Cardiac Pathophysiology - USMLE Step 1

SECTION 1

Historical Context & Motivation

The study of cardiac pathophysiology is rooted in centuries of anatomical observation and physiological experimentation. Early physicians recognized that the heart was central to life, yet the mechanisms linking structural cardiac disease to clinical symptoms remained opaque for most of medical history. The transition from purely descriptive anatomy to a mechanistic understanding of cardiac dysfunction required breakthroughs in hemodynamics, electrophysiology, and molecular biology that collectively form the foundation of modern cardiovascular medicine.

1628 • Harvey's Circulation Theory

William Harvey published De Motu Cordis, establishing that blood circulates in a closed loop propelled by the heart, overturning Galenic physiology and providing the conceptual framework necessary for understanding cardiac output and heart failure.

1816 • Invention of the Stethoscope

René Laënnec invented the stethoscope, enabling clinicians to auscultate heart sounds and murmurs at the bedside. This tool linked valvular pathology to characteristic acoustic findings and transformed cardiac diagnosis from autopsy-based to bedside-based.

1903 • Einthoven's Electrocardiogram

Willem Einthoven developed the string galvanometer electrocardiograph, allowing the first reliable recordings of cardiac electrical activity. The ECG became indispensable for diagnosing arrhythmias, ischemia, and conduction abnormalities.

1958 • Starling's Law Revisited

Sarnoff and Berglund refined the Frank-Starling relationship with ventricular function curves, providing a quantitative framework for understanding how preload modulates stroke volume and how this mechanism fails in systolic heart failure.

1990s • Neurohormonal Model of Heart Failure

Large clinical trials (CONSENSUS, SOLVD, Val-HeFT) validated that heart failure is fundamentally a neurohormonal disease. ACE inhibitors and beta-blockers targeting the RAAS and sympathetic axes became cornerstones of therapy, shifting the paradigm from hemodynamic to neurohormonal management.

The central question that cardiac pathophysiology addresses is: How do perturbations in cardiac structure, electrical conduction, coronary perfusion, and neurohormonal regulation translate into the clinical syndromes of heart failure, arrhythmia, valvular disease, and ischemic heart disease? Mastering these mechanisms is essential for the USMLE Step 1, where integrating pathophysiology with pharmacology and clinical presentation is the foundation of high-yield cardiovascular questions.

SECTION 2

Core Principles of Cardiac Pathophysiology

Cardiac pathophysiology rests on several interrelated principles that govern how the heart generates output, adapts to stress, and ultimately fails. A firm grasp of these foundational concepts allows you to predict clinical manifestations from underlying mechanisms and to reason through unfamiliar vignettes on examination day.

Frank-Starling Mechanism

Increased ventricular end-diastolic volume (preload) stretches sarcomeres toward their optimal length, increasing the force of contraction and thus stroke volume. This intrinsic autoregulation allows the heart to match output to venous return beat-by-beat, but it has a ceiling—once sarcomeres are overstretched, further volume loading reduces contractility.

Cardiac Output Determinants

Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV). Stroke volume is itself determined by preload, afterload, and contractility. Pathology in any of these four determinants—rate, preload, afterload, contractility—can reduce effective cardiac output and produce symptoms of forward failure (fatigue, hypotension) or backward failure (congestion).

Myocardial Oxygen Supply-Demand Balance

The myocardium extracts ~75% of delivered oxygen at rest, leaving little reserve. Oxygen demand is driven by wall stress, heart rate, and contractility. When demand outstrips supply—as in coronary artery disease or aortic stenosis—ischemia and potentially infarction ensue.

Neurohormonal Compensation

When cardiac output falls, the sympathetic nervous system, renin-angiotensin-aldosterone system (RAAS), and antidiuretic hormone (ADH) activate to maintain arterial pressure and organ perfusion. Short-term, these are adaptive; long-term, they drive maladaptive remodeling—fibrosis, hypertrophy, and progressive ventricular dilation.

Ventricular Remodeling

Chronic hemodynamic overload triggers structural changes in the myocardium. Pressure overload (e.g., aortic stenosis, hypertension) leads to concentric hypertrophy with increased wall thickness. Volume overload (e.g., aortic regurgitation, mitral regurgitation) causes eccentric hypertrophy with chamber dilation. Both patterns ultimately predispose to heart failure.

