Virtual conductance, real hypotension: what happens when we stand up too fast?
2007; American Physiological Society; Volume: 103; Issue: 2 Linguagem: Inglês
10.1152/japplphysiol.00544.2007
ISSN8750-7587
Autores Tópico(s)Atomic and Subatomic Physics Research
ResumoINVITED EDITORIALSVirtual conductance, real hypotension: what happens when we stand up too fast?John R. HalliwillJohn R. HalliwillPublished Online:01 Aug 2007https://doi.org/10.1152/japplphysiol.00544.2007This is the final version - click for previous versionMoreSectionsPDF (38 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat cardiovascular physiologists have long been curious about the mechanisms of fainting, and the goal of understanding why people faint under a variety of conditions has taken researchers to the ends of the Earth, and beyond. Some of these investigations have focused on adaptation to extreme environments such as microgravity, and the deleterious effects on orthostatic tolerance that are seen upon return to earth (1). At another end of the spectrum is work that has focused on the various pathophysiologies that predispose “Earthbound” individuals to repeated syncopal attacks, and finding the means to alleviate these problems (5). Perhaps at the center of our collective interests lies the intrigue of studying a system, generally very robust and reliable, right at the moment that it happens to fail. Add to that, it is often those we believe to be most fit (6), or having the “right stuff,” who collapse from something as innocuous as “getting up too fast.” In our everyday experience, we are largely unaware of the extreme variation in gravitational force vectors and their pull on blood in compliant vessels, or the multitude of local vascular and reflex responses that together minimize the impact of these forces on the delivery of oxygenated blood to our brains to support our consciousness. When these responses fail, and blood pressure is compromised, the results can range from disconcerting signs and symptoms of cerebral hypoperfusion to dangerous loss of consciousness.A particular set of circumstances that brings together many of these interesting ideas is what is known among pilots as the “push-pull effect,” a term that has its origins in the series of control-stick maneuvers that produce a steep dive and head-ward gravitational forces followed by a steep climb and foot-ward forces, potentially leading to loss of consciousness. The study by Sheriff et al. (9) in the Journal of Applied Physiology provides an exquisite beat-by-beat picture of how these gravitational forces play out in the intact human volunteer when push precedes pull and provides insight into why the system sometimes fails when we stand up quickly.Of fundamental importance to many such investigations is the tenet that the cardiovascular system plays by well-defined rules of fluid dynamics that are often boiled down to the hydraulic resistance equation: pressure equals the product of flow and conductance. Along these lines, Sheriff and colleagues (9) demonstrate that the marked fall in perfusion pressure to the brain that occurs when upright posture is preceded by a brief period of modest head-down tilt is due to an exaggerated rise in the rate of blood flowing into the legs. Recognizing that the hydraulic resistance equation was developed for steady-state conditions, the authors calculated a “virtual vascular conductance,” borrowing a concept from the muscle pump literature that they helped establish (10). Thus it becomes apparent from these new findings that a short-term head-ward gravitational force will modify the subsequent vascular response in the legs during the transition to foot-ward forces. The nature of this exaggerated rise in virtual vascular conductance is unclear at present. It could be evidence of a myogenic vasodilation (i.e., a “real” change in conductance), activated by falling transmural pressure in leg vessels during head-down tilt, or it could be a rapid refilling of leg veins that were emptied during the “push” phase, leading to accelerated arterial inflow (i.e., a “virtual” change in conductance analogous to some muscle pump theory) (8). Whether the conductance is virtual or real, it leads to real hypotension at the level of the brain. Future studies will need to partition the active, or real, part of this vascular conductance from the purely mechanical, or virtual, component.It is noteworthy that when humans move from supine to upright, there is a rapid rise in arterial inflow to the dependent limbs that is not initially matched by the volume of venous blood leaving the limbs. Because of this, the legs remain a key site of intervention via anti-shock/anti-G suits (3) and with physical maneuvers such as leg-crossing and muscle tensing (5) in the prevention of fainting responses. Put in this greater context, it is clear that changes in venous compliance of the legs with exercise training (7), microgravity (2), and other conditions may predispose some individuals to being more susceptible to the push-pull effect and related phenomenon, although demonstrating this connection has often been elusive (4).The work by Sheriff and co-workers (9) also makes a compelling case that the pulmonary circulation plays a protective role when humans are exposed to the series of forces involved in the push-pull effect, such that cardiac output increases more in the upright position when the lungs have been “prefilled” with blood by slight head-down tilt. This highlights the fact that both the systemic and pulmonary circulations are compliant and, as a result, do not strictly adhere to a simple hydraulic resistance equation on a beat-by-beat basis. It also brings to mind the classic studies at the Mayo Clinic during World War II, in which Drs. Edward J. Baldes and Earl H. Wood developed a grunting maneuver for pilots that capitalized on this pulmonary reservoir to boost arterial pressure during high-G turns. This stands as yet another example of how forces external to the cardiovascular system can be used to produce virtual conductances with real impact.REFERENCES1 Buckey JC Jr, Lane LD, Levine BD, Watenpaugh DE, Wright SJ, Moore WE, Gaffney FA, Blomqvist CG. Orthostatic intolerance after spaceflight. J Appl Physiol 81: 7–18, 1996.Link | ISI | Google Scholar2 Convertino VA, Doerr DF, Mathes KL, Stein SL, Buchanan P. Changes in volume, muscle compartment, and compliance of the lower extremities in man following 30 days of exposure to simulated microgravity. Aviat Space Environ Med 60: 653–658, 1989.PubMed | Google Scholar3 Halliwill JR, Lawler LA, Eickhoff TJ, Joyner MJ, Mulvagh SL. Reflex responses to regional venous pooling during lower body negative pressure in humans. J Appl Physiol 84: 454–458, 1998.Link | ISI | Google Scholar4 Hernandez JP, Franke WD. Age- and fitness-related differences in limb venous compliance do not affect tolerance to maximal lower body negative pressure in men and women. J Appl Physiol 97: 925–929, 2004.Link | ISI | Google Scholar5 Krediet CT, de Bruin IG, Ganzeboom KS, Linzer M, van Lieshout JJ, Wieling W. Leg crossing, muscle tensing, squatting, and the crash position are effective against vasovagal reactions solely through increases in cardiac output. J Appl Physiol 99: 1697–1703, 2005.Link | ISI | Google Scholar6 Levine BD, Buckey JC, Fritsch JM, Yancy CW Jr, Watenpaugh DE, Snell PG, Lane LD, Eckberg DL, Blomqvist CG. Physical fitness and cardiovascular regulation: mechanisms of orthostatic intolerance. J Appl Physiol 70: 112–122, 1991.Link | ISI | Google Scholar7 Monahan KD, Dinenno FA, Seals DR, Halliwill JR. Smaller age-associated reductions in leg venous compliance in endurance exercise-trained men. Am J Physiol Heart Circ Physiol 281: H1267–H1273, 2001.Link | ISI | Google Scholar8 Sheriff D. Point: The muscle pump raises muscle blood flow during locomotion. J Appl Physiol 99: 371–372; discussion 374–375, 2005.Link | ISI | Google Scholar9 Sheriff DD, Nådland IH, Toska K. Hemodynamic consequences of rapid changes in posture in humans. J Appl Physiol In press; doi:10.1152/japplphysiol.01190.2006.Google Scholar10 Sheriff DD, Rowell LB, Scher AM. Is rapid rise in vascular conductance at onset of dynamic exercise due to muscle pump? Am J Physiol Heart Circ Physiol 265: H1227–H1234, 1993.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformationCited BySystemic and cerebral circulatory adjustment within the first 60 s after active standing: An integrative physiological viewAutonomic Neuroscience, Vol. 231Lower limb-localized vascular phenomena explain initial orthostatic hypotension upon standing from squatMichael E. Tschakovsky, Kristine Matusiak, Catherine Vipond, and Lisa McVicar1 November 2011 | American Journal of Physiology-Heart and Circulatory Physiology, Vol. 301, No. 5“He’s Dizzy When He Stands Up”: An Introduction to Initial Orthostatic HypotensionThe Journal of Pediatrics, Vol. 158, No. 3Retrograde arterial leg blood flow during tilt-back from a head-up posture: importance of capacitive flows when arterial pressure changesDon D. Sheriff, Inger Helene Nådland, and Karin Toska1 March 2010 | Journal of Applied Physiology, Vol. 108, No. 3Role of sympathetic responses on the hemodynamic consequences of rapid changes in posture in humansDon D. Sheriff, Inger Helene Nådland, and Karin Toska1 March 2010 | Journal of Applied Physiology, Vol. 108, No. 3Myogenic origin of the hypotension induced by rapid changes in posture in awake dogs following autonomic blockadeBrett J. Wong, and Don D. Sheriff1 December 2008 | Journal of Applied Physiology, Vol. 105, No. 6 More from this issue > Volume 103Issue 2August 2007Pages 421-422 Copyright & PermissionsCopyright © 2007 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00544.2007PubMed17540838History Published online 1 August 2007 Published in print 1 August 2007 Metrics
Referência(s)