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Cardiovascular regulation in women with vasovagal syncope: With special reference to the venous system
Linköping University, Department of Medical and Health Sciences, Division of Cardiovascular Medicine. Linköping University, Faculty of Medicine and Health Sciences.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Although vasovagal syncope (VVS) is a common clinical condition the mechanisms behind VVS remain elusive. Upright posture is the major trigger of VVS and lower limb blood pooling affecting cardiac output has been proposed as a major determinant. The overall aim of this thesis was twofold. First, to develop new methodology for calculating limb venous compliance. Second, to study lower limb venous volume load and cardiovascular responses during hypovolemic circulatory stress caused by lower body negative pressure (LBNP) in healthy women and women with VVS, emphasizing compensatory mechanisms to maintain central blood volume.

Net fluid filtration was associated with an underestimation ofvenous compliance. This could be accounted for with a correctionmodel. Further, a new venous wall model made it possible to adopt thevenous pressure-volume curve through the entire pressure range andthus provide a valid characterization of venous compliance.

Calf blood pooling was similar between the groups and was not associated with tolerance to hypovolemic circulatory stress. Venous compliance was reduced at low venous pressures in VVS and correlated with decreased tolerance to circulatory stress. VVS women displayed attenuated sympathetic vasoconstrictor responses during graded circulatory stress, and mobilization of arm capacitance blood as well as capillary fluid absorption from extra- to intravascular space were reduced. Accordingly, more pronounced reductions in cardiac output were found in VVS. Thus, reduced compensatory mechanisms to maintain cardiac output could contribute to the pathogenesis oforthostatic VVS.

In healthy women, rapid pooling in the lower limb was associated with higher tolerance to circulatory stress and more efficient cardiovascular responses, in part due to speed-dependent baroreflex-mediated sympathetic activation. In VVS however, rapid lower limb blood pooling was associated with lower tolerance and deficient cardiovascular responses. No speed-dependent baroreflexmediated sympathetic activation was found in VVS, indicating welldefined differences in cardiovascular regulation already in the initial responses to orthostatic stress.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. , 86 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1519
National Category
Physiology Anesthesiology and Intensive Care Other Clinical Medicine Cardiac and Cardiovascular Systems
Identifiers
URN: urn:nbn:se:liu:diva-126942DOI: 10.3384/diss.diva-126942ISBN: 978-91-7685-793-9 (print)OAI: oai:DiVA.org:liu-126942DiVA: diva2:917913
Public defence
2016-05-13, Berzeliussalen, ingång 65, Campus US, Linköping, 13:00 (Swedish)
Opponent
Supervisors
Funder
Futurum - Academy for Health and Care, Jönköping County Council, SwedenMedical Research Council of Southeast Sweden (FORSS)Region ÖstergötlandSwedish Heart Lung Foundation
Available from: 2016-04-08 Created: 2016-04-08 Last updated: 2017-03-27Bibliographically approved
List of papers
1. Calf venous compliance measured by venous occlusion plethysmography: methodological aspects.
Open this publication in new window or tab >>Calf venous compliance measured by venous occlusion plethysmography: methodological aspects.
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2015 (English)In: European Journal of Applied Physiology, ISSN 1439-6319, E-ISSN 1439-6327, Vol. 115, no 2, 245-56 p.Article in journal (Refereed) Published
Abstract [en]

PURPOSE: Calf venous compliance (C calf) is commonly evaluated with venous occlusion plethysmography (VOP) during a standard cuff deflation protocol. However, the technique relies on two not previously validated assumptions concerning thigh cuff pressure (P cuff) transmission and the impact of net fluid filtration (F filt) on C calf. The aim was to validate VOP in the lower limb and to develop a model to correct for F filt during VOP.

