Cerebral Perfusion Pressure (CPP) Calculator

Variables and formula

CPP measures the pressure gradient that drives blood flow in the brain and can be determined from mean arterial pressure determination and either intracranial or central venous pressure. Therefore the two formulas are:

Cerebral Perfusion Pressure = MAP - ICP

OR

Cerebral Perfusion Pressure = MAP - CVP

Where:

■ MAP - Mean arterial pressure;

■ CVP - Central venous pressure;

■ ICP - Intracranial venous pressure.

Brain perfusion is a very fine indicator of the integrity of blood flow to the brain and remains constant over the range of MAP 50 to 150 mmHg.

There is not a specific normal range for CPP, however, it should not go lower than 70 - 80 mmHg and any changes need to be treated seriously.

Low CPP (hypoperfusion) could cause brain tissue to become ischemic whilst high CPP (hyperperfusion) may lead to elevated intracranial pressure (ICP).

Cerebral pressure explained

The resistance of cerebral vessels constantly regulates how much blood arrives to the brain. Cerebral blood flow remains when the mean arterial pressure (MAP) is between 50 to 150 mmHg.

When these mechanisms are altered or after brain trauma, abnormally low or high cerebral perfusion may have significant consequences.

Less pressure means that inadequate amounts of blood are arriving to the brain and this can cause cerebral ischemia of different degrees. Intervention needs to be rapid, especially in severe head injuries, to reduce mortality risk.

It is considered that by maintaining cerebral perfusion above 70 mmHg, the mortality risk is lowered to 35%.

In severe traumatic brain injury, although the optimal cerebral perfusion rate may be obtained and kept, the patient may still suffer from severe cerebral hypoxia due to the level and/or location of the brain injury.

CPP can be raised by increasing MAP or by decreasing intracranial pressure. The former is normally below 20 mmHg. Increased ICP leads to an increase in the interstitial hydrostatic pressure, compression of cerebral arteries and a decrease in the force of the capillaries.

Brain perfusion monitoring is required in:

■ Severe traumatic brain injury;

■ Trauma patients aged over 40;

■ Unilateral or bilateral motor posturing;

■ OR systolic blood pressure lower than 90 mmHg.

In cases where intracranial pressure is higher than 20 - 25 mmHg, intervention is required.

Beside ICP and CPP monitoring, there are other ways to check blood flow and brain oxygenation, that sometimes accompany traditional methods, such as: PtiO₂ or cerebral microdialysis.

References

1. Steiner LA, Andrews PJ. Monitoring the injured brain: ICP and CBF. Br J Anaesth; 97(1):26-38. Epub 2006 May 12.

2. Kirkman MA, Smith M. Intracranial pressure monitoring, cerebral perfusion pressure estimation, and ICP/CPP-guided therapy: a standard of care or optional extra after brain injury? Br J Anaesth. 2014; 112(1):35-46.

3. Rosner MJ, Rosner SD, Johnson AH. Cerebral perfusion pressure: management protocol and clinical results. J Neurosurg. 1995; 83(6):949-62.

4. White H, Venkatesh B. Cerebral perfusion pressure in neurotrauma: a review. Anesth Analg. 2008; 107(3):979-88.

5. Gobiet W, Grote W, Bock WJ. The relation between intracranial pressure, mean arterial pressure and cerebral blood flow in patients with severe head injury. Acta Neurochir. 1975; 32(1-2):13-24.