Acid-Base Questions: Go Back to Basics Before Your USMLE
- Jul 20, 2016
Without a doubt, at some point on test day, you will be asked to answer a question whose answer choices look like this:
pH CO2 HCO3–
- A) 2 30 12
- B) 4 19 20
- C) 6 20 26
Is this the kind of question that makes you shudder? Or does it fill you with delight because you’ve got your acid-base questions down pat? Either way, it’s always a good idea to revisit the basic, systematic approach to answering these types of questions, making sure your thought process it spot on. Acid it once and I’ll say it again: This stuff is basic. #I’mSoPunny #SorryNotSorry
Step 0: Get The Full Story
While we can almost always figure out what is going on physiologically with our hypothetical patient simply based on the numbers, it will help us immensely to figure out what is going on from the history and physical exam. Keeping in mind that all acid-base problems are driven by the kidneys and lungs, these are the systems to focus on most.
Is the patient anxious and hyperventilating (respiratory alkalosis)? Are they over-narcotized with a respiratory rate of 6/min (respiratory acidosis)? Is their week of missed dialysis driving them into a state of uremia (metabolic acidosis)? Maybe they are a heart failure patient who got over-diuresed (contraction/metabolic alkalosis). Understanding the backstory will provide an indispensable and often necessary part of the story in the interpretation of our values.
For the following algorithm, we will use the above acidotic example: pH 7.2, CO2 30, HCO3–12
Step 1: Analyze the pH to see if it is acidotic (< 7.35), alkalotic (> 7.45), or normal (7.35-7.45).
By knowing the patient’s general acid-base status, we can appropriately label them with a condition. Are they in a state of acidosis, alkalosis, or just plain normal? With a pH of 7.2, this patient is acidotic. Couldn’t be easier…
Step 2: Analyze whether it is the bicarbonate (metabolic) or carbon dioxide (respiratory) that is driving this change.
Here lies the trickiest part, and the crux of figuring out what is wrong with our patient: Is her acidosis driven by a low bicarb (as bicarb is a base, low levels are congruent with acidosis), or a high CO2 (as CO2 is an acid, high levels are congruent with acidosis)? The corollary: we want to find out if this acidotic patient is acidotic because their blood has (a) too much acid (CO2) or (b) not enough base (HCO3–).
With a CO2 of 30, which is lower than normal, we have less acid than normal, so this cannot drive the acidotic process. Therefore, we move onto bicarbonate ion.
With a bicarb of 12, which is lower than normal, we have less base than normal. This CAN drive our acidosis. We can now confidently say that our patient has a metabolic acidosis.
Step 3: Analyze whether the other value is compensating for the primary change.
Our primary change here is a low bicarb, generating a metabolic acidosis. The body’s response is to try and get rid of an acid and do what it can to compensate and balance/raise the pH. The other value, CO2 (an acid), has gone down, as the body hyperventilates to quickly dispose of this acid, and try, even if slightly, to bring the pH closer to normal.
The body should always try to compensate for the primary change. For the purposes of your test, it will never fully compensate and bring the pH back to normal. It is key to remember that respiratory compensation can occur almost instantaneously, and renal compensation takes 2-3 days. You can hyperventilate right now and get you pH pretty high by getting rid of all that CO2. However, the kidneys are less than speedy when it comes to clearing the blood of bicarb, so this compensation takes some time.
Let’s take a look at one more example to make sure we’ve got it down.
A 23-year old man with a history of poorly controlled schizophrenia and substance abuse disorder is found unresponsive in squalid apartment. He is brought into the ED by EMS. Vitals are P 62 bpm, BP 90/65, RR 4/min, T 98.0 F. On exam he has pinpoint pupils and is unarousable. EMS reports that used needles and syringes are found around his bed. Which of the following is his acid-base status most likely to be?
pH CO2 HCO3–
- A) 2 30 10
- B) 2 70 24
- C) 4 40 24
- D) 6 20 22
- E) 6 51 40
Step 0: The vignette sounds a lot like a heroin/opiate overdose. As evidenced by the low respiratory rate, he probably has opioid-induced respiratory depression.
Step 1: Depressing the respiratory drive will make him retain lots of [acidotic] CO2, so he should be in an acidotic (high acid) state. A and B look good, based upon pH’s in the acidotic range.
Step 2: Which choice agrees with a high acid, high CO2 state? Only choice B gives us a respiratory acidosis, driven by high levels of acid.
Step 3: For confirmation, we check the bicarb. In this acute incident, we wouldn’t expect the kidneys to have had time to respond to his respiratory depression, so this normal bicarb makes sense.
Stick with this algorithmic approach and you will never trip up on an acid-base question again!
Are you still having questions about problems of this type? Ask us below and we will see how we can help you out.