The Triple-Barreled Shotgun
What is the role of arterial blood gas (ABG) in a patient with undifferentiated cardiopulmonary failure?
The interpretation of an ABG depends largely on the diagnosis that the patient has. For example, a normal ABG in an asthmatic patient would be highly concerning. Attempting to interpret an ABG in a patient with unknown diagnosis is difficult. When accepting patients in transfer from outside hospitals, I sometimes receive calls where the transferring clinician will quickly launch into a recitation of a series of ABGs. Without context this is inscrutable. On occasion I have requested to video-conference with the patient via i-phone or Skype, which is invariably more helpful.
Unfortunately, the concept that we should order an ABG on a crashing patient remains dogmatically engrained in much of medical culture. There is usually one member of the ICU team who will indignantly demand to know the ABG values. There seems to be a belief that the ABG reveals some sort of divine truth about the patient, if we could only be smart enough to decrypt its meaning.
- Most crashing patients are suffering from some form of cardiopulmonary failure (cardiac failure, pulmonary failure, or both). When in doubt it is safest to maintain a broad differential and evaluate both organs simultaneously.
- EKG, CXR, and POCUS can each reveal a wealth of information immediately at the bedside. For a patient with undifferentiated cardiopulmonary failure, immediately obtaining all three tests may be a safe approach to avoid missing anything.
- EKG, CXR, and POCUS should not be thought of as competitive with one another, but rather as synergistic and complementary. Integrating data from all three studies at the bedside is a very powerful approach.
- ABG may be useful to answer a specific clinical question. However, when applied without a specific indication to a patient with an unknown diagnosis, it usually adds little useful information.
- The diagnostic power of ABG is minimal compared to studies such as POCUS. There is no evidence supporting the use of ABG for a patient with undifferentiated respiratory failure, compared to impressive evidence supporting POCUS (e.g. Lichtenstein's Blue Protocol). A diagnostic approach to cardiopulmonary failure incorporating ABG without POCUS is outdated.