Cyanosis and Oxygen Curve

[Ihateverbal]

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Can anyone explain this to me?


Oxygen-hemoglobin dissociation curve. Do you know why "cyanosis"
would cause a left-ward shift? (one of the review books said this)
and hypoxemia would be a right ward shift, which makes sense to me. However, a little confused on the leftward shift for cyanosis though

 

[Mortal_Lessons]

Can anyone explain this to me?


Oxygen-hemoglobin dissociation curve. Do you know why "cyanosis"
would cause a left-ward shift? (one of the review books said this)
and hypoxemia would be a right ward shift, which makes sense to me. However, a little confused on the leftward shift for cyanosis though

I've never heard of cyanosis causing a leftward shift; I've heard of carbon monoxide poisoning adjusting hemoglobin's affinity for oxygen, resulting in a leftward shift and cyanosis. I think the causal relationship you're putting forward might need to be re-evaluated.

EDIT: I apologize but carbon monoxide does not result in the blue cyanosis as I just re-learned; it causes a "cherry-red" blue hue according to Wikipedia.

 

[Ihateverbal]

Page 478 of FA 2009 says Cyanosis, leftward shift...

I looked at the error sheet and this wasn't one of the listed errors.

 

[future_dr_house]

rr path chapter 1

 

[Ihateverbal]

have a page # by any chance?

 

[Mortal_Lessons]

Page 478 of FA 2009 says Cyanosis, leftward shift...

I looked at the error sheet and this wasn't one of the listed errors.

Haha, I'm doing respiration today as well! You're a page ahead of me! I'm looking at it now and it appears to be stating that when your oxygen saturation is around 75 %, you are cyanotic, whereas when it is closer to 90 % you are hypoxemic. Does that make sense?

 

[Ihateverbal]

I see what you are saying, but on the corrections it stated that the cyanosis is the (L) dotted curve and the Hypoxemia is the (R) dotted curve...in that case, it's the entire curve represending cyanosis.. (from what i interpreted it to mean)

=/

 

[Mortal_Lessons]

I see what you are saying, but on the corrections it stated that the cyanosis is the (L) dotted curve and the Hypoxemia is the (R) dotted curve...in that case, it's the entire curve represending cyanosis.. (from what i interpreted it to mean)

=/

Ah, I see-I haven't looked at the corrections for this section yet...I think in general, cyanosis is a result of a decreased Hb saturation, whereas hypoxemia is a result of a decreased oxygen content in the blood.

 

[future_dr_house]

I think it's an example where FA simplifies the concept too much. Mortal Lessons has it right. SaO2 < 80% (rr path pg 2) = cyanosis, whereas decreased O2 content = hyposemia

A left shift can be caused by CO which binds hemoglobin, decreasing that amount of Hb that can bind O2 and therefore decreasing the SaO2 leading to cyanosis (see methemoglobinemia in rr path).

A right shift, occurs in situations like lactic acidosis, one cause of which might be hypoventilation. In this example, hypoventilation will decrease the O2 content of the blood, causing hypoxemia. The right shift is to try and compensate.

So, yeah...that's FA being very vague.

 

[Ihateverbal]

👍

 

[nogolfinsnow]

I think it's an example where FA simplifies the concept too much. Mortal Lessons has it right. SaO2 < 80% (rr path pg 2) = cyanosis, whereas decreased O2 content = hyposemia

A left shift can be caused by CO which binds hemoglobin, decreasing that amount of Hb that can bind O2 and therefore decreasing the SaO2 leading to cyanosis (see methemoglobinemia in rr path).

A right shift, occurs in situations like lactic acidosis, one cause of which might be hypoventilation. In this example, hypoventilation will decrease the O2 content of the blood, causing hypoxemia. The right shift is to try and compensate.

So, yeah...that's FA being very vague.

CO causes a left shift, but not cyanosis. Carboxyhemoglobin gives CO-poisoned people a pinkish look. But if SaO2 was decreased say from a R-->L shunt or diffusion defect then you'd be blue

 

[sanj238]

I just had the urge to reply to an old thread as I too had the same question.

I read what you guys wrote but I feel like this explanation may be best.

in goljan page 2, it states that SaO2 <80% leads to cyanosis
however, at the bottom it shows that almost all SaO2 is less than 80%....does that mean cyanosis all the time? obviously not.

I think the main point is that

1. <80% is reflecting how much oxygen can first bind to hemoglobin at all once it leaves the lungs. If its less than 80% you're in trouble...this is where CO can lead to cyanosis since not enough oxygen can bind to hemoglobin once it leaves the lungs...

2. However, once it has travelled into circulation, oxygen binding to hemoglobin is partly dependent on the amount of oxygen available in tissue. As oxygen in tissue increases it forces greater binding of oxygen to hemoglobin-this makes sense due to le chatliers (sp).

The diagram shows that this is reflective of tissue O2 only. That is, in tissue, normal O2 binding is actually at 50% SaO2 (again this SEEMS to contradict goljan's comment about <80% SaO2 leads to cyanosis until you realize that one is dealing with hemoglobin binding immediately at lungs and the other one relates to tissue...)

 

[Phloston]

With hypoxaemia:

Lesser pO2 means greater pCO2. Since pCO2 is greater, right-shift with hypoxaemia. Goljan talks about this.

With cyanosis:

There's an elevated absolute quantity of deoxyhaemoglobin (I think it's >2.5 g/dL). This can happen with severe polycythaemia or with cardiopulmonary pathology. This means that the net summation of all of the circulating haemoglobin would be slightly deoxygenated relative to in a non-cyanotic state (i.e. all of the fully oxygenated Hb + all of the deoxyhaemoglobin = an increased number of open O2-binding sites on Fe2+ across all Hb = more taut- vs relaxed-form [given the synergistic nature of O2-binding]).

Therefore, if a single curve is drawn depicting all of the Hb circulating, it is shifted left, since there is less O2 bound overall.

Hope the helps,