How to Read ABGs Like a Boss: Master Acid-Base Disorders!

Acid-base imbalances are an important concept of nursing practice, not to mention something you’ll need to learn in school.  Teachers have different ways to teach students how to interpret ABGs, but here’s my preferred method.

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What are ABGs?

ABG stands for Arterial Blood Gas.  You will need to stick an artery to get ABG values.  ABGs will include a few different lab values including pH, pCO2, and HCO3.

Here are the normal values:

  • pH:  7.35-7.45
  • pCO2:  35-45 mmHg
  • HCO3:  22-28 mEq/L

 

Acidosis vs. Alkalosis

Now that we have the normal values, we first will need to decide if the patient has some kind of acidosis versus alkalosis.

  • Acidosis is defined as a pH lower than 7.35.  The lower the pH, the more acidic the solution.
  • Alkalosis is defined as a pH higher than  7.45.  The higher the pH, the more alkalotic the solution.

 

Pop Quiz

Question:  Ms. Jones has a pH of 7.3.  Does she have some sort of acidosis or alkalosis?

 

Let’s try another one…

Question:  Ms. Jones has a pH of 7.5.  Does she have some sort of acidosis or alkalosis?

 

 

Respiratory vs. Metabolic

Ok great, you know how to tell if the patient is in acidosis or alkalosis, but now we need to know what has caused this change in pH.

You have two causes:  respiratory or metabolic.  Respiratory concerns the fluctuation of the lab carbon dioxide (CO2).  This should be easy to remember since we exhale carbon dioxide when we breathe out air.  Metabolic concerns the fluctuations of the lab bicarbonate (HCO3).

  • CO2 is acidic.
    • ↑ CO2 = ↓ pH
    • ↓ CO2 = ↑ pH
  • HCO3 is alkalotic.
    • ↑ HCO3 = ↑ pH
    • ↓ HCO3 = ↓ pH

So how will I remember which is causing the problem?

 

R.O.M.E.

One of my favorite mnemonics is R.O.M.E.  That stands for Respiratory Opposite, Metabolic Equal.

How do I use this mnemonic?  First, you need to determine the direction of your lab imbalances.

  1. Determine if pH is up or down from normal like we just learned.  Draw an arrow pointing up or down near your pH value.
  2. Determine if the respiratory lab (CO2) is up or down from normal.  Draw an arrow pointing up or down near your CO2 value.
  3. Determine if the metabolic lab (HCO3) is up or down from normal.  Draw an arrow pointing up or down near your HCO3 value.

Based off of your pH value alone, you already know if you’re looking at acidosis or alkalosis, so start with that point.  If the CO2 arrow is pointing in the opposite direction as your pH, then you know it is a respiratory issue.  Remember, R.O. stands for Respiratory Opposite.

Now flip the script and imagine you have a pH arrow that is pointing in the same direction as your HCO3.  That would mean this is a metabolic issue because M.E. stands for Metabolic Equal.

Confused yet?  Let’s look at some examples to help you see what I mean.

 

Respiratory Acidosis

  • pH:  7.3 ↓
  • pCO2:  50 mmHg ↑
  • HCO3:  24 mEq/L (normal)

See how the arrows are pointing in opposite directions?  This indicates a respiratory issue.  Since the pH was low, this would be respiratory acidosis.

Respiratory acidosis is characterized by an elevated arterial pCO2 and a low arterial pH.

 

Respiratory Alkalosis

  • pH:  7.5 ↑
  • pCO2:  25 mmHg ↓
  • HCO3:  24 mEq/L (normal)

Once again, the arrows are pointing in opposite directions, so this indicates a respiratory issue.  Since the pH was high, this would be respiratory alkalosis.

Respiratory alkalosis is characterized by low arterial pCO2 and an elevated arterial pH.

 

Metabolic Acidosis

  • pH:  7.3 ↓
  • pCO2:  40 mmHg (normal)
  • HCO3:  18 mEq/L ↓

See how the arrows are pointing in the same direction?  This indicates a metabolic issue.  Since the pH was low, this would be metabolic acidosis.

Metabolic acidosis is characterized by a low serum HCO3 and a low arterial pH.

