Mechanism of venous valve closure and role of the valve in circulation: a new concept

Mechanism of venous valve closure and role of thevalve in circulation: A new conceptFedor Lurie, MD, PhD, RVT,a,b,c Robert L. Kistner, MD,a,b,c Bo Eklof, MD, PhD,a,b,c and DarcyKessler, RVT,b Honolulu, Hawaii

Purpose: The purpose of this study was to investigate the blood flow changes and venous wall movements that occur in theperivalvular area during venous flow, to learn how these physiologic events influence the movements of the valve cusps,and to learn how the movements of the valve cusps influence the venous flow.Materials and methods: Twenty healthy volunteers (10 male, 10 female, age 18 to 52) were subjects of this study. Eachvolunteer was examined in semi-recumbent and standing positions at rest and during active foot movements. Ultrasoundexaminations were performed in the B-flow mode supplemented by B-mode and pulsed-wave Doppler scanning.Results: Four phases of the valve cycle are described. During the opening phase (0.27 � 0.05 s), the cusps move from theclosed position toward the sinus wall. After reaching a certain point, the valves cease opening and enter the equilibriumphase. During this phase (0.65 � 0.08 s), the leading edges remain suspended in the flowing stream and undergoself-excited oscillations with an amplitude of 0.01 to 0.16 cm. During the closing phase (0.41 � 0.07 sec), the leaflets movesynchronously toward the center of the vein. The subsequent closed phase has a duration of 0.45 � 0.05 seconds when thecusps remain closed. During the equilibrium phase, flow separation occurs at the leading edge of the cusp withreattachment at the wall of sinus. At this point, flow splits into two streams at each valve cusp. Part of the flow is directedinto the sinus pocket behind the valve cusp, forming a vortex along the valve cusp before re-emerging in the main streamin the vein. When the valve is maximally open, the two cusps create a narrowing of the lumen about 35% smaller than thevein distal to the valve. In this narrowed area flow accelerates, forming a proximally directed jet.Conclusions: The valve cusps undergo the four phases constituting the valve cycle. The local hemodynamic events, such asflow separation and reattachment, and vortical flow in the sinus play important roles in the valve operation. In additionto prevention of retrograde flow, the valve acts as a venous flow modulator. The vortical stream behind the valve cuspsparticipates in the operation of the valve, and prevents stasis inside the valve pocket. The central jet possibly facilitatesoutflow. (J Vasc Surg 2003;38:955-61.)

Although William Harvey observed almost 400 yearsago that the competent venous valve is pivotal in one-waycirculation of the blood,1 and this observation underliescontemporary understanding of the circulation, little hasbeen proved about the actual mechanism of valve closureduring the ensuing years to the present. Support for thecurrent concept of venous circulation came mainly fromultrasonographic and venographic investigations.2,3 Re-cent developments in ultrasound technology have made itpossible to visualize the valve with its cusps and flow of theblood through the valve in situ with purely noninvasivemethods.

Conventional ultrasound techniques, includingB-mode, and color and pulsed-wave Doppler scanning,provide information about the direction and velocity offlow in the venous segments, but do not routinely visualizethe valves themselves, and only indirectly identify valvefunction. The recent introduction of B-flow ultrasoundpresents real-time flow in gray scale4 and provides simulta-neous visualization of the valve cusps and blood flow events

that constitute the venous flow cycle. This permits obser-vation and measurement of the effects of flow on intralu-minal structures, particularly the valve cusps, under purelynormal conditions of venous flow. Thus the need to intro-duce artificial maneuvers of augmentation or reflux provo-cation to prove closure of the valve is eliminated. A newappreciation of cyclical flow events that occur in the peri-valvular area of the extremity veins arises from visualizationof flow stream, and cusp and wall responses during actualflow conditions.

