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DIP Episode 417 - Pulmonary Pathophysiology Series 8

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Topic

Pulmonary vascular resistance; Pulmonary hypertension pathophysiology; Cardiac hemodynamics (P=Q R); Cor pulmonale.

Key Takeaway

The development of pulmonary hypertension (PH) is governed by the relationship P = Q R, meaning elevated pressure ( P) can result from increased flow (Q), increased resistance (R), or both, and understanding this physics principle is crucial for diagnosing PH etiology.

Runtime: 36 min

Published: 2022-10-11

Completed audio: Yes

Completed notes: Yes

Anki made: Yes

Section: RESPIRATORY, SPECIAL TOPICS/RISK FACTORS

Open Notion notes Episode source

Episode Notes

Source / episode info

Episode: 417
Title: Divine Intervention Episode 417: Pulmonary Pathophysiology Series 8
Published: 2022-10-11
Source: Episode page

One-liner

This episode provides a deep dive into pulmonary pathophysiology, detailing how lung volumes affect alveolar vs. extra-alveolar vessels, establishing the hemodynamic relationship P=Q R, and outlining the three primary mechanisms (increased Q, increased R, increased LAP) that lead to pulmonary hypertension.

High-yield summary

Pulmonary Vascular Dynamics: Pulmonary vascular resistance is determined by two vessel types: alveolar vessels (supply nutrients to alveoli; dilated at low lung volumes) and extra-alveolar vessels/pulmonary arteries (send blood for oxygenation; dilated at high lung volumes).
Hemodynamic Principle: Blood flow through the pulmonary circuit follows Ohm's Law analogy: P (Pressure Gradient) = Q (Cardiac Output from RV) R (Pulmonary Vascular Resistance). PH can result from increased Q, increased R, or increased Left Atrial Pressure.
Cor Pulmonale: This term specifically describes right heart failure resulting from a pulmonary cause (e.g., COPD, sleep apnea), distinguishing it from right heart failure caused by left heart disease.
PH Etiologies: The three main causes of PH are: 1) Increased Q (VSD/ASD); 2) Increased R (COPD, PE, Hypoxia); 3) Increased LAP (Mitral Stenosis).
Primary PAH Pathophysiology: Primary Pulmonary Arterial Hypertension is often associated with decreased levels of vasodilators like Nitric Oxide (NO) and increased vasoconstrictors like Endothelin.

Learning objectives

Differentiate between alveolar vessel function and extra-alveolar vessel function in lung mechanics.
Apply the hemodynamic principle \Delta P = Q \times R to understand the causes of pulmonary hypertension.
Identify the specific clinical signs and underlying pathophysiology associated with Cor Pulmonale versus PH from left heart failure.
Recognize the biochemical markers (e.g., Endothelin, NO) associated with Primary PAH.
Understand the rationale for drug classes used in treating different types of PH.

Board exam buzzwords

Condition	Key Finding	Association	Board Exam Tip
Cor Pulmonale	Right heart failure signs (JVD, edema)	Chronic lung disease/Hypoxia (COPD, OSA)	Remember: The cause must be pulmonary to qualify as Cor pulmonale.
Pulmonary Hypertension	Mean Pulmonary Artery Pressure > 25 mm Hg	Increased Q, Increased R, or Increased LAP	Always think of the three mechanisms when diagnosing PH.
Primary PAH	High Endothelin; Low Nitric Oxide (NO)	BMPR2 mutation (genetic cause)	Test questions often focus on the chemical mechanism (Endothelin/NO) rather than the gene itself.
VSD/ASD	Increased RV flow to pulmonary circuit	Increased Cardiac Output (Q)	High Q requires higher P, leading to PH.

Rapid review table

Topic	Key Point	Context	Exam Relevance
Pulmonary Vessels	Alveolar vs. Extra-alveolar vessels	Lung mechanics/Lung volumes	Low lung volume dilates alveolar vessels; High lung volume dilates extra-alveolar vessels.
PH Hemodynamics	P = Q R	Ohm's Law analogy for circulation	PH can be caused by high flow (Q), high resistance (R), or high LA pressure ( P).
Cor Pulmonale	Right heart failure from lung disease	COPD, OSA, Chronic Hypoxemia	Crucial distinction: LHF -> RHF is NOT cor pulmonale.
PAH Treatment	PDE-5 inhibitors (e.g., Sildenafil)	Vasodilation/cGMP pathway enhancement	These drugs increase cGMP, mimicking the effect of increased NO.

Board-speak -> diagnosis

Board-speak / Vignette phrase	Diagnosis / Concept	Why it fits
A patient presents with COPD, severe OSA, and chronic hypoxemia, leading to right ventricular hypertrophy and failure.	Cor Pulmonale (PH due to pulmonary cause)	Chronic hypoxia causes hypoxic pulmonary vasoconstriction -> increased R -> PH/RHF. This is a pulmonary cause of RHF.
A patient with mitral stenosis develops signs of right heart failure, including jugular venous distention and peripheral edema.	Increased Left Atrial Pressure (PH due to LAP)	Mitral stenosis restricts flow from the LA -> increased pressure backs up into the pulmonary veins/arteries -> PH.
A neonate with a large VSD presents with signs of right heart failure, including hepatomegaly and peripheral edema.	Increased Cardiac Output (PH due to Q)	The large shunt increases flow (Q) through the pulmonary circuit, requiring higher pressures ( P) to maintain flow -> PH.
A young female patient is diagnosed with PAH; testing reveals elevated Endothelin levels and decreased Nitric Oxide bioavailability.	Primary Pulmonary Arterial Hypertension (PAH)	These are classic biochemical markers of PAH pathophysiology: high vasoconstrictor/low vasodilator state.
The primary mechanism causing pulmonary edema in a patient whose right heart failure was secondary to chronic left atrial fibrillation.	Left Heart Failure -> Right Heart Failure -> Pulmonary Edema	PH is caused by the source of the backup pressure (LHF), not just the presence of RHF itself.
A physical exam reveals signs of severe pulmonary hypertension, and echocardiogram shows a large shunt from the right ventricle to the left atrium.	Increased Cardiac Output (VSD/ASD)	The increased volume load (Q) passing through the lungs elevates pulmonary pressures ( P).

