Chapter 21: Local and General Anesthetics

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome back to The Deep Dive.

Today we are tackling something that I think we all take for granted until we desperately desperately need it.

Oh absolutely.

It feels like magic but as it turns out it's actually just very very precise and and maybe slightly terrifying chemistry.

We're talking about the off switch.

The off switch.

That's a good way to put it.

The off switch for the human body.

We are diving deep into chapter 21 of Brenner and Stevens pharmacology.

It's all about local and general anesthetics and honestly reading through this it struck me how wild it is that we can just selectively turn off a sensation like I don't want to feel my left thumb right now or even crazier turn off consciousness entirely and then just turn it back on.

It really is one of the most significant advancements in medical history.

I mean the text actually sets the scene beautifully.

It mentions that before 1846 surgery was limited to things like you know rapid limb amputations with strong men holding the patient down.

That is such a visceral and terrifying image.

It really puts a you know a root canal into perspective.

It certainly does but now thanks to these drugs we have everything from painless dental fillings to open heart surgery.

Right.

But to really get it we have to look at the machinery.

Right.

We have to look at the sodium channels the GABA receptors and weirdly enough the physics of how gas dissolves in blood.

Okay so let's start with the basics then.

The text makes a pretty hard distinction right at the beginning between two words that I admit I have probably used interchangeably my entire life.

I think most people do.

Anesthesia and analgesia.

If I pop an ibuprofen for a headache am I am I anesthetizing myself?

No and that distinction is absolutely critical for the rest of this chapter.

If you take ibuprofen or even something much stronger like morphine what you're achieving is analgesia.

Analgesia.

And that's the selective loss of pain sensation.

Yeah.

You might still feel pressure.

You definitely still feel touch and most importantly you are awake and aware.

Okay okay so analgesia is just turning down the volume on the ouch track.

Exactly.

Anesthesia by the definition in this text is the total loss of all sensation.

Not just pain.

We're talking touch, temperature, pressure and even knowing where your body is in space.

All of it.

Gone.

And then we split anesthesia itself into two main camps right.

Local in general.

Which sounds straightforward but they are mechanically worlds apart.

Local anesthetics block nerve impulse conduction in peripheral nerves or even the spinal cord.

So they're used for a specific region.

A very specific region.

A patch of skin, a joint, your eye or you know for labor.

So the brain is totally fine but the phone line from the hand to the brain is cut.

Precisely.

General anesthetics on the other hand they block cortical neuronal activity.

They go right for the source.

They target consciousness itself.

So local helps you get stitches without freaking out.

General helps you not remember that someone took out your appendix.

That's the key General anesthetics have to ensure two things.

Amnesia so, no recall and unconsciousness.

It's a chemically induced coma that we hope and we plan is reversible.

Well when you put it that way I'm very glad we're doing this deep dive.

Let's unpack the local anesthetics first because the chemistry here is surprisingly specific.

I was looking at figure 21 .1 in the book and it describes the molecule as having this two -part structure.

It almost looks like it's fighting with itself.

It's a bit of a chimera molecule isn't it?

Yeah.

That figure is really helpful because it shows that every single local anesthetic molecule essentially has two ends.

There's a lipophilic portion.

Lipophilic so fat loving.

Fat loving exactly.

Usually an aromatic ring.

It's hydrophobic so it hates water.

Okay the oil loving side.

Got it.

And then on the other side connected by a little chain there's a hydrophilic portion.

This is usually an elamine group and hydrophilic means?

Water loving.

Water loving.

Why do we need a drug that loves both oil and water?

That feels like I don't know poor design or something.

It's actually brilliant design born of pure necessity.

Think about the journey the drug has to take.

To move from the syringe through all the tissue fluids to even get to the nerve it needs to dissolve in water.

Tissue fluid is mostly water so it needs that hydrophilic water loving amine side to act as its vehicle.

Okay so the water loving side gets it to the destination.

Right but once it arrives at the nerve it faces a wall.

The nerve membrane.

And what are nerve membranes made of?

Lipids.

Fats.

Fats.

A water loving molecule would just bounce right off.

It can't get in.

It needs that lipophilic oil loving side to slip right through the membrane wall.

Okay so it needs to be water soluble to travel but then it has to be fat soluble to break and enter.

You've got it.

That's the whole game.

And that little chain between the two parts is actually how we classify them.

If the bond is an ester bond it's an ester type drug.

If it's an emimide bond it's an emade type drug.

Esters and emides.

Okay I remember seeing those in the table of contents.