This study reports observations of the valve cusps andperivalvular wall in response to venous flow events in thesuperficial and deep veins in healthy subjects. An attemptwas made to identify and measure some of the componentsof this physiologic process. The report concludes with ahypothesis of the factors that result in valve closure undernormal physiologic conditions, and with the concept thatflow events influence cusp and wall movements, and thatcusp and wall movements are critical to proper venous flow.

MATERIAL AND METHODS

Twenty healthy volunteers (10 male, 10 female; ages18-52 years) with no history of venous disease were thesubjects of this study. Each volunteer was examined in thesemirecumbent (15-degree head up) and standing posi-tions during normal respiration and during sequential ac-tive dorsal and plantar flexion of the foot to simulatewalking. One randomly selected extremity of each volun-

From Straub Foundation,a Straub Clinic & Hospital,b and Department ofSurgery,c John A. Burns School of Medicine, University of Hawaii.

Competition of interest: none.Reprint requests: Fedor Lurie, MD, PhD, Straub Foundation, 1100 Ward

Ave, Ste 1045, Honolulu, HI 96814 (e-mail: [email protected]).Copyright © 2003 by The Society for Vascular Surgery.0741-5214/2003/$30.00 � 0doi:10.1016/S0741-5214(03)00711-0

955

teer was studied. The most proximal superficial femoralvein (SFV) valve and one proximal greater saphenous vein(GSV) valve were chosen for study, because they were moreconsistently present,5 were located in both the superficialand the deep venous circulation, and were well-situatedanatomically for analysis. Only valves with clear visualiza-tion of both leaflets were included in the study. A total of13 GSV valves and 12 SFV valves were included in thisseries.

Ultrasound examination. All studies were per-formed with the Logic 700 ultrasound system with 739L7.5 MHz linear array probe (GE Medical Systems, Milwau-kee, Wis).

After identification of the valve, the shape of the vein inthe area under study was determined in the transverseplane, to confirm a circular or near-circular shape. Thetransducer was then returned to the longitudinal plane forthe remainder of the study. In this series, we did notobserve any cases with tributaries opening into the sinus orclose to the valve station.

Ultrasound examinations were performed in the B-flowmode, supplemented by both conventional B-mode andcolor Doppler scanning as needed for clarity. The B-flowmode provides visualization of the flowing stream in realtime as it approaches and passes through the valve cusps,and has the advantage of showing the effect of flow eventson the valve cusps, because both flow and structures arevisualized in gray scale. Variations in flow velocity anddirection were identified below, through, and around thevalve cusps under conditions of quiet breathing and exer-cise, with the patient in both the recumbent and standingpositions.

The direction of flow and changes in velocity withineach stream over time observed in the B-flow mode werevalidated with pulsed-wave Doppler scanning. The samplevolume of the pulsed-wave Doppler scanning was placedwithin the area of interest, and its size was adjusted to thewidth of the particular stream under study. The insonationangle was adjusted to reflect the direction of the stream atthe point of interest. If the angle of insonation exceeded 60degrees, velocity measurements were not accepted, but thedirection of the flow and relative changes in velocity over

time were documented. In all cases, pulsed-wave Dopplerscanning data for direction of flow, spatial and temporaldistribution of velocity, and changes in velocity were inaccord with B-flow observations.

Recordings were performed during five valve cycles, ata rate of not less than 30 frames per second, and then wereprinted and analyzed, frame by frame. All measurementswere performed with a planimeter or digitizer (LASICOseries 1282; Los Angeles Scientific Instruments Co Inc, LosAngeles, Calif). This allowed analysis of the time sequenceof changes in multiple aspects of flow, including velocity,position of valve cusps, distance between valve cusps, andvein size. Measurements performed at each 1/30 secondtime point are depicted in Fig 1. Cross-sectional area cal-culations assumed that the flow lumen of the vein wascircular above and below the valve, and that the flow lumenthrough the open valve orifice was elliptic, with the longeraxis equal to the diameter of the sinus and the shorter axisequal to the distance between the cusps (Fig 2). Thisassumption was validated with a pilot study of five valvesobserved in the transverse plane, which demonstrated thatthe areas calculated on the basis of these assumptions werewithin 5% difference from areas measured planimetrically.

