ABG Interpretation, part 5: Respiratory Alkalosis
In this article, we'll teach you all about respiratory 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 respiratory 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 respiratory 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. The value for partial pressure of carbon dioxide (PaCO₂) represents the respiratory system. So if PaCO₂ is under 35 mmHg, which means it's also alkalotic, this means the respiratory system is to blame for the acidosis.
If pH is over 7.45* and PaCO₂ is under 35 mmHg, it's respiratory alkalosis.
*Is the pH technically normal but on the alkalotic side? It may still be alkalosis—fully compensated!
Respiratory alkalosis with metabolic compensation
Once you have identified respiratory alkalosis, you need to determine if the metabolic system is compensating or not. The metabolic system can be represented by bicarbonate (HCO₃), so we can evaluate HCO₃ to determine if the metabolic system is compensating, and if so, by how much.
If HCO₃ is in the normal range (22 - 26 mEq/L), the metabolic system is not trying to compensate for the alkalosis. If the HCO3 level is acidic (under 22 mEq/L), it means the metabolic system is trying to compensate for respiratory alkalosis. But how do you know the difference between partially and fully compensated respiratory alkalosis?
Alkalosis with a "normal" pH
If HCO3 is acidic (under 22 mEq/L), the metabolic system is doing some level of compensation for the respiratory alkalosis. You might remember this from our previous article on respiratory acidosis, and the same is true in situations of respiratory alkalosis: The metabolic system's goal with compensation is to get the body's pH to the normal range.
If the pH is not in the normal range, the metabolic system has not completed its job all the way, so the respiratory alkalosis is only partially compensated.
If the pH has been pushed into the normal range, the metabolic system has succeeded and the respiratory alkalosis has been fully compensated.
In step 1, we evaluated pH to determine if alkalosis was occurring; we stated that a pH over 7.45 indicates alkalosis. That rule is still true, however: in cases of fully compensated respiratory alkalosis, the pH will be normal but on the alkalotic side(7.4 - 7.45).
Uncompensated respiratory alkalosis
Uncompensated respiratory alkalosis occurs when respiratory alkalosis is present, with pH alkalotic (over 7.45) and PaCO2 alkalotic (under 35 mmHg); but the metabolic system does not act to correct it, marked by HCO3 in the normal range (22 - 26 mEq/L).
Partially-compensated respiratory alkalosis
Partially compensated respiratory alkalosis occurs when respiratory alkalosis is present, with pH alkalotic (over 7.45) and PaCO2 alkalotic (under 35 mmHg); and the metabolic system acts to correct it, marked by an HCO3 level that's out of range and acidic (under 22 mEq/L).
Fully compensated respiratory alkalosis
Fully compensated respiratory alkalosis occurs when respiratory alkalosis is present, with pH normal but closer to alkalotic (7.4 - 7.45) and PaCO2 alkalotic (under 35 mmHg); and the metabolic system acts to correct it, marked by an HCO3 level that is out of range and acidic (under 22 mEq/L).
Again, the metabolic system's goal with compensation is to get the pH to the normal range. In the case of fully compensated alkalosis, the metabolic system has succeeded in its goal of correcting the alkalosis, because the pH was pushed back into the normal range.
If you have been following this series, you might be getting the hang of this, but if you're wondering...pH in the normal range? I thought we can tell if it's acidosis or alkalosis in the first place by the pH being outside the normal range?
That's the trick to remember with full compensation. The pH may be technically within the normal range. But the other two blood gases, PaCO2 and HCO3, are outside the normal range.
What causes respiratory alkalosis?
A key cause of respiratory alkalosis is hyperventilation and the resulting loss of CO₂, which can be due to fear, anxiety, high altitude, pain, shock, trauma, or a salicylate toxicity (e.g., aspirin overdose).
When we breathe, we inhale oxygen and exhale carbon dioxide; this process is called gas exchange. When breathing is too fast, we expel too much carbon dioxide, and the pressure of the carbon dioxide in our blood drops. Partial pressure of carbon dioxide is PaCO₂, which is low in cases of respiratory alkalosis!
If you recall from our previous article on respiratory acidosis, hypoventilation was a key cause, with respiratory alkalosis, we have hyperventilation, which is the opposite.
