Analyzing blood gasses is straightforward in the new paradigm of acid base physiology.
Assume that the patient is there in front of you and you have the clinical information not just blood gas data. This is always the case in clinical medicine. The artificial situation of analyzing blood gasses without the clinical setting as in a classroom is a different situation.
If you want to analyze blood gasses in the classical way, acidosis is pH etc. with only the blood gas and no clinical information, the first thing to do is start banging your head on a nearby wall. You may think that you understand it but eventually things won’t make sense and you will need a wall to bang your head into. Get used to it.
PH – A low pH is acidemia, a high pH is alkalemia. This is not the same as acidosis or alkalosis which are processes not the state of the blood or body fluids.
Metabolic acidosis – A low HCO3− is metabolic acidosis. The pH could be normal. This is not acidemia. Metabolic acidosis is not a diagnosis, not necessarily the primary process affecting pH and not necessarily abnormal. We only know that the amount of metabolic acid in the system is low.
Metabolic alkalosis – A high HCO3- is metabolic alkalosis. The pH could be normal. This is not alkalemia. Metabolic alkalosis is not necessarily an abnormal condition. It could be an adaptive response to a high PaCO2 as is commonly the case in COPD. We only know that the amount of metabolic acid in the system is high.
Respiratory acidosis – A high arterial PaCO2 is respiratory acidosis. The pH could be normal. This is not acidemia. This is not necessarily the primary process and not necessarily abnormal. Although it would be a very rare occurrence, a high arterial PaCO2 could be a normal adaptive response to a low HCO3-
If the PaCO2 is higher than normal the conclusion is that respiratory acidosis is present without making any calculations or examining other values. The value alone determines respiratory acidosis or alkalosis.
Respiratory alkalosis – A low arterial PaCO2 is respiratory alkalosis. The pH could be normal. This is not alkalemia. Not necessarily abnormal, not a billing diagnosis.