Descriptive statistics, paired t test, and repeated mea-sures analysis of variance were performed with SPSS version10.1 statistical software (SPSS Inc, Chicago, Ill). All valuesin the text and tables represent mean � SD. Statisticalsignificance was set at P � .05.

RESULTS

A consistent pattern of flow events occurs as the bloodpasses through a valve station during rhythmic opening andclosing of the valve cusps. The specific details of this patternrelate to variations in flow direction and flow velocity in andaround the valve cusps. These details vary with the positionof the subject and the level of activity of the limb.

Although flow events and movements of the valveleaflets appear to be two parts of the same physiologic

Fig 1. Measurements performed at each of 1/30 second timepoint. 1, Diameter distal to valve station; 2, diameter proximal tovalve station; 3, maximal diameter of valve sinus; 4, minimaldistance between two leaflets of valve; 5, distance between wall ofsinus and mural surface of leaflet.

Fig 2. Cross-sectional area measurements. See text for details.

JOURNAL OF VASCULAR SURGERYNovember 2003956 Lurie et al

process, for descriptive purposes we separated them intothe “valve cycle,” that is, movement of the leaflets, and the“flow cycle,” or changes in flow.

Valve cycle. The time between two consecutive clo-sures of the valve was defined as the valve cycle, which wearbitrarily divided into four phases: opening phase, equilib-rium phase, closing phase, and closed phase.

During the opening phase, the cusps move from theclosed position toward the sinus wall. This phase lasts onaverage 0.27 � 0.05 seconds when the patient is in the

horizontal position (Table I). The leaflets move with aradial velocity of 0.56 � 0.12 cm/s (GSV in horizontalposition) to 1.3 � 0.38 cm/s (SFV in standing position;Table II).

After a certain point the valves cease opening and enterthe equilibrium phase, which lasts 0.65 � 0.08 seconds.During this phase, the leading edges remain suspended inthe flowing stream and undergo self-excited oscillationsthat resemble the flutter of flags in the wind (Fig 3). Theamplitude of the oscillations ranges from 0.01 to 0.16 cm

Table I. Duration of four phases of valve cycle in subjects in supine and standing positions at rest and while performingfoot movements

Vein/position

Duration (s)

Opening phaseEquilibrium

phase

Closing phase

Closed valveRest Foot movements P Rest Foot movements P

GSV (n � 13)Supine 0.27 � 0.05 0.24 � 0.05 .07 0.65 � 0.08 0.41 � 0.07 0.29 � 0.07 �.001 0.45 � 0.05Standing 0.27 � 0.05 0.27 � 0.05 .97 1.41 � 0.25 0.43 � 0.09 0.33 � 0.10 .009 1.09 � 0.12P .79 .096 �.001 0.45 0.26 �.001

SFV (n � 12)Supine 0.28 � 0.06 0.25 � 0.04 .14 0.69 � 0.09 0.24 � 0.07 0.17 � 0.04 .005 0.44 � 0.09Standing 0.25 � 0.05 0.27 � 0.06 .54 1.41 � 0.25 0.29 � 0.07 0.27 � 0.10 .53 1.04 � 0.21P .16 .46 �.001 .09 .005 �.001

GSV, Greater saphenous vein; SFV, superficial femoral vein.

Table II. Radial velocity of valve leaflets during openingphase of valve cycle in subjects in supine and standingpositions at rest and while performing foot movements

Vein/position

Leaflet radial velocity (cm/s)

Rest Foot movements P

GSV (n � 13)Supine 0.56 � 0.12 0.56 � 0.11 .86Standing 0.80 � 0.23 0.64 � 0.15 .054P .003 .15

SFV (n � 12)Supine 0.86 � 0.31 0.90 � 0.27 .77Standing 1.3 � 0.38 1.09 � 0.37 .19P .006 .17


Fig 3. Equilibrium phase. Self-excited oscillations of leadingedges of cusps. During this phase, leading edges remain suspendedin flowing stream and undergo self-excited oscillations that resem-ble flutter of flags in wind. Amplitude of oscillations ranges from0.01 to 0.16 cm.

