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Advice on how to do Toe Brachial Index

Discussion in 'Diabetic Foot & Wound Management' started by Warts, Oct 23, 2012.

  1. Warts

    Warts Member


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    Hi All
    Can someone please post specific instructions on doing a toe brachial index.
    I did buy a systoe machine- but the results I got were not consistent (I know it was probably the user).
    I have now got a hadeco doppler, but I am still unable to get a TBI.
    I have tried utube, but only ABI demostrations.
    I live in a rural area and the medical company is 12 hours away, a hard option.
    Frustrated that cant seem to get it.
    HELP :deadhorse:
     
  2. David Smith

    David Smith Well-Known Member

    Warts

    I've never used a TBI so take this with caution but if you think about what your trying to measure then can you actually take a TBI unless you had a little BP cuff around the toe? To listen to the arterial flow in a toe then ideally I think you would need a 10mhz probe because the arteries are shallow and small. But if the cuff is around the ankle you still really only testing the cut off pressure at the ankle although there may be some correlation between the cut off pressure time and the finish of audible pulse signal from the toe doppler??

    Regards Dave
     
  3. toughspiders

    toughspiders Active Member

    Doesn't the Systoe come with everything you need, the toe cuff and the ppg probe thing??? Did you watch the demo on Briggates site?
    I guess the resting period is the same as that for an ABI?
     
  4. David Smith

    David Smith Well-Known Member

    Wart

    Attached is a paper that may be useful to you

    Dave
     
  5. Here is an article that I authored half a lifetime ago on digital pressures. And people thought I only did biomechanics papers......:cool:

    Kirby KA, Arkin DB, Laine W: Digital systolic pressure determination in the foot. JAPMA, 77: 340-342, 1987.
     
  6. Warts

    Warts Member

    Yes it does, but getting the pressure cuff on the digit and then the sensor on a small hallux or 2 digit is hard. If you do not have the room, double sided tape is used to hold the sensor, but it falls off and very,very,very sensitive. No reading could be established, false readings.
     
  7. David Smith

    David Smith Well-Known Member


    Thanks for that paper Kevin written over 25 years ago. After reading I wondered if today you would change the statement about the reason why there is reduced pressure and impulse in digital arteries and arterioles.

    [​IMG]

    I was thinking about that and was a little puzzled, the basic haemodynamic equation (or any fluid flow dynamic) is: blood pressure = cardiac ouput x Systemic vascular resistance (P=QxR ). So this is saying an increase in resistance = an increase in pressure, where the output is constant..
    However friction in a hose causes a reduction in pressure over its length.

    1. Pressure drop is directly proportioned to the length of the hose. i.e. the frictional force is applied for longer in the opposite direction to the force of the flow.

    2. Friction is independent of pressure and proportional to velocity. Therefore high pressure with low flow rate (i.e. a large diameter hose with a small bore outlet) = less friction losses = less reduction in flow rate.

    3) Pressure change in inversely proportional to the diameter of the hose. Larger diameter (bore) hose results in less frictional losses. I.e. there is less fluid in contact with the wall of the hose therefore less friction = less force resisting the flow.

    [​IMG]

    So if you had one long blood vessel of uniform diameter the pressure over the length would reduce but decreasing the pressure will decrease the flow rate (e.g. litres / minute - L/m) so it appears that the impossible would happen i.e. less fluid out than fluid in. P=QR => Q=P/R, e.g. Q=10/2 = 5 L/m and Q=6/2 = 3 L/m. Hmmm! a conundrum to come back to later?

    If you have a single blood vessel but with a reducing diameter as it gets longer then pressure reduces with length but increases with reduction in bore. But friction is not relative to pressure but rather to velocity and so increased pressure (or force per unit area) in the same size bore, = increased velocity and so increased friction in resistance to flow. So although you can say smaller diameter arteries = increased resistance to blood flow this is because of the increased flow velocity. The flow rate i.e. L/m would remain the same through out the system i.e. there must be the same volume out same volume in.
    This is analogous to inertia i.e. acceleration increases proportional to applied force but the resistance to acceleration increases as the applied force increases.

    Gonna post this now so as not to loose it accidentally but got to get back to work.

