Three-step approach to arterial blood gas analysis
Briefly, the three-step approach includes: Step 1, is the patient acidotic or alkalotic? In the second step, is the acid/alkaline toxicity respiratory or metabolic? In the third step, if it is respiratory acid/alkalosis, is it a purely respiratory factor or is there a metabolic component?
The specific methods are as follows.
Step 1, look at the PH value, the normal value is 7.4 ± 0.05. PH ≤ 7.35 is acidosis, PH ≥ 7.45 is alkalosis.
In the second step, look at the direction of PH and PCO2 change. Isotropic change (PCO2 increases and PH also increases and vice versa) is metabolic and anisotropic change is respiratory.
In the third step, if it is respiratory, then look at the ratio of PH and PCO2 change. Normal PCO2 is 40±5 mmHg, and for simple respiratory acid/alkalosis, for every 10 mmHg change in PCO2, the PH value changes by 0.08±0.02 in the opposite direction. For example, if PCO2 is 30 mmHg (10 mmHg decrease), then the PH value should be 7.48 (0.08 increase); if PCO2 is 60 mmHg (20 mmHg increase ), then the PH should be 7.24 (decrease of 2 × 0.08).
If this ratio is not met, it indicates that a second factor, the metabolic factor, is also present. At this point, the third step should be to compare the theoretical PH value with the actual PH value. If the actual PH value is lower than the theoretical PH value, it indicates that there is also metabolic acidosis, and vice versa, if the actual PH value is higher than the theoretical PH value, it indicates that there is also metabolic alkalosis. It should be noted that the PH value derived from the formula can have a fluctuation of ±0.02.
Practical examples
Example 1: The patient’s PH value is 7.58, PCO2 is 20 mmHg, and PO2 is 110 mmHg.
Analysis.
In the first step, the PH value is greater than 7.45, suggesting alkalosis.
In the second step, the PCO2 and PH values changed anisotropically, suggesting respiratory.
In the third step, the PH value should increase by 2×0.08(±0.02) when PCO2 decreases by 20mmHg, which is 7.56±0.02, which is consistent with the actual PH value, therefore this patient is a simple respiratory alkalosis.
Conclusion: This patient has simple respiratory alkalosis.
Example 2: The patient’s PH is 7.16, PCO2 is 70 mmHg, and PO2 is 80 mmHg.
Analysis.
In the first step, the PH value is less than 7.35, suggesting acidosis.
In the second step, the PCO2 and PH values change anisotropically, suggesting respiratory.
In the third step, a 30 mmHg increase in PCO2 should decrease the PH value by 3 × 0.08 (±0.02) which is 7.16 ± 0.02, while the actual PH value of this patient is exactly 7.16.
Conclusion: This patient has simple respiratory acidosis.
Example 3: The patient’s PH is 7.50, PCO2 is 50 mmHg, and PO2 is 100 mmHg.
Analysis.
In the first step,the PH value is greater than 7.45, suggesting an alkalosis.
In the second step, the PCO2 and PH values change in the same direction, suggesting metabolic.
Step 3, not necessary, as this patient is not a respiratory acid-base imbalance.
Conclusion: This patient has metabolic alkalosis.