivity derived using the more invasive modified Oxford approach (i.e., bolus intravenous administration of sodium nitroprusside and phenylephrine) (22). Briefly, the diastolic BP for each cardiac cycle within each analysis region of interest was grouped into 2-mmHg pressure bins.…”

Attenuation of sympathetic baroreflex sensitivity during the onset of acute mental stress in humans

“…Recently, a reliable nonpharmacological methodology has emerged to allow the determination of sympathetic BRS during mental stress. Specifically, the relations between spontaneous fluctuations of diastolic arterial blood pressure (DAP) and MSNA can be analyzed in an approach that provides an accurate slope assessment of sympathetic BRS (5,(11)(12)(13)(14)21).…”