We're going to come back to that because apparently that difference matters a whole lot for allergies later on.

A huge amount.

It's the main clinical distinction between them.

But before we even get to allergies we have to talk about how these things actually get into the nerve.

And this brings us to what the text calls the weak base concept.

Okay.

This is probably the most technical bit of chemistry in the chapter but it explains so much like why the dentist sometimes has trouble numbing you for a filling.

I was just going to ask about that.

I've had that happen.

The text says local anesthetics exist in two forms.

This gave me a bit of a headache reading it so uh walk me through it slowly.

It's an equilibrium.

It's a balancing act.

Yeah.

Because they are weak bases they exist in two states at the same time in your body.

You have the ionized form which has picked up a proton.

It's charged.

Like a little magnet.

Yep it has a positive charge.

And then you have the non -ionized form which is unprotonated.

It's neutral.

Ionized versus non -ionized.

Why does the nerve care which one it is?

It cares about the barrier.

Remember the nerve is wrapped in that fatty lipid membrane.

The ionized form, the charged form cannot get through that membrane.

It's too polar.

It just bounces off like trying to mix oil and water.

Okay.

Only the non -ionized neutral lipophilic form can penetrate the neuronal membrane to get inside the nerve.

So the drug has to be in its non -ionized ghost mode to walk through the wall.

I love that.

Ghost mode exactly.

But here is the catch and this is the paradox of it all.

Once it gets inside the nerve cell it has to bind to the sodium channel to actually do its job.

Right.

But the binding site on that channel, it only accepts the ionized form.

Wait what?

So it has to be non -ionized to get in but then it has to switch back to ionized to do the job.

Precisely.

It enters as a neutral molecule, slips through the wall in ghost mode, and then once it's inside the cell, which has a different pH, it picks up a proton to become ionized again.

Then and only then can it bind to the receptor and block the pain.

That is a Rube Goldberg machine of pharmacology.

That's incredible.

It is.

And this brings us right to that clinical implication you alluded to, inflammation.

The text explains that inflammation causes acidosis.

Acidosis.

So it lowers the pH of the tissue.

It lowers the pH.

And low pH means acid, which means lots of free protons, right?

Protons everywhere.

Exactly.

The tissue is swimming in excess protons.

So if you inject a local anesthetic into an infected inflamed tooth where there are tons of protons, that drug equilibrium shifts immediately.

The drug grabs a proton outside the nerve.

It puts on its armor too early.

It becomes charged before it even gets to the door.

Yes.

It becomes ionized right there in the tissue fluid.

And because it's ionized, it cannot penetrate the membrane.

It's stuck outside the wall.

It can't get into ghost mode.

So it's not that the infection makes the nerves stronger or more sensitive.

It's that the drug physically cannot get inside to do its job because of the pH.

That's it.

You've nailed it.

That is why local anesthetics are so much less effective in the presence of inflammation or infection.

The chemistry just doesn't allow the drug to reach its target.

That is fascinating.

It's purely a chemical blockade.

Now, speaking of formulation, the text mentions they usually add something else to the shot.

Epinephrine.

Why on earth are we adding adrenaline to a numbing shot?

It sounds totally counterintuitive, right?

One wakes you up.

The other is supposed to put the nerve to sleep.

But it's all about plumbing.

It's about blood flow.

Most local anesthetics are naturally vasodilators.

They open up blood vessels.

Okay.

So if I inject lidocaine, the blood vessels around it open up.

Yes, they dilate.

And that increases blood flow, which then washes the drug away from the nerve and carries it off into the rest of the body.

And that's bad.

That's bad for two reasons.

One, the numbness wears off way too fast.

And two, you get higher potentially toxic levels of the drug floating around your system.

So the epinephrine.

What does it do?

Epinephrine is a potent vasoconstrictor.

It clamps down the blood vessels in that specific area.

It acts like a chemical tourniquet.

So it traps the anesthetic right where you injected it.

Yes.

It slows the absorption away from the site.

This prolongs the duration of action.

So you stay numb longer and it reduces the peak blood levels of the drug, which significantly lowers the risk of systemic toxicity.

That is really smart.

But there's a Do not use in tissues with end arteries.

This is a golden rule in medicine.

Yeah.

An absolute never ever.

Because epinephrine cuts off blood flow so effectively, you never ever use it in tissues that don't have a backup blood supply.

What exactly are end arteries?

They're tissues where there's only one way in for the blood.

There's no collateral circulation.

The text lists them specifically and every student should memorize this list.

Fingers, toes, the nose, the ears, and the penis.

