Partial Pressure of Carbon Dioxide (PCO₂)

The partial pressure of carbon dioxide (PCO₂) measures the pressure or tension exerted by dissolved CO₂ in the blood (10% [0.10] of CO₂ is carried in plasma and 90% [0.90] in red blood cells) and is proportional to the partial pressure of CO₂ in the alveolar air. The test is commonly used to detect a respiratory abnormality and to determine the alkalinity or acidity of the blood. To maintain CO₂ within normal limits, the rate and depth of respiration vary automatically with changes in metabolism. This test is an index of the effectiveness of alveolar ventilation; it is the most physiologically reflective blood gas measurement. An arterial sample directly reflects how well air is exchanged with blood in the lungs.

CO₂ tension in the blood and in cerebrospinal fluid is the major chemical factor regulating alveolar ventilation. When the CO₂ tension in arterial blood (Pa) rises from 40 to 45 mm Hg (5.3–6.0 kPa), it causes a threefold increase in alveolar ventilation. A PaCO₂ of 63 mm Hg (8.4 kPa) increases alveolar ventilation 10-fold. When the FICO₂ is >0.05 (>5%), the lungs can no longer be ventilated fast enough to prevent a dangerous rise of CO₂ concentration in tissue fluids. Any further increase in CO₂ begins to depress the respiratory center, causing a progressive decline in respiratory activity rather than an increase.

1. Obtain an arterial blood sample (or venous sample if requested) according to protocols.

2. Label the sample with patient’s name, identification number, date, time, mode of O₂ therapy, and flow rate.

3. Place the sample on ice and transfer it to the blood gas laboratory.

1. An increase in PaCO₂ (hypercapnia) usually is associated with hypoventilation (CO₂ retention); a decrease is associated with hyperventilation (“blowing off” CO₂). A reduction in PaCO₂, through its effect on the plasma bicarbonate concentration, decreases renal bicarbonate reabsorption. For each 1–mm Hg (0.133 kPa) decrease in the PaCO₂, the plasma bicarbonate will decrease by approximately 1 mEq/L (1 mmol/L). Because HCO₃⁻ and PaCO₂ bear this close mathematical relationship, and this ratio, in turn, defends the hydrogen ion concentration, the outcome is that the steady-state PaCO₂ in simple metabolic acidosis is equal to the last two digits of the arterial pH (pHa). Also, addition of 15 to the bicarbonate level equals the last two digits of the pHa. Failure of the PaCO₂ to achieve predicted levels defines the presence of superimposed respiratory acidosis on alkalosis.

2. Causes of decreased PaCO₂ include:

   1. Hypoxia

   2. Nervousness

   3. Anxiety

   4. Pulmonary emboli

   5. Pregnancy

   6. Pain

   7. Other cause of hyperventilation

3. Causes of increased PaCO₂ include:

   1. Obstructive lung disease

      1. Chronic bronchitis

      2. Emphysema

   2. Reduced function of respiratory center

      1. Overreaction

      2. Head trauma

      3. Anesthesia

   3. Other, less common causes of hypoventilation (e.g., Pickwickian syndrome)

Pretest Patient Care

1. Explain the purpose, benefits, and risks of the invasive arterial blood sampling procedure. Assess the patient’s ability to cooperate.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Normal

• PaCO₂ (arterial blood): 35–45 mm Hg (4.7–6.0 kPa)

• PvCO₂ (venous blood): 41–57 mm Hg (5.4–7.6 kPa)