The force of cardiac contraction is directly influenced by preload (the end-diastolic stretch of myocardial fibers) and contractility (the intrinsic strength of contraction at a given preload).
1. Frank-Starling Mechanism: Preload and Contraction Strength
- The force of contraction is proportional to the end-diastolic length of cardiac muscle fibers.
- Increased preload (EDV) → Increased stroke volume (SV) → Increased cardiac output (CO).
- This physiological response allows the heart to adapt to changes in venous return.
🔹 Example:
- IV fluids increase preload, enhancing contraction strength.
- Venous vasodilators (e.g., nitroglycerin) decrease preload, reducing contraction force.
2. Factors That Increase Contractility (Positive Inotropes)
✅ Catecholamines (e.g., Epinephrine, Norepinephrine)
- Activate β1 receptors, increasing intracellular Ca²⁺ availability.
✅ Dobutamine (β1-agonist) – Used in heart failure and cardiogenic shock.
✅ Milrinone (PDE-3 inhibitor) – Increases cAMP, boosting Ca²⁺ entry into cells.
✅ Digoxin – Inhibits Na⁺/K⁺ ATPase, increasing intracellular Ca²⁺ levels.
💡 Clinical Use:
- Used in acute heart failure to improve cardiac output.
- Caution: Excessive inotropy can increase myocardial O₂ demand, worsening ischemia.
3. Factors That Decrease Contractility (Negative Inotropes)
❌ Loss of Functional Myocardium (e.g., Myocardial Infarction – MI)
- Damaged heart muscle reduces contractility permanently.
❌ Beta-Blockers (Acute Effect) - Reduce cAMP, decreasing Ca²⁺ influx and slowing HR.
- Long-term use improves cardiac function by reducing myocardial O₂ demand.
❌ Nondihydropyridine Ca²⁺ Channel Blockers (e.g., Verapamil, Diltiazem) - Inhibit Ca²⁺ entry, weakening contraction strength.
❌ Heart Failure (Systolic Dysfunction) - Weakened myocardium cannot generate sufficient contraction force.
💡 Clinical Use:
- Beta-blockers & Ca²⁺ blockers are used in conditions like hypertension, arrhythmias, and heart failure to reduce workload.
Key Takeaways:
- Force of contraction is preload-dependent (Frank-Starling mechanism).
- Increased contractility improves cardiac output but raises O₂ demand.
- Decreased contractility occurs in MI, acute HF, and with certain drugs.
- Managing inotropy is crucial in conditions like heart failure, cardiogenic shock, and ischemic heart disease.