A very, very specific list.

An unforgettable one.

If you clamp that artery shut in a finger, there is no other way for blood to get there.

You risk ischemia and then necrosis.

The tissue literally dies and turns black.

Okay.

So lidocaine with epinephrine is great for a laceration on your arm, but you keep it far, far away from the fingers and toes.

Exactly.

You have to respect the plumbing.

Let's move to metabolism then.

The body has to get rid of these drugs eventually.

You mentioned the Esther versus Amid split earlier.

Does that matter here?

This is where that classification becomes the metabolic fork in the road.

It's everything.

Esther type local anesthetics are metabolized right there in the plasma in the blood itself by enzymes called plasma cholinesteroses.

And they get broken down into P -bed derivatives.

Baba.

Yeah.

P -benzoic acid.

I feel like I've seen that on sunscreen bottles.

P -bay free.

You likely have.

And we'll discuss why that matters so much for allergies in just a second.

Now the ME type drugs are totally different.

They're much more stable in the blood.

They have to travel all the way to the liver to be metabolized by those cytochrome P450 enzymes.

So esters break down in the blood.

Amides have to make it to the liver.

Correct.

And that simple difference affects their half -life, their potential for toxicity, and that critical allergy profile.

Okay.

Let's get into the mechanism.

Section two of the outline.

We've managed to get the drug inside the cell.

It's in its ionized form now.

It finds the sodium channel.

What happens next?

The sodium channel is the engine of the nerve signal.

When you feel pain or anything, it's because these channels are opening, letting sodium ions rush in and creating an electrical spike.

The action potential.

The signal?

The signal.

The local anesthetic binds to the intracellular side of that channel, the side that's inside the nerve, and it effectively decreases the permeability to sodium.

It plugs the hole.

Sort of.

It's a bit more elegant than that.

Figure 21 .2 in the text shows it nicely.

A channel has gates, and they cycle through different states.

Resting, open, inactivated.

The drug binds to the channel and prolongs the inactivation state.

It basically locks the gate in the closed position.

So no sodium entry means no action potential propagation.

Zero.

So the pain signal starts at my cut finger, travels up the arm, hits the blockade where the dentist injected me, and just silence.

Silence.

The signal just stops dead.

It never reaches the brain.

But here's where it gets really, really interesting.

The blockade isn't random.

It exhibits something the text calls used dependence.

Used dependence.

Okay, I read this part a few times.

It sounds like the drug binds better to channels that are already open.

Exactly.

Imagine the sodium channel is a door.

The anesthetic can get into its binding site much, much easier when the door is open or has just been opened.

So wait, the more a nerve is firing, the more susceptible it is to being blocked.

Yes.

Active nerves, nerves that are firing rapidly, are blocked much faster and more effectively than resting nerves.

Hold on.

So the nerves that are screaming pain, pain,

pain because of the surgery are actually the ones that get shut up the fastest.

Exactly.

It's an incredibly elegant selective inhibition of the very fibers that are being stimulated by the procedure.

Nature is actually working in our favor here for once.

That is unbelievably convenient.

It's like the drug has a heat seeking missile for pain signals.

It really is.

And there's more.

There's also a size factor.

Small diameter fibers are blocked much more easily than large diameter fibers.

And myelination matters.

Two unmyelinated or lightly myelinated fibers go down first.

The text lists an order of loss, a specific sequence.

Right.

And it makes perfect sense now.

Because pain fibers,

the C fibers and A delta fibers, are small and lightly myelinated.

They are the very first to be anesthetized.

So pain goes first.

What's next?

Next go the autonomic fibers, things that control blood vessel tone.

And the last to go are the large heavily myelinated fibers that control touch, pressure and motor function.

Which explains exactly why when the dentist numbs you, the sharp pain goes away first, but you can still feel the pressure of them pushing on your tooth and you can still move your jaw.

Exactly.

Motor function is the last thing to be blocked.

And importantly, recovery happens in the complete reverse order.

Reverse order?

How so?

As the drug wears off, the motor function comes back first.

You can move your lip again, then touch and pressure sensation return.

And unfortunately, or fortunately, depending on how you see it, pain sensation is the last thing to fully return.

I'm going to go with fortunately.

You want that pain relief to last as long as possible.

That's very true.

But these drugs aren't perfect.

They have side effects.

What happens if they get into the system too high a dose if that tourniquet fails?

Right.

Systemic toxicity.

It primarily affects two places.

The central nervous system, CNS, and the cardiovascular system.

