CNS Ischemic Pressor Response (Cushing Response)
The correct answer is option 2: the CNS ischemic pressor response, also known as the Cushing response or ischemic response. This is a fundamental compensatory mechanism where severe cerebral ischemia triggers massive sympathetic activation, dramatically increasing blood pressure to restore cerebral perfusion pressure 1.
Physiological Mechanism
The CNS ischemic pressor response occurs when cerebral blood flow drops critically low, triggering brainstem vasomotor centers to initiate profound sympathetic outflow 1. This response:
- Increases arterial pressure by up to 89% through peripheral vasoconstriction to compensate for reduced cerebral perfusion pressure 2
- Directs sympathetic activation primarily toward peripheral vasculature rather than the heart, explaining why heart rate changes are modest or variable despite massive blood pressure elevation 2
- Serves as a protective mechanism in intracranial hypertension by restoring normal cerebral perfusion pressure when it decreases due to mass-expanding lesions, brain edema, or hydrocephalus 1
Clinical Relevance in Stroke Management
This compensatory response explains why permissive hypertension is recommended in acute ischemic stroke when patients are not receiving reperfusion therapy 3. The elevated blood pressure represents the body's attempt to maintain cerebral perfusion to potentially salvageable tissue in the ischemic penumbra 4, 3.
Blood pressure should not be treated unless systolic >220 mmHg or diastolic >120 mmHg during the first 48-72 hours in patients not receiving thrombolysis, as the hypertension may enhance collateral flow to ischemic tissue 3. Cerebral autoregulation is impaired in the ischemic penumbra, making systemic perfusion pressure critical for blood flow and oxygen delivery 3.
Why Other Options Are Incorrect
- Bainbridge reflex: Involves increased heart rate in response to atrial stretch from increased venous return
- Head paradoxical reflex: Not a recognized cardiovascular reflex
- Marey's reflex: Refers to heart rate slowing in response to increased blood pressure (baroreceptor-mediated)
Critical Clinical Pitfall
Treating elevated blood pressure reflexively without recognizing it as a compensatory response can be harmful 3. Rapid blood pressure reduction can extend the infarct by reducing perfusion pressure to the penumbra, converting potentially salvageable tissue into irreversibly damaged brain 3. If treatment is absolutely required, reduce mean arterial pressure by only 15% over 24 hours 3.