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

This chapter stretches the intepretation of the meaning of Section J2(ii) of the 2023 CICM Primary Syllabus, which asks the candidates to "describe the methods of measurement of pH in blood". The pH being affected by the temperature means that the measurement will be affected by the temperature of the sample, and specifically by whether you intend to measure the temperature of a sample that has been heated to some standard temperature, or that has been allowed to remain at whatever temperature it has drifted to. As the ensuing discussion will demonstrate, each option has its implications. 

In summary:

Scientific basis:

  • The lower the temperature, the higher the gas solubility.
  • The higher the solubility, the lower the partial pressure.  This is Henry's Law - dissolved gas and free gas are in a temperature-dependent equilibrium. (a great deal more detail on this matter is available in the chapter on partial pressure and the solubility of gases).
  • Warming a hypothermic blood sample to 37°C releases more gas, and the partial pressures will appear higher than they actually are in the hypothermic patient.

Alpha of imidazole residues 

  • Ratio of protonated to total imidazole residues among the protein molecules.
  • The pKa of imidazole at body temperature is about 6.8, which means at normal body fluid pH the alpha is about 0.55 (i.e. approximately half of the imidazole and histidine residues are protonated).
  • This  remains stable with temperature, even though the pH changes, because the pKa of imidazoline also changes with temperature.

Alpha stat approach

  • Warm (or correct) all samples to 37°C, no matter how cold the patient, and aim for a reheated pH of 7.40 and PaCO2 of 40 mmHg. 
  • Why?
    • We have no normal reference ranges for hypothermic pH PaO2 and PaCO2
    • Normal ranges at 37°C don't apply to hypothermic samples
    • Cellular physiology remains the same- 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 histidine residues) is also temperature dependent, and changes in parallel with body temperature. (Some considerable detail is offered on the physics and chemistry underlying these issues in the chapter on neutrality and the influence of temperature and pressure on pH.)

pH stat approach:

  • Interpet all the ABG samples at their current temperature, using normal reference ranges
  • This makes all the samples appear alkalotic
  • You are then tempted to hypoventilate or add CO2.
  • Why?
    • The addition of CO2 counteracts the increased solubility and decreased partial pressure of CO2 at low temperature
    • The added CO2 counteracts the hypothermic leftward shift of the oxygen dissociation curve, resulting in better oxygen delivery
    • Increased CO2 improved cerebral blood flow by vasodilating the cerebral vessels.


Kerry Brandis' Chapter 1.6 - Alpha-Stat Hypothesis should be the first port of call for the exam candidate who wishes to revise this with the absolute minimum of pointless bullshit. A similarly strong recommendation can be made for the LITFL chapter dealing with this subject, as it must surely represent some kind of leanest most minimalist approach, with all the fat trimmed and only the absolute essentials left behind. 

What is this "alpha"

The ‘Alpha’ is the degree of dissociation of imidazole, a little ring-shaped molecule found on the edges of many organic molecules, in particular of the amino acid histidine.  Specifically the alpha is a ratio of protonated to total imidazole residues among the protein molecules. The pKa of imidazole at body temperature is about 6.8, which means at normal body fluid pH the alpha is about 0.55 (i.e. approximately half of the imidazole and histidine residues are protonated). At a much more alkaline pH  the alpha is closer to 1, i.e. all the imidazole molecules are protonated; and at an acidic pH none of them are protonated and the alpha is closer to 0.

Why do we (the cellular "we") care about any of this? Well. Alpha, as described here, was first defined by Reeves and Wilson in 1969, when they described the acid-base balance of extracellular fluid among amphibians (specifically bullfrogs). Amphibians and reptiles are ectotherms (i.e. dependent on ambient temperature) and therefore represent an open system in which pH and pCO2 are not regulated but rather determined basically by the weather. As temperature rises, pH decreases and PaCO2 increases. This could end up with some wild values; pCO2 ends up being rather low at low temperatures, and the pH ends up rather high. For example in a widely quoted series of experiments by Howell et al (1970), the extracellular fluid pH of a frog at 5 C° ends up being 8.08, and their pCO2 ends up being about 5mmHg. 

This might seem bizarre, but these values are fine and normal, and the frog is generally happy like this, to the extent that they can manifest happiness.  Cellular function remains uninterrupted. This is because the definition of neutrality (going back to Arrhenius) 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. And 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°). It holds up experimentally - for example Jackson et al (1971) used this amazing apparatus to measure the minute ventilation of exercising freshwater turles and concluded that they regulate their ventilation according to the temperature, to maintain whatever (incredibly low) PaCO2 was best suited to sustain a stable protein charge state of blood proteins, which is in turn mostly a function of imidazole dissociation. In short, these organisms defend the alpha. 

What is this "pHstat"

The reader, understandably enraged by the paragraphs above, will immediately object that they are in fact a a homeothermic organism (i.e. capable of thermoregulation) and that frog or turtle studies should not be applied to their physiology. Indeed, those organisms that can control their temperature tend to be "pHstat" regulators. The "stat" in "alphastat" and "pHstat" actually means nothing specific, as "pHstat" is actually the name of a piece of chemistry equipment that automatically titrates a solution to maintain a stable pH during an experiment. The term is therefore perfectly applicable to warm-blooded organisms which regulate other things (eg. ventilation and renal tubular function) to maintain a stable pH in the face of changing conditions.

