Acing The Nursing Exam

Before you continue: Can you explain (in words) how a change in preload vs afterload changes stroke volume and what that does to blood pressure and perfusion?

Key Terms

• Cardiac output (CO) - Amount of blood pumped per minute; CO = HR x SV.

• Stroke volume (SV) - Blood ejected per beat; influenced by preload, afterload, and contractility.

• Preload - Initial stretch of ventricular muscle (often approximated by venous return/EDV).

• Afterload - Load the ventricle must overcome to eject blood (closely related to SVR).

• Frank-Starling mechanism - Increased preload (up to limits) increases SV due to increased fiber stretch.

• Mean arterial pressure (MAP) - Average arterial pressure driving perfusion; MAP ≈ DBP + (SBP-DBP)/3.

• Systemic vascular resistance (SVR) - Resistance in systemic circulation; ↑ SVR raises afterload and MAP.

• Perfusion pressure - Pressure gradient that supports blood flow to tissues (MAP is a key exam proxy).

Active Recall

• Cardiac output (CO) __________________________________________________

• Stroke volume (SV) __________________________________________________

• Preload __________________________________________________

• Afterload __________________________________________________

• Frank-Starling mechanism __________________________________________________

• Mean arterial pressure (MAP) __________________________________________________

• Systemic vascular resistance (SVR) __________________________________________________

• Perfusion pressure __________________________________________________

Worked Examples

Example 1: Compute MAP and predict perfusion risk

A patient: SBP 90, DBP 60.

• Apply MAP rule: MAP ≈ DBP + (SBP-DBP)/3

• Calculate: MAP ≈ 60 + (90-60)/3 = 60 + 30/3 = 70

• Interpret: MAP 70 is reduced versus normal (~70-100 target range), so perfusion risk rises-especially for organs dependent on MAP.

Now you try:

A patient: SBP 80, DBP 50. What is MAP (use MAP ≈ DBP + (SBP-DBP)/3)?

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Example 2: Hemodynamics-hypovolemia pattern

A patient has heavy bleeding. HR 120, SV falls, MAP drops.

• Hypovolemia → ↓ preload (less venous return)

• Preload ↓ → SV ↓ (Frank-Starling direction)

• CO = HR x SV: even if HR rises, SV drop often dominates → ↓ CO

• CO ↓ → MAP ↓ → perfusion decreases.

Now you try:

If preload decreases and contractility is unchanged, what happens to SV and CO (assume HR can’t fully compensate)?

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Example 3: Septic shock-vasodilation vs pump failure

A patient with sepsis has warm extremities and hypotension. Labs suggest vasodilation.

• Vasodilation → ↓ SVR → ↓ afterload

• With lower afterload, early SV may be maintained; CO can be normal/high early

• However, if myocardial depression develops, contractility ↓ → SV ↓ → CO ↓

• Outcome: MAP falls due to low SVR (early) and later due to low CO (late).

Now you try:

List two hemodynamic drivers that can cause MAP to fall in septic shock.

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Practice Questions

• (Easy) Define cardiac output and state the equation used in nursing hemodynamics.

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• (Easy) A patient’s SBP is 110 and DBP is 70. Calculate MAP using MAP ≈ DBP + (

SBP-DBP)/3, then state whether MAP is likely adequate for tissue perfusion.

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• (Medium) A patient with hemorrhage has HR 130, SV 40 mL, BP trending down. Explain the likely hemodynamic chain from hypovolemia to reduced MAP.

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• (Medium) Compare preload vs afterload:

• Give one definition for each

• State one common clinical situation where each increases

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• (Hard) Septic shock: warm extremities, hypotension, and later rising lactate. Explain two different mechanisms that can reduce MAP over time (include the roles of SVR, preload, and/or contractility).

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• (Hard) A patient has normal SBP but low urine output and cool extremities. Provide two reasons why “normal BP” can still mean inadequate perfusion.

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Answer Key

• Cardiac output (CO) is the amount of blood pumped by the heart per minute; CO = HR x SV.

• MAP ≈ 70 + (110-70)/3 = 70 + 40/3 ≈ 83. MAP ~83 is usually within an acceptable range for perfusion.

• Hemorrhage → hypovolemia → ↓ preload → ↓ SV → CO = HR x SV drops or cannot keep up → ↓ MAP → ↓ tissue perfusion.

• Preload = ventricular end-diastolic volume/pressure (e.g., fluid loss decreases preload; fluid bolus increases preload). Afterload = resistance the ventricle must overcome (e.g., hypertension/increased SVR increases afterload; vasoconstriction increases afterload).

• Early septic shock: vasodilation → ↓ SVR → ↓ MAP; CO may be maintained initially. Later: myocardial depression/contractility ↓ → ↓ SV → CO ↓ → MAP falls further (lactate rises from poor perfusion).

• Examples: (a) MAP may be adequate but microcirculatory dysfunction (distribution/perfusion mismatch) still causes low tissue oxygen delivery; (b) early “compensated” BP can hide inadequate effective perfusion due to low cardiac output or impaired oxygen extraction/utilization.

Exam Tips & Common Mistakes

• Confusing MAP with SBP/DBP: use the MAP rule (MAP ≈ DBP + (SBP-DBP)/3) for quick exam calculations.

• Mixing up CO vs SVR: CO is pump output (HR x SV), SVR is resistance (drives afterload and MAP when CO is limited).

• Forgetting the direction

End of chapter one. 4 more chapters in the full book.