❤️ Frank-Starling Law: Cardiac Output, Stretch, and Step 1 Integration
Why does a healthy heart pump harder when filled more? The answer lies in the Frank-Starling law—one of the most high-yield cardiac physiology concepts tested on Step 1. Today, we’ll break it down using clear graphics, exam-relevant vignettes, and memory aids to help you lock in preload physiology for good.
📈 What Is the Frank-Starling Law?
The Frank-Starling mechanism explains how increased end-diastolic volume (EDV) leads to increased stroke volume (SV)—up to a point.
“The more you fill the heart, the harder it contracts (within limits).”
It’s a length–tension relationship of cardiac muscle fibers: stretching them enhances force of contraction due to optimal actin–myosin overlap.
🫀 Frank-Starling Curve
EDV ↑ → Fiber Stretch ↑ → Sarcomere Alignment ↑ → Stroke Volume ↑
🧠 KOTC Tip: The curve eventually plateaus—excess stretch (e.g., in CHF) impairs contraction.
🔬 Clinical Applications
| Scenario | MAP Trend | Impact on Curve |
|---|---|---|
| IV fluids (↑ preload) | ↑ SV | Upward/right shift |
| Hemorrhage (↓ preload) | ↓ SV | Downward/left shift |
| Sympathetic stimulation | ↑ contractility | Steeper curve slope |
| CHF (dilated cardiomyopathy) | ↓ contractility | Flat or depressed curve |
🎯 USMLE Integration Examples
Case: Young adult with vomiting → ↓ preload → ↓ SV → ↓ BP
Case: Septic shock → vasodilation + leaky capillaries → ↓ preload
Case: Patient on norepinephrine → ↑ contractility → steeper FS curve
Case: CHF → no improvement despite fluid bolus (curve is flat)
📌 KOTC’s Q-bank includes Starling curves with draggable annotations
🧠 Mnemonic: “Stretch to Squeeze”
Stretch (preload) = Squeeze (output)
But too much stretch = Sloppy squeeze (CHF)
📌 Before You Go…
Preload. Stretch. Squeeze. That’s the rhythm of success on Step 1.
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Frequently Asked Questions (FAQs)
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