Alpha Stat vs pH Stat

[UW in 2002]

Looking for some help with understanding this issue. Below is my understanding for a pt made hypothermic for cardiopulmonary arrest, so correct me if I'm wrong.

pH-stat (temp corrected): when the pt is cold and you draw the gas, the ABG will come with a higher pH and a lower PaCO2 than normal.
1. Requires changes in minute ventilation or during bypass requires the addition of CO2 into the oxygenator of the bypass pump in order to maintain normal pH
2. Advocates of the pH-stat method argue that since hemoglobin's affinity for oxygen increases as temperature decreases (hemoglobin-dissociation curve is shifted left) the addition of CO2 or decrease in ventilation (resulting in a respiratory acidosis) will counter the left shift and therefore oxygen release at the tissues will be improved.
3. Some studies show that this is better in animals and infants.

α-stat (temp uncorrected): when the pt is cold and you draw the gas, the ABG will come back with the normal expected results of 7.4/40/100.
1. Based on the fact that the SO2, O2 Content and HCO3- do not change with changes in temperature, it is argued that acid-base status does not change when temperature changes under the alpha-stat regulation mechanism. So if you make a change in ventilation or add CO2 to the CPB circuit, you are causing a respiratory acidosis (as dictated by the pH-stat method).
2. This is the method most adult centers use. Studies indicate that myocardial function and neurological function is better preserved when this strategy is employed.


OK, so what I don't understand is: if when you're cold and the solubility of gases is supposed to increase, wouldn't this mean that your PaCO2 should increase, why would the pH stat show a respiratory alkalosis? My chemistry is so far behind me that in order to understand properly, I can't get past this part.

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[IMED4Life]

PaCO2 is a reflection of gas NOT dissolved in blood. Therefore, as temperature drops, solubility of CO2 in blood increases and the PaCO2 decreases resulting in a rise in pH (alkalosis).

 

[UW in 2002]

I always learned that the PaO2 represented the partial pressure of oxygen dissolved in the blood and therefore always thought that the PaCO2 represented the partial pressure of carbon dioxide dissolved in the blood. So is only the former state correct or have I been misunderstanding things all this time?

 

[rsgillmd]

I always learned that the PaO2 represented the partial pressure of oxygen dissolved in the blood and therefore always thought that the PaCO2 represented the partial pressure of carbon dioxide dissolved in the blood. So is only the former state correct or have I been misunderstanding things all this time?

The pressure is generated by the part not dissolved.

Think of a shaken soda bottle. The liquid is under greater pressure when it has been agitated to bring the CO2 out of solution. Leave it alone in the refrigerator and it will be like nothing ever happened. The CO2 didn't go anywhere, but the PCO2 decreased by cooling the bottle and letting the CO2 go back into solution.

 

[proman]

The gasses are in equilibrium. PaO2 or any gas does reflect what is dissolved in plasma (as opposed to bound to hemoglobin). The dissolved gas cannot be measured directly. The Clark electrode measures the amount of charge created by diffusion of oxygen across a semi-permeable membrane. The greater the amount of dissolved oxygen, the more will be in gas form and able to diffuse across the membrane, the greater the electron flow, the higher the reading. So technically, the electrode measures non dissolved oxygen, but is directly proportional to dissolved oxygen. The Severinghaus electrode (for carbon dioxide) works on a similar principle. As mentioned above, temperature impacts solubility of the gasses, which alters the equilibrium point. The blood gas analyzer warms the sample to 37 degrees then measures. Temperature correction utilizes a nomogram to adjust the values. These topics appear on the ABA exams.

 

[pgg]

Therefore, as temperature drops, solubility of CO2 in blood increases and the PaCO2 decreases resulting in a rise in pH (alkalosis).

As the blood gets colder, more CO2 is in solution, which would lower the pH.


However, colder blood has a higher pH primarily because there is less dissociation of H2O into H+ and OH-, and because the pKa of imidazole groups increases with decreasing temperature.