    (while doing some more thinking:confused:)


    Regards Dave
     
  8. Dave:

    As the arteries and then arterioles and then capillaries become smaller in diameter, they also become much more numerous. Therefore, there is the same volume in as the same volume out. Pressure will reduce as the vessels get smaller due to increased friction on the vessel walls as one gets further away from the heart.

    I don't really see a problem with the statement made in our paper from 25 years ago, even though I have not really studied this subject in detail since my first year surgery residency (when I was 27 years old) when we wrote this paper.

    Actually, I iniitially wrote a much longer paper for publication in JAPMA that was rejected by the Journal (the only paper that I have ever had rejected for publication) on doppler, segmental pressures, etc. Then my co-resident (Dave Arkin, DPM) and I worked together to submit this much shorter paper taken from my original paper that was eventually published.

    Lesson to be learned.....when at first you get rejected as an author..... keep on trying!!
     
  9. David Smith

    David Smith Well-Known Member

    Kevin

    Just about to go home so this is a quick reply:

    I was coming to this but as there are more capillaries etc then the total lumen is greater and so the resistance reduces as long as they are parallel to the blood flow circuit i.e. just like an electrical circuit, resistors in parallel reduce the total resistance, resistors in series increase the total resistance. Arterioles and capillaries are parallel so resistance reduces and pressure drops. This is consistent with P=QR

    But - P=QR = Increased friction or resistance (R) = increased pressure (P)
    So smaller bore vessels = increased resistance and so increased pressure but increased number of vessels in parallel increases total lumen and so pressure decreases.

    Back after dinner

    Dave
     
  10. David Smith

    David Smith Well-Known Member

    AAHH!

    I knew this but forgot to account for it - in a word 'venturi' when there is a narrowing in a flow the velocity increases but the pressure decreases!! This is how a carburetor works the venturi causes a pressure depression which allow atmospheric pressure to push the fuel into the air flow and into the combustion chambers. This maintains the continuity of the mass flow rate i.e. the same amount of mass of fluid flowing past a given point remains constant in time. This is called Bernoulli's principle - you can't leave a problem unsolved you know, it just keeps nagging :D

    Dave Smith
     
  11. Dave:

    None of this ever seemed to be a problem to me. I wouldn't try to over-analyze it. It seems quite logical to me that the pressure decreases as the vessel becomes further distant from the heart and don't think you need, or would even need to consider, the venturi effect to explain it.
     
  12. David Smith

    David Smith Well-Known Member

    Kevin

    I understand what your saying and I 'understand' this on one level but if you try and explain it mathematically and it doesn't add up then it needs investigation (that's just me of course).

    I may not be thinking right but I considered that

    1) When standing upright the pressure at the vessels furthest from the heart will be at the initial pressure of the heart pump output, minus the intrinsic loss due to friction plus the force unit area due to column mass x acceleration due to gravity. Therefore pressure at the distal end would be higher than the pump end.

    2) If the system where closed, i.e. a hydraulic system, and the person was lying down then the pressure in all the system would be equal. In this case, because pressure is equal, there is no pressure gradient and so there is no tendency to flow.

    3) If it were not significant to understand the concept of pressure drop due to increased velocity then it might be difficult to understand how the blood would flow as the system approached a closed system.

    What I mean by this is that if I consider a single vessel and the vessel gets smaller and smaller then this would equal an almost closed system and if pressure did not reduce or even increased (as might seem intuitive) with increased velocity then pressure at distal end would be similar or close to or even greater than the heart pump pressure i.e. small pressure differential and so the flow would be very slow, stopped or reversed. However increased velocity as the fluid flows and maintains mass flow continuity results in a relatively low local pressure and a high pressure differential and so equals good total flow. This is particularly important as the blood enters the ever enlarging venous system because in the equal pressure slow flow scenario, as the principle works in reverse, the blood would have no chance of making it back to the heart, which would be catastrophic.
    This increasing pressure problem is further exasperated by the consideration of the principle in 1) For this principle then it seems fortunate then that distal vessels do decrease in diameter. Unless of course I'm all mixed up.

    This throws up even more questions like how much friction i.e. what is the coefficient of friction between blood and the vessel wall? If there is significant friction and friction increases proportionally to velocity of flow then how does this effect total mass flow. Does increased mass velocity i.e. greater momentum result in increased potential to vessel damage?

