CPP = MAP – ICP
- Amount of pressure available to perfuse the brain
- Mean Arterial Pressure (MAP) = systemic blood pressure pushing upward
- Intracranial Pressure (ICP) = pressure in cranium resisting flow
- MAP – ICP = the net pressure upward
- Goal = CPP > 70 mmHg
- Can only calculate if you have direct ICP monitoring
- Monitor hourly
- Assess for signs of Cushing’s Triad
- If ICP too high or MAP too low
- Decreased blood flow to brain
- Brain Death
- Reduce ICP
- Medications (mannitol, steroids)
- External Ventricular Drain
- Increase MAP
- Avoid Vagal Maneuvers
- Keep MAP > 80
- See ICP lesson for detailed interventions to keep ICP low
- Purpose for permissive hypertension (some families get concerned about high blood pressure)
- Safety issues surrounding EVD / Increased ICP
So when we are caring for a patient with increased intracranial pressure, one of the most important things that we consider and monitor is the cerebral perfusion pressure or CPP.
So what is CPP? CPP is the amount of pressure available to get the blood from the heart or the systemic circulation up into the brain to perfuse it. We calculate CPP by taking the Mean Arterial Pressure, or MAP, and subtracting the Intracranial Pressure, or ICP. The MAP is the average pressure in the systemic circulation down here pushing its way upward toward the brain. The ICP is the pressure within the skull causing resistance against that blood pressure. So we want to make sure that the leftover pressure upwards is enough to perfuse the brain. Now, we know that normal ICP is about 5-15 mmHg, and normal MAP is above 65 mmHg. Our goal for these patients is actually going to be a CPP greater than 70 mmHg. This will make sure that there’s enough perfusion pressure upwards. So if we have a patient whose ICP is sitting at about 10 mmHg, we actually need to get their blood pressure up so their MAP is greater than 80. That would make their MAP (80) minus their ICP (10) equal 70 mmHg. So what we’ll see is that sometimes we allow, or even cause, hypertension – because it’s SO important that we get that blood flow up to the brain.
The problem for the patients will come when the ICP is too high or the MAP is too low. That would mean that the CPP or the available pressure to perfuse the brain is getting lower and lower. The closer your MAP and ICP get to each other, the less blood flow you are getting to your brain and the closer you are to brain death. A CPP of 0 would indicate NO flow. So we do everything we can keep their blood pressure up, including vasopressors. We usually aim for a MAP above 80, but we would go higher if our patient’s ICP was higher. We’ll also be working hard to get that ICP to stay low – refer back to the ICP lesson for details of that, but it may include inserting an EVD or giving Mannitol. Now, once your CPP gets to 0 and we say “no flow”, that’s it – we technically don’t say there’s a negative CPP – but I’ve actually had a patient who was herniating whose ICP kept going up and up – right before she finally herniated, her ICP was 85 and her MAP was 80. You’ll get to read more about her story in the Case Study attached to this lesson.
So remember that CPP is the amount of pressure available to perfuse the brain – without a good CPP, the brain isn’t getting blood flow. Tissues that don’t get blood flow will die. We calculate CPP by using MAP minus ICP. That difference is our available perfusion pressure upwards toward the brain. It’s like two people pushing on opposite sides of a door – the stronger one will win and get through faster. If they’re equal, neither one is going anywhere. Our goal in this case is usually a CPP of greater than 70 mmHg. We will intervene as necessary to get their MAP higher or to get their ICP lower so we can maximize our Cerebral perfusion pressure.
Now, CPP can only be calculated in someone with an ICP monitor in place, which is usually in the ICU. So if you see this in the clinical setting, you’ll look super smart if you do this calculation yourself and talk about the patient’s CPP! Now, go out and be your best self today. And, as always, happy nursing!