01.04 Cerebral Metabolism

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Outline

Overview

The brain has very high perfusion and energy (glucose) requirements.

Nursing Points

General

  1. Perfusion requirements
    1. The brain uses:
      1. 20% of total cardiac output (CO)
      2. 20% of total delivered oxygen (DO2)
    2. VERY sensitive to changes in CO or DO2
    3. Monitor Mean Arterial Pressure (MAP), Cerebral Perfusion Pressure (CPP), PaO2 and SpO2
  2. Energy requirements
    1. Massive energy needs
    2. Uses large amount of glucose
    3. Doesn’t require insulin to uptake glucose
    4. VERY sensitive to changes in blood glucose levels
      1. Hypoglycemia
        1. 50-70 mg/dL = neuro changes (AMS, ↓ LOC)
        2. <20 mg/dL = coma
      2. Hyperglycemia
        1. Hyperosmolar
        2. Osmotic shift of fluid into vessels
        3. Cellular dehydration in brain cells
    5. If patient presents AMS or ↓ LOC, check a blood glucose level

Transcript

When we talk about cerebral metabolism, we’re referring to the perfusion and energy requirements of the brain. So let’s look at this closer.

The brain is a very greedy organ. I like to think of it like a toddler. It wants all of your time and attention. In this case, the brain actually uses about 20% of our cardiac output. It also uses about 20% of the oxygen delivered to the body, we write that DO2. It’s only like 2% of our total body weight, but it uses 20% of our cardiac output and oxygen delivery! Talk about greedy. So what you’ll see, just like a toddler, is that as soon as you even remotely shift that time and attention away, it’s very sensitive to that and you’ll start to see neuro changes.

The same high need applies when it comes to energy. The brain has a super high requirement for glucose for energy. What makes it different from the rest of the body is that the brain cells don’t require insulin in order to uptake glucose for energy. BUT, what we do see is that changes in blood glucose will quickly cause neurological problems for your patient. Even a slightly low blood sugar of 50-70 mg per deciliter can cause neuro changes like confusion or lethargy. I’ve experienced this before, I actually got really dizzy and almost passed out. If the patient’s sugar manages to get below 20 mg per deciliter, we will actually see them more in a comatose or unresponsive state. A LOT of our rapid responses in the hospital are patients whose blood sugars are super low and they aren’t responding. So if your patient is having a significant neuro change, make sure you check their blood sugar!

Now, high blood sugar can also cause some problems neurologically. That’s because it puts the blood in what’s called a hyperosmolar state. What that means is that there’s way more solute in the blood than there is in the tissues and cells. We know the body likes to be in homeostasis, right? So to balance that out, water and fluid from the tissues and cells will begin pouring into the bloodstream. This pulls it out of the cells and causes cellular dehydration. When the brain cells get dehydrated, the patient can get very confused, restless, and even agitated. So again, with any mental status change, make sure you’re also checking a blood sugar because it can have a big effect.

So remember that the brain is like a greedy toddler – it wants 20% of our cardiac output and oxygen delivery and has a super high energy requirement. If it doesn’t get what it wants, it’s very sensitive and will throw a temper tantrum. You can see anything from confusion, to restlessness, to a full-out coma. Make sure you consider these things when assessing a patient with a neuro change.

You guys are gonna be awesome nurses and take such good care of your patients! Go out and be your best selves today! And, as always, happy nursing!