Open AccessVariability in integration of mechanisms associated with high tolerance to progressive reductions in central blood volume: the compensatory reserve
Author(s) -
Carter Robert,
HinojosaLaborde Carmen,
Convertino Victor A.
Publication year - 2016
Publication title -
physiological reports
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.918
H-Index - 39
ISSN - 2051-817X
DOI - 10.14814/phy2.12705
Subject(s) - presyncope , medicine , microneurography , baroreflex , heart rate , cardiology , blood pressure , vascular resistance , stroke volume , anesthesia , cardiac output
High tolerance to progressive reductions in central blood volume has been associated with higher heart rate ( HR ), peripheral vascular resistance ( PVR ), sympathetic nerve activity ( SNA ), and vagally mediated cardiac baroreflex sensitivity ( BRS ). Using a database of 116 subjects classified as high tolerance to presyncopal‐limited lower body negative pressure ( LBNP ), we tested the hypothesis that subjects with greater cardiac baroreflex withdrawal (i.e., BRS > 1.0) would demonstrate greater LBNP tolerance associated with higher HR , PVR , and SNA . Subjects underwent LBNP to presyncope. Mean and diastolic arterial pressure ( MAP; DAP ) was measured by finger photoplethysmography and BRS (down sequence) was autocalculated (Win CPRS ) as ∆R‐R Interval/∆ DAP . Down BRS ( ms/mmHg) was used to dichotomize subjects into two groups (Group 1 = Down BRS > 1.0, N = 49, and Group 2 = Down BRS < 1.0, N = 67) at the time of presyncope. Muscle SNA was measured directly from the peroneal nerve via microneurography ( N = 19) in subjects from Groups 1 ( n = 9) and 2 ( n = 10). Group 1 (Down BRS > 1.0) had lower HR (107 ± 19 vs. 131 ± 20 bpm), higher stroke volume (45 ± 15 vs. 36 ± 15 mL), less SNA (45 ± 13 vs. 53 ± 7 bursts/min), and less increase in PVR (4.1 ± 1.3 vs. 4.5 ± 2.6) compared to Group 2 (Down BRS < 1.0). Both groups had similar tolerance times (1849 ± 260 vs. 1839 ± 253 sec), MAP (78 ± 11 vs. 79 ± 12 mmHg), compensatory reserve index ( CRI ) (0.10 ± 0.03 vs. 0.09 ± 0.01), and cardiac output (4.5 ± 1.2 vs. 4.7 ± 1.1 L/min) at presyncope. Contrary to our hypothesis, higher HR , PVR , SNA , and BRS were not associated with greater tolerance to reduced central blood volume. These data are the first to demonstrate the variability and uniqueness of individual human physiological strategies designed to compensate for progressive reductions in central blood volume. The sum total of these integrated strategies is accurately reflected by the measurement of the compensatory reserve.