Table III. Radial velocity of valve leaflets during closingphase of valve cycle in subjects in supine and standingpositions at rest and while performing foot movements

Vein/Position

Leaflet radial velocity (cm/s)

Rest Foot movements P

GSV (n � 13)Supine 0.38 � 0.13 0.51 � 0.21 .82Standing 0.49 � 0.14 0.58 � 0.27 .27P .07 .44

SFV (n � 12)Supine 1.08 � 0.45 1.32 � 0.31 .13Standing 1.16 � 0.44 1.18 � 0.57 .91P .66 .46


Fig 4. Flow pattern during late opening phase and early equilib-rium phase, just before and immediately after valve cusps assumetheir position. See text for details.

JOURNAL OF VASCULAR SURGERYVolume 38, Number 5 Lurie et al 957

(mean, 0.04; SD, 0.03 cm), and does not differ significantlybetween horizontal and standing positions or between restand active foot movement.

The closing phase ensues. The leaflets move synchro-nously toward the center of the vein at a velocity of 0.38 �0.13 cm/s (GSV in the horizontal position) to 1.08 � 0.45cm/s (SFV in horizontal position; Table III). The cusps ofthe valve assume a symmetric position at an equal distancefrom the walls on both sides of the sinus. This phase lasts0.41 � 0.07 seconds when the patient is at rest and is muchshorter when foot movement is performed (Table I).

Finally, in the closed phase the cusps remain closed, for0.45 � 0.05 seconds in the horizontal position.

The duration of the valve cycle and of each of the fourphases depends on the position of the body. In the standingposition, the duration of the cycle is 2.9 to 3.2 seconds(95% confidence interval [CI]), which corresponds to fre-quency of 18.8 to 20.4 per minute (similar to respirations).In the horizontal position, the duration of the cycle is 1.7 to1.8 seconds (95% CI). This rhythm (34.2-36.1 per minute)is most likely influenced by both respiratory and cardiac

cycles. Muscle activity (dorsal and plantar flexion of thefoot) causes shortening of the closing phase (Table I).Because participants in this study performed foot move-ments repeatedly, the duration of the equilibrium phaseand closed phase reflected the frequency of movements.

Flow cycle. Our observations of the flow cycle in thearea of valve cusps were limited to the interval during theequilibrium and closing phases of the valve cycle. The shortduration of the opening phase prevented identification of aconsistent flow pattern. During the closed phase of thevalve cycle, flow was undetectable.

While the valve cusps are assuming their position atsome distance from the wall, the flow in the valve pocket(space between cusp and wall) is directed proximately (Fig4). Then the flow separation occurs at the leading edge ofthe cusp, with reattachment at the wall of sinus (Fig 5). Atthis time flow splits into two streams at each valve cusp.While the larger stream, located in the center of the vessel,is directed proximally along the axis of the vein, the smallerpart of the flow, at the point of attachment, turns into thesinus pocket behind the valve cusp. This part of the streamforms a vortex along the sinus wall and the mural side of thevalve cusp before remerging with the main stream in thevein (Fig 6). This vortex persists during equilibrium andclosing phases of the valve cycle.

In the segment distal to the valve (Fig 1), flow ispulsatile. This refers to rhythmic changes in velocity andflow rate (Fig 7). After passing through the valve, the flowremains pulsatile, but the velocity undergoes significantchanges. When the valve is maximally open (equilibriumphase), the two cusps of the valve create a funnel-likenarrowing of the lumen. The cross-sectional area betweenthe leaflets is about 35% smaller than the cross-sectionalarea of the vein distal to the valve (Table IV). The flowaccelerates in this stenotic area, resulting in a proximallydirected flow jet (Fig 8; Table V). On impact of the jetagainst a layer of much slower moving blood proximal tothe valve, reflection of flow occurs in the mural parts of thestream. These reflected streams reinforce the mural part ofthe sinus vortex, while the central part of the stream accel-erates under the axial pressure of the jet.