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KEY TAKEAWAY

Think of the heart as a demand-driven pump connected to a feedback control system. Like a thermostat that keeps ramping up a furnace when the house never warms up, the neurohormonal axes keep escalating their signals when cardiac output remains low. Initially this compensation maintains blood pressure, but over months it warps the very structure of the heart—thickening walls, dilating chambers, and replacing contractile myocytes with collagen—until the pump itself becomes the problem. This transition from adaptive compensation to maladaptive remodeling is the unifying theme of heart failure pathophysiology.

SECTION 3

Frank-Starling Curves & Ventricular Function

The Frank-Starling curve is the single most important diagram in cardiac pathophysiology for the USMLE. It relates left ventricular end-diastolic pressure (LVEDP, a proxy for preload) on the x-axis to stroke volume or cardiac output on the y-axis. A normal heart operates on the steep ascending limb, meaning small increases in preload produce significant increases in output. In systolic heart failure, the curve shifts downward and to the right—at any given preload, the heart ejects less volume, and the patient requires higher filling pressures to maintain the same output, which drives pulmonary congestion.

Point A represents normal operating conditions: moderate preload producing a robust stroke volume. Point B shows a failing heart at the same stroke volume but requiring markedly elevated filling pressures (rightward shift), producing pulmonary congestion. The dashed cyan curve demonstrates how positive inotropes or exercise shift the curve upward and to the left, improving output at any given preload.

In the diagram above, the progressive downward displacement of the curve in heart failure illustrates the fundamental problem of reduced contractility: the ventricle generates less force per unit of preload. Clinically, the patient at point B presents with dyspnea (elevated LVEDP transmitted to the pulmonary veins) despite a marginally maintained cardiac output. Diuretics reduce preload and move the patient leftward along their depressed curve, relieving congestion. Inotropes shift the entire curve upward. ACE inhibitors and beta-blockers, by attenuating neurohormonal remodeling, can gradually shift the curve closer to normal over weeks to months.

SECTION 4

Hemodynamic Equations & Physiologic Relationships

While cardiac pathophysiology is not primarily a quantitative discipline on Step 1, several hemodynamic equations appear repeatedly in vignettes and are essential for reasoning through clinical scenarios. Understanding these relationships at a mechanistic level—not merely memorizing formulas—allows you to predict the hemodynamic consequences of valvular lesions, shock states, and pharmacologic interventions.

CARDIAC OUTPUT

CO = HR × SV

CO = cardiac output (L/min); HR = heart rate (beats/min); SV = stroke volume (mL/beat). Normal resting CO ≈ 5 L/min. A decrease in either HR (bradycardia) or SV (reduced preload, increased afterload, or decreased contractility) will reduce CO unless the other variable compensates.

MEAN ARTERIAL PRESSURE

MAP = CO × SVR

MAP = mean arterial pressure (mmHg); SVR = systemic vascular resistance (dyne·s/cm⁵). This is the cardiovascular analogue of Ohm's law (V = IR). It explains why hypotension can result from either low CO (cardiogenic shock) or low SVR (distributive shock), and why the body compensates a falling CO by increasing SVR via sympathetic vasoconstriction.

LAW OF LAPLACE (WALL STRESS)

Wall Stress (σ) = (P × r) / (2 × h)

P = intracavitary pressure; r = ventricular radius; h = wall thickness. Wall stress is the primary determinant of myocardial oxygen demand. In pressure overload (e.g., aortic stenosis), increased P is offset by increased h (concentric hypertrophy). In volume overload (e.g., aortic regurgitation), increased r raises wall stress, prompting eccentric hypertrophy with proportional increase in h to normalize σ.

EJECTION FRACTION

EF = (EDV − ESV) / EDV × 100%

EDV = end-diastolic volume; ESV = end-systolic volume. Normal EF is 55–70%. Heart failure with reduced EF (HFrEF) is defined as EF ≤ 40%, while heart failure with preserved EF (HFpEF) features EF ≥ 50% with impaired diastolic filling. This distinction is critical because pharmacologic evidence bases differ significantly between the two phenotypes.