Differential diagnosis / distinguishing features

PH due to Increased Q (VSD/ASD)

Key Features	Distinguishing Findings	Next Step
Large shunt flow; RV volume overload on echo.	Large shunt flow; RV volume overload on echo.	Measure pulmonary artery pressures and estimate shunt fraction.

PH due to Increased R (COPD, PE)

Key Features	Distinguishing Findings	Next Step
Hypoxemia; Evidence of vasoconstriction/thromboembolism.	Hypoxemia; Evidence of vasoconstriction/thromboembolism.	Check for reversible causes (e.g., administering oxygen); rule out acute PE.

PH due to Increased LAP (Mitral Stenosis)

Key Features	Distinguishing Findings	Next Step
Elevated LA pressure gradient; Mitral valve pathology.	Elevated LA pressure gradient; Mitral valve pathology.	Assess the degree of mitral stenosis and estimate pulmonary venous hypertension.

Management pearls

When evaluating PH, always consider that the underlying cause dictates the treatment: if the cause is increased Q (e.g., VSD), closing the shunt may be necessary.
For PAH, therapies aim to increase vasodilators (like NO) or decrease vasoconstrictors (like Endothelin). PDE-5 inhibitors are a common class of drugs used.
Pulmonary edema can occur in right heart failure if the primary cause of RHF was left heart failure (e.g., LHF -> RAAS activation -> pulmonary congestion).
The physical signs of PH (JVD, peripheral edema) indicate elevated right atrial pressure and are common to all forms of severe PH/RHF.

Don't miss

🚨

Cor Pulmonale Definition: Must be caused by a pulmonary process (e.g., chronic lung disease, sleep apnea).

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Hemodynamic Equation: \Delta P = Q \times R. This equation is the foundation for understanding all PH mechanisms.

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PAH Markers: Primary PAH involves decreased NO and increased Endothelin.

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Vessel Dilatation Pattern: Alveolar vessels dilate at low lung volumes; Extra-alveolar vessels dilate at high lung volumes.

Integration & clinical reasoning

Physics Integration: The pulmonary circulation functions as a closed hydraulic circuit where pressure difference (\Delta P) drives flow (Q), and resistance (R) is the limiting factor, analogous to Ohm's Law.
Cardiology Integration (LHF/RHF): Left heart failure causes elevated left atrial pressure, which backs up into the pulmonary veins, leading to PH and potential edema, even if the right ventricle itself is not failing initially.
Pulmonary Mechanics: The differing responses of alveolar vs. extra-alveolar vessels to lung volumes demonstrate how mechanical forces influence vascular resistance.

Concept connections / cross-references

For a detailed understanding of cardiac output and heart failure mechanisms: [ Episode 12 ] (Cardiology/Hemodynamics).
For general principles of pulmonary function testing and gas exchange: [ Episode 37 ] (Pulmonary Function Testing).

High-yield association table

Condition	Association	Mechanism	Clinical Significance
Cor Pulmonale	Chronic Hypoxemia	Vasoconstriction in pulmonary arterioles	Leads to increased Pulmonary Vascular Resistance (R) and subsequent PH.
Mitral Stenosis	Elevated Left Atrial Pressure (LAP)	Obstruction of LA outflow -> back pressure into pulmonary veins.	Causes PH by increasing the driving pressure ( P).
VSD/ASD	Increased Cardiac Output (Q)	Shunting increases blood flow volume through the lungs.	Increases Q, which elevates pulmonary pressures and causes PH.
Primary PAH	Endothelin / NO imbalance	High vasoconstrictor (Endothelin) / Low vasodilator (NO).	Requires targeted therapies like PDE-5 inhibitors or ET receptor antagonists.

Key terms glossary

Term	Definition	Context	Example
Cor Pulmonale	Right heart failure secondary to chronic lung disease/pulmonary vascular disease.	Diagnosis of right heart failure.	A patient with severe emphysema and RHF is diagnosed with Cor pulmonale.
Endothelin	Potent, naturally occurring vasoconstrictor peptide.	Primary PAH pathophysiology.	Elevated Endothelin levels are associated with increased pulmonary vascular resistance.
PDE-5 Inhibitors	Drug class that inhibits phosphodiesterase type 5 (e.g., Sildenafil).	Treatment for Pulmonary Arterial Hypertension.	These drugs increase cGMP, leading to vasodilation and lowering pulmonary vascular resistance.
Functional Residual Capacity (FRC)	Volume of air remaining in the lungs after a maximal expiration.	Lung mechanics/Pulmonary circulation.	PH is generally lowest at FRC because the two vessel types are balanced.

Study optimization

Topic	Study Approach	Priority	Resources
PH Pathophysiology	Conceptual understanding of P = Q R	High (Board-level)	Reviewing the three mechanisms (Q, R, LAP) and their specific causes.
Pulmonary Vasculature	Memorizing vessel responses to lung volumes	Medium (Conceptual)	Understanding why alveolar vessels dilate at low volume vs. extra-alveolar vessels at high volume.
PAH Pharmacology	Linking drug class to mechanism of action	High (Step 1/2)	Knowing that PDE-5 inhibitors increase cGMP, mimicking NO's effect.

Question pattern recognition

Pattern: Chronic Hypoxemia + RHF signs -> Cor Pulmonale: Always confirm the cause of RHF is pulmonary; if so, it's cor pulmonale.
Pattern: Mitral Stenosis/Tricuspid Atresia -> PH due to LAP: Obstruction before the right heart increases pressure back into the pulmonary circuit.
Pattern: COPD/OSA + RHF signs -> Increased Resistance (R): Chronic hypoxia causes vasoconstriction, increasing resistance and leading to cor pulmonale.

