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Wednesday, October 6, 2010

ABG temperature correction

I was asked to do some research to determine if and when we should enter the correct temperature of the patient on hypothermic or hyperthermic patients. My research brought me to a great article written by Wesley Granger in Focus: Journal for Respiratory Care & Sleep Medicine, titled, "ABG temperature correction: to correct or not to correct; that is the question."

It appears he had the same interest as myself, and he did the research.

Here's the basics:

Research that goes back to the 1960s confirms that as the patient's temperature changes and the temperature in the ABG machine is not corrected:

1. The pH increases
2. The CO2 decreases
3. The PO2 decreases

The mechanism of the changes is to maintain a normal pH of 7.40, which, if you do not enter the corrected temperature into the ABG machine, you will note the pH, CO2 and PO2 will result as normal.

However, if you correct the temperature, the pH, CO2 and PO2 will change.

Should the temperature be corrected?

While it is known that the above values will change as the temperature is corrected in the ABG machine, most experts have trouble deciding on what are the normal ranges of pH, CO2 and PO2 at corrected temperatures.

Therefore, the general consensus is that, as Granger notes:

"We cannot use the traditional 'normal' ranges except at 37 degrees Celsius. Therefore, several articles I reviewed recommend that assessment of acid-base and oxygenation status be carried out on non-corrected (37 degrees Celsius) ABG values regardless of the patient's actual temperature. The 37 (degrees) Celsius ABG results will show if and what kind of acid-base disorder is present and the assessment is conducted in the same manner using the well recognized 'normal' values.
There are three exceptions. First, he notes, is when a patient is being intentionally cooled during surgery, in which case the physician will want to know the corrected pH because the goal is to maintain a normal pH.

The second exception is that if you want to calculate oxygen tension based on the A-a Gradient, PAO2/ PaO2 ratio or PaO2/FiO2 ratio (for a refresher on these formulas, click here), you will have to use the corrected ABG results.

The third exception is if you want to correlate the end tidal CO2 results and pulse oximeter results with the ABG. In this case, you will want to use the corrected ABG results.

So, as you can see, for some patients you may actually need to run both a corrected and a non corrected ABG.

But, for the most part, and for most situations, most experts do not at this time recommend that you enter the patient's correct temperature into the ABG machine, and run all ABG at the normal 37 degrees. This is recommended so you can use your normal ABG ranges to make therapeutic decisions.

I imagine, though, that once scientists are able to determine normal ranges at the corrected temperatures, new recommendations will be determined as to whether or not to correct.

So if your hospital is seeking wisdom on whether or not to enter the patient's correct temperature into the ABG machine, now you have the latest wisdom.

1 comment:

Anonymous said...

If venous blood tends to be higher in temp than arterial, and PH goes up with temp- Would it be possible that uncorrected VBG’s could be running on the high end in PH?