    If we don't understand these things then we may not be making the right assumptions even though they seem intuitive or self evidently true. In this case a measurement is meaningless if applied to an incorrect assumption. On the other hand if we do understand these things and still find it consistent with initial assumption then that is confirming proof. I like it when those two things come together, it's very reassuring.

    Regards Dave Smith
     
    Last edited: Oct 26, 2012
  13. Dave:

    You may want to chew on this nice little article Blood pressure drop across major arteries to capillaries and then see if things still don't make sense. I think that Poisseulle’s equation, ∆P=8µlQ/πr^4, pretty much gives us an idea of the magnitude of increase in the resistance to flow and pressure drop that occurs through a blood vessel as one progress from the arteries, to the smaller arteries, to the arterioles and then on to the capillaries.

    Here's another good article for you on blood pressure.:drinks
     
  14. By the way, Dave, I was an animal physiology major at UC Davis before I entered podiatry school and had a course on heart and circulatory physiology 1-2 years before I entered podiatry school. This course was much more detailed than any of the physiology courses I had in podiatry school and we covered all this material in great detail in my major. Therefore, this is probably why circulatory physiology seems second-nature to me, since this was hammered into me during undergraduate college when my brain was only 20 years old. Now, at 55 years old, I wish my brain was half as "plastic" as it is now (like dried clay).:drinks
     
  15. David Smith

    David Smith Well-Known Member

    At 55 I also know that feeling :confused:

    Dave
     
  16. David Smith

    David Smith Well-Known Member

    Kevin

    The Wiki refs you gave were a bit brief and lacking in several areas but the one here http://math.arizona.edu/~maw1999/blood/poiseuille/pressure.htm 'Physics of Blood Flow in Small Arteries', that you put a link to was excellent and had all the info required. The most encouraging thing was that all the assumptions, concepts and conclusions they use actually are consistent to the one's I was making (I was using Bernouli's principle for the equations) after I corrected for the effect of venturi action in the small vessels, i.e. smaller vessels result in increased flow velocity and decreased pressure, which allows for continuity of total flow mass.. I'm sure there is a concept there that is counter intuitive to most readers but important to understand.

    Regards Dave
     
  17. Dave:

    I don't think the Venturi effect has any place in the analysis of the pressure drop seen at the arteriole or capillary level in the arterial system since the distances of small diameter vessel restriction in the capillaries is much too long to be allow a true Venturi effect to occur.

    In the Venturi effect, there is a short, narrow restriction in an otherwise larger diameter tube where fluid velocity is increased and fluid pressure is dropped and then the pressure is increased after the restriction while fluid flow velocity returns to near pre-restriction levels. In the human cardiovascular system, such a short, narrow restriction does not occur at the arteriole and capillary level and the venous side of the cardiovascular system is low pressure, not high pressure like the arterial side of the cardiovascular system.
     
  18. David Smith

    David Smith Well-Known Member

    Kevin

    Please understand I'm not trying to contradict or prove you wrong, it's just that with your knowledge in animal physiology you will keep throwing up queries to my enquiries that I will have to research and discover for myself. So if you would please be kind enough to stick with it, where else but Pod Arena could one have the opportunity and privilege to engage in this kind of academic discussion with esteemed peers and superiors?

    The last part of that 2nd paragraph is true enough but the venous side of the vascular system is much larger in volume than the arterial side:

    At rest the blood is distributed as follows:

    60% in systemic veins and venules

    5% in systemic capillaries

    15% in systemic arteries and arterioles

    8% in heart

    12% in pulmonary vessels

    So if you imagine a pump where the high pressure outlet is connected to a small reservoir tank (A) a cylinder of 15 litres capacity with a large diameter of 20cm, which is connected by small pipes (B) that have a volume of 5 litres and a diameter of 2cm to a collection tank that has a volume cylinder of 60 litres with a diameter of 40cm, which is in turn connected to the low pressure return side of the pump.