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Clinical Pearl

On the USMLE, vignettes frequently describe a patient's hemodynamic profile (high PCWP, low CO, high SVR) and expect you to identify the type of shock. Remember: cardiogenic shock = ↑PCWP, ↓CO, ↑SVR; hypovolemic shock = ↓PCWP, ↓CO, ↑SVR; distributive shock = ↓PCWP, ↑CO (early), ↓SVR.

SECTION 5

Classification of Cardiac Pathology

Cardiac pathology can be organized by the primary structural or functional derangement. This classification system is especially useful for Step 1 because it maps directly onto the way questions present clinical vignettes: you are given symptoms, physical findings, and hemodynamic data and expected to identify the underlying pathological mechanism. The following diagram provides a high-level organizational framework that connects the major categories of cardiac disease to their hemodynamic consequences.

This organizational chart divides cardiac disease into four major categories: ischemic, myocardial, valvular, and electrical. Despite distinct mechanisms, all pathways converge on the common endpoint of heart failure or sudden cardiac death, underscoring the integrated nature of cardiac pathophysiology.

Category	Prototype Disease	Primary Mechanism	Remodeling Pattern
Ischemic	Myocardial infarction	Coronary artery occlusion → myocyte necrosis	Scar formation, thinning, aneurysm → eccentric remodeling
Myocardial	Dilated cardiomyopathy	Intrinsic myocyte dysfunction (genetic, toxic, infectious)	Four-chamber dilation, ↓ EF
Valvular	Aortic stenosis	Fixed outflow obstruction → pressure overload	Concentric LV hypertrophy
Electrical	Atrial fibrillation	Multiple re-entrant circuits → loss of atrial kick	Atrial dilation, tachycardia-mediated CMP

Comparison of major cardiac pathology categories

SECTION 6

Worked Example — Hemodynamic Reasoning in Heart Failure

A 62-year-old man with a history of two prior myocardial infarctions presents with progressive exertional dyspnea and bilateral lower extremity edema over the past 3 months. On examination, his JVP is elevated, he has bibasilar crackles, and a third heart sound (S3) is auscultated. An echocardiogram reveals a left ventricular ejection fraction of 25% with global hypokinesis. Right heart catheterization shows: PCWP 28 mmHg, CO 3.2 L/min, SVR 1800 dyne·s/cm⁵. Determine the type of heart failure, the hemodynamic profile, and the expected effect of initiating an ACE inhibitor.

Hemodynamic Analysis of Systolic Heart Failure

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Step 1 — Identify the Type of Heart Failure

The EF is 25%, which is well below the 40% threshold. This is heart failure with reduced ejection fraction (HFrEF), also called systolic heart failure. The history of prior MIs explains the etiology: ischemic cardiomyopathy secondary to myocardial scarring and loss of functional myocytes.

Diagnosis: HFrEF (ischemic cardiomyopathy), EF = 25%
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Step 2 — Interpret the Hemodynamic Profile

PCWP of 28 mmHg is markedly elevated (normal 6–12 mmHg), indicating left-sided congestion—hence the bibasilar crackles. CO of 3.2 L/min is reduced (normal ≈ 5 L/min), explaining exertional fatigue. SVR of 1800 dyne·s/cm⁵ is elevated (normal 800–1200), reflecting compensatory sympathetic and RAAS-mediated vasoconstriction. This hemodynamic profile—↑PCWP, ↓CO, ↑SVR—is the classic pattern of cardiogenic (pump failure) hemodynamics.

Profile: Wet and cold (congested with low output)
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Step 3 — Calculate MAP

Using MAP = CO × SVR (converted to mmHg): MAP ≈ CO × SVR / 80 = 3.2 × 1800 / 80 = 72 mmHg. This is at the lower limit of adequate organ perfusion, explaining why the patient's compensatory vasoconstriction (high SVR) is barely maintaining blood pressure.

