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Old 06-05-2010
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Lungs Carbon Monoxide Poisoning SaO2 and PaO2!

In Carbon Monoxide poisoning what happens to AaO2 and PaO2???
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Old 06-05-2010
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PAO2 will be normal...PaO2 remains Normal...but SaO2 reduces....

PaO2 is pressure that keep the O2 in dissolved form....its not changed in co poisoning....but SaO2 will decrease....as CO has 200 times more affinity for HB then O2....so CO binds to hemoglobin in preference to oxygen...
The binding of CO at one of the 4 sites of HB increases the O2 affinity of the remaining three sites.....which causes the HB molecule to retain O2 that would otherwise be delivered to the tissue....Because of the increased affinity between HB and O2......all the O2 stays in the HB.... none is delivered to the tissues...( lt shifting of O2 HB dissociation curve...)...This causes hypoxic tissue injury....COHB is bright red in colour...so poisoned pt. look pink cheeked and healthy...
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Old 06-05-2010
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Cool true versus relative anemias

i thought i'd add in my 2 centavos:

co poisoning is an example of a relative anemia-it appears to be an anemia but technically it's not. patient will present with a low SaO2 and a low CaO2-recall the equation CaO2=Hbx1.39 + SaO2 + PaO2x0.003. Here the Hb will not be lowered nor will be PaO2. SaO2 can be determined by O2 combined with Hb/O2 capacity x 100. Note that O2 capacity is Hb x 1.39.

true legit anemia will also have a low CaO2, but for a different reason. O2 capacity will not be normal as seen above, but rather abnormal. Hb will be depressed, thus lowering the O2 capacity. SaO2 will be normal, since we don't have any competitive gas (e.g. CO) to deal with. PaO2 will remain normal.
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SaO2 is decreased, and PaO2 remains normal
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Old 06-06-2010
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yup and is same for cyanide poisoning too sao2 decreased and pao2 normal

where as in anemia sao2 and pao 2 are normal amount of Hb itself is decreased
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Old 04-10-2012
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I have a bit of an issue with this. If Carbon monoxide poisoning, left shifts an oxygen dissociation curve, then why does, the PO2 not decrease ?

Hb=normal (ok, no Hb changes)

O2 % saturation= decrease (ok because a fraction less than 100% can be filled)

PO2= no change ( confused because it left shift an Ox dissociation curve- so shouldn't it be decreased pO2 ?).
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Old 02-07-2013
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Default Hypoxia

Quote:
Originally Posted by ecgram View Post
In Carbon Monoxide poisoning what happens to AaO2 and PaO2???
In order to understand what happens to PAO2, PaO2 or SaO2 to Carbon Monoxide poisoning or anemia or Polycythemia...you have to take a look at which what each of the measurements mean and how the disease can affects it. I tried to under the general mechanism below and within it is the explaination for Carbon Monoxide poisoning.

- Hypoxia
o Low oxygen delivery to tissue
o Normally: PAO2 → PaO2 → SaO2
• PAO2 = alveolar pressure O2
• PaO2 = arteriole pressure O2
• SaO2 = saturation O2
o Causes of hypoxia: ischemia, hypoxemia, decerased O2-carrying capacity of blood

----------------------------------------

• Ischemia
• Ischemia is decreased blood flow through an organ
o Decreased arterial perfusion (i.e. atherosclerosis)
o Decreased venous drainage (i.e. Budd-Chiari syndrome…blocking hepatic vein, so blood can’t flow to the liver leading to infarction of the liver parenchyma. Recall that the most common cause of Budd-Chiari syndrome is polycythemia vera…in which pts has high blood cell counts due to over production of RBCs thus making the blood thick & viscous. SLE pt has lupus anticoagulant which creates hypercoaguble state in that patient.)
o Shock – generalized hypotension resulting in poor tissue perfusion

----------------------------------------

• Hypoxemia
• Hypoxemia is a low partial pressure of oxygen in the blood (PaO2 < 60 mmHg, SaO2 < 90%)

o FiO2 (fractional % oxygen within the atmosphere)…the goal of the lung is to bring the FiO2 into the alveolar air sac and create PAO2 (partial pressure of oxygen within the alveolar air space)…that PAO2 squeezes oxygen across the interstitum and into the capillaries, which creates a PaO2 (partial pressure of oxygen within the arterioles)…PaO2 squeezes oxygen across the red blood cell membrane to load oxygen onto hemoglobin and creating SaO2 (what percentage of hemoglobin is bound by oxygen.)
o FiO2 → PAO2 → PaO2 → SaO2
o Even though the hypoxemia affects PaO2, however, any sequences precede PaO2 like FiO2 or PAO2 would also decrease PaO2 as well. And, when there is a decrease in PaO2, it would also decrease SaO2.

