This is a relatively “small-print” area of physiology relevant to the cardiac anaesthetist but requires a great deal of basic science understanding to fully appreciate. Concepts discussed on this page include:

• “Alpha” - what it is and why it’s important
• Henry’s Law
• Charles’ Law
• Oxyhaemoglobin dissociation curve shifts at different pH and temperatures
• Histidine
• Imidazole rings
• Buffalo curves
• Rosenthal correction factor

Summary

• Alpha-stat and pH-stat are two different ways of analysing blood gas samples in hypothermic patients.
• Overall this concept becomes more relevant at colder temperatures e.g. DHCA and < 30 degrees [1]
• Alpha-stat is recommended as the most straightforward and practical approach for adult cardiac surgery [2]
• Evidence overall is inconclusive and there are minimal “patient-centered” studies to draw conclusions from. The debate as to which method is best still rumbles on.

Key Considerations:

Solubility of a gas in solution depends the following factors

1. Henry’s Law = Solubility of a gas is proportional to the partial pressure of gas above the liquid interface (at a constant temperature). Higher partial pressure results in increased solubility of the gas in the liquid, this concept explains why fizzy drinks “hiss” when opened as the dissolved gas escapes. This relationship is true with a “fixed constant” for a given temperature.

2. Charles’ Law = Increased temperature results in gas molecules having additional kinetic energy. This energy makes them more ready to escape from solution and explains a fall in solubility as temperature increases. Changes in temperature alter the “fixed constant” in the Henry’s Law equation (above)

Other considerations for cardiac surgery:

1. Oxy-haemoglobin dissociation curve (OHDC)
• Left shift at lower temperatures = higher affinity of Hb for oxygen with O2 being more difficult to offload
• Right shift at higher temperatures = Less affinity of Hb for O2 and increased O2 offloading
2. Cerebral vasodilation
• Increasing PaCO2 causes cerebral vasodilatation. This can in theory increase embolic load in adults
• In paediatrics some evidence suggests a benefit to cerebral vasodilatation in DHCA with reduced post-operative seizures and neurological deficits seen in one study.
3. Decreased metabolic rate
• Low temperatures decrease enzyme activity and metabolic rate
• This results in reduced CO2 production together with less O2 extraction

Physiology

Normal plasma pH = 7.4, normal intracellular pH = 6.8 (At 37 degrees)

This gradient of 0.6 is maintained across all temperature ranges due to buffering compounds, predominantly found in proteins. Across temperature ranges the intracellular pH remains at pH neutral (PN) to maintain protein (especially enzyme) function.

Protein function is largely determined by the degree of molecular ionisation; maintaining the degree of ionisation within a narrow band (at varying temperatures) is achieved by intracellular buffers. pH can be “adjusted” by the buffering system to compensate for temperature changes and therefore maintain enzyme function and therefore cell volume status by maintaining fluxes of ions across the cell membrane.

In the search for a protein which had a pKa value close to 7 and a dissociation constant “alpha” close to 50:50 at physiological pH the pioneering investigators of the alpha-stat hypothesis discovered the imidazole “residues” (rings) attached to histidine moeties of intracellular proteins. At 37 degrees serum pH is normally 7.4, intracellular pH is 6.8 and the alpha is 0.55 of the imidazole rings meaning approximately half of the residues are protonated.

Histidine

• An essential amino acid
• Imidazole residues are incorporated into the histidine molecule as “side-chains”
• The imidazole side-chains play an important role in intracellular buffering of pH changes. (Works at pH ranges of 6.2-7.8)

As a side note, Imidazole Rings found in drugs include:

1. Midazolam
2. Etomidate - R+ enatiomer binds to GABA-A depressing the RAS
3. Antifungal “azole” drugs by inhibiting ergosterol biosynthesis (a key component of fungal membranes)
4. Theophylline (and caffeine!)
5. Metronidazole
6. Losartan - first A2RB with imidazole group in part of the drug structure

Imidazole Ring

(CH)3(NH)N

Imidazole as a term was first coined in 1887 by German scientist Rudolf Hanstzsch

A highly polar compound with an “electron-rich” molecular structure allowing it to easily bind to other organic and non-organic compoounds.