 

Metabolic Alkalosis

  • pH:  7.5 ↑
  • pCO2: 40 mmHg (normal)
  • HCO3:  35 mEq/L ↑

Once again, the arrows are pointing in the same direction, so this indicates a metabolic issue.  Since the pH was high, this would be metabolic alkalosis.

Metabolic alkalosis is characterized by an elevated serum HCO3 and an elevated arterial pH.

 

Pop Quiz

Question:  Interpret the following ABGs.

  • pH:  7.3
  • CO2:  40 mmHg
  • HCO3:  15 mEq/L

 

Question:  Interpret the following ABGs.

  • pH:  7.5
  • CO2:  20 mmHg
  • HCO3:  22 mEq/L

 

Compensation

Next, we need to talk about compensation.  In any acid-base disorder, it is expected that the body would attempt to remedy the situation using compensatory mechanisms.  That is, that the body will attempt to correct an acidic or alkalotic pH.

The degree to which compensation is achieved can be broken down into three categories:  uncompensated, partially compensated, or fully compensated.

No compensation is similar to what we have covered so far.  You will simply see a respiratory/metabolic acidosis/alkalosis with one normal CO2 or HCO3 value since the body is not attempting to correct the problem.

In partial compensation, the pH value will remain abnormal.  The CO2 and HCO3 labs will remain abnormal in the same direction as one another.

In full compensation, the pH will return to normal.  Note, however, that the pH will still likely favor towards acidic or alkalotic within the normal range.  This can help you determine whether it is respiratory or metabolic in nature.  The CO2 and HCO3 labs will remain abnormal in the same direction as one another.  If you’re still not sure if it is respiratory or metabolic, determine which lab differs more from normal, and that is probably your cause.

 

Respiratory Acidosis

Partially Compensated

  • pH:  7.3 ↓
  • pCO2:  50 mmHg ↑
  • HCO3:  35 mEq/L ↑

In respiratory acidosis that is partially compensated, the pH will still be acidic, but the body’s compensatory mechanisms would increase HCO3 in attempts to return the pH to normal.

Fully Compensated

  • pH:  7.35 (normal)
  • pCO2:  50 mmHg ↑
  • HCO3:  39 mEq/L ↑

In respiratory acidosis that is fully compensated, the pH will still be close to acidic, but within the normal range.  The body’s compensatory mechanisms would have increased HCO3 sufficiently to return the pH to normal.

 

Respiratory Alkalosis

Partially Compensated

  • pH:  7.5 ↑
  • pCO2:  25 mmHg ↓
  • HCO3:  20 mEq/L ↓

In respiratory alkalosis that is partially compensated, the pH will still be alkalotic, but the body’s compensatory mechanisms would decrease HCO3 in attempts to return the pH to normal.

Fully Compensated

  • pH:  7.45 (normal)
  • pCO2:  25 mmHg ↓
  • HCO3:  15 mEq/L ↓

In respiratory alkalosis that is fully compensated, the pH will still be close to alkalotic, but within the normal range.  The body’s compensatory mechanisms would have decreased HCO3 sufficiently to return the pH to normal.

 

Metabolic Acidosis

Partially Compensated

  • pH:  7.3 ↓
  • pCO2:  30 mmHg ↓
  • HCO3:  18 mEq/L ↓

In metabolic acidosis that is partially compensated, the pH will still be acidotic, but the body’s compensatory mechanisms would decrease CO2 in attempts to return the pH to normal.

Fully Compensated

  • pH:  7.35 (normal)
  • pCO2:  25 mmHg ↓
  • HCO3:  18 mEq/L ↓

In metabolic acidosis that is fully compensated, the pH will still be close to acidotic, but within the normal range.  The body’s compensatory mechanisms would have decreased CO2 sufficiently to return the pH to normal.

 

Metabolic Alkalosis

Partially Compensated

  • pH:  7.5 ↑
  • pCO2: 55 mmHg ↑
  • HCO3:  35 mEq/L ↑

In metabolic alkalosis that is partially compensated, the pH will still be alkalotic, but the body’s compensatory mechanisms would increase CO2 in attempts to return the pH to normal.

