USMLE Step 1 - Cardiovascular Physiology & Hemodynamics Flashcards

Question 1

Cardiac Output (CO) Equations

Accepted Answer

1. Standard equation: $CO = HR 	imes SV$
2. Fick Principle: $CO = \frac{	ext{O}_2 	ext{ consumption}}{	ext{Arterial O}_2 	ext{ content} - 	ext{Mixed Venous O}_2 	ext{ content}}$

Question 2

Mean Arterial Pressure (MAP) Equations

Accepted Answer

1. Based on Cardiac Output: $MAP = CO 	imes TPR$
2. Based on Blood Pressure (at normal HR): $MAP = DBP + \frac{1}{3}(SBP - DBP) = \frac{2}{3}DBP + \frac{1}{3}SBP$

Question 3

Total Peripheral Resistance (TPR) Equation

Accepted Answer

$TPR = \frac{MAP - RAP}{CO}$
Where $MAP$ = Mean Arterial Pressure, $RAP$ = Right Atrial Pressure, and $CO$ = Cardiac Output.

Question 4

Stroke Volume (SV) & Ejection Fraction (EF) Formulas

Accepted Answer

- Stroke Volume: $SV = EDV - ESV$
- Ejection Fraction: $EF = \frac{SV}{EDV} = \frac{EDV - ESV}{EDV}$
*Normal $EF \ge 55\%$.*

Question 5

Pulse Pressure (PP) Determinants

Accepted Answer

$PP = SBP - DBP$
- Directly proportional to **Stroke Volume ($SV$)**.
- Inversely proportional to **Arterial Compliance**.
*High PP seen in hyperthyroidism, aortic regurgitation, exercise, and aging (stiff arteries).*

Question 6

Poiseuille Law & Resistance ($R$)

Accepted Answer

$R = \frac{8 \eta L}{\pi r^4}$
Where $\eta$ = blood viscosity, $L$ = vessel length, and $r$ = vessel radius.
- *Key Yield:* Resistance is inversely proportional to the 4th power of radius ($r^4$). Halving radius increases resistance by $16	imes$.

Question 7

Cardiac Cycle Phase: Isovolumetric Contraction

Accepted Answer

- **Events:** Ventricles contract, raising pressure without volume change.
- **Valves:** Mitral and Tricuspid valves close (produces **S1** sound); Aortic and Pulmonic valves remain closed.
- **PV Loop:** Vertical line moving upward from End-Diastolic Volume (EDV).

Question 8

Cardiac Cycle Phase: Isovolumetric Relaxation

Accepted Answer

- **Events:** Ventricles relax, decreasing pressure without volume change.
- **Valves:** Aortic and Pulmonic valves close (produces **S2** sound); Mitral and Tricuspid valves remain closed.
- **PV Loop:** Vertical line moving downward from End-Systolic Volume (ESV).

Question 9

PV Loop Changes: Increased Preload

Accepted Answer

- **EDV:** Increases (shifts right).
- **SV:** Increases.
- **ESV:** Unchanged.
- **Graph:** Rightward expansion of the loop bottom-right corner.

Question 10

PV Loop Changes: Increased Afterload

Accepted Answer

- **Peak Systolic Pressure:** Increases.
- **ESV:** Increases (less blood ejected).
- **SV:** Decreases.
- **Graph:** Taller and narrower loop.

Question 11

PV Loop Changes: Increased Inotropy (Contractility)

Accepted Answer

- **ESV:** Decreases (more efficient contraction).
- **SV:** Increases.
- **EF:** Increases.
- **Graph:** End-Systolic Pressure-Volume Relationship (ESPVR) slope shifts up and to the left.

Question 12

Pressure-Volume Loop: Aortic Stenosis

Accepted Answer

- **Pathophysiology:** High resistance to LV outflow (increased afterload).
- **PV Loop Changes:** Increased peak LV pressure, decreased stroke volume, increased ESV.
- **Consequence:** Concentric LV hypertrophy.

Question 13

Pressure-Volume Loop: Mitral Regurgitation

Accepted Answer

- **Pathophysiology:** Blood leaks into LA during systole; no true isovolumetric phases.
- **PV Loop Changes:** Increased EDV (increased preload), decreased ESV, higher total stroke volume (though forward SV is decreased), loss of true vertical isovolumetric lines.

Question 14

Pressure-Volume Loop: Aortic Regurgitation

Accepted Answer

- **Pathophysiology:** Backflow from aorta to LV in diastole; no true isovolumetric relaxation.
- **PV Loop Changes:** Markedly increased EDV (huge preload), increased peak systolic pressure, increased stroke volume, high pulse pressure.

Question 15

Pressure-Volume Loop: Mitral Stenosis

Accepted Answer

- **Pathophysiology:** Impaired LV filling during diastole.
- **PV Loop Changes:** Decreased EDV (decreased preload), decreased ESV, markedly decreased stroke volume.

Question 16

S3 Heart Sound Pathophysiology & Causes

Accepted Answer

- **Timing:** Early diastole during rapid ventricular filling phase.
- **Mechanism:** Rapid inflow hitting a dilated, compliant ventricle or volume overload.
- **Causes:** Heart failure, dilated cardiomyopathy, mitral regurgitation; normal in children and pregnant women.

Question 17

S4 Heart Sound Pathophysiology & Causes

Accepted Answer

- **Timing:** Late diastole ("atrial kick").
- **Mechanism:** Atrial contraction forcing blood into a stiff, non-compliant ventricle.
- **Causes:** LV hypertrophy (e.g., long-standing hypertension, aortic stenosis), hypertrophic cardiomyopathy, acute MI. Always pathological.

Question 18

Aortic Stenosis Murmur Characteristics

Accepted Answer

- **Type:** Systolic crescendo-decrescendo murmur.
- **Location:** Right upper sternal border (2nd intercostal space), radiating to the carotids.
- **Features:** Delayed/weak carotid pulse (*pulsus parvus et tardus*), soft S2.

Question 19

Mitral Regurgitation Murmur Characteristics

Accepted Answer

- **Type:** Holosystolic / Pansystolic murmur.
- **Location:** Apex, radiating to the axilla.
- **Features:** High-pitched, blowing quality; unchanged with respiration.

Question 20

Mitral Valve Prolapse (MVP) Murmur Characteristics

Accepted Answer

- **Type:** Mid-systolic click followed by a late systolic murmur.
- **Location:** Apex.
- **Dynamic Maneuvers:** Earlier click/murmur onset with decreased preload (e.g., Valsalva, standing); delayed onset with increased preload (e.g., squatting).

Question 21

Aortic Regurgitation Murmur Characteristics

Accepted Answer

- **Type:** Early diastolic decrescendo murmur.
- **Location:** Left sternal border.
- **Clinical Signs:** Bounding pulses, head bobbing (De Musset sign), wide pulse pressure.

USMLE Step 1 - Cardiovascular Physiology & Hemodynamics

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