ABG Interpretation, part 4: Respiratory Acidosis
In this article, we'll teach you all about respiratory 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 respiratory 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 respiratory 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. The value for partial pressure of carbon dioxide (PaCO₂) represents the respiratory system. So if PaCO₂ is over 45 mmHg, the respiratory system is to blame for the acidosis.
If pH is under 7.35* and PaCO₂ is over 45 mmHg, it's respiratory acidosis.
*Is the pH technically normal but on the acidic side? It may still be acidosis—fully compensated!
Respiratory acidosis with metabolic compensation
Once you have identified respiratory acidosis, you need to determine if the metabolic system is compensating or not. The metabolic system can be represented by bicarbonate (HCO₃), so we can look to HCO₃ to determine how much, if at all, the metabolic system is compensating.
If HCO₃ is in the normal range (22 - 26 mEq/L), the metabolic system is not trying to compensate for the acidosis. If HCO₃ level is basic (over 26 mEq/L), it means the metabolic system is trying to compensate for respiratory acidosis. But how do you know the difference between partially and fully compensated respiratory acidosis?
When pH is "normal"
If HCO₃ is basic (over 26 mEq/L), some amount of metabolic compensation is happening for the respiratory acidosis. The metabolic system's goal with compensation is to get the 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 acidosis is only partially compensated.
If the pH has been knocked back into the normal range, the metabolic system has succeeded and the respiratory 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 respiratory acidosis
Uncompensated respiratory acidosis occurs when respiratory acidosis is present, with pH acidic (under 7.35) and PaCO₂ acidic (over 45) 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 acidosis
Partially compensated respiratory acidosis occurs when respiratory acidosis is present, with pH acidic (under 7.35) and PaCO₂ acidic (over 45 mmHg); and the metabolic system acts to correct it, marked by an HCO₃ level that's basic (over 26 mEq/L).
The metabolic system's goal with compensation is to get the pH to the normal range. In the case of partially compensated acidosis, the metabolic system has only partially succeeded in correcting the acidosis, because the pH is still acidic and outside the normal range.
Fully compensated respiratory acidosis
Fully compensated respiratory acidosis occurs when respiratory acidosis is present, with pH normal but closer to acidic (7.35 - 7.39) and PaCO₂ acidic (over 45 mmHg); and the metabolic system acts to correct it, marked by an HCO₃ level that's basic (over 26 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 acidosis, the metabolic 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 being outside the normal range?
That's the trick with fully compensated respiratory acidosis. The pH may be technically within the normal range. But the way to determine this is still respiratory acidosis is that the pH is on the acidic side of normal (7.35-7.39) and the other two blood gases, PaCO₂ and HCO₃ are outside the normal range.
What causes respiratory acidosis?
The key cause of respiratory acidosis is hypoventilation, which is breathing at an abnormally slow rate that causes excess carbon dioxide in the body.
When we breathe, we inhale oxygen and exhale carbon dioxide; this process is called gas exchange. When breathing is too slow, we don't expel enough carbon dioxide, and the pressure of the carbon dioxide builds up in our blood (partial pressure of carbon dioxide is PaCO₂ which is elevated!)
Hypoventilation can be caused by a number of pulmonary or respiratory conditions, including acute respiratory distress syndrome, asthma, chronic obstructive pulmonary disease (COPD), pneumonia, a pneumothorax, or a pulmonary embolism. If you need to learn more about these conditions, they are covered in our Med-Surg flashcards for nursing students. Patients with chest trauma may not be able to expand their chest to take a deep breath, which can result in hypoventilation and respiratory acidosis.
The signs and symptoms of respiratory acidosis can include confusion, lethargy, or dyspnea (difficulty breathing). Patients may have pale or cyanotic skin (bluish-purplish discoloration).
Chronic respiratory acidosis can lead to pulmonary hypertension, heart failure, or polycythemia.
Polycythemia is a condition marked by a high number of red blood cells in the body, which results in a high hematocrit level.
Respiratory acidosis treatments usually start with providing the patient with oxygen. Respiratory acidosis caused by obstructive airway diseases like COPD and asthma can be treated with a bronchodilator, which helps to expand the airway.
In more serious cases where the patient cannot breathe enough, or at all, on their own, they may be put on mechanical ventilation.
In terms of general nursing interventions, monitor changes in the patient's respiratory functions, and as always, maintain a patent airway.
