Chapter 22: Cardiovascular Disease

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Welcome.

You are joining us for a very special Last Minute Lecture edition of the Deep Dive.

That's right.

We know you're probably, I mean, you might be gearing up for a massive exam right now, or maybe you're stepping onto the hospital wards for the very first time.

Which is terrifying and exciting.

Exactly.

Yeah.

So our mission today is crystal clear.

We are stepping into a one -on -one tutoring session with you to absolutely master chapter 22.

Cardiovascular disease.

Right, from Clinical Biochemistry and Metabolic Medicine, 8th edition.

We are gonna make sure you have this material just completely locked down.

Yeah, and for our college student listener out there, really consider this, your dedicated distraction -free study block.

The core philosophy of our session today is all about building a logical progression.

We're gonna walk through the exact chronological sequence of the chapter.

So no jumping around.

No jumping around.

We will connect normal biochemical principles directly to pathophysiology.

Then we'll see how that pathophysiology leads to specific laboratory abnormalities.

And finally, how you use those lab results for actual clinical interpretation.

Because it's not just a list of numbers to memorize.

No, not at all.

We are building a physiological roadmap in your mind so you actually understand what is happening inside the patient.

Okay, let's unpack this, starting right at the top of the material with the very first, and honestly, arguably the most critical concept,

the definition of an acute myocardial infarction, an AMI.

Right.

Because you really can't diagnose what you can't define.

You really can't.

And the text provides a very strict universal definition that you need to commit to memory.

An AMI is defined as a typical rise and door fall of cardiac biomarkers.

And they specifically call out troponin there.

Yes, the text explicitly prefers the biomarker troponin for this.

You need to see at least one value sitting above the 99th percentile of the upper reference limit.

Okay, so a high lab value.

Well, here is the crucial part.

A high lab value alone is actually not enough to make the diagnosis.

That biochemical elevation must be combined with clear clinical evidence of ischemia.

Ah, got it.

So we're looking for that perfect marriage of the laboratory data and the clinical picture.

Exactly.

And what does that clinical evidence actually look like on the wards?

It could be classic clinical symptoms of ischemia, like severe chest pain radiating to the jaw or arm.

A classic Hollywood heart attack clock.

Right.

Or it could be electrocardiogram changes, specifically new ST segment changes, or a new left bundle branch block.

It could also be the development of pathological Q waves on the ECG later on, or even imaging evidence, like an echocardiogram showing a new area of the heart muscle that just isn't moving properly due to ischemia.

You always need the combination of the biochemical marker and the clinical evidence.

That makes total sense.

And to really illustrate how vital the timing of those biomarkers is, the chapter gives us two fantastic clinical case studies right at the beginning.

These are great.

If you're taking notes on this, pay close attention, because this is the exact kind of scenario you're gonna face.

Let's look at the diagnostic timeline.

Imagine you are the junior doctor on call.

It's almost midnight, 23 .30.

The paramedic's role in case one.

A 46 -year -old man who has a tight central chest pain.

A very standard presentation.

Very standard.

But here's the kicker.

That pain actually started way back at 03 .0 that morning.

He delayed calling for help because he thought it was just bad indigestion.

Which is an incredibly common scenario on the wards.

Right.

Now, while in the ambulance, the paramedics gave him an intramuscular injection.

An IM injection of dimorphine to help manage his pain.

His ECG comes back normal.

Then you get his live results back.

And this is where it gets tricky.

His total creatine kinase, or CK, is 565 units per liter.

The normal reference range is less than 2550.

So CK is more than double the normal limit.

But his troponin T is 1 .0 nanograms per liter, which is completely normal against a reference of less than 10.

This case is a classic clinical trap.

It perfectly highlights the application of this biochemistry.

Think about the timeline.

His chest pain started at 03 .00, and he was admitted at 23 .30.

So that's over 20 hours later.

Over 20 hours have passed.

If he had actually suffered a myocardial infarction at 3M, we would absolutely expect that troponin T to be significantly elevated by now.

Right, it wouldn't still be normal.

Exactly.

The fact that his troponin T is totally normal this late in the game makes an AMI highly unlikely.

