Oxyhemoglobin dissociation curve ?

[sn216]

hi all,

i have a question about ohdc. the curve is shifted to the left when there's an increase in o2 and ph and a decrease in temp and co2.

but will there be a shift to the left during times of stress or when hco3- levels increase?

i have in my notes that a shift to the left occurs during alkalosis. well, uncompensated metabolic alkalosis is an increase in ph and increase in hco3-. where as uncompensated respiratory alkalosis is an increase in ph and a decrease in co2.

respiratory alkalosis can occur from pain, fear, anxiety, stress, fever, etc. metabolic alkalosis occurs from vomiting, ng suctioning, hypokalemia, diuretic use. respiratory can be caused by stress and metabolic has the increased hco3-.

are both answers correct to my question?

thanks in advance!!!

[Daytonite, BSN, RN]

hco3 has nothing to do with oxyhemoglobin association or disassociation. you are missing a third element in determining ohdc.

keep this confined to dpg, acidity/alkalinity and body temperature as the only 3 determining factors of oxyhemoglobin association or disassociation. dpg is 2,3-diphosphoglycerate, a component of rbcs that controls the movement of oxygen from red blood cells to body tissues. dpg is part of a feedback loop that helps prevent tissue hypoxia and is why oxygen binds and unbinds to hemoglobin. you know what acid/base and body temperature are.

there is a shift to the left when there is an increased affinity for oxygen. conditions include acute alkalosis where the ph is elevated (alkaline). there is a decrease in 2,3-diphosphoglycerate (dpg) and decreased temperature. pco2 can also be decreased but it is more important that the other 3 elements are present.

there is a shift to the right when oxygen does not have an affinity for hemoglobion as in acute acidosis where the ph is low, an increase in 2,3-diphosphoglycerate (dpg) and elevated temperature. pco2 can also be elevated but it is more important that the other 3 elements are present. a mneumonic that might help is dat for an oxygen dissociation curve shift to the right which stands for increased dpg, acidity, temperature. the increased acidity does not mean an elevated ph.

complicated stuff. try to stay basic and don't go into all kinds of symptoms. in fact, this infor can be put on a simple little chart. if you've got ph and temperature, you've got your ohdc.

[sn216]

Thanks, Daytonite. I guess my answer would be that stress could cause a shift to the left.

The question was based on a worksheet we were given to look over this weekend. We were not given any values (DPG, acidity/alkalinity and body temperature). We were just asked when will a shift to the left occur?

During times of stress or elevated bicarb. Since you stated bicarb has nothing to do with it then the answer must be stress.

Thanks!

[Daytonite, BSN, RN]

ah, some critical thinking is needed to answer this question!

i would consider what would these two things do to acid/base or temperature since dpg wasn't discussed with you guys. with stress, which places an increase on the body's metabolic rate, temp is going to elevate so the curve is going to shift to the right.

with elevated bicarb (bicarb is a measurement of abgs) look at the rome mnemonic (https://allnurses.com/forums/2016253-post25.html) to get the acid/base clue:

• respiratory opposite
  
  • ph elevated pco2 diminished = respiratory alkalosis
    
  • ph diminished pco2 elevated = respiratory acidosis
    

  [*]metabolic equal

  • ph elevated hco3 elevated = metabolic alkalosis
    
  • ph diminished hco3 diminished = metabolic acidosis
    

so, the mnemonic tells us that an elevated hco3 gives us metabolic alkalosis would be a shift to the left.

the answer to the question "when will a shift to the left occur, during times of stress or elevated bicarb?" is arrived at by critical thinking and applying the principles of how ohdc shifts in the face of temperature and acid/base status. the answer is: a shift to the left will occur during an elevated bicarb.

do you understand how i got the answer?

[sn216]

Yes, I do see how you got that answer. I originally circled HCO3- as my answer but then your previous answer made me think stress. In my notes, stress, pain, fear is seen in respiratory alkalosis and I know alkalosis means shift to the left.

I spent about 5 or 6 hours researching this one question last night! Ughhh... But yea, I see how you came up with the answer. I also found a nice chart on wikipedia on the OHDC.

I was just really confused since it seemed that both stress and HCO3- would play a role.

Thanks for the clarification! Well, time to get some sleep... I have peds clinicals in a few hours. :zzzzz