Effects of temperature on gas solubility

The lower the temperature, the lower the corrected pCO2Solubility 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 haemoglobin 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.

Which regulatory technique is optimal?

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.

And so, as the temperature falls and the pH increases, one has two options. One may continue to tolerate the higher pH and lower CO2, pointing to the fact that the  alpha remains stable, like the frog or turtle. Or one may act as a pHstat device, trying to fight against the pH change by increasing their CO2 and targeting a normal pH (7.40). Which approach is superior?

In general, there is no agreement as to the exact rationale for this decision, but the majority of cardiac anaesthetists tend to use alphastat, and the following justifications are offered in support of this choice:

  • Correcting pH for temperature makes no physiological sense, as the pH of neutrality also changes with temperature, as mentioned above.
  • Corrrecting pCO2 for temperature can lead to misinterpretation. 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). We have no reference ranges for weird temperatures - blood gases are usually measured at 37°, which is usually close enough to our patients actual temperature. But 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.
  • Extra CO2 added to the circuit can produce unpleasant effects. Specifically, crebral blood flow can increase. That may sound like a positive step but in fact this also means the increased flow of microemboli.  
  • Oxygen delivery does not need to be corrected for the temperature. 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.

Thus: rewarm your sample, and then read the numbers as if the patient had a normal temperature. You don't know the value of the PaCO2 at the patient's actual temperature, and you don;t want to know. Trust that the alpha knows what its doing. This is the essence of the alpha stat approach.

References

Ashwood, E. R., G. Kost, and M. Kenny. "Temperature correction of blood-gas and pH measurements." Clinical chemistry 29.11 (1983): 1877-1885.

Bacher, Andreas. "Effects of body temperature on blood gases." Applied Physiology in Intensive Care Medicine. Springer Berlin Heidelberg, 2006. 33-36.

Bradley, A. F., M. Stupfel, and J. W. Severinghaus. "Effect of temperature on PCO2 and PO2 of blood in vitro." Journal of applied physiology 9.2 (1956): 201-204.

Davis, Michael D., et al. "AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry: 2013." Respiratory care 58.10 (2013): 1694-1703.

Device-specific information in all these ABG pages refers to the ABG machine used in my home unit.

Other machines may have different reference ranges and different symbols.

For my ABG analyser, one can examine this handy operations manual.

There is also an even more handy reference manual, but one needs to be an owner of this equipment before one can get hold of it. Its called the "989-963I ABL800 Reference Manual" and can occasionally be browsed accidentally.

Covington, Arthur K., R. G. Bates, and R. A. Durst. "Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)." Pure and Applied Chemistry 57.3 (1985): 531-542.

Reed, Christopher A. "Myths about the Proton. The Nature of H+ in Condensed Media." Accounts of chemical research 46.11 (2013): 2567-2575.

Palascak, Matthew W., and George C. Shields. "Accurate experimental values for the free energies of hydration of H+, OH-, and H3O+." The Journal of Physical Chemistry A 108.16 (2004): 3692-3694.

Franks, Felix. Water: a matrix of life. Vol. 22. Royal Society of Chemistry, 2000.

Yagasaki, Takuma, et al. "A theoretical study on anomalous temperature dependence of pKw of water." The Journal of chemical physics 122.14 (2005): 144504.

Ohtaki, Hitoshi. "Effects of temperature and pressure on hydrogen bonds in water and in formamide." Journal of molecular liquids 103 (2003): 3-13.

Bandura, Andrei V., and Serguei N. Lvov. "The ionization constant of water over wide ranges of temperature and density." Journal of Physical and Chemical Reference Data 35.1 (2006): 15-30.

Moon, Richard B., and John H. Richards. "Determination of intracellular pH by 31P magnetic resonance." Journal of Biological Chemistry 248.20 (1973): 7276-7278.

Roos, Albert, and Walter F. Boron. Intracellular pH. American Physiological Society, 1981.'

Davis, Michael D., et al. "AARC clinical practice guideline: blood gas analysis and hemoximetry: 2013." Respiratory care 58.10 (2013): 1694-1703.

Shapiro B.  Temperature correction of blood gas values. Respir Clin N Am 1995;1(1):69–76

Reeves, R. B., and T. L. Wilson. "Intracellular pH in bullfrog striated and cardiac muscle as a function of body temperature." Fed. Proc.. Vol. 28. 1969.

Jackson, D. C. "The effect of temperature on ventilation in the turtle, Pseudemys scripta elegans." Respiration Physiology 12.2 (1971): 131-140.

Howell, B. J., et al. "Acid-base balance in cold-blooded vertebrates as a function of body temperature." American Journal of Physiology-Legacy Content 218.2 (1970): 600-606.

Davis, Michael D., et al. "AARC clinical practice guideline: blood gas analysis and hemoximetry: 2013." Respiratory care 58.10 (2013): 1694-1703.