o High altitude – The FiO2 would decrease because there is a decreased barometric pressure results in decreased PAO2…hence, decreasing PaO2…resulting in hypoxemia.

o Hypoventilation: Increased PACO2 results in decrease PAO2…hence, decreasing PaO2…resulting in hypoxemia.
• Causes of decreased PAO2: increase PACO2 (hypoventilation: CO2 piles up in the lung bc it’s not being blown up; COPD: air gets trapped within the lung)

o Diffusion defect: PAO2 not able to push as much O2 into the blood due to a thicker diffusion barrier (i.e. interstial pulmonary fibrosis), thus, resulting in a decrease PaO2
• interstial fibroisis of the lung due to thickening of the alveolar air sacs so that PAO2 cannot squeeze blood into the capillaries)

o V/Q mismatch – Blood bypasses oxygenated lung (circulation problem, e.g., right-to-left shunt), or oxygenated air cannot reach blood (ventilation problem, i.e. atelectasis)

----------------------------------------

• Decrease O2-carrying capacity arises with hemoglobin (Hb) loss or dysfunction

• Note: the dissolved oxygen within the blood actually enters the red blood cells & binds to hemoglobin…it’s the hemoglobin that actually have the role of carrying oxygen to the tissues & unloading at the level of the tissues…the vast majority contents of the blood is actually due to oxygen bound to hemoglobin…so if you knock out the RBCs or if you knock out the ability of the hemoglobin to carry oxygen…thus resulting in hypoxia of the tissues.

• Anemia (decrease in RBC mass) – PaO2 normal; SaO2 normal
o Decrease in RBC mass → decrease in the amount of oxygen being carried in the blood → decreased amount of blood going into the tissue → hypoxia
o FiO2 → PAO2 → PaO2 → SaO2
o PaO2 is normal bc here is no problem in bringing the oxygen from the atmosphere into the alveolar…there is no problem in bringing the oxygen from the alveolar into the arteriole…there is no problem in bringing the oxygen into the blood…hence PaO2 is normal.
o SaO2 is normal because SaO2 only looks at what % of the Hb is bound by oxygen…even though we have a decrease in the amount of oxygen and less Hb…but the Hb that we still have..each of that Hb has 100% oxygen bound to the Hb. Meaning, there are enough 4 molecules of oxygen on one red blood cells. So, for example, if you have a total of 3 red blood cells in your body. Each of the red blood cells would have 4 molecules of oxygen…hence, the SaO2 is normal. Even though you have anemia, instead of having 3 red blood cells, now, you have only 2 red blood cells, but, each of the red blood cells still carry 4 molecules of oxygen, hence, you have a normal level of SaO2.

Carbon monoxide poisoning
o CO binds hemoglobin more avidly than oxygen – PaO2 normal; SaO2 decreased.
o FiO2 → PAO2 → PaO2 → SaO2
• PaO2 is normal bc here is no problem in bringing the oxygen from the atmosphere into the alveolar…there is no problem in bringing the oxygen from the alveolar into the arteriole…there is no problem in bringing the oxygen into the blood…hence PaO2 is normal.
• SaO2 is decreased…normally, one RBC can hold up to 4 Hb molecules…but now, it can only hold 2 Hb b/c the other two spaces are being bound by CO….hence, SaO2 is decreased.
o Exposures include smoke from fires & exhaust from cars w/ the garage closed or gas heaters.
o Classic findings: cherry-red appearance of skin (bc Hb is so tightly bound that it reflects the red light and creats red appearance to the skin…this is deceptive bc the patient is actually hypoxic bc not carrying oxygen to the tissues
o Early sign of exposure: headache; significant exposure leads to coma & death

• Methemoglobinemia
o Fe2+ (ferrous) normally binds to O2
o Iron in heme is oxidized to Fe3+, which cannot bind oxygen – PaO2 normal; SaO2 decreased.
• PaO2 (blood partial pressure of O2) is normal bc there is no problem in bringing the oxygen from the atmosphere into the alveolar…there is no problem in bringing the oxygen from the alveolar into the arteriole…there is no problem in bringing the oxygen into the blood…hence PaO2 is normal.
• SaO2 is decreased…normally, one RBC can hold up to 4 Hb molecules…but now, it can only hold 2 bc the other two spaces are being bound by Fe3+….hence, SaO2 is decreased
o Seen with oxidant stress (e.g., sulfa drugs & nitrate drugs) or in newborns
o Class findings: cyanosis w/ chocolate-colored blood
o Treatment: intravenous methyle blue, which helps reduce Fe3+ back to Fe2+ state
o Methemoglobinemia causes dusky discoloration to the skin (similar to cynoasis), and bc methemoglobin is unable to carry oxygen, a state of functional anemia is induced.
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