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Thread: CO2's allosteric mediation of hemoglobin's effect on capillaries?

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    CO2's allosteric mediation of hemoglobin's effect on capillaries?

    I guess what I'm asking is are there any Doctor's or Medical folks who can tell me if the finger-tip (oxygen level) analyzers used in hospitals, are using the expansion of capillaries in the extremities (due to hemoglobin's detection of higher CO2 levels) as their means of indicating a need to increase a patients oxygen levels? And what is the actual mode of operation going on here. I know it relates to an infra-red led (sending unit) and some form of receiving mechanism to convert that measurement to actual patient oxygen levels, is that correct or incorrect??? And does anyone have data on how this is affected by partial pressures of both Oxygen and CO2 gases? In other words, do these finger-tip sensors act differently, as partial pressures increase (unlike a hospital setting where partial pressure is more or less constant). Are there any Naval Dive Doctors who might have data taken using these units in a hyperbarric chamber analysis of Navy divers?

    I'm trying to understand the relationship between the (WIKIPEDIA info in blue) information below, and these finger-tip type sensors. Any help would be greatly appreciated. I'm very interested in the Haldane effect (in hyperbarric atmospheres) and these devices.

    Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr Effect.
    Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its levels are high, the capillaries expand to allow a greater blood flow to that tissue.
    Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO2 in their blood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing that is too rapid leads to hyperventilation, which can cause respiratory alkalosis.


    Thanks for any help you might offer,

    uga82grad (AKA) Jack

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    Re: CO2's allosteric mediation of hemoglobin's effect on capillaries?

    Let me rephrase this this: I'm aware of small battery-powered home versions of these types of finger-clip units for measuring O2 levels. By what mechanism are they functioning? They all have a red LED sending unit, and some type of photo cell receiving unit. I'm assuming it has something to do with light transmission/absorption through an extremity (hence the finger tip location of these sensors) and some sort of algorithm that translates this into an oxygen level. Is it really O2 levels being read, or is it detecting expanded capillaries because of excess CO2 build-up, and then extrapolating these O2 level readings via some algorithm utilizing the Haldane effect???

    There must be some Dive Docs out there who can help me with this....

    I'm interested in the possibility of adapting one of these (experimentally) to fit a divers glove which would then interface with a LCD wrist display, with the idea being to display a divers actual O2 levels & blood CO2 levels, and possibly being able to forewarn a diver of approaching CO2 hit event or blackout situation.

    Any thoughts....anybody????

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    Re: CO2's allosteric mediation of hemoglobin's effect on capillaries?

    Firstly, you cannot measure blood CO2 with one of "these types of finger-clip units" - the technology does not exist, unfortunately.

    The pulse oximeters (to which you are referring) work by measuring differential absorption of red and infrared light at wavelengths of 660 and 940 nm. All non-pulsatile elements of the signal are then subtracted out leaving (in theory) the differential absorption due to arterial blood within the finger (capillary and venous blood, skin, bone, nail polish etc all being non-pulsatile). The result is then compared with a table (or converted with an empirically derived equation) to get arterial saturation of blood. Note that it measures haemoglobin saturation, which in most divers breathing air will be 99-100%. Breathing almost any PO2 higher than 0.21 atmospheres will result in a pulse oximeter reading of 100% in a normal diver, although the actual PO2 in the blood will vary widely with inspired PO2.

    The bottom line is that you might be able to adapt a pulse oximeter for use underwater, but it would just read 100% all the time, and therefore not give you any useful information. Since blood CO2 cannot, unfortunately, be measured in the same way, we are still searching for workable physiological monitoring. End-tidal CO2 (see numerous long threads here on RBW) is probably the most likely technology to work in practice.

    Hope this helps. Here's a link you may find helpful:

    Pulse Oximetry

    Andy

    PS: The Haldane effect has no real relevance here. For those interested, it was John Scott Haldane, the same one who gave his name to the type of decompression algorithm.
    Last edited by apitkin; 2nd July 2010 at 02:37. Reason: Typos

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    Re: CO2's allosteric mediation of hemoglobin's effect on capillaries?

    Quote Originally Posted by apitkin  View Original Post
    Firstly, you cannot measure blood CO2 with one of "these types of finger-clip units" - the technology does not exist, unfortunately.

    Hey Andy, it is true, at least to my knowledge, that this technology doesn't exist in finger clip units, but it does exist in at least one ear clip unit.

    In an earlier thread I was talking about a study I found:

    http://www.rebreatherworld.com/showthread.php?t=32552

    "a study that used an experimental Pulse Oximeter combined with Carbon Dioxide Tension monitoring ear sensor..."

    Study: A Combined Ear Sensor for Pulse Oximetry and Carbon Dioxide Tension Monitoring: Accuracy in Critically Ill Children ? A & A


    At the time I searched and found the manufacturer, but I never made contact with them for further investigation...

    -Scott

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    Re: CO2's allosteric mediation of hemoglobin's effect on capillaries?

    Quote Originally Posted by Scott  View Original Post
    Hey Andy, it is true, at least to my knowledge, that this technology doesn't exist in finger clip units, but it does exist in at least one ear clip unit.

    In an earlier thread I was talking about a study I found:

    http://www.rebreatherworld.com/showthread.php?t=32552

    "a study that used an experimental Pulse Oximeter combined with Carbon Dioxide Tension monitoring ear sensor..."

    Study: A Combined Ear Sensor for Pulse Oximetry and Carbon Dioxide Tension Monitoring: Accuracy in Critically Ill Children ? A & A


    At the time I searched and found the manufacturer, but I never made contact with them for further investigation...

    -Scott
    Transcutaneous CO2 measurement has been around for a while, and is still occasionally used in some medical applications, e.g sleep studies. However, it is currently not practical for use underwater, because of movement, cooling by water, power consumption and requirement for regular calibration due to drift. It has been used in hyperbaric chambers, mostly for research purposes. If someone can come up with a reliable, efficient device that can be used underwater, we'll all be interested, but I doubt it will be easy. Measuring CO2 in gas is much easier by comparison.

    Andy

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    Re: CO2's allosteric mediation of hemoglobin's effect on capillaries?

    Thanks Scott and Andy,

    Looks kinda like this idea of mine is leading to a dead-end.

    thanks,
    Jack

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