ABG Interpretation, part 6: Metabolic Acidosis
by Cathy Parkes June 17, 2021 Updated: January 26, 2022 10 min read
In this article, we'll teach you all about metabolic acidosis, 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 acidosis; 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 acidosis on an ABG using two steps:
- Check the pH. A pH under 7.35 is acidic, and indicative of acidosis.*
- Determine which system, respiratory or metabolic, is to blame for the acidosis. The value for bicarbonate (HCO₃) represents the metabolic system. So if HCO₃ is under 22 mEq/L, which means it's also acidic, this means the metabolic system is to blame for the acidosis.
If pH is under 7.35* and HCO₃ is under 22 mEq/L, it's metabolic acidosis.
*Is the pH technically normal but on the acidic side? It may still be metabolic acidosis—fully compensated!
Metabolic acidosis with respiratory compensation
Once you have identified metabolic acidosis, 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 acidosis.
If the PaCO₂ level is basic (under 35 mmHg), it means the respiratory system is trying to compensate for metabolic acidosis. But how do you know the difference between partially and fully compensated metabolic acidosis?
When pH is "normal"
If PaCO₂ is basic (under 35 mmHg), some amount of respiratory compensation is happening for the metabolic acidosis. 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 acidosis is only partially compensated.
If the pH has been knocked back into the normal range, the respiratory system has succeeded and the metabolic acidosis is fully compensated.
In step 1, we evaluated the pH to determine if acidosis was occurring, we stated that a pH under 7.35 indicates acidosis. That is a strong rule of thumb, except in the cases of fully compensated acidosis, where the pH will be normal but on the acidic side.
Uncompensated metabolic acidosis
Uncompensated metabolic acidosis occurs when metabolic acidosis is present, with pH acidic under 7.35 and HCO₃ acidic (under 22 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 acidosis
Partially compensated metabolic acidosis occurs when metabolic acidosis is present, with pH acidic (under 7.35) and HCO₃ acidic (under 22 mEq/L); and the respiratory system acts to correct it, marked by an PaCO₂ level that's basic (under 35 mmHg).
The respiratory system's goal with compensation is to get the pH to the normal range. In the case of partially compensated metabolic acidosis, the respiratory system has only partially succeeded in correcting the acidosis, because the pH is still acidic and outside the normal range.
Fully compensated metabolic acidosis
Fully compensated metabolic acidosis occurs when metabolic acidosis is present, with pH normal but closer to acidic (7.35 - 7.39) and HCO₃ acidic (under 22 mEq/L); and the respiratory system acts to correct it, marked by a PaCO₂ level that's basic (under 35 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 acidosis, the respiratory system has succeeded in its goal of correcting the acidosis, 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 acidosis or alkalosis in the first place by the pH beingoutside the normal range?
That's the trick with fully compensated metabolic acidosis. The pH may be technically within the normal range. But the way to determine this is still metabolic acidosis is that the pH is on the acidic side (7.35 - 7.39) and the other two blood gases, HCO₃ and PaCO₂ are outside the normal range.
Common causes of metabolic acidosis include diabetic ketoacidosis (DKA), kidney failure, starvation, or anything that causes hypermetabolism.
Another cause of metabolic acidosis is diarrhea, which can lead to a loss of bicarbonate (HCO₃ under 22 mEq/L).
Other causes of metabolic acidosis include dehydration, pancreatitis, or liver failure.
Diabetic Ketoacidosis (DKA)
The key cause of metabolic acidosis is diabetic ketoacidosis (DKA). This is a life-threatening complication for diabetic patients (usually with Type 1 diabetes).
Insulin carries glucose into the body's cells where it can be used as energy. If a diabetic patient is not producing enough insulin or any insulin, the glucose can't get from their bloodstream into their cells to be used for energy. The cells want energy, so they start breaking down protein and fat to get it. The breakdown of protein and fat causes an accumulation of acid in the body, including ketones, which causes metabolic acidosis.
For the key facts you need to remember on DKA, check out our Medical-Surgical Nursing lesson on Diabetes Complications. This is one of the many important topics covered in our Medical-Surgical Nursing Flashcards.