    The pump has a flow rate of 20 litres per minute, that flow rate or flow mass must be maintained through out the system. In the first tank (A) the velocity of the flow mass is= n. Velocity of flow mass = flow rate divided by the cross sectional area (v=f/a).
    By this equation you will see that the velocity increases as the diameter of a pipe reduces. But the velocity increases as a power rate i.e. the cross sectional area of a 2cm pipe is not 10 times less than a 20 cm pipe because area is = Pi x r^2, therefore 10^2 = 100 and 1^2 = 1 so the increase in velocity in 100 fold. This graph show radius to flow rate relationship. Therefore A = 314cm^2 and B= 3.14cm^2 and C = 1250cm^2

    [​IMG]

    So flow velocity in B = n x (100 x c) and flow velocity in C = n x (0.25 x c) - where c is the constant Pi. And so flow velocity is much less in C than A so therefore there is much less pressure in C. analogous to high pressure medium flow velocity arteries, low pressure high velocity capillaries and low pressure low velocity veins.

    As we know, pressure is inversely proportional to flow velocity this is Bernoulli's principle and explains the conservation of energy since if the velocity and pressure increased there would have to be an input of extra energy from somewhere.

    So in this system the change in local pressure = change in local velocity of flow mass and change in velocity of flow mass is inversely proportional to pipe diameter. Therefore reduced pipe diameter = reduced fluid pressure regardless of the length of the small bore pipes.

    However the circulatory system is 60,000 miles long and capillaries are only 5mm long on average.

    There is much more to investigate on the effects of friction in the real vascular system but in the above model friction is disregarded because it cannot reduce flow rate because we assume a constant flow rate of 20 L/m

    However there is still a conundrum in that increased flow velocity = increased friction, which would indicate more tendency to slow flow rate in small vesssels??:confused: I continue to research:eek:

    Regards Dave
     
  19. Dave:

    No problems with the discussion. I am enjoying it and it is forcing me to reacquaint myself with concepts I learned 35 years ago.

    The reason I don't think that the venturi effect can be used to describe the decrease in pressure seen in the arterioles and capillaries in animal circulatory systems is because the venturi effect relies on only a relatively short distance of restriction in tube diameter between two larger diameter tubes. The principal won't apply in going from one tube to multiple smaller diameter tubes over a longer distance since the larger diameter tubes, in the animal circulatory system, are quite far apart and involve such a great reduction in tube diameter that flow restriction is just too great to allow a venturi effect to occur.

    In the illustrations I have attached of a venturi tube, note that the venturi tube has a relatively short distance of narrowed tube diameter where the fluid flow velocity increases and fluid pressure drops, which is then directly attached to a larger diameter tube where the fluid flow velocity decreases and fluid pressure increases. The venturi tube entry and exit angles are designed to minimize turbulence so that there is a minimum loss in fluid energy in passing through this short narrowing in the venturi tube.

    No such venturi-like structure is present within normal arteries, arterioles, capillaries, venules or veins (see illustration), even though I will admit that a short restriction in an artery due to a congenital defect or arterial wall placque may indeed create a venturi effect.

    However, the resistance to flow is so great at the capillary level, due to their very small diameter (5-10 microns in diameter) and the capillaries are so numerous when compared to their supplying arteres (aorta is about 20 mm in diameter or 2,000 times larger in diameter than the capillaries) that the venturi effect couldn't possibly be occurring at the capillary level due to severe flow restrictions and also to the multiple branching of vessels going from the artery to arteriole and then finally on to the capillary level.

    Of course, this is further compounded by the fact that the blood is not a homogeneous fluid but rather contains red blood cells (RBCs) that will, especially at the capillary level, signficantly affect blood flow including the known mechanical tendency for RBC stacking in small vessels called rouleaux. Therefore, the reduction in flow velocity and pressure seen within the animal circulatory system at the capillary level is complex and multifactorial, but, I doubt, could be explained as being due, in any way, to the "venturi effect".
     