So if it's not a heart attack, you have to ask yourself,

why on earth is his total CK elevated?

Because it's not coming from the heart at all.

It's actually from skeletal muscle damage.

Think about what an intramuscular injection physically does.

Oh, the dimorphine injection.

Yes.

You are pushing a needle deep into skeletal muscle tissue to deposit that drug.

That physical trauma to the muscle fibers releases skeletal muscle enzymes straight into the bloodstream.

Total CK measures all creatine kinase in the blood, not just the cardiac kind.

The text specifically warns us about this.

Always beware of interpreting elevated plasma CK concentrations when a patient has recently had an IM injection.

That is such a brilliant catch.

And it really shows why you can't treat lab values in a vacuum.

If the clinicians really wanted to check for a cardiac origin of that CK, what should they have done?

Assaying for the CKMB isoenzyme would be much better in that specific scenario.

CKMB is predominantly cardiac in origin, whereas total CK is a blunt instrument that picks up skeletal muscle damage too.

Let's contrast that trap with case two.

He was a 66 -year -old man who was sent into the casualty department by his general practitioner.

He had severe tight chest pain that occurred three whole days ago.

Three days.

Yeah.

The pain largely resolved after about six hours, but ever since then, he has been feeling incredibly weak and breathless.

You run his labs.

His total CK comes back totally normal at 235 units per liter.

But his troponin T is elevated at 13 .0 nanograms per liter.

And when you look at his ECG, it shows changes suggestive of an infarct in the lateral leads, specifically V4 through V6.

Here we see the exact opposite scenario regarding timing, and it perfectly demonstrates cause and effect in biochemistry.

Why is his CK normal while his troponin is high?

If they're the half -life, right.

It all comes down to the biological half -life of these markers.

Plasma CK usually starts to rise four to six hours after a myocardial infarction, but it is cleared from the body and normalizes very rapidly, usually after just a couple of days.

Since this man is three days out from his cardiac event, his CK has already returned to its baseline level.

It's kind of like looking for footprints on a beach three days after the tide has come in.

The CK evidence has already been washed away.

That is a great way to picture it.

Conversely, plasma troponin T also starts to rise at that same four to six hour mark post -infarct, but it remains elevated in the bloodstream for up to 10 days.

That's a huge window.

It is.

So troponins are incredibly useful cardiac markers, not just in those early frantic hours in the emergency room, but also for patients who present late, like this gentleman.

That makes a lot of sense.

If we connect this to the bigger picture, we have to look at the underlying pathophysiology of what we now call acute coronary syndrome, or ACS.

Clinically, the text points out that we no longer think of a heart attack as a simple binary yes or no event.

Instead, we look at it as a progressive spectrum of disease.

It starts at angina pectoris, progresses to minor myocardial injury, which is now reclassified as non -ST segment elevation myocardial infarction, or N -STEMI, and goes all the way up to a massive STEMI, and ST segment elevation myocardial infarction.

And to help you visualize that spectrum, the text describes a couple of vital clinical images you should picture in your mind.

First, think about the ECG paper.

Imagine the flat baseline of a normal heartbeat.

If a patient is experiencing ischemic angina pectoris, you'll see ST depression.

The segment of the line right after the main heartbeat actually sags or slopes downward, horizontally below the baseline.

But if you're looking at an actual infarctus STEMI, you'll see ST elevation, where that segment spikes upward above the baseline, like a tombstone.

It's very visually distinct.

The text also describes a coronary arteriogram, which is an imaging technique to look at the blood vessels.

It shows a critical right coronary artery stenosis, which basically means a severe narrowing of the vessel.

That physical blockage is the root cause of the entire spectrum we just talked about.

And whether it's an N -STEMI or a full STEMI, they share a common pathophysiological pathway.

It begins with acute coronary artery plaque rupture.

A fatty plaque inside the artery wall bursts open, which is then rapidly followed by a cascade of events resulting in thrombosis formation, a blood clot that plugs the artery.

And the final outcome depends on the collateral circulation.

Heavily depends on it.

Let's quickly define that for anyone who might be rusty.

Collateral circulation refers to the alternate backup blood vessels that can sometimes reroute blood around a blockage.

