Why get arterial blood gases?

1

Is breathing really overrated?

It may be. A Japanese yoga master survived just fine breathing once per minute for 1 hour (see reference ). But, when medical students and residents prioritize activities that they really enjoy, year after year, “breathing” consistently ranks very high.

2

Mr. O’Flaherty has just undergone an inguinal herniorrhaphy under local anesthesia. The recovery room nurse asks permission to sedate him. She says that he is confused and unruly and keeps trying to get out of bed. Is it safe to sedate Mr. O’Flaherty?

No. A confused, agitated patient in the recovery room or surgical intensive care unit (SICU) must be recognized as acutely hypoxemic until proved otherwise. This is really important!

3

Mr. O’Flaherty is moved to the SICU, and at 2:00 a.m. the SICU nurse calls to report that he has a partial pressure of oxygen (pO ₂ ) of 148 mm Hg on facemask oxygen. Is it okay to roll over and go back to sleep?

No. More information is needed.

4

You glance at the abandoned cup of coffee sitting on your well-worn copy of Surgical Secrets . What is the pO ₂ of that cup of coffee?

It is 148 mm Hg.

5

How can Mr. O’Flaherty and the coffee have the same pO ₂ ?

The abandoned coffee presumably has had time to equilibrate completely with atmospheric gas. At sea level, the barometric pressure is 760 mm Hg. To obtain the pO ₂ in the coffee, subtract water vapor pressure (47 mm Hg) and multiply by the concentration of oxygen (20.8%) in the atmosphere:

• pO ₂ = (760 − 47) × 20.8% = 148 mm Hg

6

What is the difference between Mr. O’Flaherty’s and the coffee’s pO ₂ ?

Nothing. Both represent the partial pressure of oxygen in fluid. A complete set of blood gases is necessary.

7

What constitutes a complete set of blood gases?

• pO ₂

• pCO ₂

• pH

• O ₂ Sat

• Hgb concentration