The Lay Medical Man

Tag: EEG

  • 🧠 When Raising CPP Won’t Help: The Type 2 Picture on Cerebral Microdialysis

    🧠 When Raising CPP Won’t Help: The Type 2 Picture on Cerebral Microdialysis

    Your ICP is 14. Your CPP is sitting where the autoregulation index says it should. PbtO2 is comfortable. And the lactate/pyruvate ratio from the microdialysis catheter has been climbing all night.

    This is the moment microdialysis earns its place, because it is telling you something no pressure-based monitor can. The question is what to do next — and the instinct to push the pressure up is often wrong.

    📌 The hook: High LPR with low pyruvate means no oxygen. High LPR with normal or high pyruvate means sick mitochondria. The first responds to flow and oxygen. The second does not.

    🔬 What the catheter actually measures

    A 20 kDa membrane perfused at 0.3 µL/min, giving roughly 70% recovery, with vials collected hourly. Three properties follow from that, and all three matter at the bedside: the readout is focal, it is delayed, and it is labour-intensive. You are sampling one small volume of brain, an hour behind events.

    📊 Thresholds worth knowing

    From the 2014 international consensus statement:

    AnalyteThreshold
    Lactate/pyruvate ratio> 25 abnormal; > 40 metabolic crisis
    Brain glucose< 0.8 mmol/L low; < 0.2 mmol/L critical
    GlutamateAdjunct — excitotoxicity, seizures
    GlycerolAdjunct — membrane breakdown

    🎯 Where the catheter sits decides what it means

    The tip belongs in at-risk tissue: pericontusional brain, or territory prone to vasospasm. Not in the dead core, where the numbers are meaningless, and not in distant normal brain, where they are reassuring and irrelevant. Confirm the gold tip on CT before you believe anything the analyser tells you.

    🩺 Type 1 or Type 2?

    Type 1, ischaemic. High LPR with low pyruvate. Oxygen or flow is not arriving. This one you treat with delivery.

    Type 2, non-ischaemic. High LPR with normal or high pyruvate. Substrate is arriving; the cell cannot use it. Raising CPP here buys you nothing, and the oedema and cardiac cost of chasing pressure are real. The rest of this post is about Type 2.

    1️⃣ Exclude hidden ischaemia first

    Mixed pictures exist, and a focal catheter can miss a regional supply problem. Before you accept a mitochondrial label, go back through the delivery chain: PbtO2, CPP measured against the autoregulation-guided optimum rather than a protocol number, ICP, haemoglobin, PaO2 and PaCO2.

    If you find a supply deficit, fix it regardless. It is cheap, and it is the safe default when you are not sure which picture you are looking at.

    2️⃣ Hunt for the cause of the metabolic stress

    • Seizures or non-convulsive status. Get continuous EEG. A high LPR with high glutamate points here, and non-convulsive status is easy to miss in a sedated patient.
    • Fever or hyperthermia. Treat to normothermia to cut demand.
    • Glucose. Read brain glucose alongside the LPR, never alone. Low brain glucose with tight systemic control is neuroglycopenia, and the answer is to liberalise.
    • Sepsis, or a mitochondrial toxin. This includes propofol above 4 mg/kg/h — the propofol infusion syndrome link. Reduce the dose and change agent.
    • Catecholamine- and glutamate-driven hyperglycolysis. This raises lactate without any true mitochondrial failure, and it is worth recognising before you escalate.

    3️⃣ Cut demand, support the substrate

    • Deepen sedation, or use a barbiturate if the ICP situation allows it.
    • Avoid tight glycaemic control. Aim for roughly 140–180 mg/dL, following the NICE-SUGAR logic. The injured brain tolerates tight control badly.
    • Start nutrition early.

    4️⃣ Experimental options — name them, and say so

    In a viva, knowing these exist is worth marks. Claiming they are standard is not.

    • Hypertonic sodium lactate as an alternative fuel (Ichai, Bouzat).
    • Normobaric hyperoxia. Small studies improved the LPR, but PET showed no gain in CMRO2. The evidence is genuinely mixed.
    • Cyclosporin, as a blocker of the mitochondrial permeability transition pore.

    🗣️ Viva essentials

    Q. What do the lactate/pyruvate ratio and glucose tell you that ICP cannot, and why must the catheter not sit in the contusion core?

    Say: LPR > 25 and > 40 · type 1 versus type 2 · penumbral placement · 0.3 µL/min. ICP and CPP describe the plumbing; microdialysis describes whether the cell is actually making ATP. In dead core tissue there is no metabolism left to measure, so the numbers mean nothing.

    Probe: how does tight insulin therapy show up? As falling brain glucose with a rising LPR — neuroglycopenia in a patient whose blood sugar looks respectable.