Fully Compensated

  • pH:  7.45 (normal)
  • pCO2: 60 mmHg ↑
  • HCO3:  35 mEq/L ↑

In metabolic alkalosis that is partially compensated, the pH will still be alkalotic, but the body’s compensatory mechanisms would have increased CO2 sufficiently to return the pH to normal.

 

Test your knowledge

Question:  Interpret the following ABGs.

  • pH:  7.44
  • CO2:  20 mmHg
  • HCO3:  12 mEq/L

 

Question:  Interpret the following ABGs.

  • pH:  7.37
  • CO2:  60 mmHg
  • HCO3:  36 mEq/L

 

Question:  Interpret the following ABGs.

  • pH:  7.57
  • CO2:  22 mmHg
  • HCO3:  18 mEq/L

 

Question:  Interpret the following ABGs.

  • pH:  7.51
  • CO2:  40 mmHg
  • HCO3:  32 mEq/L

 

Question:  Interpret the following ABGs.

  • pH:  7.43
  • CO2:  60 mmHg
  • HCO3:  36 mEq/L

 

Question:  Interpret the following ABGs.

  • pH:  7.31
  • CO2:  57 mmHg
  • HCO3:  32 mEq/L

 

Anion Gap

Now you have the basic idea, but let’s briefly cover the anion gap.  The anion gap is simply the difference between the cations and anions in the blood.  Here’s an example:

  • pH:  7.31
  • CO2:  57 mmHg
  • HCO3:  32 mEq/L
  • Na:  150 mEq/L
  • Cl:  100 mEq/L
  • K:  4 mEq/L

So we’ll ignore the first two points and focus on the last 4 which are cations and anions.  HCO3 and Cl are anions (negatively charged ions) and Na and K are cations (positively charged ions).

Therefore the anion gap is Na – (Cl+HCO3) or in this example, (150) – (100+32) = 18.

Note that in the U.S., standard practice omits K+ from the anion gap and treats it as an unmeasured cation.

A normal anion gap is between 7-13.

 

Clinical significance

The anion gap is primarily used for differential diagnosis of metabolic acidosis.  Metabolic acidosis can be caused by an increase in acid production and/or a loss of bicarbonate, so whether or not the anion gap is elevated can help determine the cause.

 

Elevated anion gap

Causes such as lactic acidosis and diabetic ketoacidosis (DKA) are associated with an elevated anion gap in the setting of metabolic acidosis.  This is because these conditions will generate an excess of acids.

 

Normal anion gap

Diarrhea is another cause of metabolic acidosis and is the result of a loss of bicarbonate.  Diarrhea does not create additional acid production, and generally, the anion gap will remain normal.

 

Causes & Treatments

Let’s talk causes and treatments.  You know what they’ve got, now learn WTF happened and how to fix it!

 

Respiratory Acidosis

Characterized by an excess of CO2, hypercapnia.  Common Causes:

  • Hypoventilation
  • Sedatives or drug overdose
  • Coma
  • Thoracic injury
  • Stroke
  • Encephalitis

Treatment often includes ventilation support.  If the cause is reversible (such as an overdose), give reversal agents such as naloxone (Narcan) or flumazenil.

 

Respiratory Alkalosis

Characterized by a lack of CO2, hypocapnia.  Common causes:

  • Hyperventilation
  • Fever
  • Anxiety, panic attack
  • Hypoxemia
  • Pneumothorax
  • Pulmonary embolism
  • Pulmonary edema
  • High altitude syndrome

Treatment of the underlying cause.

 

Metabolic Acidosis

Characterized by a lack of HCO3 or increased acid production.  Common causes:

  • Diarrhea
  • Diabetic Ketoacidosis (DKA)
  • Lactic Acidosis

Treatment usually involves administration of bicarbonate and correction of other mechanisms.

 

Metabolic Alkalosis

Characterized by an excess of HCO3 or decreased acid production.  Common causes:

  • Vomiting
  • Hypokalemia
  • Renal failure
  • Hypochloremia
  • Hypovolemia

Treatment involves repletion of electrolytes, administration of fluids in the case of hypovolemia.  The patient may need dialysis or CVVH.

 

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