Lecture Notes: Inotropes

Objectives

• Describe the usage and effects of inotropes
• Outline how inotropes affect practice, cardiac status, and patient care

Definition

• Inotropes: Agents influencing the contractility of muscle tissue
  • Positive Inotropes: Increase force of contractility
  • Negative Inotropes: Decrease force of contractility

Focus on Positive Inotropes

Digoxin

• Category: Inotrope

• Effects:

  • Increases force and velocity of myocardial contraction
  • Inhibits sodium pump; increases intracellular calcium
  • Decreases conduction in the AV node
  • Decreases ventricular response in atrial fibrillation
  • Prolongs slope of phase IV diastolic depolarization, decreasing heart rate
  • Acts as a vasodilator, reducing afterload

• Indications:

  • Atrial fibrillation with uncontrolled ventricular response (ventricular HR > 100 bpm)
  • Heart failure due to systolic dysfunction (NYHA class II-IV)
  • Failure of diuretics and vasodilator therapy
  • Severe heart failure with reduced EF and hypotension

• Drug Interactions:

  • Antiarrhythmics, beta-blockers, clonidine, methyldopa, certain antibiotics, atorvastatin

• Sensitivity Factors:

  • Elderly (>70), low skeletal mass, electrolyte imbalances
  • Acute MI, fibrotoxic heart disease, renal failure, COPD

• Risks:

  • Anorexia, restlessness, insomnia, drowsiness
  • Arrhythmias: PVCs, AV block, junctional/ventricular tachycardia, bradyarrhythmias

Mechanism Reflection

• Digoxin as Positive Inotrope: Increases cardiac output in heart failure
• Digoxin as Negative Chronotrope: Controls atrial fibrillation with rapid ventricular rate

Acute Heart Failure and Inotropes

Clinical Conditions

• Chronic heart failure
• Acute heart failure with pulmonary edema
• Cardiogenic shock

Importance of Acute Support

• Necessary to prevent worsening of condition
• Cardiogenic Shock: Inadequate cardiac output to maintain circulation

Catecholamine-like Agents

• Provide acute inotropic stimulation, vasodilation, or vasoconstriction

Sympathetic Nervous System Receptors

• Alpha Receptors: Vasoconstriction in blood vessels
• Beta-1 Receptors: Increase heart contractility and rate
• Beta-2 Receptors: Bronchodilation and vasodilation
• Dopaminergic Receptors: Dilation of renal and mesenteric arteries

Key Inotropic Agents

Dobutamine

• Receptors: Mostly beta-1, moderately beta-2, little alpha
• Effects: Increases heart contractility, causes vasodilation

Dopamine

• Receptors: Beta-1 and alpha
• Effect: Dose-dependent

Epinephrine

• Receptors: Stimulates all SNS receptors
• Response: Fight-or-flight-like

Norepinephrine

• Receptors: Mostly alpha and beta-1

Usage of Sympathomimetic Drugs

• Purpose: Augment SNS, especially post-depletion of endogenous catecholamines
• Result: Increased contractility, stroke volume, vasoconstriction/dilation, heart rate, BP, cardiac output, and tissue perfusion

Inotropic Effects Table

• Tool for study: Listing inotropic agents and effects on various parameters

Reflection on Inotropes and Cardiogenic Shock

• Purpose: Increase cardiac output through HR, vasoconstriction, and contraction force

Conclusion

• End of presentation on inotropes