MAP ≈ 72 mmHg (borderline low)
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Step 4 — Predict ACE Inhibitor Effects

An ACE inhibitor (e.g., enalapril) blocks angiotensin II production. Acutely, this reduces SVR (afterload reduction), which decreases the impedance to LV ejection. With lower afterload, the failing ventricle can eject a greater stroke volume, improving CO. Simultaneously, reduced aldosterone secretion promotes natriuresis, lowering preload and PCWP. Over weeks to months, attenuation of angiotensin II and aldosterone reduces myocardial fibrosis and adverse remodeling, improving long-term survival. The S3, which reflects rapid ventricular filling into a dilated, volume-overloaded chamber, may diminish as volume status improves.

Expected: ↓SVR, ↓PCWP, ↑CO, ↓ remodeling — mortality benefit

SECTION 7

HFrEF vs. HFpEF — A Critical Distinction

One of the most high-yield topics in cardiac pathophysiology for Step 1 is the distinction between heart failure with reduced ejection fraction (HFrEF) and heart failure with preserved ejection fraction (HFpEF). While both syndromes present with congestion and exercise intolerance, their underlying mechanisms, pathological anatomy, patient demographics, and evidence-based treatments differ substantially. The table below provides a systematic comparison.

Feature	HFrEF (Systolic Failure)	HFpEF (Diastolic Failure)
Ejection Fraction	≤ 40%	≥ 50%
Primary Defect	Impaired contractility (systolic dysfunction)	Impaired relaxation/compliance (diastolic dysfunction)
LV Chamber	Dilated, thin-walled	Normal or small cavity, thick-walled
Typical Patient	Younger male, post-MI, dilated CMP	Elderly female, HTN, obesity, diabetes
LV Pressure-Volume Loop	Rightward shift, decreased ESPVR slope	Upward shift of EDPVR, steep compliance curve
S3 Gallop	Common (volume overload)	Less common; S4 may be heard (stiff ventricle)
Proven Therapies	ACEi/ARB/ARNI, β-blockers, MRAs, SGLT2i, hydralazine-nitrate	SGLT2i (empagliflozin, dapagliflozin); diuretics for symptoms

HFrEF vs. HFpEF — Key Distinctions for USMLE Step 1

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KEY TAKEAWAY

Think of HFrEF as a weak pump that cannot squeeze adequately (systolic dysfunction), and HFpEF as a stiff container that cannot fill properly (diastolic dysfunction). A bicycle tire pump with a broken handle (HFrEF) fails to generate sufficient pressure on each stroke. A pump with an intact handle but a rigid, non-expansile barrel (HFpEF) cannot draw in enough air. Both result in inadequate output, but the engineering fix for each is fundamentally different—and this is exactly why the pharmacologic evidence bases diverge.

SECTION 8

Connection to Advanced Cardiovascular Physiology

The foundational hemodynamic principles covered in this lesson connect directly to more advanced topics that appear in Step 1 and become even more prominent in Step 2 CK and clinical clerkships. Understanding cardiac pathophysiology at the mechanistic level provides the scaffold for integrating pharmacology, pathology slides, and clinical management algorithms. The table below maps each foundational concept to its advanced extension.

Foundational Concept	Advanced Extension	Clinical Relevance
Frank-Starling curve	Pressure-volume (PV) loops	PV loops allow quantitative assessment of end-systolic elastance (contractility), arterial elastance (afterload), and mechanical efficiency—essential for understanding IABP and LVAD physiology
Neurohormonal activation	RAAS pharmacology cascade	ARNI (sacubitril/valsartan) enhances natriuretic peptide signaling while blocking RAAS, representing a dual-target approach that reduced mortality beyond ACEi alone (PARADIGM-HF trial)
Ventricular remodeling	Cardiac fibrosis and MMP biology	Matrix metalloproteinases (MMPs) degrade extracellular matrix during remodeling; aldosterone antagonists (spironolactone, eplerenone) reduce fibrosis and improve outcomes in HFrEF
Oxygen supply-demand	Coronary physiology and FFR	Fractional flow reserve (FFR) and coronary flow reserve (CFR) quantify the functional significance of coronary stenoses, guiding revascularization decisions
Arrhythmia mechanisms	Channelopathies and EP studies	Long QT syndrome, Brugada syndrome, and CPVT involve specific ion channel mutations (KCNQ1, SCN5A, RyR2) that are increasingly tested on Step 1

