Question about alkaline drift

[anbuitachi]

I feel like I am missing something obvious.. but why does alkaline drift occur in hypothermia? My book states "there is a natural alkaline drift in hypothermia due to increase in gas solubility".
The solubility of gases like O2 and CO2 increases so more of it are dissolved in the blood. CO2 dissolved in blood is in equilibrium with Carbonic acid, which should reduce pH? The partial pressure of CO2 (PaCO2) decreases due to lower temperature and increased solubility.

Basically question is why increased pH with more dissolved CO2..

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[AdmiralChz]

You just answered it - a lower PaCO2 leads to a higher pH by basic acid/base physiology. Think to everyone’s favorite topic: pH stat and Alpha stat for CPB ABG analysis with hypothermia. The partial pressure of CO2 is what’s important here for pH, and there’s less of it when you’re cold (it’s in solution, dissolved)

That being said - at some point there’s a decrease in oxygen delivery to peripheral tissues that reaches a critical point (left shift of O2-hgb curve) which will cause hypoperfusion and acidosis so something to keep in mind.

I think I have that right...

 

[anbuitachi]

You just answered it - a lower PaCO2 leads to a higher pH by basic acid/base physiology. Think to everyone’s favorite topic: pH stat and Alpha stat for CPB ABG analysis with hypothermia. The partial pressure of CO2 is what’s important here for pH, and there’s less of it when you’re cold (it’s in solution, dissolved)

That being said - at some point there’s a decrease in oxygen delivery to peripheral tissues that reaches a critical point (left shift of O2-hgb curve) which will cause hypoperfusion and acidosis so something to keep in mind.

I think I have that right...

That is what is confusing me. Why is the PaCO2 determining the pH, and not the amount of CO2 dissolved in the blood? Because dissolved CO2 creates carbonic acid so that should affect pH. I mean at a constant temperature the PaCO2 is proportional to how much is dissolved in the blood so that is fine, but at a lower temperature, the correlation changes and more CO2 is dissolved for a given PaCO2...

edit: now reading some other sources, seems like it is due to decreased dissociation of acid? Carbonic acid less wanting to release the H..?

 

[pgg]

Also, Kw is affected by temperature. You get less dissociation of H2O to H+ and OH- at lower temperatures. Water itself is less acidic (defined as H+ concentration) at lower temperatures.

I'd have to look it up but I think the pH difference for water alone is a few tenths between normothermia (37ish) and DHCA (18ish) temps.

 

[vector2]

That is what is confusing me. Why is the PaCO2 determining the pH, and not the amount of CO2 dissolved in the blood? Because dissolved CO2 creates carbonic acid so that should affect pH. I mean at a constant temperature the PaCO2 is proportional to how much is dissolved in the blood so that is fine, but at a lower temperature, the correlation changes and more CO2 is dissolved for a given PaCO2...

edit: now reading some other sources, seems like it is due to decreased dissociation of acid? Carbonic acid less wanting to release the H..?

As you said, PaCO2 and H2CO3 have a linear relationship based on temperature and a solubility constant is used to convert between them so ABG readings can be used to estimate pH with the henderson hasselbach equation. I believe what you read is correct regarding why pH actually increases: proton dissociation is endothermic and more HA is favored over H+ during hypothermia. Bottom line, I guess, more dissolved CO2 at low temp may shift bicarb buffering towards H2CO3 but not necessarily lead to more proton dissociation.