In the CNS, you see this really strange biphasic pattern.

First, you get stimulation.

Stimulation, like coffee jitters.

I would have expected sedation.

You'd think so, but it's more like restlessness, tremor, even euphoria.

Some patients describe metallic taste or ringing in the ears.

Euphoria from a numbing drug?

That's bizarre.

It happens because the drug blocks inhibitory neurons in the brain before it blocks the excitatory ones.

Think of it like this.

It takes the brakes off the brain first.

Okay, so the brain revs up because the stop signals are blocked.

Exactly.

But that phase is followed by inhibition.

As the drug concentration rises, it eventually blocks everything.

And that leads to drowsiness, sedation, and eventually coma.

Death usually comes from respiratory failure because the brainstem forgets to tell the lungs to breathe.

That is terrifying.

And the heart?

The heart is also a major target.

Most local anesthetics are vasodilators, so they can cause hypotension, a drop in blood pressure.

But more importantly, they depress the heart muscle itself.

They block sodium channels in the heart too.

The same channel?

The very same type of channels.

This slows electrical conduction, which can lead to

The text specifically mentions looking for a wide QRS complex on the ECG as a key sign of cardiotoxicity.

Now, you mentioned allergies earlier and that PAVA stuff.

Let's circle back to that.

Yes.

This is the main reason we distinguish esters from amides in the clinic.

As we said, ester -type anesthetics are metabolized into a PAVA -paraminophenzoic acid.

PAVA is a known and fairly common allergen.

It can cause hypersensitivity reactions in a small but significant percentage of people.

So if a patient says, I'm allergic to Novacaine, which is an ester.

They are very likely reacting to the PAVA metabolite from the pro -caine.

The good news is, and this is a critical point, there is no cross -reactivity between the classes.

So an allergy to one isn't an allergy to the other.

Correct.

If a patient is allergic to an ester, they can almost always tolerate an omni -type drug like lidocaine just fine because amides do not break down into PAVA.

That is a crucial nugget for anyone working in healthcare.

Esters cause allergies via PAVA.

Amides usually don't.

Exactly.

And one last safety note from the techs.

The FDA issued a pretty stern warning about topical anesthetics.

The creams and gels.

You mean like slathering numbing cream all over your legs before laser hair removal or something?

That is the exact scenario.

People think because it's a cream, it's totally safe.

But if you cover a large surface area, or worse, cover with plastic wrap to soak it in, the systemic absorption can be massive.

So it's like giving yourself an IV dose.

It can be.

The text notes can lead to seizures, cardiac abnormalities, and even death.

So you have to respect the cream.

Respect the cream.

Okay.

Let's talk about how we get these drugs into the body.

Section 3 covers the various routes of administration.

We have topical, obviously.

Right.

Used on skin, mucous membranes, or the cornea of the eye.

The text mentions a specific product called EMLA, which stands for eutectic mixture of local anesthetics.

EMLA.

Right.

That's the one they use for kids before a needle stick to numb the skin.

Yes.

It's a really cool bit of chemistry.

It's a mixture of two drugs, lidocaine and prilocaine.

Normally, these are solids at room temperature.

But when you mix them in just the right ratio, their combined melting point drops below room temp and they become an oil.

So the mix is a liquid.

An oily liquid that penetrates the skin much better than either drug could alone.

It gets about five millimeters deep.

Cool.

Then there's infiltration, which is just injecting it right into the tissue where you're working.

And something called iontophoresis.

That sounds futuristic.

It kind of is.

It uses a small electric current to literally push the charged anesthetic molecules into the tissue.

Dentists use it sometimes as a needle -free option.

And a device called Zingo.

That sounds like a toy.

It uses rapid helium gas pressure to blast powdered lidocaine particles right into the skin.

Again, great for kids or any needle -phobic adults.

It's a needle -free infiltration.

Now moving up to the bigger blocks.

The text mentions nerve blocks and field blocks.

This is where you get more targeted.

A nerve block is when you inject near a specific peripheral nerve or a whole plexus to numb a larger region, like a radial nerve block to numb the entire hand for surgery.

And a field block.

A field block is where you inject a wall or a perimeter of anesthesia around the area you're cutting, blocking all the nerves that enter that field.

And then the big guns.

Spinal and epidural.

People confuse these all the time.

I know, I do.

They are very, very different anatomically, and it's a critical distinction.

Spinal anesthesia, also called endrogethical anesthesia, goes deep.

The needle goes all the way into the subarachnoid space.

So it mixes directly with the cerebrospinal fluid, the CSF.