Exists as a 5-sided planar ring structure which can be readily altered in drug design

Exhibits “aromaticity” meaning the ring is not readily broken apart

Exhibits amphotericism i.e. it can exist as either and acid or a base making it an ideal buffer with buffering capability from pH 6.3-7.8 at 25 degrees C

pH changes with Temperature

Measured arterial pH changes with temperature in an almost linear manner [3,4] This can be described on a graph as a “Buffalo curve” referencing the University where it’s creators hailed from (see below)

• At higher temperatures more dissociation occurs with a given acid; HA meaning more H+ and A- will exist at a higher temperatures
• Cooler temperatures result in a trend towards HA with less H+ and A- (and so less protons) Less H+ dissociated at lower temperatures make “cool” blood gas samples more alkalotic.
• Cooler temperatures also result in more CO2 dissolved in solution so measured PaCO2 becomes lower (see Henry’s Law above)

Alpha-Stat Approach

Alpha-stat [4] involves taking the ABG and then measuring the sample at 37 degrees, regardless of what the temperature the patient’s original sample was taken at. Alpha refers to the ratio of protonated vs unprotonated histidine residues on the imidazole residues of proteins within the cell (described in detail above)

Potential Benefits

• More straightforward to analyse and doesn’t require a correction factor
• Maintains normal transmembrane pH gradients
• Considered to maintain cerebral auto-regulation which may be decoupled with a pH stat approach.
• Alkaline pH may improve resilience to the ischaemia / reperfusion insult that occurs during DHCA.

Potential Risks

• Cerebral vasoconstriction (PaCO2 levels are allowed to fall)
• Reduced SjVO2 values
• Less efficient and homogenous cooling due to vasoconstriction

pH -Stat

The pH-stat approach involves correcting the “alkaline drift” which normally occurs with hypothermia, especially at very low values < 30 degrees. The sample is “measured” at the patient’s current temperature e.g. 22 degrees for DHCA. In reality this method involves a correction factor performed by the blood gas analyser using the Rosenthal correction factor (pH changes by 0.015 units per degree celsius)

• CO2 and oxygen are more soluble at lower temperatures so PaCO2 and PaO2 are both reported as lower making the sample appear hypoxic with a respiratory alkalosis
• This “abnormal” result is then “normalised” by either reducing the sweep speed of the blender or (more rarely) by adding CO2 to the oxygenator

Potential Benefits

• Improved cerebral vasodilation = potentially improved oxygenation (Measurably higher Sjvo2 levels have been demonstrated)
• Improved, homogenous cooling due to systemic vasodilation
• Improved oxygen delivery - counteracts the left-shift of the OHDC seen with hypothermia which could improve myocardial oxygenation

Potential Risks

• More complex with a conversion factor being required (although the machine does calculate it for you)
• Dangers may come when extra CO2 is added to "correct" for the alkalosis, this causes cerebral vasodilation and potentially worse neurological outcomes in a potentially emobli-rich environment.
• Altering CO2 content disrupts the intra-cellular to intra-vascular pH gradient, potentially leading to impaired protein and enzyme function
• Uncoupling of cerebral autoregulation due to CO2-mediated vasodilation

Summary:

Evidence

The evidence for which method to use during DHCA is very mixed and certainly no clear guideline exists to adhere to.

Overall a meta-analysis looking at 16 studies relating to the topic from Glenfield (UK) in 2009 suggested in their conclusion that the method used should be determined by the age of the patient. Paediatric patients likely do better with a pH-stat approach whereas adult patients likely do better using an alpha-stat approach. [6]

References

1. Circulation 2002. Sep 24;106(12 Suppl 1):I103-8 https://pubmed.ncbi.nlm.nih.gov/12354717/
2. Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC clinical practice guideline: blood gas analysis and hemoximetry: 2013. Respir Care. 2013 Oct;58(10):1694-703
3. E R Ashwood, G Kost, M Kenny, Temperature correction of blood-gas and pH measurements., Clinical Chemistry, Volume 29, Issue 11, 1 November 1983
4. An imidazole alphastat hypothesis for vertebrate acid-base regulation: Tissue carbon dioxide content and body temperature in bullfrogs
5. Deranged physiology - https://derangedphysiology.com/main/cicm-primary-exam/acid-base-physiology/Chapter-115/alpha-stat-and-ph-stat-models-blood-gas-interpretation
6. Khairul Anuar Abdul Aziz, Ayo Meduoye, Is pH-stat or alpha-stat the best technique to follow in patients undergoing deep hypothermic circulatory arrest?, Interactive CardioVascular and Thoracic Surgery, Volume 10, Issue 2, February 2010, Pages 271–282