Miller 6th ed, page 1980 makes it clear as mud.

These topics appear on the ABA exams.

Painfully so ...


1994 ITE book A #39

An arterial blood sample is obtained from a patient undergoing hypothermic cardiopulmonary bypass at 28°C. Analysis at 37°C shows a pH of 7.40 and PaCO2 of 40 mmHg. Which of the following values are most likely if the analysis is corrected to 28°C?


(A) pH 7.40; PaCO2 40
(B) pH > 7.40; PaCO2 < 40
(C) pH > 7.40; PaCO2 > 40
(D) pH < 7.40; PaCO2 < 40
(E) pH < 7.40; PaCO2 >40



B for the reasons above

 

[epidural man]

Looking for some help with understanding this issue. Below is my understanding for a pt made hypothermic for cardiopulmonary arrest, so correct me if I'm wrong.

pH-stat (temp corrected): when the pt is cold and you draw the gas, the ABG will come with a higher pH and a lower PaCO2 than normal.
1. Requires changes in minute ventilation or during bypass requires the addition of CO2 into the oxygenator of the bypass pump in order to maintain normal pH
2. Advocates of the pH-stat method argue that since hemoglobin's affinity for oxygen increases as temperature decreases (hemoglobin-dissociation curve is shifted left) the addition of CO2 or decrease in ventilation (resulting in a respiratory acidosis) will counter the left shift and therefore oxygen release at the tissues will be improved.
3. Some studies show that this is better in animals and infants.

&#945;-stat (temp uncorrected): when the pt is cold and you draw the gas, the ABG will come back with the normal expected results of 7.4/40/100.
1. Based on the fact that the SO2, O2 Content and HCO3- do not change with changes in temperature, it is argued that acid-base status does not change when temperature changes under the alpha-stat regulation mechanism. So if you make a change in ventilation or add CO2 to the CPB circuit, you are causing a respiratory acidosis (as dictated by the pH-stat method).
2. This is the method most adult centers use. Studies indicate that myocardial function and neurological function is better preserved when this strategy is employed.


OK, so what I don't understand is: if when you're cold and the solubility of gases is supposed to increase, wouldn't this mean that your PaCO2 should increase, why would the pH stat show a respiratory alkalosis? My chemistry is so far behind me that in order to understand properly, I can't get past this part.

Anesthesiologists still talk and care about this?

 

[pgg]

Anesthesiologists still talk and care about this?

It was on my written exam ...

At least we can quit talking about methoxyflurane and copper kettles.

 

[hrmm]

Miller 7th ed. p.1415

"(1)Heating a blood sample decreases pH, gas solubility, and Hb affinity for O2
and CO2. (2)Thus, as the blood from a hypothermic patient (say
35°C) is heated and analyzed at 37°C, more gas becomes dissolved
in solution and the measured Po2 and Pco2 will be higher than
at 35°C."

These 2 sentences above seem contradictory:
1. Heating decreases gas solubility
2. Heating a sample causes more gas to become dissolved in solution

Can anyone explain this?

 

[WashMe]

Miller 7th ed. p.1415

"(1)Heating a blood sample decreases pH, gas solubility, and Hb affinity for O2
and CO2. (2)Thus, as the blood from a hypothermic patient (say
35°C) is heated and analyzed at 37°C, more gas becomes dissolved
in solution and the measured Po2 and Pco2 will be higher than
at 35°C."

These 2 sentences above seem contradictory:
1. Heating decreases gas solubility
2. Heating a sample causes more gas to become dissolved in solution

Can anyone explain this?

Must be a typo, given that the 1st sentence says heating a sample decreases gas solubility. I imagine it is supposed to say "less gas becomes dissolved in solution". The gas that was previously dissolved contributes to the elevated Po2 and Pco2, because only undissolved gases generate partial pressures.

 

[itokama]

hrmm,

I was told to read about this recently... This article was helpful: http://www.clinchem.org/content/29/11/1877.full.pdf
Maybe it will be useful to you too...