Alpha-stat and pH-stat models of blood gas interpretation

The alpha-stat hypothesis suggests we always interpret our blood gases as corrected to the same temperature (normal body temperature) irrespective of what the body temperature actually is. The pH-stat hypothesis instead recommends that we always correct the temperature to the core body temperature. Each approach has its merits and demerits.  In order to maintain one's appearance as an intelligent interpreter of blood gas data, one should decide on which approach to use, and come up with some well-articulated arguments as to why one is using it.

There are several eminent resources for this subject matter:

• Kerry Brandis' Chapter 1.6 - Alpha-Stat Hypothesis should be the first port of call.

The "Alpha" and it's meaning

The ‘Alpha’ is the ratio of protonated to total imidazole of histidine residues among the protein molecules. It is expressed as a ratio or percentage. It At 37°C, at the normal intracellular pH of 6.8, alpha is approximately 0.55; that is to say approximately half of the imidazole and histidine residues are protonated. T.J. Morgan from Oh's Manual reports to us that this 0.55 alpha value is optimal for intracellular enzyme structure and function, and that maintaining this optimal value should be a goal of therapy.

Rationale for the routine application of the alpha-stat approach

Correcting pH for temperature makes no physiological sense, as the pH of neutrality also changes with temperature.

pH changes with temperature, and this relationship is almost linear. The linked article addresses this concept with diligence, brevity and a heaping serving of complicated maths. For the calculus-averse, Kerry Brandis has gone through this with patience and passion. For those who do not appreciate brief and lucid explanations, a rambling series of digressions is also available as a part of the ABG Interpretation chapter:

Briefly, the reason for the change in pH associated with changes in temperature is explained by the fact that dissociation is an endothermic reaction - i.e. for a given acid HA, with more energy available in the system more HA molecules will dissociate into their components, H⁺ and A^-. The balance of HA and its dissociation products will therefore trend left (towards HA) if energy is subtracted from the system, eg. by a big cooling blanket. With less H⁺ activity in the system, the pH will increase (because it is a negative logarithm of H⁺ activity). Thus, the apparent alkalosis of hypothermia is observed.

Now, blood gases are measured at 37°, which is usually close enough to our patients actual temperature. However, if our patient is not normothermic, we have the option of correcting the ABG results for their actual body temperature. This will give very different results.

Say you correct the pH for 20° or 30°, whatever the patients temperature is. You get a number. But you have no normal reference ranges for pH at that temperature - only for 37°. If you start using the normal values for pH and pCO₂ at 37° to interpret the gas at 20°, you will run into all sorts of trouble. For one, the patient will appear to have a respiratory alkalosis, and you will be tempted to play with the ventilator.

But in actual fact, at all temperatures intracellular pH remains at pN- the normal pH of neutrality, required for cellular function. The reason for this is that protein buffering of intracellular pH (via imidazole residues on histidine moeties ) is also temperature dependent, and changes in parallel with body temperature. Not only that, but as intracellular pH increases (obeying the linear relationship) so must the extracellular pH increase, in order to maintain the normal pH gradient (from pH 7.4 to pH 6.8, at 37°). Thus, if the cellular machinery is already functioning under ideal conditions for that temperature, the introduction of a respiratory acidosis (by changing ventilator settings) will result in a deterioration of this function, by decreasing the transmembrane pH gradient.

Lastly, and most importantly, behold the definition of neutrality (going back to Arrhenius)- it is not "a pH of 7.0", but rather the presence of equal numbers of H⁺ and OH^- ions. Because temperature has an equal effects on the concentration of each of them, neutrality is preserved no matter the temperature.

The lower the temperature, the lower the corrected pCO₂.

Solubility of CO₂ is increased in hypothermia; it is a temperature-dependent property. The PCO₂ of your hypothermic patient, if interpreted at the actual body temperature, will appear abnormally low (even if you already expect it to be low, given the decreased metabolic rate).

The lower the temperature, the lower the corrected PaO₂.
But:
Changes in temperature do not significantly alter the oxygen content of blood.

Solubility of O₂ is also increased in hypothermia. In fact, total blood oxygen content is increased in hypothermia. Now, the solubility of oxygen in water increases only by 100% from 37° to 0°, but the affinity of hemoglobin for O₂ increases by a whopping factor of 22. The hemoglobin hungrily absorbs the dissolved oxygen, decreasing the PO₂ - thus increasing the concentration gradient and enticing more O₂ into the blood from the alveolar gas. (this doesn't mean that the oxygen-carrying capacity of the blood is dramatically increase, of course- there is still 1.37ml of oxygen per 1g of hemoglobin).

Now, you put this cooled sample into the blood gas machine, and the electrode will measure its PO₂ at 37°. The increase in temperature at the electrode will force the hemoglobin to give up a lot of its oxygen, and some of the dissolved oxygen will also come out of solution. The result is a slightly raised PaO₂.

However, this is not a realistic way of looking at the sample. Who cares what the available oxygen content of blood is at 37°? At a low temperature, all that oxygen is trapped in high-affinity bonds with hemoglobin, and is unavailable to the mitochondria.

How much oxygen is available to these cold mitochondria? There are numerous complex formulae to arrive at the corrected PO₂ value. However, a simple method is to subtract 5mmHg for every 1°C from the measured PaO₂ - thus, at 32° with a measured PaO₂ of 100mmHg the actual PaO2 is 70mmHg.

So what do you do? Does PaO₂ need to be corrected at the bedside?

In short, no.
Ashwood and colleagues present us with an excellent table (see Table 5 in their article) which demonstrates that for any given hemoglobin saturation, oxygen content of blood (in mls of gas per ml of blood) is not significantly altered, even though the calculation of PaO2 gives vastly different values.

For example, at 95% SaO₂ the hypothermic patient (at 22°) would have a PaO2 of 95mmHg when measured at 37°, and 27mmHg when measured at 22° - that difference is massive. But the calculated oxygen content of blood varies very little - from 0.1920 to 0.1933 mls/ml.

Thus, there is little point in correcting PaO₂for temperature. The American Association for Respiratory Care recommended against the routine correction of blood gas samples for temperature, quoting the analysis by Shapiro (1995) as their main reference for this recommendation.