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.
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.
Metabolic Acidosis – A low HCO3− is metabolic acidosis. The pH could be normal. This is not acidemia. This only means that the amount of metabolic acid in the system is low.
If we are making a medical diagnosis we should always include the reason such as metabolic acidosis due to lactic acidosis or metabolic acidosis due to diabetic ketoacidosis.
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. A diagnosis might be metabolic alkalosis due to chronic CO2 retention.
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. We know only that there is retention of CO2 as ventilation is not removing CO2 to a normal level.
A diagnosis might be respiratory acidosis due to narcotic induced respiratory depression.
Respiratory alkalosis – A low arterial PaCO2 is respiratory alkalosis. The pH could be normal. This is not alkalemia. This only means that the PaCO2 is low. This is not a billing diagnosis as it is not necessarily the primary abnormality. A diagnosis might be respiratory alkalosis due to psychogenic hyperventilation.