Test yourself

Common mistakes to avoid

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Mistake 1: Confusing Cor Pulmonale: Assuming any right heart failure with lung disease is cor pulmonale. (Correction: The cause must be pulmonary, not just associated.)

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Mistake 2: Misunderstanding Pulmonary Edema: Believing that pulmonary edema can only occur if the right ventricle fails. (Correction: LHF -> RHF can still cause pulmonary edema because the pressure backup originates from the left side.)

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Mistake 3: Overlooking the Physics: Treating PH as a single entity rather than recognizing it is driven by three distinct hemodynamic forces (\Delta P, Q, or R).

Common traps

⚠️

Trap 1 (The "Easy" PAH Diagnosis): Assuming that any patient with COPD and RHF has cor pulmonale. (Trap: Must confirm the cause of RHF was pulmonary.)

⚠️

Trap 2 (The Physics Trap): Confusing which variable is driving flow. Remember \Delta P is the difference between PA pressure and LA pressure.

⚠️

Trap 3 (The Vessel Dilatation Trap): Assuming alveolar vessels are always dilated in lung disease. (Trap: They dilate specifically at low lung volumes.)

Original transcript with highlights

Okay, welcome. My name is Devine. This is episode 417 of the Divine Intervention Podcast. And today's podcast will be continued on the Poetry Path of Physiology series. This is going to be series 8. Before I jump right in, I just want to send out a reminder. If you are taking your USMELY step 2 CK or step 3 exams or complex level 2 or complex level 3 exams or you are taking shelf exams, I need a bird overview of what's going to be coming up on your different shelf exams. So, I'll link what you're going to take advantage of the courses I offer. I offer this month, I'm going to be offering an MBA me test against strategy's course. It's first step 2, step 3 and shelf exams. It's going to be taking place on the 21st of this month from 5 to 7 30 PM Pacific Standard Time. We use many different questions to show you how to knock out MBA me questions. There are tons of people that have taken this class, found it to be supremely helpful, I've had many people that have had very durable scoring increases, Q Bank percentage increases as a result of taking this course. And then I have a biostatistics board camp that's going to be taking place on the 20th of this month. That's next week from noon to 4 PM Pacific Standard Time. For those of you that are taking the USML Es and this course obviously is for step 1 to step 3, complex 1 to 3 shelf exams. Basically here's the thing, the USML Es these days, the biostat questions they write, most of them are not formula based.

You plug and chug into a formula, no. Most of it is understanding based. You can memorize the formulas all you want, you very likely won't be able to tackle most of the biostat questions you see. So if you want to understand because the biostat at least you know how to give my podcast, the biostat class is not for, let me teach you how to plug and chug, let me show you all the formulas, no, you can do that on your own. What I want to teach you is how to reason with biostat. So that regardless of how hard the question is seen, you can just navigate through it very, very easily. And really having that biostatistic knowledge will also help you navigate the drug ads a lot easier. And then finally I have the 20 hour review, it will take place from the 24th to the 28th. So that's Monday to Friday from noon to 4 p.m. Pacific Standard Time on all 5 days. We'll go through more than 1,000 concepts from PEEDS, Surgery, OBGYN, Internal Medicine, Neurosyc, Biostats, Ethics, Communications, Healthcare Systems, Multi-Systems, Persises and Disorders. So touch on some biostat, but if you want a very good solid deep dive into bio stats, you should attend the biostatistics class. Okay, so let's, so if you're interested in any of these classes, just shoot me an email through the website and I'll be happy to give you some more information so that you can reserve your spot. So now let's continue. So really today, I want to, again, keep focusing on a lot of physiology.

But again, you'll notice what I'm going to try to do is discuss the physiology first, and then we're jumping to the path. Because again, a lot of people really struggle with poem. Because again, the understanding is usually not there. And many times in resources, you don't see poem clearly, carefully explained. So let's try to make that different with Lisa series. So the series 1 to 7, if you want to know the exact episodes, just go back on the website, go on their exam topics list, and then go on the step 2, step 3, or even the step 1 tab. Because really this podcast, I suspect, will help everyone. So the first step 1 to step 3, come next 1 to 3. And just do a control F for Pomonaire Pathophysiology. You can even just do a Google search, and it will take you to the exact episode on the website. Just do a Pomonaire Pathophysiology series 1, divine intervention in Google, and it will come up. Okay, so today, I want to address Pomonaire vascular resistance. Now, the thing is, if you really want to understand Pomonaire vascular resistance, you need to understand that there are two general types of blood vessels related to the lungs. There are two general types of blood vessels related to the lungs. There's the aviolar vessels. These are literally in the lungs. And their job is literally to supply nutrients that the aviolar used themselves. Remember, the aviolar is not just a bunch of bags. Now, there are two cells that live there. Those cells are really have to eat.

So, the aviolar vessels, they supply nutrients to the aviolar themselves. Now, what are the second group of blood vessels in the lungs? So, the second group of blood vessels, they are the Pomonaire arteries. Their main job is literally to send blood to the lungs for oxygenation, not for the aviolar to eat, but for oxygenation. These Pomonaire arteries, you can actually think of them as extra aviolar vessels. Extra aviolar vessels. So, again, there are aviolar vessels and there are extra aviolar vessels. The extra aviolar vessels are essentially the Pomonaire arteries. Now, the thing is, these two kinds of vessels, they have properties that they combine, that determine the Pomonaire vascular resistance. So, the combined pressures in these two is what determines the Pomonaire vascular resistance. So, whenever you have high-long volumes, it actually stretches the aviolar vessels. So, notice I'm going vessel by vessel, I'm not combining both now, I will at the end. Whenever you have high-long volumes, so I'm going to talk about how high-long volumes affect aviolar vessels and how high-long volumes affect extra-aviolar vessels. The effect is different, you got to make sure you understand this. So, high-long volumes, they stretch the aviolar vessels and they make the aluminum smaller. The aluminum becomes smaller in response to the high-long volumes, the stretch. Obviously, if the aluminum is smaller, the radius is small and obviously if radius is small, the resistance goes up.