It does.

It bathes the spinal cord directly.

And this is where the text talks about a fascinating concept, bricity.

Bricity, right.

Density.

Yes.

This is just applied physics.

Anesthesiologists can use a hyperbaric solution, which they make by mixing the anesthetic with glucose to make it heavier or denser than the patient's own CSF.

So because it's heavier, gravity affects it.

Exactly.

By tilting the patient on the operating table, the anesthesiologist can control exactly where that pool of numbness flows.

That is wild.

So you tilt the table head down and the anesthesia slides up.

Yes.

Or you sit the patient up to get a saddle block for procedures in the perineal area.

But you have to be incredibly careful.

If you tilt them the wrong way and it slides too high up toward the brain stem.

You turn off their breathing.

Right.

Respiratory depression.

It's a major risk.

Okay.

And epidural, how's that different?

Epidural is injected into the epidural space.

So it's outside the dura, that tough membrane surrounding the spinal cord.

It doesn't mix directly with the CSF.

It has to soak through.

It diffuses across the dura to reach the nerve roots.

It's often used for labor, because you can place a catheter and give continuous infusions.

But the epidural space has a lot of blood vessels.

So there's more systemic absorption of the drug.

You have to monitor the mother and the baby for things like cardiac depression.

Okay.

Let's run through the specific agents the book mentions in section four.

We've got the esters first.

Cocaine is the first one listed.

It's naturally occurring from the coca plant.

And it's unique among all local anesthetics.

Why is it unique?

Because unlike all the others that open blood vessels,

cocaine causes intense vasoconstriction.

It shuts them down.

Why does it do that?

It's a sympathomimetic.

It blocks the reuptake of norepinephrine at nerve terminals.

So you get this big adrenergic effect.

That's why it's still occasionally used, topically, in nasal surgery.

It numbs the nose and stops the bleeding at the same time.

But obviously, huge abuse potential.

Massive abuse potential and CNS stimulation.

It's a schedule two controlled substance for a reason.

Then there's procaine.

Ugh.

Auto -caine.

The old brand name everyone knows.

It was the standard for years.

But by modern standards, it's not great.

It's low -potency, has a short duration of action, and it has that Pebo allergy risk we talk about.

Is it still used?

The text notes, it's actually not available for clinical use in the U .S.

anymore.

But it's historically significant as the prototype ester.

And benzocaine.

I've seen that in stores.

Yes, it's strictly topical.

It's in sunburned creams, hemorrhoid ointments, cough lozenges.

It has very low solubility in water so it stays on the surface nicely and doesn't get absorbed systemically very well, which is good.

Okay, moving to the amides,

the more modern drugs.

And lidocaine is the king.

It is the workhorse.

Its intermediate potency has a fast onset.

You find it in everything.

Patches, jellies, injectable solutions.

It's also used as an intravenous anti -rhythmic drug for the heart, which just shows you how connected these sodium channels are throughout the body.

Next on the list is Bupivacaine.

Bupivacaine is very high potency with a much longer duration of action.

It's a favorite for obstetrics, for epidurals during labor.

Why there specifically?

Because it provides great sensory pain relief without as much motor block so the mother can still feel the urge to push and participate in labor.

But, and this is a big but, it has a higher risk of severe cardiac depression.

It's much harder to resuscitate a patient if you accidentally overdosed among Bupivacaine compared to lidocaine.

And prilocaine.

There is a very specific warning here in the text about hemoglobin.

Yes.

This is a classic board exam question.

Prilocaine is metabolized in the body into something called otilloidine.

And this metabolite is a potent oxidizing agent.

It can cause a condition called methamoglobinemia.

That's a mouthful.

What is it?

Basically, it oxidizes the iron in your hemoglobin from the ferrous state to the ferric state.

It creates methamoglobin, which cannot bind and carry oxygen.

So your blood literally stops working.

It stops carrying oxygen.

And a classic sign is that the blood turns a chocolate brown color.

Chocolate colored blood.

That's memorable.

It is.

The patient turns blue, a condition called cyanosis.

It is treatable with an antidote called methylene blue, but it's a specific and dangerous risk associated with prilocaine.

Okay.

That covers the local stuff.

We have numbed the finger.

We have blocked the nerve.

We've even blocked the whole lower half of the body.

Now we're going to switch gears completely.

We are moving from the finger to the brain.

To the big switch.

General anesthetics.

The text opens this section with a bit of history.

William Morton, 1846, at Mass General.

The demonstration of diethyl ether in what's now called the ether dome, it changed everything.