  20. David Smith

    David Smith Well-Known Member

    Kevin

    With regard to 'many branching vessels' the configuration of the branching vessels is that they are in parallel to the arterial-venous flow. This means that the more there are the less resistance to total flow there is. i.e. Total resistance = the reciprocal sum of all the capillary vessels. => 1/Rt = 1/r1+1/r2 +1/r3 ->1/rn. So if there are four capillaries with resistance 20, 30, 8, and 22, then the total resistance is 3.94, if there were hundreds of capillaries then the total resistance would approach 1 i.e. the same as the supplying artery.
    Because the relative effect of the change in variables of pressure, diameter, length velocity, viscosity etc, it difficult to consider which has the greater effect. However most texts seem to suggest that change in diameter of vessel, rather than length or velocity, has the most effect on flow and pressure. This is because there is a power relative change of flow Vs diameter curve of radius^4. Whereas I think that change in velocity has a change of pressure to diameter ratio of r^2 and length has a linear ratio.

    More thinking and reading to be done:wacko:

    Regards Dave
     
  21. David Smith

    David Smith Well-Known Member

    Here's an area of confusion -

    Bernoulli's principle states that flow mass rate is conserved and so therefore velocity increases while pressure decreases (constant energy) when the diameter of the vessel is reduced. So, if the total cross sectional area is greater then the flow mass velocity will be slower (v=Q/A or Q/A-Q/A=0) and/but frictional resistance is reduced in larger diameter vessels. This is because of lower velocity and relative reduction of fluid in contact with the vessel walls.This might suggest that there is less resistance to blood flow in capillaries when summed together and yet the literature states that flow is greatly decreased in the capillaries because of increased friction as the vessel diameter reduces. There appears to be a contradiction!?
    However in this case there is a much greater area of fluid to vessel wall contact and so friction resistance is greatly increased and so flow rate is impeded.
    Therefore we should end up with large cross sectional area and low velocity flow with high resistance to flow.
    However this is not confirmed by the parallel resistance sum 1/Rt=1/R1+1/R2+1/R3 -> +1Rn. As you add the reciprocals of the individual capillary resistance values then you end up with a total resistance (Rt) that is less than the lowest individual resistance and approaches 1 or less than 1, where 1 is equal to the resistance in the larger arterial vessel.

    Regards Dave
     
  22. David Smith

    David Smith Well-Known Member

    Eureka!:eek:

    I've cracked it Kevin: (probably ;))

    So in your original paper quote you said that:

    To paraphrase - The pressure drop from heart to foot and from ankle to toe is due to restriction of flow. Because the arteries (and capillaries) reduce in lumen size there is an increase in resistance to flow that decreases pressure wave and head.

    When I read this I felt it was not entirely correct since in a hydraulic system pressure is constant and in the case of the cardio Vascular system (CV) the total flow is constant. I.E. if there were 2 litres / min in and only 1.5 out then we would have a problem very quickly. So I thought since there is a measurable drop in pressure from proximal to distal, what then causes that drop and yet still allows constant flow rate?

    Much contemplating and reading and reasoning later-----:confused:

    Hypotheses - The pressure drop measured across the length of an arteriole or capillary is not due to the restriction of flow or the diameter of the vessel lumen.


    Proof 1)

    In any hydraulic circuit, with no flow of fluid and not under the influence of gravity, the pressure is constant and equal throughout the system.

    In a similar system that is acted on by gravity then the pressure gradient by gravity is equal in both columns of water and the pressure in the connecting pipe is equal to the level it is connected in the column level.


    Example 1).

    Two cylindrical reservoirs at equal height relative to the ground (i.e. equal gravity) of 100 litres volume and 10cm x 10cm square, and so each is 1 meter in length, filled with water and open to atmospheric pressure are joined by a pipe 0.5cm square (squares intuitively calculate more easily than circles) at the bottom of the columns of water (i.e. the high pressure end.)

    In this case the pressure in both reservoirs and the connecting pipe are all equal, the water levels are equal also and this is true regardless of the reservoir sizes or difference in size or volume or the diameter of the connecting pipe

    Therefore it is not the diameter or lumen size of the connecting pipe that determines the pressure within it.