Kidney failure and starvation can lead to metabolic acidosis for the same reason as DKA. If cells can't get glucose in, they begin to break down muscle and fat, which causes acid buildup.
Hypermetabolism is elevated metabolic activity wherein the body's basal metabolic rate is increased. If the body is burning through its glucose quickly, it may turn to protein and fat for energy, causing acid buildup. Hypermetabolism can be caused by a fever or infection, seizures, or heavy exercising.
Some medications that interfere with the reabsorption of HCO₃ can cause metabolic acidosis as a side effect. One such medication is the anti seizure medication topiramate. For your exams, however, the focus will be on the more common causes outlined above.
Signs and symptoms
Symptoms of metabolic acidosis include hypotension, tachycardia, weak pulses, GI upset (nausea, vomiting), and dysrhythmias.
Dysrhythmias and metabolic acidosis
Dysrhythmias (abnormal heart rhythms) can be a symptom of metabolic acidosis. Dysrhythmias can be caused by electrolyte imbalances. Electrolytes are minerals that have electric charge, and the heart relies on them to conduct its electrical impulses to keep itself running. Potassium is an important electrolyte for the heart.
During metabolic acidosis, the potassium levels in the body usually increase. So, metabolic acidosis leads to hyperkalemia (too much potassium), and hyperkalemia can cause life-threatening dysrhythmias.
Other signs and symptoms of metabolic acidosis can include Kussmaul's respirations. This is a key symptom of DKA in a diabetic patient. Kussmaul's respirations are distinctively deep, rapid breaths. This is respiratory compensation in action—the respiratory system working overtime to try to balance out metabolic acidosis by removing carbon dioxide (lowering PaCO₂).
Other symptoms of DKA are a fruity breath odor (that's the glucose!) and warm, flushed skin.
One treatment option for metabolic acidosis is to provide the patient with sodium bicarbonate, an alkalinizing agent. This is in an effort to lower the acid levels by creating acid-base balance.
A patient with metabolic acidosis may need IV fluids and insulin, if they have DKA. Giving the patient insulin helps their body get glucose into the cells, so the cells stop breaking down protein and fat for energy, which lowers the acid in the bloodstream.
Sodium bicarbonate and insulin are some of the important medications covered in our Pharmacology Flashcards for Nursing Students.
If a patient has renal failure (kidney failure), then hemodialysis is another option.
Metabolic vs. respiratory acidosis
The difference between metabolic and respiratory acidosis stems from which body system causes the acidosis. Metabolic acidosis is when the HCO₃ of the metabolic system is acidic and causes the body's pH to become acidic. Respiratory acidosis happens when the PaCO₂ of the respiratory system is acidic and causes the body's pH to become acidic.
Learn more about respiratory acidosis.
Metabolic acidosis vs. alkalosis
The difference between metabolic acidosis and alkalosis is how acidic or alkaline (basic) the blood is. Metabolic acidosis happens when the HCO₃ of the metabolic system is acidic and causes the body's pH to become acidic. Metabolic alkalosis happens when the HCO₃ levels are basic and cause the body's pH to become basic.
Learn more about metabolic alkalosis.
In this video, we will talk about metabolic acidosis. So we'll talk about what lab values you will see with uncompensated metabolic acidosis, partially compensated metabolic acidosis, and fully compensated metabolic acidosis. We will also cover the causes of metabolic acidosis, what symptoms you may see in your patient, and treatment options for metabolic acidosis.
Alright. So when we look at the pH, if it is under 7.35, then we know we have acidosis present, and we just have to figure out who's to blame.
When we look at PaCO2 here, if it's in the normal range between 35 and 45, then we can't blame that acidosis on the respiratory system. When we go over here to HCO3, which is bicarb which represents the metabolic system, we can see it's low, it's under 22, which means that we have metabolic acidosis and that it is uncompensated because this respiratory system's not doing anything to try to save the day. It's not trying to fix the situation, it's just chilling out in its normal range, so we have uncompensated metabolic acidosis.
Then if we go to the second row here, again, if the pH is under 7.35, we have acidosis.