So, this will increase the Pomonaire vascular resistance. But, if you're looking at things from the perspective of the extra aviolar vessels, as the long volumes get bigger, the aluminum actually gets bigger. And again, you may be like, oh, divine, because anyway I can understand this, I don't remember right. But, it makes logical sense. When your long volumes are huge, it means a lot of oxygen is coming in. A lot of oxygen is coming in. If a lot of oxygen is coming in, then you want the blood in your body to be oxygenated at that same time. Because, literally, it doesn't make any sense for your blood to be trying to get oxygenated when there's not enough oxygen. When you're in a healthy person, when your long volume is maximum, right. And there's so much oxygen around. You want so much blood from the extra aviolar vessels, your Pomonaire arteries to be coming through. So, it would make sense that on the high-long volumes, the aluminum should be bigger, right. So that they can get oxygenated more efficiently, right. Really, the radio traction of the lungs is essentially what makes the aluminum of these extra aviolar vessels bigger, right. And if it makes the lung bigger, the mob blood is going to flow through your Pomonaire arteries to the Pomonaire capillaries, they're going to get oxygenated, get into the Pomonaire veins, go to left heart, which is awesome. That's literally what you want. So, at high-long volumes, again, these extra aviolar vessels, they're dilated, right.

And that decreases the Pomonaire vascular resistance. So, again, just to, and really, like all these things, if you take the low-long volume state, everything is reversed. And again, if you understand what I just said for the high-long volume state, everything for the low-long volume state should make perfect sense. On the low-long volume conditions in a healthy individual, there's not much oxygen in the lungs at that point. Since there's not much oxygen in the lungs, it makes no sense for your Pomonaire vessels to be dilated. Your extra aviolar vessels, it literally makes no sense for them to be dilated. This should be constructed. But it will make more sense for your aviolar vessels to be dilated so that oxygen can come to the lung tissue. It's almost like, wow, because I just kind of think about it. Just, let's be logical here, right. If you work, the appearance of working, and then you get break time. Well, if you work at a place that gives break time, it's supposed to be the law, but, you know, there's probably not always the case. But you're supposed to be working, and then it's supposed to have break time. When you're working, you shouldn't be eating on the job. But when you have break time, you should be able to chill and eat. So the thing is, when you have high-long volumes, literally your aviolar vessels on the clock, they're working. That's not the time they should be eating. So it makes sense that they should be constructed at that point.

They shouldn't be trying to get nutrients then. But when they've given up oxygen and everything, and it's a low-long volume state, where, ooh, they're not working. They're not on the clock right now. They own break time. Then it makes sense that those aviolar vessels should be dilated, so that they can re-up, they can get some nutrients to keep them going. Right? But then the extra aviolar vessels, there is no point in them being dilated then. They should be constructed. Why? Because now they're not really fulfilling the purpose of giving oxygen, getting a blood oxygenated. So they should just kind of chill for a bit. So that the avioli can eat so that they are ready. They can have energy for the next quarter of the year. That's just a nice easy way to just kind of keep things in perspective. So obviously it should be obvious that again, the aviolar vessels, they prefer low-long volumes, while the extra-aviolar vessels prefer high-long volumes. Because again, at high-long volumes, etra-aviolar vessels are dilated. At low-long volumes, the aviolar vessels are dilated. Again, if you just understand these things, then you don't have to memorize it at all. You're just kind of stick it in your mind there. So, okay. And so I said at the end, I'm going to pull all of this together. So let's pull it all together.

So the thing is, obviously there must be a time where there's like some happy medium between these two counter-balancing forces of, you know, pulmonary vascular resistance in the aviolar vessels, pulmonary vascular resistance in the extra-aviolar vessels. Really, the happy medium where, you know, these two counter-balancing forces are kind of all happy and is like a perfect mix for the long story pretty efficiently. That's the FRC. That's the functional residual capacity, right? So let me tell you this. If you mix up the pressures, if you mix up the resistances in the aviolar vessels and the extra-aviolar vessels, the longs operate in general under the lowest pulmonary vascular resistance at FRC, right? That's the functional residual capacity. Again, we've kind of defined, we've kind of defined many of these pressures already. Okay, so now that you understand this pulmonary vascular resistance business, let's kind of talk about pulmonary hypertension, right? So we talk about the physiology and always bring some integrations in, right? So what is pulmonary hypertension? Well, pulmonary hypertension is whenever you have a mean pulmonary artery pressure that is consistently over 25 millimeters of mercury, okay? A mean pulmonary artery pressure over 25 millimeters of mercury, right? The thing is, if you have pulmonary hypertension for a long time, that's bad, because your right ventricle is going to undergo hypertrophy.

If you don't have the ghost hypertrophy, then you're going to have a right heart failure, right? And really, you should think of pulmonary hypertension when you have an exam question that talks about things like JVD, peripheral adema, hepato-megaly, right? Because essentially, blood is backing up, right? Because of the right heart that has failed. Blood is backing up to proximal to the field, right heart. Now, I want to say two things here, because these are things that people you routinely mess up on exams. Now, you can have pulmonary adema because you have a right heart failure. I'll say that again. Pulmonary adema can be a finding in a person with right heart failure. You see some people who are the erroneously learned at all. If a person has right heart failure, they cannot have pulmonary adema. No. What if that right heart failure was caused by the most common cause of right heart failure, which is left heart failure? If you have left heart failure causing you to your right heart to fail, you will have pulmonary adema because guess what? Those pulmonary vessels are proximal to the left heart. So, see that again. The endgame means the love to play this trick on people, because they know some people have learned some of this stuff that's just not real, right? The fact that you have pulmonary adema does not cross out right heart failure. You can literally have pulmonary adema as a component of right heart failure. That's one.

