respiratory acidosis, metabolic acidosis, respiratory alkalosis and metabolic alkalosis. Arterial blood gas analysis can be simply divided into three steps. The first step is to determine whether the patient has acidosis or alkalosis. The second step is to determine whether the acidosis or alkalosis is respiratory or metabolic. The third step is to determine whether the patient has respiratory acidosis or respiratory alkalosis due to respiratory factors alone or metabolic factors.
The specific method of arterial blood gas analysis is as follows:
Step 1: Check the patient's pH. A normal pH is 7.4 ± 0.05. If the patient is acidotic, the pH should be ≤ 7.35. If the patient is alkalotic, the pH should be ≥ 7.45.
Step 2: Observe the direction of changes in the patient's pH and PCO2. If the acidosis or alkalosis is metabolic, pH and PCO2 will change in the same direction (an increase in pH will increase PCO2, or a decrease in pH will decrease PCO2). If the acidosis or alkalosis is respiratory, pH and PCO2 will change in opposite directions.
Step 3: If respiratory acidosis or alkalosis is present, observe the ratio of changes in the patient's pH to PCO2. A normal PCO2 is 40 ± 5 mmHg. In simple respiratory acidosis or alkalosis, for every 10 mmHg change in PCO2, the pH value changes in the opposite direction by 0.08 ± 0.02. For example, if a patient has a PCO2 of 40 mmHg and a pH of 7.4, a decrease in their PCO2 by 10 mmHg to 30 mmHg would increase their pH by 0.08 to 7.48. A decrease in their PCO2 by 20 mmHg to 60 mmHg would decrease their pH by 0.16 to 7.24.
If the changes in pH and PCO2 do not conform to this ratio, it indicates that the patient also has a second factor (metabolic factor). In this case, the medical staff should compare the patient's theoretical pH value with the actual pH value. If the actual pH value is lower than the theoretical pH value, it indicates that metabolic acidosis is also present. Conversely, if the actual pH value is higher than the theoretical pH value, it indicates that metabolic alkalosis is also present. It should be noted that the pH value calculated according to the formula can fluctuate by ± 0.02.
