Contemporary ReviewCardiac output and vasodilation in the vasovagal response: An analysis of the classic papers
Introduction
Syncope is defined as a transient loss of consciousness secondary to global cerebral hypoperfusion and is characterized by rapid onset, short duration, and spontaneous complete recovery.1 During vasovagal syncope (or fainting), the fall in blood pressure (BP) is mediated initially by decreased cardiac output (CO) with or without vasodilation. The decrease in CO is secondary to stroke volume reduction. When bradycardia and withdrawal of sympathetic vasoconstrictor activity occur, they are relatively late events, by which time BP is already low. There has been much debate as to whether decreased CO or vasodilation is the dominant hypotensive mechanism preceding vasovagal syncope.2, 3
Tilt testing was first used by Weiss et al4 for investigating orthostasis and by Graybiel and McFarland5 as a means of studying tolerance to decreased venous return as a determinant of fitness for military aviation. In 1986, Kenny, Sutton, and coworkers6 introduced tilt testing as a means of diagnosing vasovagal syncope. Since then, cardiologists, physicians, and researchers have adopted this approach to evaluate the hemodynamics of vasovagal syncope and other conditions related to orthostatic intolerance.1 In the 1980s, Penaz and Wesseling introduced the Finapres or volume clamp method, which allowed continuous noninvasive measurement of finger arterial pressure. Thus, rapid changes in arterial pressure could be obtained accurately. Pulse wave analysis subsequently allowed the computation of relative changes in stroke volume (SV) and thereby calculated relative changes in CO and systemic vascular resistance (SVR). These scientific developments enabled clinicians and researchers to study noninvasively the vasovagal responses on a beat-to-beat basis.7, 8 In addition, direct nerve recordings from the leg using the microneurographic technique enabled continuous monitoring of efferent vasoconstrictor sympathetic activity. Observations obtained from relatively small numbers of cases have demonstrated changes in sympathetic vasoconstrictor tone during vasovagal syncope, but this is only activity directed toward the leg.3
Despite these developments, the mechanisms underlying vasovagal syncope are still being debated.2, 3 Most arguments rely on the classic experiments performed before the 1980s. These results emphasized that vasodilation is the dominant mechanism initiating the vasovagal response
We performed an in-depth critical overview of the classic literature related to the mechanisms underlying vasovagal syncope to provide a comprehensive historical summary.
Section snippets
Methods
Referenced papers were selected by hand searches of our own databases. For focused searches, PubMed was used as the preferred database. All available studies were checked for relevance to the present review. For the mechanisms involved in orthostatic BP control in healthy subjects, we refer to standard texts.2, 9
Historic overview
During World War I, Cotton and Lewis10 studied recruits with “the irritable heart” or “the effort syndrome,” in whom fainting was frequent. Findings from 8 soldiers (age 21–28 years) who fainted during medical examinations were presented with vivid descriptive precision and with great care. Heart rate (HR) was measured by counting the radial pulse or auscultation of the heart, and BP was determined using an arm cuff and stethoscope.
In 6 soldiers, the faint was elicited during blood sampling
Summary
- 1.
Fewer than 10 young adult healthy males without a fainting history were studied in early laboratory studies (Table 1). Females, seniors, and patients were not studied. In addition, subjects were instrumented, which greatly influences fainting responses. Thus, the classic studies included only a selected healthy male population under highly invasive conditions.
- 2.
The circulatory changes during a faint are rapid, and the time between onset of hypotension and loss of consciousness is crucial.
Acknowledgments
We gratefully acknowledge the critical comments of Jacques Lenders, Paul Fadel, Roland Thijs, and Mike Joyner.
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