It allowed surgery to evolve from, you know, a desperate, horrific last resort into a sophisticated science.

Before this, speed was the only metric that mattered for a surgeon.

But the drugs we use today are different.

We mostly use inhalational agents gases.

And the text spends a lot of time on the physics of this.

There are two main concepts we really need to nail down.

MC and the blood.

Gas partition coefficient.

Right.

Let's start with MSE, minimal alveolar concentration.

This is the measure of potency.

Potency.

Exactly.

And it's inversely related, which can be confusing.

Think of it like a golf score.

A lower number is better, or in this case, stronger.

MA is the concentration of gas in the lungs, in the alveoli needed to prevent movement in 50 % of patients in response to a surgical incision.

So a low MAC means you need very little gas to knock the patient out.

Exactly.

Low MA equals high potency.

Can you give me an example?

Sure.

Halothane has a low MA, about 0 .75%.

It's very potent.

Nitrous oxide, on the other hand, has a huge MAC, over 100%.

I think it's 105%.

Wait, how can you have over 100 % of something?

You can't.

You can't breathe 105 % gas.

And that's the point.

Nitrous is so weak, it has such low potency that even if you were breathing pure nitrous, which would kill you from lack of oxygen, you still wouldn't be fully anesthetized for surgery.

You cannot achieve surgical anesthesia with nitrous alone.

Got it.

Low MAC, strong drug.

Now, the blood gas partition coefficient.

This determines the speed of induction, how fast you fall asleep.

And this is the famous counterintuitive rule.

The text says it clearly.

High solubility in blood means slow W induction.

Low solubility in blood means fast induction.

That just feels wrong.

If it's soluble in blood, shouldn't it get to the brain faster?

You would think so, but you have to think of the blood as a giant sponge.

Okay, a sponge.

If the anesthetic gas is highly soluble in blood, like the older drug, halothane, the blood acts like a giant dry sponge.

You breathe the gas in and the blood instantly soaks it all up.

It loves the gas and holds onto it tightly.

It hoards it.

It absolutely hoards it.

The brain doesn't see any significant out of the gas until that entire blood sponge is saturated in full.

You have to fill up that massive reservoir of blood before the partial pressure of the gas can rise enough to push it out of the blood and into the brain.

And that takes time.

So the blood is effectively stealing the gas from the brain until it's had its fill.

That is a perfect way to put it.

Now consider a drug with low solubility like nitrous oxide or desflurane.

The blood is like a wet rock.

It doesn't soak up the gas at all.

It just bounces off.

Right.

So as soon as you breathe it in, the partial pressure in the blood shoots up instantly because the blood isn't holding onto it.

That high pressure forces the gas into the brain immediately.

So spongy blood means slow sleep.

Non -stick blood means fast sleep.

That's the rule.

Fast in, fast out.

Now, what are these gases actually doing to the brain?

The text mentions an old theory and a new theory.

The old theory was called the Meyer -Overton principle.

They noticed way back when that the potency of an anesthetic correlated perfectly with how oily it was, its lipid solubility.

So the more fat soluble, the stronger it was.

Exactly.

So they thought the gas just dissolved into the lipid membrane of the neuron and physically messed it up sort of non -specifically, like putting gum in a lock, just gunking up the OK, so just physical disruption.

What's the new theory?

The new theory, as shown in Figure 21 .3, is much more precise.

We now know these gases interact with specific protein receptors in the membrane, primarily the GABA receptor.

GABA is the brain's main inhibitory neurotransmitter.

It's the BRAKEs.

It is the BRAKEs.

These drugs are positive allosteric modulators.

They potentiate GABA.

They bind to the GABA receptor and make it work better, increasing the influx of chloride ions into the neuron.

Negative ions.

Why does this even happen?

Why the excitement?

It's that same principle we saw with local anesthetics in the brain.

The anesthetic gas blocks the small inhibitory neurons in the brain before it blocks the larger excitatory ones.

The brain loses its filter before it loses consciousness.

So the goal is to get through stage two as fast as humanly possible.

The goal is to skip it entirely.

That's a major reason why we usually induce anesthesia with a rapid acting IV drug like propofol.

It works in seconds, bypasses stage two completely, and lands the patient directly and safely into stage three.

And stage three is?

Surgical anesthesia.

This is the goal.

Unconsciousness, loss of reflexes, regular respiration, muscle relaxation.

This is where the surgeon can work.

Which leaves stage four.

It's a medullary depression.

This is overdose.

