Cerebral Perfusion Pressure (CPP) Calculator

How does this Cerebral Perfusion Pressure (CPP) calculator work?

This is a health tool that determines the pressure of cerebral perfusion based on mean arterial pressure determination and either intracranial or central venous pressure. Given that brain perfusion is a very fine indicator of the integrity of blood flow to the brain, should be monitored accurately, especially in cases of trauma.

The Cerebral Perfusion Pressure (CPP) calculator measures the pressure gradient that drives blood flow and there are only two fields to fill in according to the data necessary in the formula:

Following the legend, the formulas are:

■ MAP - Mean arterial pressure;

■ CVP - Central venous pressure;

■ ICP - Intracranial venous pressure.

Cerebral Perfusion Pressure = MAP - ICP

OR

Cerebral Perfusion Pressure = MAP - CVP

Cerebral pressures

The brain is set to constantly regulate the blood it receives through the resistance of cerebral vessels but there are cases in which the way these normal mechanisms work is altered and the brain becomes unable to support changes in perfusion pressure. Cerebral blood flow remains constant over the range of MAP 50 to 150 mmHg.

There is no optimum registered range for CPP as evidence has yet to show whether there is an ideal value, but values should not go lower than 70 - 80 mmHg and any slight change or imbalance should be treated seriously. Often, clinicians are let to make a judgment call as every estimation needs to account for patient data and type of trauma or underlying cause as well. Some severe TBIs, although maintaining optimal cerebral perfusion rates, will still suffer from severe cerebral hypoxia due to the level and/or location of the brain injury.

Less pressure can lead to brain tissue damage, cerebral ischemia to different degrees, due to inadequate blood arriving to the brain and as well a dangerous increase in intracranial pressure (ICP). The faster the intervention is in these cases, the more CPP evaluation can prevent and reduce mortality in severe head injuries. It is considered that by maintaining cerebral perfusion at above 70 mmHg, the mortality risk is lowered to 35%.

CPP can be raised by either raising the mean arterial pressure or by trying to lower the intracranial pressure. ICP normal range should be under 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 to filter blood.

Beside ICP and CPP monitoring, there are other modalities to check blood flow and brain oxygenation such as: PtiO₂ or cerebral microdialysis and these are often used in conjunction to the traditional methods.

The most common criteria that require monitoring include: severe traumatic brain injury, age over 40, unilateral or bilateral motor posturing or systolic blood pressure lower than 90 mmHg. In cases where intracranial pressure is found to be higher than 20 - 25 mmHg, intervention is required.

Example calculation

Let’s take the case of a patient with the following measured determinations:

■ Systolic blood pressure = 114 mmHg;

■ Diastolic blood pressure = 86 mmHg;

■ Intracranial pressure = 32 mmHg.

The value of MAP is derived from the SBP and DBP values above and equals = 95 mmHg.

(Learn how to calculate the mean arterial pressure)

CPP = 95 - 32 = 63 mmHg

References

1) Kirkman MA, Smith M. (2014) 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; 112(1):35-46.

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

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

4) Czosnyka M, Pickard JD. (2004) Monitoring and interpretation of intracranial pressure. J Neurol Neurosurg Psychiatry; 75(6):813-21.