Exactly.

The more collateral circulation you have and the less severely that artery is occluded, the better the outcome.

The text uses a powerful phrase that every medical student needs to memorize.

Minutes are myocardium.

Minutes are myocardium.

It is absolutely critical to diagnose an AMI promptly because the patient may be a candidate for thrombolytic therapy to chemically dissolve the clot in situ or for an angioplasty to mechanically open the artery with a balloon.

Time is muscle.

The faster the blood flow is restored, the more heart muscle you save from irreversible death.

That is why our biochemical markers need to be highly specific, highly sensitive, and change rapidly in the blood.

Here's where it gets really interesting as we take our deep dive into the specific biochemistry of those cardiac biomarkers, starting with the absolute gold standard,

troponins.

Ahead of hitters.

Right, these are muscle regulatory proteins, and there are three types you need to know.

Troponin C, troponin I, and troponin T.

What I found fascinating in the text is how we actually measure these.

There are no structural differences between the cardiac and skeletal muscle forms of troponin C.

None at all.

However, the cardiac and skeletal forms of troponin I and troponin T are structurally different from one another.

Because of that unique structural difference, we can create very specific immunological assays to distinguish the cardiac damage from regular muscle damage.

While they are the gold standard, there are crucial clinical caveats you need to be aware of.

As we established with the clinical cases, troponins appear in the plasma four to eight hours after symptoms begin, or even a bit earlier with some of the newer high sensitivity assays, and they are best measured 12 hours after the start of chest pain.

So they aren't instant.

Right, they aren't the earliest markers to show up, but they stay elevated for seven to 10 days.

But here are the variables the text throws at us that you need to watch out for.

Troponin T can be elevated in patients with chronic kidney disease.

This means it might not be strictly cardio -specific in those specific renal patients.

The kidneys just aren't clearing it normally.

That's a big one.

On the other hand, troponin I has its own quirk.

It can be extensively modified by proteolysis after it is released into the plasma.

Let's break down proteolysis.

It basically means that enzymes in the bloodstream are actively chopping up the troponin I proteins into smaller fragments.

Because it gets chopped up, different commercial laboratory assays might give you varying results depending on which specific antigenic epitopes, which specific protein fragments, they're actually designed to detect.

Which is frustrating for standardizing results.

The text actually notes there isn't universal agreement in the medical community on whether troponin I or troponin T is definitively the better test.

You also have to remember that a raised troponin is a highly sensitive marker of occult myocardial damage.

Occult meaning hidden or secondary damage, even in non -ischemic conditions.

Like what?

We are talking about seeing elevated troponins during a subarachnoid hemorrhage due to a massive release of vasoactive peptides putting stress on the heart.

You can also see it in severe hypertension, tachyarrhythmias, cardiac surgery, sepsis, congestive cardiac failure, pulmonary embolism, and even hypothyroidism.

It's used heavily for risk stratification too.

If you have a patient with unstable angina who has raised cardiac troponins, they are statistically much more likely to have an acute myocardial infarction or even die within the next six months compared to someone with normal levels.

Now, even though troponins are the primary choice today, the chapter outlines a historical and chronological timeline of other markers that you still need to know.

Sometimes a broader panel of markers is useful.

Think of these enzymes as runners in a relay race.

I like that analogy.

First out of the gate is myoglobin.

It's a low molecular weight ham containing protein.

Because it is so physically small, it leaks out of damaged cells very rapidly.

It typically rises very early, just two to four hours after the onset of an AMI, and peaks at 12 to 24 hours.

So myoglobin is your early warning system.

It alerts the clinician fast, but the major downside is that it is not cardiac specific at all.

You find myoglobin at all skeletal muscle, so a heavy workout could theoretically raise it.

Exactly, following myoglobin, you have the middle distance runners, the classic cardiac enzymes, total CK, and aspartate immunotransferase, or AST.

CK takes the baton and starts to rise at four to six hours, peaks at 24 to 48 hours, and lasts two to four days.

AST rises just a bit later, at six to eight hours, peaks at 24 to 48, and lasts four to six days.

