A practical framework
An arterial blood gas is one of the most information dense tests in medicine, and one of the most misread. The trick is to interpret it the same way every time, in the same order, so the answer falls out instead of being guessed at.
Step 1: look at the pH
Start with the pH. Below 7.35 is acidemia. Above 7.45 is alkalemia. The pH tells you which side of normal you are on. It does not tell you why.
If the pH is normal and the rest of the numbers are abnormal, you are almost certainly looking at a mixed or compensated picture, and you will have to work harder.
Step 2: identify the primary disturbance
Now look at PaCO2 and bicarbonate together. PaCO2 is the respiratory component. Bicarbonate is the metabolic component.
A low pH with a high PaCO2 is a respiratory acidosis. The patient is not ventilating enough. Think opioid overdose, severe asthma or COPD exacerbation, neuromuscular weakness, a tiring patient on the ventilator.
A low pH with a low bicarbonate is a metabolic acidosis. Something is generating acid or losing base. Think lactic acidosis from shock or sepsis, diabetic ketoacidosis, renal failure, toxic ingestions, severe diarrhea.
A high pH with a low PaCO2 is a respiratory alkalosis. The patient is blowing off CO2. Think pain, anxiety, sepsis, pulmonary embolism, salicylate toxicity, an over ventilated patient.
A high pH with a high bicarbonate is a metabolic alkalosis. Often vomiting, nasogastric suction, diuretics, or volume contraction.
Step 3: check for compensation
The body tries to defend the pH. In a respiratory acidosis, the kidneys retain bicarbonate over hours to days. In a metabolic acidosis, the lungs blow off CO2 within minutes by increasing minute ventilation. Compensation never fully normalizes the pH. If the pH is completely normal in the face of a clear primary disturbance, suspect a mixed problem.
For a metabolic acidosis, Winter's formula gives the expected PaCO2: 1.5 times bicarbonate plus 8, plus or minus 2. A measured PaCO2 higher than expected means a concurrent respiratory acidosis. A measured PaCO2 lower than expected means a concurrent respiratory alkalosis.
Step 4: oxygenation tells a different story
PaO2 and the oxygenation status are a separate question from the acid base interpretation. Look at PaO2 in the context of the FiO2 the patient is on. A PaO2 of 80 on room air is fine. A PaO2 of 80 on 100 percent oxygen is a serious gas exchange problem and points to a high A a gradient, usually from shunt or significant V to Q mismatch.
The PaO2 to FiO2 ratio is a quick screen. Below 300 suggests acute lung injury. Below 200 meets criteria for moderate ARDS. Below 100 is severe.
Step 5: tie it back to the patient
The numbers are only useful in the context of the patient at the bedside. A pH of 7.20 in a young asthmatic who is finally tiring is an emergency. The same pH in a chronic CO2 retainer who walked in talking is the patient's baseline and does not need intubation.
Always finish an ABG interpretation by asking what is going to change because of these numbers. Adjust the ventilator. Treat the underlying problem. Recheck in an hour. If nothing is going to change, the test did not need to be drawn.
A worked example
A patient in septic shock has a pH of 7.18, PaCO2 of 22, bicarbonate of 8, PaO2 of 92 on 50 percent FiO2.
The pH is low, so this is an acidemia. The bicarbonate is the abnormal value driving it, so it is a metabolic acidosis. Expected PaCO2 by Winter's formula is 1.5 times 8 plus 8, which equals 20. The measured PaCO2 of 22 is right in range, so respiratory compensation is appropriate. The PaO2 to FiO2 ratio of 184 puts the patient in moderate ARDS territory.
The plan writes itself. Aggressive resuscitation, source control, lung protective ventilation, and a recheck after the next round of interventions.
ABG interpretation gets faster with reps. The framework does not change.