ABG Interpretation, part 7: Metabolic Alkalosis
by Cathy Parkes June 22, 2021 Updated: January 18, 2023 4 min read
Want to learn about metabolic alkalosis? In this article you'll learn all about metabolic alkalosis, including: the difference between uncompensated, partially-compensated, and fully compensated, and the lab values you can expect to see with each of those; what causes metabolic alkalosis; and its symptoms and treatment.
The Arterial Blood Gas video article series follows along with our Arterial Blood Gas Flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX. You can also check out our ABG cheatsheet and practice questions.
You can identify metabolic alkalosis on an ABG using two steps:
- Check the pH. A pH over 7.45 is alkalotic, and indicative of alkalosis.*
- Determine which system, respiratory or metabolic, is to blame for the alkalosis. The value for bicarbonate (HCO₃) represents the metabolic system. So if HCO₃ is over 26 mEq/L, which means it's also alkalotic, this means the metabolic system is to blame for the alkalosis.
If pH is over 7.45* and HCO₃ is over 26 mEq/L, it's metabolic alkalosis.
*Is the pH technically normal but on the alkalotic side? It may still be metabolic alkalosis—fully compensated!
Metabolic alkalosis with respiratory compensation
Once you have identified metabolic alkalosis, you need to determine if the respiratory system is compensating or not. The respiratory system can be represented by partial pressure of carbon dioxide (PaCO₂), so we can look to PaCO₂ to determine how much, if at all, the respiratory system is compensating.
If PaCO₂ is in the normal range (35 - 45 mmHg), the respiratory system is not trying to compensate for the alkalosis.
If the PaCO₂ level is acidic (over 45 mmHg), it means the respiratory system is trying to compensate for metabolic alkalosis. But how do you know the difference between partially and fully compensated metabolic alkalosis?
When pH is "normal"
If PaCO₂ is acidic (over 45 mmHg), some amount of respiratory compensation is happening for the metabolic alkalosis. The respiratory system's goal with compensation is to get the pH to the normal range.
If the pH is not in the normal range, the respiratory system has not completed its job all the way, so the metabolic alkalosis is only partially compensated.
If the pH has been knocked back into the normal range, the respiratory system has succeeded and the metabolic alkalosis is fully compensated.
In step 1, we evaluated the pH to determine if acidosis was occurring, we stated that a pH over 7.45 indicates alkalosis. That is a strong rule of thumb, except in the cases of fully compensated alkalosis, where the pH will be normal but on the alkalotic side.
Uncompensated metabolic alkalosis
Uncompensated metabolic alkalosis occurs when metabolic alkalosis is present, with pH basic (over 7.45) and HCO₃ basic (over 26 mEq/L); but the respiratory system does not act to correct it, marked by PaCO₂ in the normal range (35 - 45 mmHg).
Partially compensated metabolic alkalosis
Partially compensated metabolic alkalosis occurs when metabolic alkalosis is present, with pH basic (over 7.45) and HCO₃ basic (over 26 mEq/L); and the respiratory system acts to correct it, marked by an PaCO₂ level that's acidic (over 45 mmHg).
The respiratory system's goal with compensation is to get the pH to the normal range. In the case of partially compensated metabolic alkalosis, the respiratory system has only partially succeeded in correcting the acidosis, because the pH is still basic and outside the normal range.
Fully compensated metabolic alkalosis
Fully compensated metabolic alkalosis occurs when metabolic alkalosis is present, with pH normal but closer to basic (7.40 - 7.45) and HCO₃ basic (over 26 mEq/L); and the respiratory system acts to correct it, marked by a PaCO₂ level that's acidic (over 45 mmHg).
Again, the respiratory system's goal with compensation is to get the pH to the normal range. In the case of fully compensated metabolic alkalosis, the respiratory system has succeeded in its goal of correcting the alkalosis, because the pH was pushed back into the normal range.
If you're following closely, you might be thinking...pH in the normal range? I thought we can tell if it's alkalosis or acidosis in the first place by the pH beingoutside the normal range?
That's the trick with fully compensated metabolic alkalosis. The pH may be technically within the normal range. But the way to determine this is still metabolic alkalosis is that the pH is on the alkalotic side (7.40 - 7.45) and the other two blood gases, HCO₃ and PaCO₂ are outside the normal range.
Common causes of metabolic alkalosis
A common cause of metabolic alkalosis is antacid overdose. If a patient is popping TUMS beyond the recommended amount, it can throw their acid-base balance off and they could end up with metabolic alkalosis.
A loss of body acids is another key cause of metabolic alkalosis. If a patient has prolonged vomiting due to something like hyperemesis, that can cause metabolic alkalosis.