METHODS: Strain-gauge technique was used to study calf volume changes in 15 women and 10 age-matched men. A thigh cuff was inflated to 60 mmHg for 4 and 8 min with a subsequent decrease of 1 mmHg s(-1). Intravenous pressure (P iv) was measured simultaneously. C calf was determined with the commonly used equation [Compliance = β 1 + 2β 2 × P cuff] describing the pressure-compliance relationship. A model was developed to identify and correct for F filt.

RESULTS: Transmission of P cuff to P iv was 100 %. The decrease in P cuff correlated well with P iv reduction (r = 0.99, P < 0.001). Overall, our model showed that C calf was underestimated when F filt was not accounted for (all P < 0.01). F filt was higher in women (P < 0.01) and showed a more pronounced effect on C calf compared to men (P < 0.05). The impact of F filt was similar during 4- and 8-min VOP.

CONCLUSIONS: P cuff is an adequate substitute for P iv in the lower limb. F filt is associated with an underestimation of C calf and differences in the effect of F filt during VOP can be accounted for with the correction model. Thus, our model seems to be a valuable tool in future studies of venous wall function.

National Category
Clinical Medicine
Identifiers
urn:nbn:se:liu:diva-113393 (URN)10.1007/s00421-014-3009-4 (DOI)000347725800003 ()25272971 (PubMedID)
Available from: 2015-01-17 Created: 2015-01-17 Last updated: 2017-12-05
2. Reduced venous compliance: an important determinant for orthostatic intolerance in women with vasovagal syncope
Open this publication in new window or tab >>Reduced venous compliance: an important determinant for orthostatic intolerance in women with vasovagal syncope
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2016 (English)In: American Journal of Physiology. Regulatory Integrative and Comparative Physiology, ISSN 0363-6119, E-ISSN 1522-1490, Vol. 310, no 3, R253-R261 p.Article in journal (Refereed) Published
Abstract [en]

The influence of lower limb venous compliance on orthostatic vasovagal syncope (VVS) is uncertain. The most widespread technique to calculate venous compliance uses a nonphysiological quadratic regression equation. Our aim was therefore to construct a physiologically derived venous wall model (VWM) for calculation of calf venous compliance and to determine the effect of venous compliance on tolerance to maximal lower body negative pressure (LBNP). Venous occlusion plethysmography was used to study calf volume changes in 15 women with VVS (25.5 +/- 1.3 yr of age) and 15 controls (22.8 +/- 0.8 yr of age). The fit of the VWM and the regression equation to the experimentally induced pressure-volume curve was examined. Venous compliance was calculated as the derivative of the modeled pressure-volume relationship. Graded LBNP to presyncope was used to determine the LBNP tolerance index (LTI). The VWM displayed a better fit to the experimentally induced pressure-volume curve (P &lt; 0.0001). Calf blood pooling was similar in the groups and was not correlated to the LTI (r = 0.204, P = 0.30). Venous compliance was significantly reduced at low venous pressures in women with VVS (P = 0.042) and correlated to the LTI (r = 0.459, P = 0.014) in the low pressure range. No correlation was found between venous compliance at high venous pressures and the LTI. In conclusion, the new VWM accurately adopted the curvilinear pressure-volume curve, providing a valid characterization of venous compliance. Reduced venous compliance at low venous pressures may adversely affect mobilization of peripheral venous blood to the central circulation during hypovolemic circulatory stress in women with VVS.

Place, publisher, year, edition, pages
AMER PHYSIOLOGICAL SOC, 2016
Keyword
vasovagal syncope; orthostatic intolerance; venous compliance; venous capacitance
National Category
Clinical Medicine
Identifiers
urn:nbn:se:liu:diva-125296 (URN)10.1152/ajpregu.00362.2015 (DOI)000369058900005 ()26561647 (PubMedID)
Note

Funding Agencies|Futurum-The Academy of Health Care; Jonkoping County Council; Medical Research Council of Southeast Sweden; Heart and Lung Foundation

Available from: 2016-02-24 Created: 2016-02-19 Last updated: 2017-04-26

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