Number two, and I just literally said that the most common cause of right heart failure is what? Is left heart failure? Again, I'm going really slowly just to really make sure that you understand these things that we're talking about. Now, what's the second thing I want to highlight? Is this whole term of corpomonali? Many people, I don't know. I don't know. It still strikes me as bizarre to this day. Many met students get questions wrong on corpomonali for no good reason. Let me explain. I just said that the most common cause of right heart failure is left heart failure. Okay. So that means, oh, besides this most common cause, there must be other things that cause right heart failure. Right? You can get right heart failure not just because you have a left atrial problem, but because you have a pulmonary problem. Whenever you have a pulmonary problem, that pulmonary problem can over time give rights to right heart failure. Whenever you have a pulmonary cause of right heart failure, that's what's called corpomonali. I'll say that again. Whenever you have a pulmonary cause of right heart failure, that's what's known as corpomonali. Corpomonali. So if you have left heart failure causing a right heart failure, we do not call that corpomonali. But if you have a pulmonary cause like obstructive sleep apnea, like idiopathic pulmonary atrial hypertension, like cystic fibrosis, like sarcoidosis, if those corpomonali problems, right, look at this.

I'm trying to give you the money here. If those corpomonali, so problems from the lungs, not the left heart, if those corpomonali problems cause right heart failure, then that's what's called corpomonali. Okay. That's what's called corpomonali. Please don't get this stuff wrong with exams. It's very disheartening when you see students get this stuff wrong on tests. So what are the causes of pulmonary hypertension? Well, let's use an equation. Or it's probably going to be useful for a person. It's very good for studying for step one. It's also going to help you the step two, step three, folks, right. So many of you from college physics, probably remember the equation V equals IR, right. V equals IR, V is voltage, I stands for current, I stands for resistance, right. So what in the world is V? The voltage here is the energy that is needed to achieve flow through a circuit. If basically a thing that drives, that actually literally pushes that electricity through a circuit is the voltage. And then what's I? I is the current, right. So current is a measure of how fast electricity is moving through a circuit, right. It's literally a measure of how fast electricity is moving through a circuit. And then R, right, R is a measure of the resistance to flow offered by that circuit. So V is the thing that pushes the electricity through. R is the resistance to that thing being pushed through.

And then if you combine like, wow, this V pushing things through this R preventing things from moving through while determining the speed that I, right, the current, the speed which things flow through said circuit. Your blood vessels are literally a circuit, they literally will be the laws of physics, right. So let me be translating this physics to physiology, right. So from the physiology perspective, you can literally take V equals IR to mean P equals QR, P equals QR. Because if you think about it, the P here, because I said voltage is what literally provides the driving force for current through a circuit, then a push is going through a circuit. So let's say your pulmonary vessels are circuit. So, and okay, let me, let me take a step back here a bit. Let me really make sure you understand the stuff. So obviously if you want currents to move through a circuit, you need to go from a place of high voltage, current has to flow from high voltage to low voltage. That's like a central principle, right. Because that higher voltage, there's a spread between, let's say, oh, a voltage of 20 volts and voltage of 10 volts. That's spread of 10 volts will provide the driving force for current in the circuit. Well, if you look at the pulmonary vessels as a circuit, you're going from pulmonary artery to pulmonary capillary to pulmonary vein, that's a circuit. Okay. So what's at the beginning of that circuit is the pulmonary artery.

What's at the end of that circuit is the left-eatrium because literally you're trying to get that blood from the pulmonary artery to the left-eatrium. So, which one has to be at the higher pressure gradient? Again, just don't try to memorize stuff. Just think about it logically. Which one has to have the higher pressure? It has to be the pulmonary artery. The pulmonary artery has to have the higher pressure so that, and the left-eatrium has to have the lower pressure. So there is a pressure gradient. There's a, whenever you hear the word gradient means difference. There's a pressure gradient. There's a pressure difference. There's a pressure spread between the pulmonary artery and the left-eatrium. You want the pulmonary artery to be at higher pressures than the left-eatrium so that blood can literally flow from the pulmonary artery to the left-eatrium. Right? So that's the P in this equation. And then the Q is the speed of flow. Right? The flow. Right? That Q in this case, we can actually call it the cardiac output. Remember, some of them may be like cardiac output, but divine. You know, cardiac output doesn't even refer to the left-eatrical only. No. Because if you think about it, how did the blood in the left-eatrical get there? Where did it come from? Okay, it came from the left-eatrium. Okay, where did the blood in the left-eatrium come from? The pulmonary vessels. Right? Where did the blood in the pulmonary vessels come from? The right ventricle. Right?

The stuff you see in the left ventricle is the reflection of what is in the right ventricle. So the cardiac output can be a sore gate for what's in the coming out of the right ventricle. Right? And then the R in the equation, the P equals Q times R, is the pulmonary vascular resistance. Right? Is the pulmonary vascular resistance? So basically the P equals Q times R. The P, right? You can see the delta P, the pressure difference. The P, the thing that's driving flow through the pulmonary vessels, is the difference in pressure between the pulmonary artery and the left-eatural pressure. Right? Equals your cardiac output, which is your Q multiplied by your pulmonary vascular resistance. If you understand this, whenever I start talking about, when I start talking about pulmonary hypertension, which is like right now, or none of this stuff will be like a foreign language to you at all. So, let's look at this. Right? So, let's look at cardiac output. Right? So again, remember we said that P is equal to Q times R. Right? P is equal to Q times R. P is equal to Q times R. Right? P is equal to Q times R. So, let's look at things from the perspective of cardiac output. If you have anything that markedly increases flow through the long vasculature, that's going to cause pulmonary hypertension. Remember we said earlier in this series, right? Or if I've not said I'm saying it now, the pulmonary vessels they love to operate at low pressures. Right?