Who's to blame? If we look at PaCO2, it is under 35 which means it's basic. So we definitely know that the respiratory system is not to blame for the acidosis.
However, with HCO3, we see that it is low under 22. So the metabolic system is to blame for the acidosis. And the respiratory system is trying to fix the situation, right? It's becoming more basic to help counteract this acidosis.
So we have partially compensated metabolic acidosis. The metabolic system is to blame for the acidosis. Respiratory system's trying to save the day but hasn't done so completely because that pH is still out of range.
Then if we go down to this last row, if we have a pH that is within the normal range, so it's between 7.35 and 7.45, but it's a little on the low side like if it's 7.35 or 7.36, then you can guess there may be some compensation that is occurring.
And sure enough, if we look here at PaCO2, we'll see that it's basic. It is under 35.
And if we look at HCO3, it is acidic. It is under 22.
So we can deduce that we had metabolic acidosis that was fully compensated by the respiratory system because our pH is in normal range. So again the metabolic system caused the acidosis - they're to blame - and the respiratory system came in and fixed the situation, got that pH in the normal range.
Alright. Let's talk about some of the causes of metabolic acidosis. One big cause is something called diabetic ketoacidosis, or DKA. This is a life-threatening complication for, typically, Type 1 diabetic patients that we see fairly regularly in the hospital. So with DKA, these Type 1 diabetic patients, they don't produce enough insulin or any insulin, and so the glucose can't get from the patient's bloodstream into their cells. They can't get in there because it needs insulin to get in there. So these cells feel like they're starving to death, right, so they start breaking down protein and fat to try to get energy. And with that breakdown of protein and fat, we get an accumulation of acid in the body, including ketones, and that's what causes the metabolic acidosis.
Other causes of metabolic acidosis include kidney failure, so that's another really common cause, as well as starvation, for kind of the same reasons as DKA, right? If your cells are starving, they can't get glucose in. They start breaking down that muscle and fat, the protein and fat, which is how we have that accumulation of acid.
Other causes include diarrhea. So one way to remember that diarrhea causes metabolic acidosis is that you lose bicarb out the butt. Another tip that someone gave me is that with diarrhea, you have diarrhea out the acidosis. So if either of those little tricks help you, that's great.
Other causes include dehydration, pancreatitis, liver failure, and then anything that causes hypermetabolism. So this could be something such as a fever, seizures, or heavy exercising.
And then in terms of symptoms, symptoms of metabolic acidosis include hypotension, tachycardia, weak pulses, GI upset such as nausea and vomiting, dysrhythmias because we often have hyperkalemia or too high of our potassium levels when it comes to metabolic acidosis. It happens almost all the time. So when you think of metabolic acidosis, I want you to think of hyperkalemia. And when you think of hyperkalemia, I want you to know that there is a high risk of life-threatening dysrhythmias when a patient's potassium levels are out of whack.
Other signs and symptoms can include Kussmaul's respirations. So you would see this with a diabetic patient who has DKA. They have this deep rapid breathing due to respiratory compensation for that metabolic acidosis. So just like we were talking before, the metabolic system is to blame for the acidosis, and the respiratory system often tries to compensate. And it will do that by increasing the depth and the rate of breathing, so you have this really deep, rapid breathing that you see with Kussmaul respirations.
Other things include a fruity odor or breath, so that's definitely a sign of DKA and of metabolic acidosis, and then warm, flushed skin.
In terms of treatment, we can provide the patient with sodium bicarbonate. So again, we're trying to eliminate the acid, trying to create acid-base balance. So we can give them bicarbonate.
We can also give them IV fluids as well as insulin for DKA. So by giving them insulin, we can get that glucose into the cells so that the cells will stop trying to break down that fat and that protein.
And then, if a patient has renal failure, like kidney failure, then hemodialysis is another option.
Okay. So that is it with metabolic acidosis. I spent a little more time on the causes, signs and symptoms, and treatment, just because it's really important for you to know as a nursing student and as a nurse. In my next video, we'll go over metabolic alkalosis, all of that same information. And we're on the home stretch, so stick with me. We're almost there.
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