Hypertension, bradycardia and irregular respiration
Mnemonic
Cushing’s triad, the response to critically raised intracranial pressure:
- Hypertension, with a widening pulse pressure
- Bradycardia
- Irregular respiration, often Cheyne-Stokes
“Up, down, and irregular.”
The mechanism is a chain: rising intracranial pressure reduces cerebral perfusion, so the brainstem drives a sympathetic surge to raise systemic pressure; the resulting hypertension triggers a baroreceptor mediated bradycardia; brainstem compression then disturbs the respiratory centres.
Cerebral perfusion pressure = mean arterial pressure - intracranial pressure, which is why the reflex exists and why lowering blood pressure in this setting is dangerous.
It is a late and pre-terminal sign, usually indicating imminent herniation, and it demands immediate treatment of the pressure rather than of the blood pressure.
Expansion
The triad:
- Hypertension, with a widening pulse pressure
- Bradycardia
- Irregular, depressed respiration
The mechanism follows from cerebral perfusion pressure = mean arterial pressure - intracranial pressure. As intracranial pressure rises, perfusion pressure falls and the brainstem becomes ischaemic. A massive sympathetic discharge raises systemic pressure to restore perfusion.
The resulting hypertension then stimulates the baroreceptors, producing reflex vagal bradycardia. Distortion of the brainstem disturbs the respiratory centre.
Note the contrast with hypovolaemic shock, where hypotension and tachycardia occur together. Hypertension with bradycardia in a head-injured patient is the opposite pattern and signals impending herniation, requiring immediate measures to lower intracranial pressure.