ABG interpretation is an important skill for doctors, especially Internists, Intensivists & Pulmonologists. ABG interpretation steps are made easy to understand in this post.
Normal values
pH 7.35 – 7.45 – ( < 7.35 Acidemia , > 7.45 Alkalemia)
pCO2 35 – 45 mmHg – (< 35 mmHg – Respiratory alkalosis , > 45 mmHg Respiratory acidosis)
[HCO3–] 22 – 28 mmol/L – (< 22 mmol/L – Metabolic acidosis , > 28 mmol/L – Metabolic alkalosis)
Important Considerations
- When pH is lower or higher than normal range, the correct term is Acidemia and Alkalemia, respectively.
- The terms, Acidosis and Alkalosis denotes the processes that has lead to Alkalemia or Alkalemia, as the case may be.
- When the primary acid-base disorder is Respiratory, there is renal compensation in the form of increase or decrease in bicarbonate. And, when the primary acid-base disorder is Metabolic, there is respiratory compensation in the form of increase or decrease in pCO2.
- Respiratory compensation is rapidly visible while renal compensation takes time.
- In case of metabolic alkalosis, the compensation is through hypoventilation and retaining CO2. But it is important to note that respiratory rate cannot decrease beyond a certain limit because hypoxia, and hypercapnia itself, would stimulate respiratory center.
Steps in Interpreting Blood Gases
1. Check pH
Check pH and decide about normal pH, acidemia or alkalemia
2. Check pCO2
If pH is abnormal and pCO2 going opposite to the direction of pH, then the pathologic process leading to acid-base abnormality is Respiratory e.g., pH is 7.28 (acidemia), & you observe that pCO2 is high e.g., 60 mmHg (opposite to the direction of pH i.e., pH is low and pCO2 is High), it suggests that there is Respiratory acidosis leading to acidemia. On the contrary, if pCO2 was also on the lower side with this low pH, then the pathologic process is not respiratory but is metabolic. Check [HCO-3] and it shall go in the direction of pH to be able to call the pathologic process to be Metabolic.
Respiratory = pCO2 opposite to pH direction
Metabolic = HCO3 along the direction of pH
3. Check for Compensation
Check for compensation whether it is adequate. Once decided about metabolic/respiratory acidosis/alkalosis (as the case may be), check adequacy of compensation.
If primary acid-base disorder is Metabolic in nature, compensation occurs through respiratory mechanism & a change in pCO2 is observed. Similarly, if primary acid-base disorder is Respiratory in nature, compensation occurs through renal system & a change in HCO3 occurs to bring pH back to normal/near normal.
Calculate the compensation through following equations. If the compensation is not as expected, consider mixed acid-base abnormalities. Please note that in cases Respiratory acidosis/alkalosis, the calculation of compensation depends upon whether the disease causing the acid-base disorder is acute or chronic, because renal compensatory mechanisms are slower and in acute process these mechanisms, HCO3 change is not dramatic.
Metabolic acidosis
Expected pCO2 = 1.5 (HCO3) + 8
Metabolic alkalosis
Expected pCO2 = 40 + [0.7 (HCO3 – 24)]
Respiratory acidosis
Is the disease process Acute? – Expected HCO3 = 24 + ( pCO2 – 40 / 10 )
Is the disease process Chronic? – Expected HCO3 = 24 + 4 ( pCO2 – 40 / 10 )
Respiratory alkalosis
Is the disease process Acute? – Expected HCO3 = 24 – 2( 40 – pCO2 / 10 )
Is the disease process Chronic? – Expected HCO3 = 24 – 5 ( 40 – pCO2 / 10 )
4. Determine Anion Gap (AG), if there is Metabolic Acidosis
- If the abnormality is Metabolic acidosis, calculate anion gap to see whether it is Elevated anion gap or normal anion gap metabolic acidosis. This is important because differential diagnosis of causes and treatment of both types are different.
Anion gap = Na+ – (Cl‑ + HCO-3) – Reference range 6-12
- Adjust Anion gap for hypoalbuminemia by using this equation:
Adjusted Anion Gap = Measured Anion Gap + 2.5 [ 4 – serum albumin]
- If there is high anion gap, check ‘Delta Ratio’ for the presence of other metabolic defects.
Delta Ratio = AG adjusted – AG normal / HCO3– normal – HCO3–measured
(If delta ratio is more than expected, there is additional Metabolic alkalosis. If it is less than expected, then there is additional Normal anion gap metabolic acidosis.)
- Look if Metabolic acidosis can be explained by the Lactic acidosis, Ketoacidosis or Renal failure in your patient. If not, proceed to check for serum Osmolal gap to check for osmotically active substances. If the gap is >20, consider alcohol ingestion.
Osmolal Gap = Measured Serum Osmolality – Calculated Serum Osmolality
Where Measured is Serum Osmolality is laboratory value.
Calculated Serum Osmolality = 2 x Na + BUN/2.8 + Glucose/18
Now the Last Step FINALLY
5. Determine Anion Gap (AG), if there is Metabolic Acidosis
Once the abnormality/abnormalities determined, search for the cause.
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Check out this Video on ABGs
This is one of the best series on acid-base analysis by Dr Eric Strong on his YouTube channel ‘Strong Medicine’. I’d strongly recommend the series to those who would like to know about blood gas interpretation in depth.