Fig 5. Flow pattern during equilibrium phase. Flow separation atleading edge of cusp and reattachment at wall of sinus, withreflection into sinus pocket. Flow separation is a phenomenon offlow detaching from vessel wall and forming region of “deadwater” in which unsteady motion occurs.15

Fig 6. Flow pattern during equilibrium phase. Vortex forms insinus pocket. See text for details.

Fig 7. Rhythmic flow pattern distal to valve, as observed in supineposition at rest.

Table IV. Degree of valvular stenosis expressed aspercent of diameter distal to valve

Vein/Position

Degree of valvular stenosis (%)

Rest Dorsiflexion P

GSV (n � 13)Supine 37 � 6 40 � 5 .18Standing 35 � 4 37 � 6 .26P .34 .16

SFV (n � 12)Supine 33 � 5 39 � 5 .013Standing 32 � 5 37 � 5 .017P .6 .41



DISCUSSION

Operation of venous valve The repeatedly observedpattern of valve cusp movements and the consistent patternof flow dynamics in the valve area were found in all studiedveins in the lower extremities. These events are clearlypurposeful. Experiments in vitro6,7 and in the animal mod-el8 showed that valve leaflets do not open all the way out totouch the sinus wall, as was confirmed with B-flow imagingin the present study. This position of the cusps at a distancefrom the wall is the result of two factors in the flowdynamics around the valve: formation of vortical flow be-hind the valve cusp, and acceleration of flow in the centralportion of the vein between the leaflets.

Mathematical modeling has shown that flow separationshould occur at the leading edge of the valve cusps, with thereattachment point at the sinus wall. Part of the stream thenshould turn back into the sinus and form a vortex thatdilates the sinus with increased pressure on the inside of thesinus wall and along the mural surface of the valve cusp.9

This flow pattern is similar to that in the aortic valve, firstdescribed by Leonardo da Vinci.10

The geometric shape of the vein is important in forma-tion of the vortex and therefore in operation of the venousvalve.11-13 The mechanical properties of the sinus wall aredifferent from the other parts of the vein in that the sinus ismore distensible under pressure.11,12 This distensibilityfacilitates formation of vortical flow by reflection of the flowat the point of reattachment toward the sinus pocket. At thesame time, the sinus is enlarging as a result of the pressureof vortical flow.14

As vortical flow persists, it also applies pressure to themural surface of the valve cusps. When the pressure on themural side of the cusp and the pressure on the luminal sideof the cusp are in equilibrium, the valve remains open andthe cusps float in the stream. This dynamic equilibrium issustained by equilibrium in velocity of the two streams,vortex on the mural side and axial flow on the luminal sideof the valve cusps. Changes in any of these streams can leadto closure of the valve. Self-excited oscillations of theleading edges of the leaflets that occur during this equilib-rium phase make this balance unstable and sensitive to smallchanges in flow.

When venous flow rate increases distal to the valve, asduring foot movement, the axial velocity of flow betweenthe valve cusps increases rapidly. This velocity increasecauses decreased pressure on the luminal side of the cusp,per Bernoulli’s law: �V2/2 � P � const15 (Fig 9). Thevortical flow on the mural side of the cusp remains un-changed until the accelerated part of the stream reaches theedge of the cusp, and is reflected into the sinus. At thispoint the cusps start moving toward the axis of the vessel,which further constricts the lumen. With increasing pres-sure on the mural side and decreasing pressure on theluminal side of the cusps, valve closure is favored. Thispattern was observed consistently in the SFV valve duringrepeated dorsal and plantar flexion (Fig 10, A and B). Eachfoot movement causes significant increase in velocity andclosure of the valve. The valve cusps move much fastertoward closure, compared with the rate observed duringrest (Table I).

