Cushing's triad and the Cushing reflex
Cushing's triad is the combination of high blood pressure, a slow pulse and irregular breathing, a late sign of dangerously raised pressure inside the skull.
Remember
When the brain is short of blood, the body pushes the blood pressure up, the pulse then slows and the breathing becomes irregular, a late warning of very high pressure inside the skull.
Memory aid
pressure up, pulse down, breathing ragged
Each phrase gives the direction of one change in the triad: blood pressure up, pulse down and breathing irregular.
Cushing's triad, also called the Cushing reflex, appears late when intracranial pressure (the pressure inside the skull) is dangerously high. It is a set of three findings: systolic hypertension, bradycardia (a slow heart rate) and irregular breathing. It can mean that herniation (the brain being pushed through an opening in the skull) is about to happen, so the patient needs emergency treatment.
The chain starts with the cerebral perfusion pressure, the pressure that drives blood through the brain. It falls when the pressure inside the skull rises, and the brain, especially the brainstem, becomes ischaemic (short of blood). That triggers a sympathetic surge (the fight or flight response) that narrows the arteries and drives the blood pressure up, mostly the systolic pressure, so the pulse pressure (the gap between systolic and diastolic) widens. The body is trying to force blood into the brain.
Baroreceptors (pressure sensors in the arteries) detect the high pressure and slow the heart. So the bradycardia is a reflex response to the high blood pressure, not the first event. Pressure on the brainstem, where the breathing centres sit, then makes the breathing irregular.
Key facts
- The triad
- systolic hypertension, bradycardia (a slow heart rate) and irregular breathing.
- The trigger
- a falling cerebral perfusion pressure, which leaves the brain ischaemic (short of blood).
- Blood pressure
- the systolic pressure rises much more than the diastolic, so the pulse pressure widens.
- Heart rate
- the slowing is a reflex response to the high blood pressure, not the first event.
- Breathing
- it becomes irregular because of pressure on the brainstem, where the breathing centres sit.
- Meaning
- a late sign that herniation may be imminent, needing emergency treatment.
Exam traps
- Low blood pressure, a fast pulse and shallow breathing describe shock (a failing circulation), as in major blood loss. The triad is the reverse for the blood pressure and the pulse: the blood pressure is high and the pulse is slow.
- A raised blood pressure with a fast pulse and fast breathing is a sympathetic stress response, as in pain or anxiety. In the triad the heart slows and the breathing is irregular rather than fast.
- The slowing of the heart is not the primary trigger: it is a secondary reflex to the high blood pressure, so the sequence starts with ischaemia and sympathetic activation.
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