The signs and symptoms of respiratory alkalosis can include shortness of breath, dizziness, anxiety, chest pain, or numbness in the hands and feet.
Respiratory alkalosis treatments usually focus on the underlying conditions causing the respiratory alkalosis.
It may help the patient to breathe into a paper bag to help retain more CO₂.
Respiratory vs. metabolic alkalosis
The difference between respiratory and metabolic alkalosis stems from which body system causes the alkalosis. Respiratory alkalosis happens when the PaCO₂ of the respiratory system is alkalotic and causes the body's pH to become alkalotic. Metabolic alkalosis is when the HCO₃ of the metabolic system is alkalotic and causes the body's pH to become alkalotic.
Learn more about metabolic alkalosis.
Respiratory alkalosis vs. acidosis
The difference between respiratory alkalosis and acidosis is how acidic or alkalotic (basic) the blood is.
Respiratory alkalosis happens when the PaCO₂ levels are alkalotic and cause the body's pH to become alkalotic.
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.
In this video, I'm going to talk about respiratory alkalosis and what ABG values you will see with uncompensated respiratory alkalosis, partially compensated respiratory alkalosis, and fully compensated respiratory alkalosis. We will also cover the causes, symptoms, and treatment of respiratory alkalosis as well.
Okay. When we look at the pH here, if it is over 7.45, we know we have alkalosis present and then we just need to see who to blame for the alkalosis.
And when we look at PaCO2, if that is low, under 35, then we know the respiratory system is to blame for the alkalosis, so we have respiratory alkalosis.
And then, when we look at HCO3, which is bicarb, that really represents the metabolic system. If that value is within the normal range, between 22 and 26, that means the metabolic system is chilling out. They're not trying to fix the situation, right? They're not trying to compensate for this respiratory alkalosis.
So in this case, we have uncompensated respiratory alkalosis.
Then, if we go to the second row, if our pH is over 7.45, again we have alkalosis.
Our PaCO2 is low, under 35, so we know we have respiratory alkalosis. But let's look what's going on here.
Bicarb is under 22, which means it's acidic. It is trying to compensate for this respiratory alkalosis by becoming more acidic. Has it fully compensated for the respiratory system messing up? No. Because our pH is still out of range, right? It's still over 7.45. But it's trying.
So here we have partially-compensated respiratory alkalosis. The respiratory system's to blame for the alkalosis. Metabolic system's trying to compensate, but has not fully compensated because that pH is not within normal range.
Third row here.
If we have a pH that is within normal range -- so it is between 7.35 and 7.45 -- then that's normal. But if it's a little bit on the basic side, meaning it's like 7.44 or 7.45, then we may suspect we have some compensation going on.
And sure enough, when we go to the next row here, we can see that PaCO2 is low, which means we probably have respiratory alkalosis here. And I suspect that the metabolic system has saved the day.
And sure enough, when we look at bicarb, HCO3, it is low. It is acidic. So in this situation, we had some respiratory alkalosis that the metabolic system has compensated for. It has become a lot more acidic to help counteract the basic nature of the respiratory system.
So we have fully compensated respiratory alkalosis. That metabolic system was able to come in, fix the situation, and get the pH into a normal range.
Now let's talk about the causes of respiratory alkalosis.
If you recall, with respiratory acidosis, the big issue was with hypoventilation. The patient has some kind of respiratory condition and they're not breathing effectively. Well, with respiratory alkalosis, the issue is with hyperventilation. It could be hyperventilation due to fear, due to anxiety, possibly due to a salicylate toxicity situation, like if they overdosed on aspirin. It could also be due to high altitude, pain, shock, or trauma.
In terms of the signs and symptoms, a patient who has respiratory alkalosis may be short of breath. They may feel dizzy. They may have anxiety. They may have chest pain and they may also have numbness in their feet and their hands.
In terms of treatment, we're going to want to treat whatever the underlying condition is. If the patient has anxiety, we can give them antianxiety medications. We can also have the patient breathe into a paper bag to help retain more CO2 to help resolve this respiratory alkalosis as well.
Okay. So that's it with respiratory alkalosis. In my next video, we will talk about metabolic acidosis, which is super important to know, and we'll go through all the same good information for that condition.
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