And finally you have the marathon runners, the late markers, these are lactate dehydrogenase, or LDH, and hydroxybutyrate dehydrogenase, or HPD.

These don't even start to rise until 12 to 24 hours after the infarct, they peak at 48 to 72 hours, and they stay elevated for a long seven to 12 days.

The text notes a practical tip for the wards.

You really shouldn't bother drawing blood for these standard enzyme assays until at least four hours after the onset of chest pain, because you simply won't see anything yet.

Let's focus on creatine kinase for a moment, because the biochemistry here is elegant, we know total CK lacks specificity.

But CK exists as three major isoenzymes, BB, MB, and MM.

Most of the CK released after an infarct is actually CKMM, which is frustrating, because CKMM is found in both skeletal and myocardial muscle.

Which brings us back to that first case study.

Exactly.

But what we care about early on is that CKMB fraction.

If the simultaneous measurement of plasma CKMB activity exceeds about 5 % of the total CK activity, it strongly helps confirm an early cardiac diagnosis.

And the text goes even deeper into the biochemical fingerprint of CKMB.

CKMB actually exists as two further isoforms, CKMB1 and CKMB2.

Normally, in a healthy person, CKMB1 predominates in the blood plasma.

But after an acute myocardial infarction, this ratio actually flips.

It flips.

Yes, CKMB2 is the specific isoform that is predominantly released from the damaged dying myocardium.

Assaying for that specific isoform ratio flip requires specialized laboratory technology, but it's a brilliant definitive sign of heart damage.

And what about AST?

Why does an enzyme we usually associate with the liver rise during a heart attack?

The text explains the physiological ripple effect.

Even a relatively small myocardial infarct causes some degree of hepatic congestion.

Because the right side of the heart isn't pumping effectively, blood backs up into the venous system and that pressure backs up directly into the liver.

Ah, so it's a backup in traffic.

Exactly.

That secondary liver congestion causes the hepatocytes to leak, contributing to the rise in plasma AST activity.

You are seeing a systemic consequence of the heart damage showing up in the liver panel.

Okay, let's shift gears from acute emergency events like an AMI to chronic conditions.

Chronic heart failure is a clinical condition defined by symptoms of left or right ventricular dysfunction.

If you are looking at a patient, this means dyspnoea or shortness of breath, severe fatigue and clinical signs of fluid overload like pulmonary crepitations, crackles in their lungs when you listen with a stethoscope or a visibly raised jugular venous pressure in their neck.

And while echocardiography is fantastic for physically measuring how much blood the heart is pumping, the cardiac ejection fraction biochemically, we rely heavily on a group of hormones called the natriuretic peptides.

There are three major ones you need to know for your exam.

Atrial natriuretic peptide or AMP, brain natriuretic peptide, BNP and C -type natriuretic peptide, CNP.

And they have specific receptors they bind to as well.

Right, A -type receptors bind AMP and BNP.

B -type receptors bind CNP and C -type receptors bind all of them and are primarily involved in clearing these peptides out of the bloodstream.

To understand how to use them, you need to understand their physiological triggers.

AMP is produced in the atria of the heart and is released when there is increased atrial wall tension.

Think of a balloon being stretched too tight because the intravascular fluid volume is too high.

BNP was originally found in the brain, hence the name, but is primarily secreted by the cardiac ventricles in response to cardiac failure and ventricular hypertrophy or thickening of the heart muscle.

And their job, as the name natriuretic suggests, is to promote natriuresis.

For those new to the term, natriuresis essentially means telling the kidneys to pee out more sodium.

Simple as that.

Waterfall is sodium, so this reduces the overall extracellular fluid volume and takes the pressure off the heart.

CNP, on the other hand, is a bit different.

It exists in the brain, kidneys and endothelial cells.

It acts mostly as a potent local vasodilator, widening the blood vessels, rather than acting as a circulating cardiac hormone like the other two.

Clinically, according to NICE guidelines, plasma BNP and its precursor, pro -BNP, are incredibly useful tools.

Why?

Because their negative predictive value is fantastic.

Meaning if it's normal.

A completely normal plasma concentration of BNP virtually rules out chronic cardiac failure as the cause of a patient's breathlessness.