    🧾 Take-home

    Pyruvate is the discriminator. Low pyruvate sends you to the delivery chain; normal pyruvate sends you looking for seizures, fever, sepsis, propofol and tight glycaemic control. The temptation with a rising LPR is always to push the pressure higher. In the Type 2 picture that is effort spent on the wrong organ system.

    For education only. Follow your local neurocritical care protocols.

  • 🧠 The Sleep Switch: Why a Patient on Dexmedetomidine Wakes When You Call Them

    A patient on a dexmedetomidine infusion looks asleep. You say their name. They open their eyes, squeeze your hand, answer a question, and drift off again. Nothing else in the sedation cupboard behaves like that.

    The reason is anatomical. Dexmedetomidine does not blanket the cortex the way propofol does. It flips a switch the brain already owns.

    📌 In one line: Dexmedetomidine silences the locus coeruleus, which releases the VLPO from inhibition and tips the sleep–wake flip-flop towards sleep. Everything clinically distinctive about the drug follows from that one step.

    🔀 The sleep switch

    The VLPO — ventrolateral preoptic nucleus — is a small cluster of GABA and galanin neurons in the anterior hypothalamus. It is the brain’s off switch for wakefulness.

    It sends inhibitory projections to the arousal nuclei: the locus coeruleus (noradrenaline), the tuberomammillary nucleus (histamine), the raphe nuclei (serotonin), and the orexinergic neurons of the lateral hypothalamus. Those nuclei inhibit the VLPO straight back.

    Mutual inhibition of that kind makes a flip-flop circuit: two stable states, awake or asleep, with fast transitions between them and no comfortable middle ground. That is the design intent. Nobody benefits from being half asleep while crossing a road.

    💉 Where dexmedetomidine gets in

    Dexmedetomidine is an alpha-2A agonist, and the locus coeruleus is dense with those receptors. Agonism there cuts noradrenergic output. The LC falls quiet, the VLPO is released from inhibition, and the switch tips towards the sleep state.

    In other words, dexmedetomidine borrows an endogenous pathway. Propofol and the volatile agents do something quite different: they act directly on GABA-A receptors across the cortex and thalamus — a broader and far less physiological suppression.

    That is why dexmedetomidine sedation resembles natural stage-2 NREM sleep rather than GABAergic hypnosis, and why a sufficient stimulus can flip the switch back. The rousable patient is not a quirk of dosing. It is the mechanism showing itself.

    📈 What it means on the EEG

    A stage-2 NREM-like state has stage-2 features: sleep spindles and K-complexes, not the deep delta slowing or burst suppression of heavy GABAergic hypnosis.

    The practical consequence is that interictal epileptiform discharges stay visible. This is what makes dexmedetomidine suited to MRI-EEG in epilepsy surgery planning, where propofol risks suppressing the very activity you came to record. The same property lets you rouse the patient for a neurological check and then allow them to settle again — useful in awake craniotomy and in the neuro-ICU.

    🎯 The drug needs an intact target

    If the mechanism depends on switching off the locus coeruleus, then the LC has to be there to switch off. It sits in the dorsal pontine tegmentum, close to the floor of the fourth ventricle — a crowded neighbourhood, and one we operate in.

    • Pontine infarct or haemorrhage
    • Surgery on the floor of the fourth ventricle
    • Brainstem traumatic injury
    • Infiltrative lesions, such as diffuse midline glioma, H3 K27-altered (formerly DIPG)

    The two you will actually meet are the fourth-ventricle tumour resection and the pontine stroke. The glioma is an instructive example rather than a common anaesthetic encounter. Where the target is damaged, the sedation you predicted may simply not arrive — and the answer is a different agent, not a larger dose.

    🌙 REM rebound and the sleep apnoea patient

    REM is the respiratorily unsafe stage of sleep: atonia of the pharyngeal dilators and of the accessory muscles. Opioids and volatile agents suppress REM during and immediately after surgery, and that debt is repaid later as REM rebound, classically on postoperative nights 2 to 4.

    That is precisely when the patient with obstructive sleep apnoea desaturates — well after the recovery unit, usually on a general ward, often unmonitored, and frequently on the night everyone has stopped worrying.

    Dexmedetomidine suppresses REM far less than the GABAergic sedatives do. Together with its minimal respiratory depression, that is a substantial part of the argument for it in this group.

    🗣️ A viva probe

    Q. Your patient on dexmedetomidine looks asleep but obeys commands. Which sleep stage does that resemble, and what does it tell you about the EEG?

    A. Stage-2 NREM, with spindles on the trace. The practical point is that epileptiform activity stays readable, which is why the drug suits MRI-EEG in epilepsy work-up.

    🧾 Take-home

    Propofol imposes unconsciousness on the cortex. Dexmedetomidine persuades the hypothalamus to do it for you. The rousable patient, the readable EEG, the preserved REM and the occasional disappointing response after a pontine lesion are all the same fact, seen from four directions.

    For education only. Follow your local sedation and anaesthesia protocols.