A 19-year-old man is brought to the emergency department after sustaining a traumatic brain injury during a motor vehicle collision. On arrival, the patient is comatose and has a Glasgow Coma Scale score of 3. His blood pressure is 110/70 mm Hg, and pulse is 114/min. Pupils are miotic, equal, and reactive to light. Ecchymosis is present behind the ear, but the remainder of the trauma survey is unremarkable. Rapid-sequence intubation is performed, and the patient is mechanically ventilated. A noncontrast head CT scan demonstrates bifrontal contusions and a basilar skull fracture. One day later, a repeat head CT scan reveals diffuse cerebral edema. The ventilator respiratory rate is adjusted to achieve a PaCO₂ level of 26-30 mm Hg. Which of the following is the most likely effect of this intervention?

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11ec160b_3cd5_35ed_9249_ebe58b6f807e_MD0001_00 The brain has little room for expansion as it is encased in the cranium. A small volume change in any of the intracranial components (eg, brain tissue, blood, cerebrospinal fluid [CSF]) can produce significant changes in intracranial pressure (ICP). Therefore, even small increases in cerebral blood volume can raise ICP and cause brain compression. The main factors influencing cerebral circulation are systemic blood pressure and arterial blood gas levels. When systemic blood pressure is 60-140 mm Hg, it has little effect on cerebral blood volume because autoregulation (via cerebral blood vessel dilation and contraction) keeps blood flow constant. Blood pressure >150 mm Hg increases cerebral vascular volume and blood flow, causing a corresponding increase in ICP. In contrast, blood pressure <50 mm Hg causes cerebral hypoperfusion and potential ischemia. Arterial blood gases also have a powerful effect on cerebral blood flow, with PaCO₂ being the most important regulator. A drop in PaCO₂ due to hyperventilation causes vasoconstriction. The resulting reduction in cerebral blood volume (Choice C) leads to decreased ICP. Lowering PaCO₂ is one of the measures employed to reduce ICP in mechanically ventilated patients with cerebral edema. (Choice A) The brain has a very large oxygen requirement, about 20% of all O₂ consumed by the body. Induced sedation and therapeutic hypothermia can decrease brain metabolic demand, exerting a neuroprotective effect and improving ICP by reducing cerebral blood volume. (Choice B) The vasoconstriction induced by hyperventilation can cause a decrease in local cerebral perfusion that can worsen neurologic injury in patients with acute traumatic brain injury or stroke. Therefore, hyperventilation should be used with caution in these patients, and the PaCO₂ should be maintained >30 mm Hg. (Choice E) The thecal sac is a sheath of dura mater that contains CSF and surrounds the spinal cord. Increased ICP can cause displacement of CSF from the brain into the thecal sac. However, the drop in ICP caused by hyperventilation reduces CSF displacement. Educational objective: Carbon dioxide is a potent vasodilator of cerebral vasculature. Tachypnea causes hypocapnia and cerebral vasoconstriction, thereby decreasing cerebral blood volume and intracranial pressure.

A 19-Year-Old Man Is Brought to the Emergency Department After