Prolonged nasogastric tube suctioning and excess diuretic therapy can also result in metabolic alkalosis. Diuretics are just one of the many important medications covered in our Pharmacology Flashcards for Nursing Students
Symptoms of metabolic alkalosis include tachycardia, dysrhythmias, muscle weakness, and lethargy.
Treatment of metabolic alkalosis usually involves treating the underlying cause.
If the underlying cause of metabolic alkalosis is prolonged vomiting, the patient will likely need an anti-emetic to control their vomiting.
If a patient has gotten metabolic alkalosis from taking too many TUMS, they will require some patient education to not do that anymore.
With metabolic alkalosis, it's important to replace the patient's lost fluid and electrolytes, so you will want to pay attention to the related lab values.
The lab values that pertain to fluid and electrolytes are Sodium, Calcium, Magnesium, Potassium, Chloride, Phosphorous, Urine Output, Urine Specific Gravity, Urine Osmolality. If you need to learn & retain the normal ranges for these lab values, they are covered in our Lab Values Flashcards for Nursing Students!
In this video, we're going to talk about metabolic alkalosis and what lab values you would see with uncompensated metabolic alkalosis, partially-compensated metabolic alkalosis, and fully compensated metabolic alkalosis. And we'll also cover the causes of this disorder, what symptoms you would see with metabolic alkalosis, and what treatment options exist.
Okay. So when we see a pH over 7.45, that means we have alkalosis, and we have to figure out who's to blame.
If we look at PaCO2, if that's within normal range - so between 35 and 45 - then we know the respiratory system isn't to blame.
And if we look at HCO3, or bicarbonate, which represents the metabolic system, if it is over 26, then that is basic. And then we know that we are dealing with metabolic alkalosis. So in this case, we have uncompensated metabolic alkalosis. We have alkalosis that is caused by the metabolic system. And the respiratory system is chilling out and not doing anything to try to fix the situation.
Alright. On this next row, again if pH is over 7.45, that means we have alkalosis.
If we look at the PaCO2, and it's over 45, then that is acidic. So we know that the respiratory system is not causing the alkalosis It looks like it's trying to compensate, though.
Right? And then, if we go over here to bicarb, or HCO3, if it is over 26, then we know for sure that we have metabolic alkalosis that the respiratory system is trying to compensate for. It's becoming acidic to try to compensate for this basic, or alkalosis.
So in this case, we have partially compensated metabolic alkalosis. So we've got alkalosis caused by the metabolic system, and the respiratory system is trying to fix the situation, but they haven't fixed it completely because our pH is still out of range.
Okay. Down here, if we have a pH that is within the normal range - so it is between 7.35 and 7.45, but it is on the high side of things - like if it's 7.44 or 7.45, then we may suspect that there was some alkalosis that was being compensated.
So let's go over here to PaCO2, which is the respiratory system. And we see that it's acidic. It is over 45. So that's definitely not anything that would be causing alkalosis.
But if we go over here to HCO3, we see that it's over 26, which means it is to blame for the alkalosis. But luckily for the metabolic system, the respiratory system came in and saved the day and compensated, right? It got so acidic that it was able to compensate for this alkalosis and get this pH into a normal range.
So in this case, we have fully compensated metabolic alkalosis. The metabolic system is to blame for the alkalosis; respiratory system has fixed the situation.
Alright, now let's talk about some of the common causes of metabolic alkalosis. So one really common cause that I've definitely seen a lot in the hospital is an antacid overdose. So if a patient is just popping those TUMS over and over, they can really throw their acid-base balance out of whack, so they could definitely end up with metabolic alkalosis if they take too many antacids.
Also, loss of body acids is a key cause of metabolic alkalosis. So if a patient has prolonged vomiting due to something like hyperemesis, which I had - and I had a lot of vomiting - that can definitely cause metabolic alkalosis.
In addition, prolonged NG tube suctioning and excess diuretic therapy can also result in metabolic alkalosis.
In terms of symptoms, a patient who has metabolic alkalosis may have tachycardia. They may have dysrhythmias as well as muscle weakness and lethargy.
In terms of treatment, we're really going to want to treat the underlying cause. So if the underlying cause is prolonged vomiting, then we're definitely going to give the patient anti-emetics for that vomiting.
If the patient is just taking too many TUMS, we're going to have to really educate them and make sure they don't do that again.
And then it's going to be really important that we replace lost fluid and electrolytes, so definitely you want to pay attention to those labs and replace the patient's fluid and electrolytes as ordered.
So that is it with the deck [of flashcards]. Like I said, it's a nice thin deck, 15 cards. And in my next video, I'm going to start running through a variety of practice problems to really help you get comfortable with ABG interpretation. Thanks so much for watching!
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