U01.07.009 Starling curves

The Frank-Starling mechanism explains how the heart adapts to changes in preload, affecting stroke volume and cardiac output. Understanding these curves is crucial for USMLE Step 1, especially when evaluating conditions like heart failure (HF) and the effects of drugs like digoxin.

Starling Curves Overview

The Frank-Starling law states that the force of contraction is proportional to the end-diastolic length of the cardiac muscle fibers (preload). This relationship is represented by Starling curves, which help assess cardiac function under different conditions.

Condition	Effect on Starling Curve	Mechanism
Normal	Standard curve	Stroke volume (SV) increases with ventricular end-diastolic volume (preload) due to optimal sarcomere stretch.
Exercise	Curve shifts upwards	Increased sympathetic stimulation enhances contractility and cardiac output (CO).
Heart Failure (HF)	Curve shifts downwards	Decreased contractility reduces stroke volume despite increased preload.
HF + Digoxin	Curve shifts upwards compared to HF alone	Digoxin increases contractility by inhibiting Na⁺/K⁺ ATPase, leading to more intracellular Ca²⁺.
Increased Contractility	Curve shifts upwards	Catecholamines and positive inotropes (e.g., digoxin) enhance cardiac muscle performance.
Decreased Contractility	Curve shifts downwards	Loss of functional myocardium (e.g., MI), β-blockers (acute use), non-dihydropyridine Ca²⁺ channel blockers, and dilated cardiomyopathy reduce contractile force.

Factors Affecting Contractility

Contractility is the heart’s ability to generate force at a given preload.

↑ Contractility (shifts curve up)
- Catecholamines (sympathetic stimulation)
- Positive inotropes (e.g., digoxin)
↓ Contractility (shifts curve down)
- Myocardial infarction (loss of functional myocardium)
- Acute β-blocker use
- Non-dihydropyridine Ca²⁺ channel blockers (e.g., verapamil, diltiazem)
- Dilated cardiomyopathy

Clinical Implications

Heart Failure: Reduced contractility leads to lower stroke volume despite increased preload. Treatment focuses on improving contractility (e.g., digoxin) or reducing afterload (e.g., ACE inhibitors).
Exercise Physiology: Increased preload and contractility boost stroke volume and cardiac output, improving oxygen delivery to tissues.
Pharmacology Considerations: Drugs like β-blockers can reduce contractility, while digoxin can enhance it—critical for managing heart failure patients.

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USMLE Step 1 Review Series

Curriculum

U01.07.009 Starling curves

Starling Curves Overview

Factors Affecting Contractility

Clinical Implications

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