Step 1. 

Look at the Pao₂ level, and answer this question: Does the Pao₂ show hypoxemia? The Pao₂ is a measure of the partial pressure (P) of oxygen dissolved in arterial (a) blood plasma. Sometimes, Pao₂ is shortened to Po₂. It is reported in millimeters of mercury (mm Hg). Pao₂ reflects 3% of total oxygen in the blood.¹

The normal range of Pao₂ values for persons breathing room air at sea level is 80 to 100 mm Hg. However, the normal range is age dependent for infants and for persons 60 years old or older. The normal level for infants breathing room air is between 50 and 70 mm Hg.² The normal level for persons 60 years old or older decreases with age as changes occur in the ventilation/perfusion (V/Q) matching in the aging lung.^1,3 The correct Pao₂ for older persons can be ascertained as follows: 80 mm Hg (the lowest normal value) minus 1 mm Hg for every year of age above 60 years. Using this formula, a 65-year-old individual can have a Pao₂ as low as 75 mm Hg (80 mm Hg − 5 mm Hg = 75 mm Hg) and still be within the normal range. An acceptable range for an 80-year-old person (20 years older than 60 years) is 60 mm Hg (80 mm Hg − 20 mm Hg = 60 mm Hg).

At any age, a Pao₂ lower than 40 mm Hg represents a life-threatening situation that necessitates immediate action.¹ A Pao₂ value less than the predicted lowest value indicates hypoxemia, which means that a lower-than-normal amount of oxygen is dissolved in plasma.¹

The Pao₂ level should be analyzed before those of other blood gas components. A Pao₂ of less than 40 mm Hg severely compromises tissue oxygenation and calls for the immediate administration of supplemental oxygen or mechanical ventilation, or both. The test results for the Pao₂ level can be analyzed quickly. If the Pao₂ level is more than the lowest value for the patient’s age, it is normal.

Step 2. 

Look at the pH level, and answer this question: Is the pH on the acid or alkaline side of 7.40? The pH is the hydrogen ion (H⁺) concentration of plasma. Calculation of pH is accomplished by using the partial pressure of carbon dioxide (Paco₂) and the plasma bicarbonate level (HCO₃⁻). The formula used is the Henderson-Hasselbalch equation (see Appendix B).⁴

The normal pH of arterial blood is 7.35 to 7.45, and the mean is 7.40. If the pH level is less than 7.40, it is on the acid side of the mean. A pH level less than 7.35 is known as acidemia, and the overall condition is called acidosis. If the pH level is greater than 7.40, it is on the alkaline side of the mean. A pH level greater than 7.45 is known as alkalemia, and the overall condition is called alkalosis.1,4,5

Step 3. 

Look at the Paco₂ level, and answer this question: Does the Paco₂ show respiratory acidosis, alkalosis, or normalcy? The Paco₂ is a measure of the partial pressure of carbon dioxide dissolved in arterial blood plasma, and it is reported in millimeters of mercury (mm Hg). It is the acid–base component that reflects the effectiveness of ventilation in relation to the metabolic rate.⁴ In other words, the Paco₂ value indicates whether the patient can ventilate well enough to rid the body of the carbon dioxide produced as a consequence of metabolism.

The normal range for Paco₂ is 35 to 45 mm Hg. This range does not change as a person ages. A Paco₂ value greater than 45 mm Hg defines respiratory acidosis, which is caused by alveolar hypoventilation. Hypoventilation can result from chronic obstructive pulmonary disease (COPD), oversedation, head trauma, anesthesia, drug overdose, neuromuscular disease, or hypoventilation with mechanical ventilation.⁶

Ventilatory failure results when the Paco₂ level exceeds 50 mm Hg. Acute ventilatory failure occurs when the Paco₂ level is greater than 50 mm Hg and the pH level is less than 7.30. It is referred to as acute because the pH is abnormal, not allowing enough time for the body to compensate by returning the pH to the normal range. Chronic ventilatory failure is defined as a Paco₂ value greater than 50 mm Hg and a pH level greater than 7.30.1,7

A Paco₂ value that is less than 35 mm Hg defines respiratory alkalosis, which is caused by alveolar hyperventilation. Hyperventilation can result from hypoxia, anxiety, pulmonary embolism, pregnancy, and hyperventilation with mechanical ventilation or as a compensatory mechanism for metabolic acidosis.⁴

Step 4. 

Look at the HCO₃⁻ level, and answer this question: Does the HCO₃⁻ show metabolic acidosis, alkalosis, or normalcy? Bicarbonate (HCO₃⁻) is the acid–base component that reflects kidney function. The bicarbonate level is reduced or increased in the plasma by renal mechanisms. The normal range is 22 to 26 mEq/L.4,5

A bicarbonate level of less than 22 mEq/L defines metabolic acidosis, which can result from ketoacidosis, lactic acidosis, renal failure, or diarrhea. The cumulative effect is a gain of acids or a loss of base. A bicarbonate level that is greater than 26 mEq/L defines metabolic alkalosis, which can result from fluid loss from the upper gastrointestinal tract (vomiting or nasogastric suction), diuretic therapy, severe hypokalemia, alkali administration, or steroid therapy.4,5,6

Step 5. 

Look again at the pH level, and answer this question: Does the pH show a compensated or an uncompensated condition? If the pH level is abnormal (less than 7.35 or greater than 7.45), the Paco₂ value or the HCO₃⁻ level, or both, will also be abnormal. This is an uncompensated condition because the body has not had enough time to return the pH to its normal range.4,5,6,8 Box 19-2 provides two examples of uncompensated ABGs. If the pH level is within normal limits and the Paco₂ value and the HCO₃⁻ level are abnormal, the condition is compensated because the body has had enough time to restore the pH to within its normal range.^4,5,6,8

Differentiating the primary disorder from the compensatory response can be difficult. The primary disorder is the abnormality that caused the pH level to shift initially. It is determined according to the pH level; the primary disorder is considered to be the one on whichever side of 7.40 the pH level occurs.4,8 Box 19-3 provides two examples of compensated ABGs. Partial compensation may be present and is evidenced by abnormal pH, Paco₂, and HCO₃⁻ levels, indications that the body is attempting to return the pH to its normal range.^4,8

Table 19-1 summarizes the changes in the acid–base components that accompany various acid–base disorders.^4,5,8 In addition to the parameters previously discussed, other factors must be considered when reviewing a patient’s ABGs, including oxygen saturation, oxygen content, base excess and deficit, and anion gap analysis. Table 19-2 summarizes conditions that may potentiate acid–base abnormalities.^4,5,8