Role of valve in venous flow dynamics. The exis-tence of valves in the veins is typically viewed solely as amechanism to guide direction of flow and to prevent reflux.Observations presented in this report and published byothers indicate that venous valves have other importantfunctions.

The valve presents anatomic stenosis of the flow lumen.The degree of stenosis was measured as 61% � 12% in GSV7

and 50% in reversed venous grafts.16 Our data (Table IV)show that the degree of valve-associated stenosis is in therange of 35%. This stenosis leads to acceleration of flow and

Fig 8. Changes in axial velocity of flow while passing valve.Superficial femoral vein, supine position.

Table V. Velocity change while flow passing valve (distalto proximal) in subjects in supine and standing positionsat rest

Vein/Position

Axial flow velocity (cm/s)

Distal Between leaflets Proximal P

GSV (n � 13)Supine 9.7 � 6.8 18.2 � 12.6 12.2 � 8.4 �.001Standing 6.5 � 2.0 12.2 � 3.7 8.7 � 2.4 �.001

SFV (n � 12)Supine 7.0 � 3.3 13.1 � 6.4 9.8 � 4.7 �.001Standing 3.6 � 2.1 6.8 � 4.1 5.5 � 2.9 .002


Fig 9. Forces acting on valve leaflet. Pi , pressure applied to muralsurface of valve leaflet generated by vortical flow in valve pocket,with velocity Vvortical ; Po , pressure applied to luminal surface ofleaflet, generated by axial flow between leaflets, with velocityVaxial .


development of the prograde jet (Fig 8; Table V). Mea-sured increase in velocity in the space between leaflets wasmore than would be predicted by area reduction. The

difference between expected (from degree of stenosis) andmeasured values of velocity was on average 33% � 13%.One possible explanation for this discrepancy is the geo-metric shape of the valve orifice. However, it can also bedue to measurement error, because a fraction of a millime-ter difference in diameter measurement resulted in a differ-ence of 10% or more in expected velocity.

The combination of easily extensible wall of the sinusand stiff material of the valve cusps,11 along with distinctgeometry of the valve cusps, creates conditions for flowseparation and formation of vortical flow behind the valvecusps. This constitutes a self-sustained mechanism of clo-sure of valve cusps. Rhythmic closure and opening of thevalves act in concert with pulsatile venous flow. One impor-tant effect of this mechanism is prevention of blood stasisinside the valve pockets. This has been indirectly confirmedby Hamer et al.17 These authors found that localizedhypoxemia occurs in the venous valve pockets only duringsteady flow, but not during pulsatile flow. The pulsatility ofvenous flow might be an important natural protectivemechanism that prevents thrombus formation in venousvalve pockets.

The ongoing force of flow in the veins is multifactorialand is influenced by cardiac impulses, respiratory cycle,venous resistance, right heart cycle, pressure in the inferiorvena cava, anatomy of veins, and patient position andactivity level. At the same time, specific valve cusp move-ments are governed by the anatomic structure of the valveand of the wall in the valve sinus, which causes lumenstenosis, and the dynamic changes in local pressure on thetwo sides of the valve during the flow cycle.

CONCLUSIONS

On the basis of our observations and analysis of pub-lished data, we propose a revised concept of venous valveoperation and its hemodynamic functions. We postulatethat movements of the valve cusps consistently undergofour phases that constitute the valve cycle. Local hemody-namic events, such as flow separation and reattachment,and vortical flow in the sinus pocket are important in valveoperation. These hemodynamic events are predeterminedby the shape and mechanical properties of the sinus and thevalve cusps, and they constitute a self-sustained mechanismfor competent valve operation.