The drug concentration gets so high it shuts down the brain stem centers that control breathing and blood pressure.

This means cardiovascular collapse and death.

The anesthesiologist's entire job is to keep you in stage three and keep you far, far away from stage four.

Let's look at the specific gases in section six.

First up, the only non -halogenated one we use, nitrous oxide.

Laughing gas.

It's unique.

As we said, it has very low potency, that high MSAC.

So you can't use it alone for major surgery.

But it has very high analgesia.

It kills pain extremely well.

And because of that super low blood solubility?

It works super fast and wears off super fast.

The moment you take the mask off, it's gone.

But there's a toxicity note here in the text about vitamin B12.

Right.

What's that about?

Chronic exposure to nitrous oxide.

Like a dentist who is around it all day or someone abusing it, can irreversibly oxidize the cobalt atom in vitamin B12.

This inactivates the vitamin.

And what does that lead to?

It can lead to megaloblastic anemia and severe neuropathy.

So it's not as benign as people think, especially with chronic exposure.

Then we have the halogenated agents.

Halothane is listed as the prototype.

The old standard.

It's potent, but it's a slow induction because of that high solubility, that blood sponge effect.

But the big, big problem with halothane is the liver.

Halothane hepatitis.

It's a severe, often fatal, immune reaction to a metabolite of halothane that binds to liver proteins.

The body sees its own liver cells as foreign and attacks them.

It can cause massive liver necrosis.

So it's an allergic reaction to your own liver?

In essence, yes.

Because of this, the text says a patient shouldn't be re -exposed to halothane for at least 6 to 12 months.

It's very rarely used in the U .S.

now.

What about the newer ones?

Isoflurane, sevoflurane, desflurane.

They sound similar.

They're all improvements.

Isoflurane is faster than halothane, but has a very pungent odor.

Sevoflurane is described in the text as near -ideal.

It's a rapid, smooth induction.

It smells decent, so kids tolerate it for a mass conduction.

And it has very low toxicity.

It's a go -to for many situations.

And desflurane.

Desflurane has an extremely low blood.

Gas coefficient, so it's super, super fast.

On or off like a light switch.

But it's very irritating to the airways.

You can't use it to put a patient to sleep because they will cough, gag, and hold their breath.

You use it to maintain anesthesia once they are already under it with something else.

Now, there is a text box in this chapter box 21 .3 that reads like a medical thriller.

It's called a case of the escalating appendectomy.

It describes a nightmare scenario called malignant hyperthermia.

This is a life -saving emergency.

Our true oh -my -god moment in the operating room.

It's a genetic condition.

The classic scenario is a patient gets a halogenated gas, like sevoflurane, plus a specific muscle relaxant called succinylcholine.

And what happens?

Suddenly, their muscles become rock -hard rigid.

Their heart rate spikes to over 150, and their temperature skyrockets.

We're talking fevers of 105, 107, 109 degrees Fahrenheit.

It's a hypermetabolic crisis.

What is causing that?

What's the defect?

It's a genetic defect in the ryanodyne receptor in the muscle cells.

This receptor is a calcium channel.

The anesthetic drug triggers this faulty receptor to get stuck in the open position.

So calcium just floods out.

Uncontrolled.

Massive calcium release into the muscle cells.

This causes the muscles to

sustainably, which generates incredible heat and leads to the breakdown of muscle tissue.

It cooks the body from the inside out.

That is a very accurate description.

And the cure.

Is there one?

Yes.

There is a specific antidote, a drug called dantrolene.

Dantrolene works by blocking that calcium release at the ryanodyne receptor.

It's the only thing that works.

Every single operating room in the country has a malignant hyperthermia cart with vials of dantrolene ready to be mixed and given immediately.

Okay.

Final section.

Let's talk about the parenteral or RVV anesthetics.

These are the drugs we use to induce anesthesia or for sedation in procedures like a colonoscopy.

Right.

We usually start with one of these to knock you out quickly, bypassing that nasty stage two.

And then we switch to an inhalational gas to keep you under for the duration of the surgery.

The barbiturates are mentioned first, thiopentol and methohexidol.

Phi one tall is the classic truth serum you see in old movies, though it's not really used for that.

It's ultra fast acting because it's so lipid soluble.

It hits the brain in one circulation, but it redistributes out of the brain into fat muscle and then leaks back out slowly, causing a prolonged hangover effect.

So you feel groggy for a long time.

For a very long time.

It's largely been replaced for that reason.

Replaced by propofol.

The milk of amnesia.

Why do they call it milk?