    Example 2). If there is a flow of fluid through the system:
    With the same set up as Ex1. Except that the one of the two cylinders, call them A&B, are filled (say cylinder A) and cylinder B is empty and the connecting pipe has a ¼ turn valve that is turned off. Open the 1/4 turn valve and the water will flow.
    Considerations:
    The flow rate is a function of the resistance of the cross sectional area of the connecting pipe. The resistance is a function of the cross sectional area Pi*r^2 and increases as the pipe lumen becomes smaller the ratio is radius^4 => (r1/r2)^4 - so radius 10 = 10,000 but radius 5 = 625 and so flow is 16 times less even though radius is only 2 times less. Flow is also a function of pressure but pressure / flow ratio is linear and proportional i.e. double pressure = double flow rate so lumen diameter is far more significant. However if the pressure is constant then the flow rate is constant. NB Viscosity also affects flow and blood viscosity is about 3 times that of water but as this is more or less constant it need not be considered. (Although some research indicates that blood can be considered a non Newtonian fluid and so its stiffness changes with force impulse it is not assumed here) Friction in line also reduces flow the longer the pipe the greater the friction resisting flow, so if a hose of uniform diameter is long enough eventually the flow will be stopped along its whole length. Poiseuilles flow equation is Flow rate (Q ) = [Pi*r^4*(p1-p2)]/8*viscosity (n) * vessel length (L) – Therefore the Pi*r^4 part is accounted for in the consideration of resistance as a function of change in radius (smaller vessel = greater resistance) the last part 8*n*L is tiny 8*3*0.0005 (0.5mm) => 0.012. However, in this case we are considering a constant flow rate (Q) and the vessel length is constant and so the friction is constant and need not be considered.
    The flow velocity is also a function of the radius and this is in the ratio of r^2 so r1^2/r2^2 so radius 1 is 10 and radius 2 is 2 then the product is and 5^2 is 25 therefore if the if the radius changes by a factor of 5 then the velocity increases 25 times.
    Ex2.1
    In the Ex2. the connection pipe length is fixed and so flow rate is determined by the pressure and/or the connection pipe diameter. The pipe diameter is fixed at 0.5cm and the length is constant therefore any change in flow rate must be due to pressure change. If we open the ¼ turn valve then there is a flow into cylinder B. The flow rate in the cylinder A the connection pipe and Cylinder B are all equal. This must be so because A cannot empty faster than B fills. The pressure will change as the water head drops in A and therefore the flow rate slows as the pressure difference between A and B equalises or returns to value 1 (p1/p2). Since the flow rate is constant but the diameter of the connection pipe is small relative to Cylinder A then the velocity is much greater and so the relative pressure is much reduced.
    Ex2.2
    Consider that there are the same conditions as Ex2.1 but with the addition of a lift pump in cylinder B that is pumping at the same flow rate that flows through the connection pipe and then redirects the water back into the cylinder A. In this case there is a constant flow with low pressure in the connection pipe due to increased velocity and low pressure in the Cylinder B due to the low head of water below the pump. The pipe from the pump has a series of one way valves that stops back pressure acting on the column of water in B and stops back flow of water if the pump stops.
    This would be very similar to the case of the heart pump replacing the pressure due to gravity, the arteries are cylinder A the connection pipe is the capillaries and cylinder B is the venous system. The lift pump is the leg muscle pump and the valves are the one way valves in the deep and superficial veins of the leg.
    Counter argument:
    It might be postulated that because there are many capillaries and arterioles that the total lumen is approaching equal to the major vessels. Therefore if the lumen is not significantly smaller then the blood flow velocity would not be significantly increased and so there would be no concomitant and significant decrease in pressure across the capillaries.
    This is a possibility because if the small vessels total lumen sums to the equal of the large vessel lumen then while the lumen is equal the resistance is much higher because of the larger surface area in contact with the fluid flow.
    Example 3)
    Vessel diameter 2cm, capillary diameter 0.02mm. Resistance of capillary is 2/0.02^4 = 10*10^6.
    The cross sectional area of large vessel is Pi*r^2 = 3.14*1=3.14cm^2
    The cross sectional area of the capillary is 3.14*(0.02*0.02) =0.00126cm^2 - so 3.14/0.00126=2492.
    The lumen area of 2492 capillaries = the lumen area of the large vessel so:
    The total resistance, assuming uniform diameter of all capillaries and a parallel configuration, is therefore Rt=>1/rt=1/r1+1/r2->1/r2492 = 40000. Therefore if the total capillary lumen cross sectional area is the same as the large vessel lumen then the resistance is 40,000 times higher. (That’s if I’ve done the maths right!? But the concept is good)
    Conclusion
    This might lead to the conclusion that the resistance of the capillaries reduces the flow and the pressure. But there is a problem i.e. constant flow rate is a requirement of the CV system or any closed pumped flow system i.e. you can’t have less flow mass out than flow mass in. Also for the total capillary lumen size to equal or approach the low resistance to flow of the large vessels then there would have to be many more capillaries resulting in a much larger volume in the capillaries than the large vessels but the literature indicates that the capillaries only hold 5% of the total blood volume.
    It seems to me that the only way to achieve reduced pressure while keeping constant flow rate (flow mass) and constant pressure is to reduce the lumen size and increase the flow velocity.
    It might seem reasonable to assume that the total lumen area of all the capillaries in the CV system is significantly smaller than the total lumen of the large arteries and the result is increased flow velocity and low distal pressure with constant flow rate at the optimum system pressure allowed by the system resistance.
    Comment:
    It has been assumed that the pressure is constant but in the real CV system this is not so. The heart sends out pulses of pressure as the heart beats. This means that, without any consideration of the fluid dynamics in the distal CV system, there will be a pressure rise and fall between diastolic and systolic pressures of about 50mm Hg or 1psi or 7KPa. Therefore within that range flow rates and velocities will vary. The elasticity of the individual vessels and the ability of the body to dilate and constrict vessels and vessel systems in organs will also affect the actual pressure, flow rate and flow velocity. Therefore the assumptions made earlier may not fit well into the real life CV system but I believe it may need further investigation unless there is a huge hole in my reasoning or maths. Please feel free to point these out and optimise my learning and understanding. (probably need to do a degree in fluid mechanics to understand this properly, it probably easier just to accept that blood flow and pressure drop distally because of increased system resistance):confused::D