Bridging foundational and advanced cardiovascular concepts

As you progress through your cardiovascular studies, you will find that the pressure-volume loop integrates virtually all of the concepts from this lesson into a single graphical framework. The slope of the end-systolic pressure-volume relationship (ESPVR) quantifies contractility independent of loading conditions, while the end-diastolic pressure-volume relationship (EDPVR) captures ventricular compliance. Learning to manipulate PV loops—shifting the ESPVR leftward with inotropes, shifting the EDPVR rightward with volume loading—will consolidate your understanding of every cardiac pathology discussed here.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A patient with chronic aortic stenosis develops concentric left ventricular hypertrophy. Using the Law of Laplace, explain why this remodeling pattern is an adaptive response to the hemodynamic stress imposed by the stenotic valve. At what point does this adaptation become maladaptive?

Show answer

According to the Law of Laplace, wall stress σ = (P × r) / (2h). Aortic stenosis increases the pressure (P) the LV must generate to eject blood across the narrowed valve. To normalize wall stress, the ventricle increases wall thickness (h) by adding sarcomeres in parallel—concentric hypertrophy. This is initially adaptive because it reduces per-sarcomere stress, lowering myocardial oxygen demand toward baseline. However, this adaptation becomes maladaptive when hypertrophy leads to impaired diastolic relaxation (the thickened wall becomes stiff, raising filling pressures and causing HFpEF symptoms), subendocardial ischemia (the increased muscle mass outstrips coronary perfusion capacity, especially during tachycardia), and eventually systolic failure if myocyte apoptosis and fibrosis supervene.

Question 2: BASIC CALCULATION

A patient's echocardiogram shows an end-diastolic volume (EDV) of 180 mL and an end-systolic volume (ESV) of 120 mL. Calculate the stroke volume and ejection fraction. Classify the type of heart failure based on EF.

Show answer

Stroke volume (SV) = EDV − ESV = 180 − 120 = 60 mL. Ejection fraction (EF) = SV / EDV × 100% = 60 / 180 × 100% = 33.3%. Because EF is ≤ 40%, this is classified as heart failure with reduced ejection fraction (HFrEF). This patient has evidence of systolic dysfunction: the ventricle is dilated (high EDV) and unable to eject an adequate fraction of its volume. Clinically, this patient would benefit from guideline-directed medical therapy including ACE inhibitors (or ARNI), beta-blockers, mineralocorticoid receptor antagonists, and SGLT2 inhibitors.

Question 3: INTERMEDIATE

A 55-year-old woman with longstanding hypertension presents with exertional dyspnea and an S4 gallop. Her echocardiogram shows an EF of 62%, LV wall thickness of 14 mm (normal < 11 mm), and impaired relaxation on Doppler assessment. Her BNP is mildly elevated at 250 pg/mL. Explain the pathophysiologic mechanism linking her hypertension to her symptoms, and describe where she would fall on a Frank-Starling curve relative to normal.

Show answer

Chronic hypertension creates sustained pressure overload on the LV, driving concentric hypertrophy (wall thickness 14 mm). The hypertrophied myocardium has impaired lusitropy (relaxation), requiring elevated filling pressures to achieve the same end-diastolic volume. This is HFpEF—the EF is preserved at 62%, but the ventricle is stiff and non-compliant. On a Frank-Starling curve, this patient's curve has a normal or near-normal peak (contractility is intact), but the operative point sits at a higher LVEDP for any given stroke volume compared to a normal individual. The S4 gallop reflects forceful atrial contraction against a stiff ventricle. The elevated BNP reflects the elevated wall stress from increased filling pressures. The patient's position on the curve might appear normal in terms of SV, but the x-axis coordinate (LVEDP) is pathologically shifted rightward, producing pulmonary congestion and dyspnea.

Question 4: APPLIED

A patient in the coronary care unit following a large anterior STEMI develops cardiogenic shock. Swan-Ganz catheter data show: PCWP 30 mmHg, CO 2.8 L/min, SVR 2200 dyne·s/cm⁵. The team initiates dobutamine infusion and nitroprusside. For each agent, predict the expected changes in PCWP, CO, and SVR, and explain the mechanism.