In addition to prevention of retrograde flow, the valveacts as a venous flow modulator. The vortical stream formsbehind the valve cusp, and axial jet forms at the center ofthe vein. The vortex participates in operation of the valve,and prevents stasis inside the valve pocket. The central jetpossibly facilitates outflow.

REFERENCES

1. Harvey W. Exercitato anatomica de motu cordis et sanguinis in animali-bus. The Keynes English translation of 1928. Birmingham (England):The Classics of Medicine Library, 1978. p 86-7.

2. Kistner RL, Ferris EB, Randhawa G, Kamida C. A method of perform-ing descending venography. J Vasc Surg 1986;4:464-8.

Fig 10. A, Changes in flow velocity during foot movement. A,Distal to valve (distance 1 in Fig 1); B, between cusps (distance 4 in Fig1). B, Changes in cross-sectional area and flow velocity during footmovement. A, Distal to valve (distance 1 in Fig 1). Heavy line,Cross-sectional area; thin line, velocity. B, Between cusps (distance 4in Fig 1). Heavy line, Area between cusps; thin line, flow velocitybetween cusps; 1, Opening phase; 2, equilibrium phase; 3, closingphase.


3. van Bemmelen PS, Beach K, Bedford G, Strandness DE Jr. The mech-anism of venous valve closure: its relationship to the velocity of reverseflow. Arch Surg 1990;125:617-9.

4. Chiao RY, Mo LY, Hall AL, Miller SC, Thomenius KE. B-mode bloodflow (B-flow) imaging. IEEE ultrasonics symposium proceedings.2000; San Juan, Puerto Rico. p 1469-1472.

5. Maros T. Data regarding the typology and functional significance of thevenous valves. Morphol Embryol (Bucur) 1981;27:195-214.

6. Thubrikar MJ, Robicsek F, Fowler BL. Pressure trap created by vein valveclosure and its role in graft stenosis. J Thorac Surg 1994;107:707-16.

7. McCaughan JJ, Walsh DB, Edgcomb LP, Garrett HE. In vitro obser-vations of greater saphenous vein valves during pulsatile and nonpulsa-tile flow and following lysis. J Vasc Surg 1984;1:356-61.

8. Carrier EB. Observation of living cells in the bat’s wing. In: Physiolog-ical papers dedicated to Prof August Krogh. London, England: W. M.Heinemann Ltd; 1926. Cited in Franklin KJ. The valves in veins: amonograph on veins. Springfield, Ill: Charles C Thomas; 1937. p 72.

9. Karino T, Motomia M. Flow through a venous valve and its implicationfor thrombus formation. Thromb Res 1984;36:245-57.

10. Kele KD. Leonardo da Vinci on moment of the heart and blood.London, England: Harvey and Blythe; 1952. Cited in Bellhouse BJ,

Belhouse FH. Mechanism of closure of the aortic valve. Nature 1968;217:86-7.

11. Ackroyd JS, Pattison M, Browse NL. A study of the mechanical prop-erties of fresh and preserved human femoral vein wall and valve cusps.Br J Surg 1985;72:117-9.

12. Qui Y, Quijano RC, Wang SK, Hwang NHC. Fluid dynamics of venousvalve closure. Ann Biomed Eng 1995;23:750-9.

13. Lee D, Abolfathi AH, DeLaria GA, Phifer TJ, Nashef AS, Quijano RC.In vitro testing of venous valves. ASAIO Trans 1991;37:M266-8.

14. Lurie F, Kistner RL, Eklof B. The mechanism of venous valve closure innormal physiologic conditions. J Vasc Surg 2002;35:713-7.

15. Fung YC. Biomechanics: circulation. 2nd ed. New York, NY: Springer-Verlag, 1997. p 229-33.

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17. Hamer JD, Malone PC, Silver IA. The PO2 in venous valve pockets: itspossible bearing on thrombogenesis. Br J Surg 1981;68:166-70.

Submitted Feb 17, 2003; accepted Apr 29, 2003.


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Mechanism of venous valve closure and role of the valve in circulation: a new concept