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Muscle Sympathetic Nerve Activity (MSNA) at Presyncope: Influence of Tolerance to Central Hypovolemia
Author(s) -
Ryan Kathy L,
Rickards Caroline A,
HinojosaLaborde Carmen,
Cooke William H,
Convertino Victor A
Publication year - 2011
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.25.1_supplement.647.5
We have shown previously that MSNA is not reduced in all subjects at the point of presyncope induced by lower body negative pressure (LBNP). Because individual tolerances to LBNP vary widely, in this study we extend our characterization of presyncopal MSNA responses of individuals by classifying them as either high (HT) or low tolerant (LT) to LBNP. Subjects (14M/4F) were instrumented to record beat‐by‐beat arterial pressure (Finometer) and MSNA (peroneal nerve) during a progressive LBNP protocol taken to the point of tolerance (i.e., presyncope). Subjects were classified as HT if they completed at least the −60 mmHg level of LBNP (n=13) or LT if they did not complete this level (n=5). MSNA burst frequency and total MSNA increased in both groups during LBNP. At presyncope, burst frequency was higher in HT than in LT (p=0.02), while total MSNA did not differ (p=0.35). Immediately before presyncope, burst frequency and total MSNA were reduced in 9 HT subjects, while 4 HT subjects exhibited a further increase in MSNA. While MSNA was reduced in all 5 LT subjects at this point, this result was not different from HT subjects ( P = 0.28; Fisher Exact test). None of the 18 subjects demonstrated complete cessation in MSNA, indicating that total sympathetic withdrawal does not occur at presyncope regardless of tolerance. These data suggest that LT and HT subjects may not profoundly differ in their pattern of MSNA reduction at presyncope.