Step 1: Analyze the pH
pH < 7.35 = acidosis
pH > 7.45 = alkalosis |
Determine if the pH is within the normal range, or reflects acidosis or alkalosis. |
Step 2: Analyze the PaCO2
PaCO2 > 45 = acidosis
PaCO2 < 35 = alkalosis |
Carbon dioxide is produced in the tissues of the body and eliminated in the lungs. Changes in the PaCO2 level reflect lung function. |
Step 3: Analyze the HCO3-
HCO3- < 22 = acidosis
HCO3- > 26 = alkalosis |
Bicarbonate is produced by the kidneys. Changes in the HCO3- level reflect metabolic function of the kidneys or bicarbonate loss (e.g., from diarrhea). |
| Step 4: Match the PaCO2 or HCO3- with pH |
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If pH < 7.35 and PaCO2 > 45 and HCO3- level is normal, the patient has respiratory acidosis.
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Causes of respiratory acidosis include hypoventilation, respiratory infection, severe airflow obstruction as in chronic obstructive pulmonary disease (COPD) or asthma, neuromuscular disorders, massive pulmonary edema, pneumothorax, central nervous depression, spinal cord injury, and chest wall injury.
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If pH < 7.35 and HCO3- < 22 and PaCO2 level is normal, the patient has metabolic acidosis.
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Causes of metabolic acidosis include renal failure, DKA, lactic acidosis, sepsis, shock, diarrhea, drugs, and toxins such as ethylene glycol and methanol.
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If pH > 7.45 and PaCO2 < 35 and the HCO3- level is normal, the patient has respiratory alkalosis.
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Causes of respiratory alkalosis include hyperventilation, pain, anxiety, early stages of pneumonia or pulmonary embolism, hypoxia, brainstem injury, severe anemia, and excessive mechanical ventilation.
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If pH is > 7.45 and HCO3- > 26 and the PaCO2 level is normal, the patient has metabolic alkalosis.
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Causes of metabolic alkalosis include diuretics, corticosteroids, excessive vomiting, dehydration, Cushing syndrome, liver failure, and hypokalemia.
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Step 5: Assess for compensation by determining whether the PaCO2 or the HCO3- go in the opposite direction of the pH.
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When a patient has an acid-base imbalance, the respiratory and metabolic systems try to correct the imbalances the other system has produced. There is full or partial compensation if the PaCO2 or HCO3- go in the opposite direction of the pH. |
| If pH 7.35-7.40 (compensated acidosis), PaCO2 > 45 (acidosis), and HCO3- > 26 (alkalosis), the patient has compensated respiratory acidosis. |
To compensate for respiratory acidosis, the kidneys excrete more hydrogen ions and elevate serum HCO3-, in an effort to normalize the pH. |
| If pH 7.35-7.40 (compensated acidosis), PaCO2 <35 (alkalosis), and HCO3- <22 (acidosis), the patient has compensated metabolic acidosis. |
To compensate for metabolic acidosis, the patient's respiratory center is stimulated and the patient hyperventilates to blow off more CO2, raising the pH. |
| If pH 7.40-7.45 (compensated alkalosis), PaCO2 <35 (alkalosis), and HCO3- < 22 (acidosis), the patient has compensated respiratory alkalosis. |
To compensate for respiratory alkalosis, the metabolic system is activated to retain hydrogen ions and lower serum HCO3-, in an effort to raise the pH. |
| If pH 7.40-7.45 (compensated alkalosis), PaCO2 > 45 (acidosis), and HCO3- > 26 (alkalosis), the patient has compensated metabolic alkalosis. |
To compensate for metabolic alkalosis, the patient’s respiratory center is suppressed; decreased rate and depth of respiration causes CO2 to be retained, lowering the pH. |
Step 6: Analyze the PaO2 and SaO2
If PaO2 < 80 mm Hg or SaO2 < 95%, the patient has hypoxemia.
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Causes of hypoxemia include COPD, pneumonia, atelectasis, acute respiraory disease syndrome (ARDS), certain medications, high altitudes, interstitial lung disease, pneumothorax, pulmonary embolism, pulmonary edema, pulmonary fibrosis, anemia, heart disease, intracardiac shunt and sleep apnea. |