When evaluating an ABG from a client with a subdural hematoma, the nurse notes the PaCO2 is 30 mm Hg. Which of the following responses best describes this result?
A. Appropriate; lowering carbon dioxide (CO2) reduces intracranial pressure (ICP).
B. Emergent; the client is poorly oxygenated.
C. Normal
D. Significant; the client has alveolar hypoventilation.
Correct Answer: A. Appropriate; lowering carbon dioxide (CO2) reduces intracranial pressure (ICP).
A normal PaCO2 value is 35 to 45 mm Hg. CO2 has vasodilating properties; therefore, lowering PaCO2 through hyperventilation will lower ICP caused by dilated cerebral vessels. A subdural hematoma forms because of an accumulation of blood under the dura mater, one of the protective layers to the brain tissue under the calvarium.
Option B: Oxygenation is evaluated through PaO2 and oxygen saturation. The clinician must begin immediate medical management. These measures include sedation, neuromuscular blockade when appropriate, moderate hyperventilation to a Pc02 (32 to 36), adequate oxygenation to maintain Sp02 greater than 95%, head elevation, and avoidance of hyperthermia.
Option C: Often, the bleeding is undetected initially, discovered as a chronic subdural hematoma. When there is a sufficient accumulation of blood to occupy a large intracranial space, the brain midline shifts toward the opposite side, encroaching on the brain structures against the inner surface of the calvarium after decreasing the volume of the lateral third and fourth ventricles. As the intracranial space becomes limited, the volumetric forces push the uncal portion of the temporal lobe toward the foramen magnum causing herniation of the brain.
Option D: Alveolar hypoventilation would be reflected in an increased PaCO2. The infusion of hypertonic saline or mannitol serves to decrease intracranial pressure by promoting osmotic changes in the brain and transiently affecting the rheological properties of the cerebral blood flow, respectively.
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