Acid-base disturbances: Pathology review

Two people came into the Emergency Department one day. The first one is 33 year old Muriel who came in with abdominal pain, a severe headache and hyperventilation. One of Muriel’s friends said that she caught her drinking antifreeze. The other one is 35 year old Eustace who came in with confusion and hypoventilation. Eustace also has duodenal ulcers, for which he has been taking antacids. Among other tests, an ABG was done for both individuals. The results showed that Muriel had low pH, along with low levels of bicarbonate and low levels of pCO2, while Eustace had high pH, along with high levels of bicarbonate and high levels of pCO2.

Okay, based on lab results, both individuals seem to have acid-base disturbances. Now, let’s go back to the basics for a bit. So, in plasma you can find carbon dioxide or CO2 and water or H2O. They are constantly mixing together in order to make bicarbonate ion or HCO3− and hydrogen ion or H+. Similarly, HCO3− and H+ can form CO2 and H2O.

Now, HCO3 − is mostly regulated by the kidneys and metabolism, while CO2 is regulated by the lungs. The blood pH which corresponds to the hydrogen ion concentration needs to stay in a very narrow range, between 7.37 and 7.42. Basically, the more hydrogen ions, the more acidic the blood is and the lower the pH. Less hydrogen ions means the blood is more alkaline, and the higher the pH. So, let’s say that HCO3− levels decrease for some reason. In this case, the equation shifts to the right and more HCO3− and H+ will be produced and as a result the blood becomes more acidic, so pH levels decrease. On the other hand, if HCO3− levels rise, less H+ will be produced and the pH rises. Now, if CO2 increases, then the equation shifts to the right and the pH drops. If CO2 decreases, then the equation shifts to the left and the pH rises. Stay with us here. In practice, the Henderson-Hasselbalch equation is used to calculate the pH based on HCO3 and pCO2 values, where pCO2 represents the partial pressure of carbon dioxide. Now, In order not to overcomplicate things here, just remember, If HCO3 goes up or if pCO2 goes down, then pH increases and if HCO3 goes down or if pCO2 goes up, then pH decreases.

Now, let’s start talking about acid-base disturbances, which are divided into four types: metabolic acidosis, respiratory acidosis, metabolic alkalosis and respiratory alkalosis. In order to determine which is which, the 4 high yield parameters are: pH, pCO2, bicarbonate levels and compensatory response.