However, the caveat is that BNP is nonspecific when elevated.

It can be raised independently in patients with obesity or severe renal disease, so a high number doesn't automatically mean heart failure.

Moving on from the heart itself, the chapter briefly touches on a crucial blood test called D -dimers in the context of deep vein thrombosis, or DVT.

Diagnosing a DVT, a blood clot in the deep veins of the leg or pelvis, can be notoriously tough just by looking at the patient clinically.

It really is.

But missing it is a huge deal, because that clot could break off, travel to the lungs, and cause a fatal pulmonary embolus.

Biochemically, when a thrombus forms anywhere in the body, the coagulation cascade converts fibrinogen into a tough protein mesh called fibrin.

As the body naturally tries to break down that clot, it releases degradation products into the plasma, and those are the D -dimers.

So it's evidence of clot breakdown.

Right.

So a raised plasma D -dimer concentration is a crucial sensitive marker of active thrombosis.

It's the biochemical red flag that tells the clinician, hey, there's a clot forming somewhere.

This prompts them to immediately order an ultrasound to actually find the DVT and treat it before it becomes a pulmonary embolism.

Now let's step back and look at prevention and the underlying causes of all these cardiovascular diseases.

The chapter outlines the major cardiovascular risk factors, and you need to divide these into non -modifiable and modifiable categories.

Keep them separated in your head.

Exactly.

Non -modifiable factors are the cards you are dealt, age, sex, family history, and ethnicity.

The text notes, for instance, that some South Asian groups are at a statistically higher risk.

The modifiable factors, the ones we actually try to treat, include abnormal lipids like hypercholesterolemia,

hypertriglyceridemia, and low HDL cholesterol, plus the big lifestyle factors, smoking, hypertension,

obesity, and diabetes mellitus.

But the text zooms in on two specific biochemical risk factors you need to understand deeply.

The first is homocysteine.

Biochemically, this is a sulfur -containing amino acid.

It is derived from another amino acid, methionine, by a process called demethylation.

The body can recycle it, remethylating it back to safe levels using betaine or methyl tetrahydrofolate.

So if a patient has elevated plasma homocysteine, specifically above 10 micromoles per liter, it confers a significantly increased cardiovascular risk.

This elevation can be due to simple nutritional deficiencies in vitamin B12, folate, or vitamin B6, which are needed for that recycling process.

Therefore, it can be genetic.

Right, caused by a single base pair substitution mutation,

C677T, in the MTHFR gene.

And while homocysteine levels naturally increase with age, with impaired renal function, and with certain drugs, treating these patients with vitamin supplements does successfully lower the blood levels.

However, the text points out an interesting caveat.

Large -scale studies haven't firmly proven that lowering the homocysteine with vitamins actually reduces the hard clinical cardiovascular risk.

The second specific biochemical risk factor is high -sensitivity C -reactive protein,

or HSCRP.

It is vital to remember that atherosclerosis, the hardening of the arteries, is fundamentally an inflammatory process.

In those atheromatous plaques inside the arteries, you will find inflammatory cells, macrophages, monocytes, and T lymphocytes.

And these immune cells are active.

They release chemical messengers called cytokines, specifically interleukin -6 or IL -6.

That IL -6 then travels through the blood to the liver where it stimulates the hepatic synthesis of acute phase proteins, including CRP.

So it's not just a generic marker?

No, measuring plasma CRP isn't just a generic marker of feeling unwell.

It actually reflects the inflammatory state of the plaque itself, and the likelihood of that plaque rupturing.

High -sensitivity lab assays measure this very accurately.

Desirable low -risk values are less than one milligram per liter.

If you have a patient who presents with acute coronary syndrome, and their HSCRP is more than three milligrams per liter at onset,

they have a significantly higher likelihood of going on to suffer a full, acute myocardial infarction, or even cardiovascular death.

It is a powerful prognostic tool.

Extremely powerful.

Finally, we need to discuss hypertension and clinical biochemistry's role in managing it.

Hypertension is defined as a sustained systolic blood pressure of more than 140 millimeters of mercury, and or a diastolic pressure of more than 90.