Respiratory vs. metabolic acidosis
The difference between respiratory and metabolic acidosis stems from which body system causes the acidosis. Respiratory acidosis happens when the PaCO₂ of the respiratory system is acidic and causes the body's pH to become acidic. Metabolic acidosis is when the HCO₃ of the metabolic system is acidic and causes the body's pH to become acidic.
Learn more about metabolic acidosis.
Respiratory acidosis vs. alkalosis
The difference between respiratory acidosis and alkalosis is how acidic or alkaline (basic) the blood is. Respiratory acidosis happens when the PaCO₂ of the respiratory system is acidic and causes the body's pH to become acidic. Respiratory alkalosis happens when the PaCO₂ levels are basic and cause the body's pH to become basic.
Learn more about respiratory alkalosis.
Okay. In this video, we are going to talk about respiratory acidosis. We're going to talk about what values you will see for uncompensated respiratory acidosis, partially compensated respiratory acidosis, and fully compensated respiratory acidosis. We will then talk about some causes of respiratory acidosis, what symptoms you can expect to see in your patient with this type of imbalance and how we can treat respiratory acidosis.
Okay. So when we look at the pH, and if the pH is under 7.35, that means we have acidosis. Remember that's the first step of our process.
So now we're going to figure out who's to blame. Well, when we look at PaCO2, if that is over 45, that is indicative of respiratory acidosis, so we know that the respiratory system is to blame. We have respiratory acidosis.
The next thing we need to check is for compensation. So when we look at HCO3, if it is within normal range, that means that the metabolic system is not trying to compensate for this respiratory acidosis. So if we have values that fall within these ranges, we have uncompensated respiratory acidosis.
The respiratory system is to blame for the acidosis, and the metabolic system is not currently trying to fix the situation.
Let's look at this next row down. Again, pH is under 7.35, so we have acidosis. Our PaCO2 is over 45. So we have respiratory acidosis. Now let's look at bicarb. If the bicarb level, HCO3, is over 26, that is basic, which means that the metabolic system is trying to compensate for this respiratory acidosis. But because the pH is still under 7.35, it hasn't done the job completely. So in this case, we have partially-compensated respiratory acidosis, which means that the respiratory system is to blame for the acidosis. The metabolic system is trying to fix the situation, right, by becoming more basic. But it hasn't completely fixed the situation because our pH is still out of range.
Let's look at the third row down. If the pH is within the normal range, but is a little bit on the acidic side-- so if the pH, for example, is 7.35 exactly, or 7.36, well those numbers are within the normal ranges, but they're a little on the acidic side. So if our pH is normal, but on the acidic side, and we look here and the PaCO2 levels are over 45, we can guess that we have respiratory acidosis that is likely being compensated.
And sure enough, when we look over here at HCO3, it is over 26, which is basic. So in this situation, we have a normal pH but a little on the acidic side, we have the respiratory system that is causing the respiratory acidosis, and we have the metabolic system, which is coming in to save the day by becoming more basic and balancing out that pH. So we have fully compensated respiratory acidosis in this case.
The key cause for respiratory acidosis is really hypoventilation, and this hypoventilation can be due to a number of pulmonary or respiratory conditions, including acute respiratory distress syndrome, asthma, COPD, pneumonia, a pneumothorax, a pulmonary embolism. If the patient has had some kind of chest trauma and they can't expand their chest to take in a nice, deep breath, that can result in respiratory acidosis. Also, if a patient has had too many opioid analgesics, that definitely results in respiratory depression, as will taking too many benzodiazepines for anxiety.
So in terms of symptoms, some of the symptoms you may see with respiratory acidosis include confusion, lethargy, difficulty breathing, which is dyspnea.
The patient may also have pale or cyanotic skin. And then, with prolonged respiratory acidosis, the patient may have something called polycythemia, which means that they have a lot of red blood cells in their blood. So the percentage of red blood cells there is quite high, which results in a high hematocrit level. In addition, with prolonged respiratory acidosis, the patient may end up with pulmonary hypertension or heart failure as well.
In terms of treatment, we can provide the patient with oxygen. We can also give them bronchodilators for obstructive airway diseases, such as COPD or asthma. So we can give them a bronchodilator to help expand that airway.
We can also give naloxone for an opioid overdose and we can give flumazenil for a benzodiazepine overdose.
And then, lastly, for really serious conditions, we can do mechanical ventilation as well.
Okay, so that's it for respiratory acidosis. In my next video, we will cover respiratory alkalosis and go over all of this same information for that condition.
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