Learning Objectives

By the end of this section, the learner should be able to:

• Explain the Frank–Starling mechanism and its physiological basis.
• Interpret Starling curves under normal conditions (rest and exercise).
• Describe how heart failure and positive inotropic agents alter the Starling curve.
• Relate changes in preload and contractility to stroke volume and cardiac output.

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The Frank–Starling mechanism describes the intrinsic ability of the heart to adjust its force of contraction in response to changes in venous return. Within physiological limits, the force of myocardial contraction is directly proportional to the end-diastolic length of cardiac muscle fibers, which is determined by ventricular end-diastolic volume (EDV, preload).

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Axes of the Starling Curve

• X-axis: Ventricular end-diastolic volume (preload)
• Y-axis: Stroke volume (or cardiac output)

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Normal Starling Curves

• Normal (rest): Represents baseline cardiac performance at rest.
• Normal (exercise): The curve shifts upward and to the left, reflecting increased stroke volume at any given preload due to enhanced contractility and venous return during exercise.

Physiological states along the normal curve:

• Rest → Walking → Running
  Progressive increases in preload and contractility result in higher stroke volume and cardiac output.

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Heart Failure

In heart failure, the Starling curve is shifted downward and to the right:

• For any given preload, stroke volume is reduced.
• This reflects impaired myocardial contractility due to loss of functional myocardium (e.g., myocardial infarction) or chronic ventricular dysfunction.

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Effect of Positive Inotropes

In patients with heart failure, positive inotropic agents (e.g., dobutamine, milrinone, digoxin) shift the curve upward, partially restoring stroke volume and cardiac output at a given preload.

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Myocardial Contractility

Contractility refers to the intrinsic strength of myocardial contraction independent of preload.

Increased contractility occurs with:

• Catecholamine stimulation (e.g., sympathetic activation)
• Positive inotropic drugs (e.g., dobutamine, milrinone, digoxin)
• Exercise

Decreased contractility occurs with:

• Loss of functional myocardium (e.g., myocardial infarction)
• Acute β-blocker administration
• Nondihydropyridine Ca²⁺ channel blockers
• Heart failure

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Clinical Correlation

• Increasing preload alone has limited benefit in heart failure because the flattened Starling curve limits stroke volume response.
• Improving contractility (e.g., with inotropes) is more effective in acutely decompensated heart failure.

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Activity