Most cases you see will be essential hypertension, meaning the root cause is unknown.

But you absolutely must know the secondary causes.

Diabetes mellitus, renal diseases like polycystic kidney disease, or pylonephritis, Cushing syndrome, Kahn syndrome, pheochromocytoma, acromegaly, primary hyperparathyroidism, and renal artery stenosis.

This raises an important question.

How exactly does the clinical biochemistry lab help manage a patient with hypertension?

Beyond diagnosing those rare endocrine causes you just listed, biochemistry is absolutely vital for monitoring the safety of antihypertensive therapies.

Because the treatments have metabolic costs.

Precisely.

For example, if a patient is treated with non -potassium sparing thiazide diuretics to lower their blood pressure, the lab needs to monitor them closely.

Why?

Because these drugs can cause a whole host of metabolic derangements.

Hypokalemia, hyperruricamia, hypercalcamia, hyperglycemia, and hyponatramia.

And the monitoring becomes even more critical when you put a patient on ACE inhibitors or angiotensin II, receptor blockers, ARBs.

These drugs can cause hyperkalemia, dangerously high potassium.

But more importantly, they can cause a 10 to 20 % rise in plasma creatinine, particularly in patients who happen to have underlying renal artery stenosis.

Let's trace the physiological cause and effect there because this is a classic exam question.

In a patient with renal artery stenosis, the main blood flow into the kidney is severely compromised by a blockage.

To compensate for this low flow and maintain the glomerular filtration rate, or GFR, the kidney's ability to filter blood, the body uses the hormone angiotensin II to constrict the efferent arterial.

That is the exit pipe of the kidney filter.

Okay, so it pinches the exit.

By pinching the exit pipe, pressure builds up inside the filter and the kidney keeps working.

Therefore, if you give that patient an ACE inhibitor or an ARB, you are chemically blocking that angiotensin II.

You remove the pinch on the exit pipe, the pressure inside the filter suddenly drops, and you can severely acutely impair their renal function.

That is exactly why clinicians must closely monitor a patient's renal function, specifically looking at plasma urea and creatinine within days of initiating these drugs or increasing the dose.

That makes perfect sense.

Let's summarize the core lessons from chapter 22 for your exam and your future clinical practice.

First, prompt diagnosis of an acute myocardial infarction relies on identifying the universal criteria, primarily using troponins, which are more specific to the heart and stay elevated much longer than CK.

Second, understand the enzyme timeline.

Myoglobin is your early sprinter, but lacks specificity.

CK and AST follow, with the CK -MB fraction offering crucial early diagnostic help.

Third, utilize the natriuretic peptides, especially BNP, to confidently rule out chronic heart failure in a breathless patient.

And lastly, remember the vital role of the biochemistry lab in not just identifying risk factors like homocysteine and HCRP, but in actively protecting the patient by monitoring the metabolic side effects of hypertension treatments.

So what does this all mean?

If you're staring at this textbook right now, why should you care about memorizing the exact hours CK peaks, or which specific receptor binds C and P?

Because understand the physiological why behind these lab values is the difference between simply memorizing a textbook to pass a test and actually saving a patient's life on the wards.

Absolutely.

When minutes are myocardium, knowing instantly that a normal troponin at 20 hours confidently rules out an infarct, or knowing you must check a creatinine level three days after starting an ACE inhibitor, makes you an incredibly vital, dangerous mistake -catching part of the diagnostic team.

I will leave you with a final thought to mull over as you close your books today.

We've seen how incredibly sensitive markers like high sensitivity troponin and HSCRP can risk stratify patients and detect hidden occult damage long before a massive event.

If these biochemical tools are becoming this precise at detecting microscopic changes, could the future of clinical biochemistry move entirely away from these universal 99th percentile cutoffs?

Perhaps one day we will rely on personalized baseline biomarker profiling, tracking every individual's unique cardiac signature year over year, detecting an impending plaque rupture years before they ever reach your emergency room.

A fascinating look at where the science is heading.

You've got the knowledge, now go ace that material.

Thank you for studying with us, from all of us here at the Last Minute Lecture Team on the Deep Dive.