91 - 95 SBAs for the Final FRCA
Question 91
A 72 year old man is currently at 30°C on cardiopulmonary bypass for elective CABG.
You do an ABG using α-stat.
Which of the following best describes the main physiological advantage of α-stat ABG interpretation during hypothermic cardiopulmonary bypass?
- It increases cerebral blood flow and improves homogeneous cerebral cooling
- It maintains intracellular enzyme function by maintaining a constant pH at the patient's actual temperature
- It maintains autoregulation of cerebral blood flow by preserving electrochemical neutrality of histidine residues
- It reduces oxygen consumption by increasing carbon dioxide solubility
- It minimises the leftward shift of the oxyhaemoglobin dissociation curve during hypothermia
Answer
- It maintains autoregulation of cerebral blood flow by preserving electrochemical neutrality of histidine residues
What do you do if the patient is really cold and you want to do a blood gas analysis of their acid base balance?
There are two options for interpreting blood gases when it comes to temperature compensation:
- α-stat
- pH stat
α-stat:
You accept that pH and PaCO₂ will naturally change with changes in body temperature.
- As body temperature drops, pH naturally rises and PaCO₂ decreases as CO₂ becomes more soluble in water at lower temperatures
- Blood pH naturally rises by about 0.015 per °C
- You are not trying to "correct" these changes because they are a normal part of how the body adapts to cooling
- The blood gas sample is warmed to 37°C by the machine
- You interpret the results as if the patient were at 37°C, even if they are actually colder
pH-stat:
You aim to keep pH at 7.4 and PaCO₂ at 5.3 kPa, even when the patient’s temperature changes.
- You actively intervene to normalise these values as if the patient were at 37°C, regardless of their actual body temperature
- The blood gas is measured at the patient’s actual temperature, and not warmed to 37°C
- The results are interpreted in the context of what the reference values would be at 37°C, but adjustments (like adding CO₂) are made to bring the patient's pH back to 7.40
So for this question:
- If you're using pH stat interpretation, you would see a decent-going alkalosis and hypocarbia
- you would then add CO₂ to the bypass circuit to normalise the PaCO₂ value and bring the pH back down
This induces cerebral vasodilatation and increases blood flow and more even cooling of the brain (the first two options are therefore an effect of pH stat interpretation).
- For α-stat the machine just warms the sample up and analyses it at 37°C
- This causes the PaCO₂ and pH to correct automatically, so you don't see the same alkalosis in the results
By not adding CO₂ to the circuit, and accepting a hypothermic alkalosis, you maintain the dissociation state of imidazole (histidine) residues on intracellular proteins and maintain electrochemical neutrality (hence the third option is correct).
We think this means better preservation of cerebral autoregulation and coupling of cerebral blood flow to metabolic demand.
(The fourth option is an effect of the cooling, not the ABG interpretation method, and the fifth is going to happen at low temperatures regardless of how you interpret your ABG.)
It seems that α-stat is generally accepted as providing better autoregulation and reduces cerebral microembolism by avoiding excessive cerebral blood flow (preferred in adults). However pH-stat may provide more effective and even cooling and oxygen delivery (often preferred in children).