When do CO2 levels impact K?

This question was emailed to me a little while ago. Its a tough one, but a good one!

How are CO2 and K related?

According to this article, blood hydrogen concentration is one of the regulatory mechanisms that regulates how much potassium is inside and outside the cells. Remember that only ~2% of our bodies potassium is outside the cell (or in the blood, which is the part we measure). The other ~98% is inside the cell.

For every 0.1 unit decrease in blood pH there is 0.6mEq/L increase in serum potassium concentration. On the flip side, for every 0.1 unit increase in blood pH there is a 1mEq/L decrease in serum potassium concentration.

In the outpatient kidney setting, we use CO2 as a marker of acidosis. And, it is recognized that a low CO2 level may be the cause of hyperkalemia. In these instances, it is recommended to treat the acidosis rather than focus on low potassium diet restrictions.

But how do we translate CO2 levels into a pH level?

The easy answer, we can’t.

While we know that low CO2 levels can be indicative of acidosis (lower blood pH than normal), unfortunately CO2 levels alone can’t be used to figure out the pH level of the blood.

According to this article, acid base balance in our bodies involve back and forth reactions between bicarbonate, carbonic acid, H2O and CO2. So CO2 is considered a “first-clue to an acid-base disturbances”, but isn’t sufficient on it’s own to enable us to know the exact pH of the blood.

What other factors impact CO2 levels?

That same article highlights that CO2 levels is sensitive to two things.

1. CO2 from blood off gases at a rate of 6mEq/L per hour. This means if the sample sits too long the CO2 level won’t be accurate.
2. CO2 levels can also be impacted by volume status. Volume overload can lead to decreased levels of CO2 and dehydration can lead to lower levels of CO2 levels.

So back to the original question – how low does CO2 need to be before potassium levels are impacted?

Easy answer, I don’t know. While there is a very clear link between pH and serum K levels, CO2 is too fuzzy of an acidosis marker.

In order to determine pH we need both the CO2 levels and the arterial carbon dioxide partial pressure. A reading we seldom have in outpatient clinics.

My tips for interpretation

When the K is high and the CO2 levels are low, your patient may have some level of acidosis. This could mean there is a shift of potassium extra-cellularly.

I start to consider this anytime I see a CO2 value outside the lab reference range. For mild hyperkalemia (K less than 5.5mmol/L), in many cases correction of acidosis will be my most important intervention.

If K is above 5.5mmol/L, I would be more likely to ensure acidosis is being corrected and ensure a patient is avoiding highly bioavailable potassium food sources.