But the thing is, if you really raise the cardiac output less in the context of a VSD or an ASD, where you're putting more blood in the right side of the heart compared to normal, and more blood is flowing through those blood vessels, that's going to cause a lot of problems. That's literally going to cause a lot of problems. Right? That's literally going to cause a lot of problems. Because again, remember, the, you may be like, divine, why would more cardiac in the context of all these equations for the more equation-minded, the more physics-minded folks? Why would I'm increasing cardiac output cause, so again, remember this cardiac output I'm referring to is cardiac output from the right ventricle. Right? As I give that relationship earlier, a few minutes ago here. So, if you think about it, I literally said that P is equal to Q times R. Right? So, the pressure that's driving blood flow through the pulmonary vessels is equal to the Q that's the cardiac output from the right ventricle multiplied by R. So, that means that PM, the Q, they're directly related. If there's a greater pressure gradient, there's going to be more flow. If there's more flow, you need higher pressures to ensure that more flow. So, if you have an ASD or VSD, we're putting a lot of blood in the right ventricle, or in the right side of the heart, the more blood is going to be flowing through that pulmonary vessels.

For that more blood to be flowing through those pulmonary vessels, there has to be a higher pressure to support that flow. I just kind of think of it this way. Let's say you're trying to push a sedan. Okay, fine. Maybe not a big deal for a very strong person. But if you have a massive truck, are you going to use the same strength used to push a sedan and think that, oh, I'll use the exact same amount of strength to push a truck. No, that's not going to work. For you to be able to push that massive load that is coming, that truck, you need to use more pressure and to apply more force to the back of that truck. Remember, pressure is force over the area. So, you need to apply more force to the back of that truck. So, it's just the same thing. So, you may be like, wow, define ASDVSD is not a big deal. And it is a big deal because if you have those problems, you're putting more blood in the pulmonary, you're putting more blood in the right ventricle. For the right ventricle to push that more blood out, it needs to apply, it needs to push you more force. Like pushing a truck, it's not pushing a sedan. That's going to raise your pulmonary pressure. That's going to raise your pulmonary pressure. That's going to cause burn air hypertension. Now, what if you have an increase in pulmonary vascular resistance? And many times, whenever a person has an increase in pulmonary vascular resistance, you're basically making it harder.

It's almost like, wow, going from driving on a smooth road to driving on a lot of potholes. So, whenever you have that big increase in pulmonary vascular resistance, that can cause it to be really hard for blood to flow through pulmonary artery. That's going to cause pulmonary hypertension. And then, also, if you have an increase in left-itre pressure, remember I said that the pressure of rigging that drives flow through the pulmonary vessels is the pressure difference between the pulmonary artery, which should have pre-done higher pressures, and the left-itre pressure, we should have pre-done lower pressures relative to the pulmonary artery, because again, is that difference that drives flow. So, for example, a person has the unfortunate circumstance of like a mantra stenosis. If you have a mantra stenosis, well, your left-itre pressure is going to go up a lot. Well, if your left-itre pressure is going up a lot, then over time, your left-itre pressure will exceed the pressure in the pulmonary artery. If that happens, then you're not going to have flow. Literally, the pulmonary artery may be like, wow, this left-itre pressure is really open its pressures. So, that means I need to increase my pressures, too. Let me give you an example. So, let's say normally, again, I'm just making up numbers here, they're not accurate. Let's say normally, your pulmonary artery is at 20 millimeters of mercury. Your left-itre pressure is at 10 millimeters of mercury.

Okay, you have that spread of 10 to drive blood flow through the pulmonary vessels. But then, let's say your left-itre, because of my stress stenosis, the pressure rises to 20. Well, the pressure rises to 20. Your body is going to be like, we need to find a way to get blood to the left-itre. If not, body is not going to get any blood. So, your pulmonary artery is going to be like, ah, okay, well, let's say what we're going to do here. So, your pulmonary artery is going to bump up its pressures to like 30 to keep that spread. Well, guess what? You've just got to give it yourself, pulmonary hypertension. So, again, you'll see that all these things from that equation I gave you, gives you a very solid basis to go down to understand, oh, this is how these problems actually arise. So, again, remember, if you understand these factors, you understand the things that cause a pulmonary hypertension. So, what are some causes of pulmonary hypertension? Well, if you think about it, if you have like COPD, right? If you have COPD, or you have like obstructive sleep apnea, or you go to a high altitude, or think about it, all those things cause hypoxia. Well, what does your body do? What do your pulmonary vessels do to your response to hypoxia? The viso-constrict. You've probably heard of this term called hypoxic pulmonary viso-constriction. Whenever there is hypoxia, your pulmonary vessels constrict. If they constrict, that's going to raise your pulmonary vascular resistance, right?

You also see some people, they may have chronic pulmonary embolite. They love to test these on shelf exams, and on step two, step three. If you have pulmonary embolite, that can cause pulmonary hypertension, right? If you're young female, if you see a young female pulmonary hypertension, think of a BMPR2 mutation, right? Bone morphogenic protein receptor 2 mutation, right? BMPR2 mutation, basically, it's a mutation that can happen, unfortunately, in some young women that can cause them to have this thing we call idiopathic pulmonary arterial hypertension. But again, the MBM is, they're smart. They know that most people that have the job description, medical students have memorized that. Oh, you know, idiopathic pulmonary arterial hypertension arises from a BMPR2 mutation. They know that you've all memorized that. They know that Anki is like mainstream in medicine these days. So how can they test that same thing? This is slightly different format. Well, they can say, they can, instead of writing BMPR2 mutation, they can put increased levels of endothelialin, or they can put decreased levels of nitric oxide. It's literally the same thing because this mutation gives rise to many of these problems. These mutations gives rise to many of these problems, right? But they are now going more to the chemical mechanisms behind many of these things. So remember, you have a primary pulmonary arterial hypertension, right? Remember that it's associated with high levels of endothelialin.