Because it literally looks like milk.

It's a white lipid emulsion.

It's a phenol compound suspended in soybean oil, glycerol, and aglesithin.

So you have to worry if you have an egg or soy allergy.

You do have to be cautious, yes.

Propofol is incredibly popular because it has a rapid onset, about 40 seconds, a short duration, and a very clear headed recovery.

Much less nausea, much less hangover than the barbiturates.

It's the standard for outpatient procedures.

Then there's a drug called etomidate.

Etomidate is the friend of the cardiac patient.

Its main claim to fame is that it has very little effect on blood pressure or heart rate.

So if you have a trauma patient who is bleeding out or someone with a failing heart, etomidate is often the safest choice for induction.

And then there's ketamine.

The book makes it clear this one is different from everything else.

Completely different class, different mechanism.

It blocks NMDA receptors, which are glutamate receptors, rather than working on GABA.

And because of this, it produces what's called dissociative anesthesia.

What does that look like in a patient?

The patient might look like they're awake, their eyes might be open, often with a slow side -to -side movement called nystagmus.

They can be catatonic, but they feel no pain, and they'll have no merry of the event.

They are chemically dissociated from their environment.

What are the pros and cons of that?

Pros.

It actually increases blood pressure and heart rate, which is great for a patient in shock who is crashing.

It also doesn't depress respiration very much, so they often keep breathing on their own.

It's a fantastic battlefield and trauma drug.

And the cons.

Emergence phenomena.

That sounds ominous.

It can be.

As adults wake up from ketamine, they can experience profound delirium, vivid and sometimes terrifying hallucinations, and out -of -body experiences.

A bad trip.

A very bad trip.

This is much less common in children, so ketamine is widely and safely used in pediatric medicine.

But in adults, you often have to code -minister a benzodiazepine with it to sort of smooth out the landing.

And finally, the text mentions benzodiazepines like metazolam and opioids like fentanyl in this context.

Right.

Metazolam, brand name Verst, is great for preoperative sedation.

It doesn't really knock you out, but it causes powerful antiregrade amnesia.

You'll be awake and chat with the doctor.

They give you the Verst, and you wake up in the recovery room with no memory of the trip to the OR or anything after the shot.

And fentanyl.

Fentanyl is an extremely potent opioid.

It's used for the analgesic component of what we call balanced anesthesia.

Balanced anesthesia.

Yeah, you use a mix of drugs for their specific effects.

Propofol for sleep and amnesia, fentanyl for pain relief, and inhalational gas for maintenance, and maybe a muscle relaxant for paralysis.

A little of each is safer than a lot of one.

There's a term here.

Neuroleptanesthesia.

That's a specific combination, usually fentanyl plus an antipsychotic like droperdol.

It produces a state called twilight sleep.

You are intensely analgesic and sedated, but you're conscious enough to answer questions and cooperate.

Why would you want that?

It's useful for certain types of brain surgery where you need the patient to respond.

Like, can you move your hand to ensure you aren't cutting into vital areas?

Wow.

We have covered a lot of ground.

I mean, from the protons fighting to get into a nerve, to the sponge -like quality of blood holding onto gas, to the dissociative state of ketamine.

It really is a tour of the nervous system's vulnerabilities.

We're just using chemistry to exploit the fundamental physics of our own biology to our advantage.

So to summarize the big takeaways for our listeners, local anesthetics block sodium channels, but they need to be in their non -ionized ghost mode to even get inside the nerve.

And they work best on small, active pain fibers first.

Correct.

And in general, anesthetics primarily work by potentiating GABA to shut down the cortex.

Their speed depends on blood solubility.

The less soluble the gas is, the less sponge -like the blood is, and the faster they work.

And always, always check for end arteries before using epinephrine, and make sure you know where the dantoline card is just in case of malignant hypothermia.

Safety first.

Always.

Here's my final thought for you to chew on.

The text mentioned the dissociative state of ketamine and that paradoxical excitation in stage 2 anesthesia where the brain goes haywire before it shuts down.

It really begs the question,

how delicate is the line between consciousness, unconsciousness, and hallucination?

It seems like our entire reality is just a very specific balance of partial pressures and receptor binding, a tiny shift in chemistry, a few more molecules on a GABA receptor, and the entire world disappears or changes completely.

It is a fragile thing, consciousness.

We're all just a few molecules away from the dark.

On that existential note, thank you for listening to this deep dive into chapter 21.

Stay curious and stay safe out there.

This has been the Last Minute Lecture Team.

We'll see you next time.