    Regards Dave
     
    Last edited: Oct 31, 2012
  23. I don't have time now to go through all your points, Dave. To summarize in a sentence or two, what is the pressure drop in capillaries due to then?
     
  24. David Smith

    David Smith Well-Known Member

    Answer = The increase in velocity so that flow mass/rate is conserved

    Well that was my hypothesis and I sat up till early in the morning working thru the logic and maths and writing it up :wacko::wacko: and then I went to bed at 0130 and as my head hit the pillow I realised there were two gaping holes in my logic.:bang:

    1) The sum of the capillary lumen may be much larger than the larger vessel lumen but because they are very short it may not amount to a large volume

    2) And this is the killer- my example 2.2 actually disproves my proposition i.e. returns the null hypotheses. With one small connecting pipe the flow would be very slow but the pressure between A & B would not equalise because of the lift pump (and the greater volume in the real venous system). This would maintain a pressure gradient to allow flow. So therefore the flow rate can be controlled by the number of small connecting pipes i.e. capillaries. The pressure gradient can be maintained by regulating the flow rate of the return / lift pump.The pressure in A can also be maintained by adjusting the return rate of the lift pump which in reality are the leg muscles and the right atrium. This is further proved by the fact that the right ventricle has an output pressure at 1/7th of the left ventricle, which is because the plumonary system is at heart level and is highly vascular and so requires low circulation pressure and has low resistance to flow due to the effect of reduction in resistance due to parralel configuration of millions of small vessels.

    OK I give in You win Prof Kirby.

    Conclusion: Pressure and flow in the arterial system are reduced distally by the resistance of the small vessels (and this is allowed because of the low pressure conditions in the venous system).

    Hey I didn't loose though coz now I have a much better understanding of the CV system and some of the theory of haemodynamics win win win eh!?:drinks

    Regards Dave
     
    Last edited: Oct 31, 2012
  25. foxdenise

    foxdenise Member

    Hi Warts
    I am using the Hadeco Smartdop to obtain TBIs. There was a 'Quick Guide' instructions included with machine. Kellie from Briggate Melbourne also emailed me another document with a little more detail. The results seem to be very variable especially if the patient moves/talks/breaths!!
    I find taping the PPG on with micropore tends to keep the best contact if the toe is big enough. Colleagues have the Systoe system and the instructions were to use double sided tape and micropore if required. I can email you the documents if you need and other info I have found.
    Regards
    Denise
     
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