Show answer

Dobutamine is a β₁-adrenergic agonist that increases myocardial contractility (positive inotropy) and mildly decreases SVR via β₂-mediated vasodilation. Expected effects: CO increases (↑ contractility → ↑ SV), PCWP decreases (improved LV emptying reduces residual volume), and SVR mildly decreases. On the Frank-Starling curve, dobutamine shifts the operative point upward and leftward—more output at lower filling pressures. Nitroprusside is a direct arteriolar and venous vasodilator that releases nitric oxide. It reduces both preload (venodilation → ↓ venous return → ↓ PCWP) and afterload (arteriolar dilation → ↓ SVR). The afterload reduction allows the failing ventricle to eject more efficiently, increasing CO. Combined, these agents improve forward flow while relieving pulmonary congestion—though careful monitoring of blood pressure is essential because both agents can precipitate hypotension in a heart with minimal contractile reserve.

Question 5: CRITICAL THINKING

A 28-year-old man collapses during a basketball game and is resuscitated from ventricular fibrillation. Echocardiography reveals asymmetric septal hypertrophy with systolic anterior motion (SAM) of the mitral valve and a resting LVOT gradient of 60 mmHg. His father died suddenly at age 35. Integrate the pathophysiology of hypertrophic cardiomyopathy (HCM) to explain: (a) why dynamic LVOT obstruction worsens with exercise, dehydration, or Valsalva; (b) why beta-blockers are first-line therapy; and (c) why this condition produces diastolic dysfunction despite supranormal ejection fraction.

Show answer

(a) In HCM, the hypertrophied septum narrows the LVOT, and SAM of the anterior mitral leaflet creates dynamic obstruction. Anything that decreases LV volume or increases contractility worsens the obstruction because the septum and mitral leaflet come into closer proximity: exercise (↑ contractility, ↑ HR), dehydration (↓ preload → smaller LV cavity), and Valsalva (↓ venous return → ↓ preload) all reduce LV internal dimensions and intensify the Venturi effect that drags the mitral leaflet toward the septum. (b) Beta-blockers reduce heart rate (prolonging diastolic filling time), decrease contractility (reducing ejection velocity through the LVOT), and slow conduction (suppressing arrhythmias). By reducing contractility and allowing greater filling, they increase LV volume and reduce the dynamic gradient. (c) The massively hypertrophied myocardium is stiff and has impaired relaxation—classic diastolic dysfunction. Despite an EF that may exceed 70–80% (because the hypertrophied muscle generates supranormal contractile force and the cavity is small), filling pressures are elevated because the non-compliant ventricle resists passive diastolic filling. This produces exertional dyspnea even in the absence of systolic failure, and the elevated filling pressures can precipitate pulmonary edema and atrial fibrillation. The paradox of HCM is that it is simultaneously a disease of excessive systolic function (obstruction, supranormal EF) and inadequate diastolic function.

SUMMARY

Cardiac Pathophysiology — Summary

Summary

Cardiac pathophysiology integrates the Frank-Starling mechanism, the determinants of cardiac output (heart rate, preload, afterload, contractility), the Law of Laplace governing wall stress, and the neurohormonal compensation pathways (sympathetic nervous system, RAAS, ADH) into a unified framework for understanding heart disease. When myocardial oxygen supply-demand balance is disrupted—whether by coronary occlusion, pressure overload, volume overload, or intrinsic myocyte dysfunction—the heart initiates compensatory mechanisms that are adaptive in the short term but drive maladaptive ventricular remodeling (concentric hypertrophy, eccentric dilation, fibrosis) over weeks to months.

The critical clinical distinction between HFrEF (impaired contractility, EF ≤ 40%) and HFpEF (impaired compliance, EF ≥ 50%) determines the therapeutic approach: HFrEF has robust evidence for ACE inhibitors/ARNI, beta-blockers, MRAs, and SGLT2 inhibitors, while HFpEF management centers on SGLT2 inhibitors and diuretics for symptom relief. Hemodynamic reasoning using MAP = CO × SVR and the Frank-Starling curve allows you to predict the effects of pharmacologic interventions and classify shock states (cardiogenic, hypovolemic, distributive) based on catheterization data. Mastering these interrelated principles provides the mechanistic foundation for every cardiovascular question on USMLE Step 1.