Endothelialin is a powerful visual constrictor. It's one of the most powerful visual constrictors in the body. Endothelialin also causes smoke muscle to proliferate. Obviously, if you have smoke muscle proliferation, that's going to decrease the lumen size of your pulmonary arteries. That's going to cause pulmonary hypertension. Also, one other thing that's associated with pulmonary arterial hypertension is having decreased levels of nitric oxide. Like literally, they will not give you the BMPR2 business. They will not give you the endothelialin business, but they will give you the nitric oxide business. So they can see which of the phones associated with the individual having primary pulmonary arterial hypertension. And they will put decreased levels of nitric oxide. They'll be like, what? But again, it's literally one of the pathophysiologies behind the problem. Remember, nitric oxide is a powerful visual dilator that raises your cyclic GMP. So if you have lower levels of visual dilator, that's not going to be a good thing for your pulmonary vessels. So if you understand all these things, again, you can predict the causes of pulmonary arterial hypertension. And can predict the treatments for pulmonary arterial hypertension. Like the endothelialin receptor antagonist, they all aim in sentan, like bousentan, ambercentan. These are drugs used for pulmonary arterial hypertension. The first four diastere is five inhibitors, like cell dentophilant, adalaphyl. What do they do?

Well, they're going to inhibit phosphodiesterase five. That's going to raise your cyclic GMP. That's giving you like an indirect nitric oxide effect. That's going to help, right? Because again, we said decreased levels of nitric oxide cause these problems. What increased levels of nitric oxide is obviously going to help this problem, right? And also remember, prostaglandins are very good visual dilators. So anything that's a prostaglandin analog that has prosting the name, like ILO prost, like a prostimial, right? These things are all drugs that can dilate your pulmonary vessels. They can help you in pulmonary arterial hypertension. So I think since this has gone for 28 minutes, I think I'm going to go ahead and stop here. I kind of want to start talking about like ventilation and perfusion. But I'm going to go after that in series nine. Again, if you liked the kind of teaching that I give here, with this podcast, this reflects a lot of the kind of teaching that I do in my review courses. So again, if you're looking for a review course that really helps you understand that I just memorized stuff, you want to attend my review courses. I really take pins to make sure I really explain things. I mean, there are certain things that it just makes more sense to memorize. Instead of just trying to go through the one hour of understanding it, where like it may provide very little yield on your test, those things will go through them.

But a lot of the stuff I try to make sure that you truly, truly understand what's going on. So I do offer one or one tutoring as I wrap up. I do offer one or one tutoring for all the USMN exams. Step one to step three, complex one to three pre-clinical med school exams, third year shelf exams. The only thing I don't do to is women, but I tutor to really every other exam. And then I have these podcasts on Apple Podcasts or Google Podcasts on Spotify. I also have a You Tube channel, Divine Intervention, USMN, podcast and videos. If you search for that You Tube channel, you'll find the videos that I have. And then I know many people are going through interviews season right now. I do offer a lot of interviews. I do a lot of mock interviews. Again, I've been on an admissions committee for a top three med school. And I've interviewed tons of people that are now residents, now attendants. So if you want guidance, if you want help with mock interviews and your application, reach out to me through the website and I'll give you some more information. And then I offer this thing called longitudinal tutoring. So some people do this where they work with me for like the entire pre-clinical years. And then that prepares them extremely well for step one. Or they work with me all throughout the third years. And that prepares them extremely well for step two. If you're interested in any of those things, these are just things that need to be set up far ahead of time.

Because again, my schedule gets super busy. So if you just set it up ahead of time, then you can get those guaranteed hours. And then finally, I have a new website called divineinterventionallifelessons.com. Many people say, oh wow, divine, I love the life lessons that you put at the end of your podcasts. So I decided to start a website, divineinterventionallifelessons.com. Right now we have, I think, more than 120 podcasts. I make two every week. And I use a biblical perspective to address a life lesson. Just a common problem that's faced by people. Most of them are between 10 to 20 minutes long. They're very short, very sweet. Go straight up to the point. Okay. Now, and I also have Apple podcasts. So shit with that. It's called the divine intervention life lessons podcast. So you can check that out on Apple podcasts. Now, I want to share a quick life lesson today. Right. So what's my quick life lesson? My quick life lesson today is about being in the minority. Right. The thing is, we live in a world. There's this common term strength in numbers. The thing is, there's nothing wrong with being in a group of people if you're on the right track. But again, the fact that everyone is doing something. Does not necessarily make it the right thing to do. That's a common error many people kind of fall for in life. You see them, they're like, oh, wow, everybody is doing their soymos be right. No, the fact that everyone is doing something. Does not mean it's right.

The fact that everyone is using a resource. Does not mean it's right. You see some people, they're like, oh, if I don't use this particular book, or you see some people, they make these book claims online. Now you know what? If you do not use this book, I'm speaking to a student. Now, if you do not use this book, you're going to fill you exam. No, there are many different words to success on the USMLA exams. In fact, some of the methods I used to study in med school are probably quite different from what was very popular at that time and what is even very popular now. So again, I'll just encourage you. Do what works for you. You don't always have to be in the majority to succeed. Or let's look at another example. You see like, now the real estate bubble is popping in the US, right? The houses were just beat up like crazy. And then the prices of houses are dropping like No Man's Business. Now you see homely stands where people are like, bring it down their prices. Just to try to get someone to buy those homes. Well, guess what? Everyone was buying homes like No Man's Business over the last year or whatnot. And everyone was like, oh, you see like this herd mentality is one of the most common causes of failure in the stock market. Everybody's doing this.

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Practice questions — USMLE style

Question 1 — Cardiology/Pulmonary Pathophysiology

A 35-year-old male presents with progressive fatigue, peripheral edema, and jugular venous distention. Physical examination reveals signs of right heart failure. Initial workup suggests a history of chronic obstructive pulmonary disease (COPD) and severe sleep apnea. The patient's echocardiogram shows evidence of right ventricular hypertrophy and elevated mean pulmonary artery pressure. Which statement best describes the underlying pathophysiology in this patient?

A) The right heart failure is secondary to left atrial hypertension, which has caused back-pressure into the pulmonary circulation.
B) The primary cause of right heart failure is increased systemic vascular resistance due to chronic hypoxia.
C) The elevated mean pulmonary artery pressure and subsequent right ventricular hypertrophy are directly attributable to chronic lung parenchymal disease.
D) The patient's condition represents a primary left heart failure that has secondarily caused the observed signs of right heart failure.

