Hi! This helped me understand it a little better...
CO2 = acid, makes things acidic
HCO3 = base, makes things alkalotic
Ok always look at the pH first...
pH<7.35 = acidosis
pH>7.45 = alkalosis
Then, if the CO2 is high or low, then it is respiratory...If the HCO3 is high or low then it is metabolic. How you remember that is that the respiratory system is involved with CO2 (blowing air off or slowing RR), and the kidneys (metabolic) are involved with HCO3 (excreting or not excreting).
Example (Here's how you think thru it):
pH = 7.25 CO2 = 40 HCO3 = 17
Ok, first, the pH is low so think acidosis. CO2 is WNL. HCO3 is low. Draw arrows if it helps. The abnormal values are both low (think Equal). Metabolic imbalances are equal. So, this must be metabolic acidosis!
Now, for compensation...
If you have a metabolic imbalance, the respiratory system is going to try to compensate. Respiratory = CO2. If the CO2 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will decrease the CO2 because you want to get rid of the acid (CO2). In alkalosis, it will increase because you want to add more acid (CO2)
If you have a respiratory imbalance, the kidneys will try to compensate. Kidneys = HCO3. If the HCO3 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will increase HCO3 because you want to hold on to the base to make it more alkalotic. In alkalosis, it will decrease because you want to excrete the base to make it more acidic.
I know this can be confusing just reading it but it has always helped me to look at it on paper and draw little up/down arrows and rationalize the changes. Work several ABGs for practice. Hope this helps and good luck!