Answer: C. Explanation: This scenario describes Cor Pulmonale, which is right heart failure resulting from chronic pulmonary disease (like COPD). The key distinction emphasized in the transcript is that when the cause of RV failure originates from a pulmonary problem (e.g., COPD), it is termed cor pulmonale. Option A and D describe typical left-to-right or systemic causes, while option B incorrectly attributes the resistance to the systemic circulation rather than the pulmonary vasculature itself.

Question 2 — Physiology/Hemodynamics

A patient with a large Ventricular Septal Defect (VSD) is admitted for evaluation of signs of pulmonary hypertension and right heart failure. The underlying hemodynamic principle governing this complication relates to the relationship between pressure, flow, and resistance in the pulmonary circuit ($P = Q \times R$). Which mechanism best explains why the VSD leads to increased pulmonary vascular resistance and subsequent pulmonary hypertension?

A) Increased left atrial pressure forces blood into the right ventricle, elevating overall pulmonary artery pressure.
B) The high volume of blood shunted from the left side to the right side increases cardiac output (Q), requiring a higher driving pressure (P).
C) Chronic hypoxia secondary to the VSD causes generalized hypoxic pulmonary vasoconstriction, increasing resistance (R).
D) The increased flow rate through the pulmonary vessels reduces the mean pulmonary artery pressure gradient, leading to compensatory vasoconstriction.

Answer: B. Explanation: The relationship $P = Q \times R$ dictates that if cardiac output ($Q$) increases significantly—as happens when a large shunt like VSD forces high volume blood into the right side of the heart—the driving pressure ($P$) must increase proportionally, assuming resistance ($R$) remains constant. This increased required pressure elevates pulmonary artery pressures and can lead to secondary pulmonary hypertension.

Question 3 — Pulmonary Pathophysiology/Molecular Mechanisms

A young woman is diagnosed with Primary Pulmonary Arterial Hypertension (PAH). Genetic testing reveals a mutation in the Bone Morphogenic Protein Receptor Type II ($BMPR2$). The underlying pathophysiology of PAH involves an imbalance between potent vasoconstrictors and vasodilators. Which combination of molecular findings is most consistent with the pathogenesis of this condition?

A) Increased nitric oxide levels and decreased endothelin activity.
B) Decreased cyclic GMP synthesis and increased prostaglandin production.
C) High levels of endothelialin combined with reduced bioavailability of nitric oxide.
D) Elevated atrial natriuretic peptide (ANP) and low plasma aldosterone concentration.

Answer: C. Explanation: Primary PAH is characterized by a profound imbalance in vasoactive mediators. The transcript highlights that the condition is associated with high levels of the powerful vasoconstrictor, Endothelin, and decreased levels of the potent vasodilator, Nitric Oxide (NO). Option C correctly identifies this critical molecular imbalance.

Question 4 — Respiratory Physiology

A patient who has been exposed to a high altitude environment develops acute symptoms suggestive of pulmonary vascular compromise. Physiologically, what is the immediate response of the pulmonary vasculature to systemic hypoxia?

A) Generalized vasodilation in all lung regions to maximize oxygen diffusion across the alveolar membrane.
B) Increased cardiac output from the right ventricle to maintain adequate blood flow despite low partial pressure of oxygen.
C) Localized vasoconstriction within the pulmonary arterioles, leading to increased overall pulmonary vascular resistance.
D) Dilatation of the alveolar vessels and constriction of the extra-alveolar vessels to optimize nutrient exchange.

Answer: C. Explanation: The body's immediate response to hypoxia (low $\text{PO}_2$) is Hypoxic Pulmonary Vasoconstriction (HPV). This mechanism causes pulmonary arterioles to constrict, which increases overall pulmonary vascular resistance ($\text{R}$). While this aims to divert blood flow away from poorly ventilated areas toward better-ventilated areas, if generalized or chronic, it leads to severe pulmonary hypertension.

Quick fire review

What is the threshold for diagnosing pulmonary hypertension?

A mean pulmonary artery pressure consistently over $25 \text{ mm Hg}$.

Which type of lung vessel tends to dilate during high lung volumes, thereby decreasing PVR?

The extra-alveolar vessels (or pulmonary arteries).

What is the primary mechanism that causes increased pulmonary vascular resistance in response to hypoxia?

Hypoxic pulmonary vasoconstriction.

If a patient has right heart failure due to COPD, what condition is this called?

Cor pulmonale.

In the physics model $P = Q \times R$, which variable represents the pressure difference that drives blood flow through the pulmonary circuit?

The difference between the Pulmonary Artery pressure and the Left Atrial pressure ($\text{PA} - \text{LA}$).

What is a key clinical trap regarding pulmonary edema in right heart failure?

A patient with RHF can still have pulmonary edema, especially if the underlying cause of the RHF was left heart failure.

Name two factors that increase Pulmonary Vascular Resistance (R) and lead to PH.

Hypoxia/COPD (vasoconstriction), Pulmonary Embolism (increased resistance).

What is the physiological difference between alveolar vessels and extra-alveolar vessels regarding lung volume changes?

Alveolar vessels constrict at high lung volumes; Extra-alveolar vessels dilate at high lung volumes.

Which specific mutation is associated with Idiopathic Pulmonary Arterial Hypertension (PAH)?

BMPR2 mutation.

What class of drugs acts by increasing $\text{cGMP}$ and helps treat PAH?

PDE-5 inhibitors (e.g., Sildenafil).

If a patient has RHF due to left heart failure, what is the most common cause of that underlying LHF?

Mitral regurgitation or systemic cardiac disease leading to elevated LA pressure.

What substance, highly associated with PAH, acts as a powerful vasoconstrictor and promotes smooth muscle proliferation?

Endothelin.

Quick recall / Anki-style questions