Positive inotropic agents

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Mechanism of Inotropy - The Heart's Power-Up

  • Primary Driver: ↑ intracellular Calcium ($Ca^{2+}$) is the final common pathway for inotropy.
  • Key Pathway: Sympathetic stimulation of β1 receptors activates the cAMP-PKA pathway.
    • PKA phosphorylates L-type $Ca^{2+}$ channels → ↑ $Ca^{2+}$ influx.
    • PKA also enhances sarcoplasmic reticulum (SR) $Ca^{2+}$ release.

Cardiomyocyte Ca2+ release & beta-adrenergic pathway

Digoxin's Unique Mechanism: Unlike sympathomimetics, Digoxin inhibits the Na⁺/K⁺-ATPase pump. This leads to ↑ intracellular Na⁺, which then reduces Ca²⁺ efflux via the Na⁺/Ca²⁺ exchanger, ultimately increasing intracellular $Ca^{2+}$ and contractility.

Cardiac Glycosides - Digoxin's Digital Grip

  • Mechanism: Directly inhibits the Na⁺/K⁺ ATPase pump in myocardial cells.
    • This increases intracellular Na⁺, reversing the Na⁺/Ca²⁺ exchanger.
    • Result: ↑ intracellular Ca²⁺ → ↑ contractility (positive inotropy).
    • Also stimulates the vagus nerve → ↓ AV nodal conduction & ↓ heart rate.

Digitalis mechanism: Na/K ATPase inhibition & inotropy

  • Clinical Use:

    • Heart Failure (HFrEF): For symptomatic control; no survival benefit.
    • Atrial Fibrillation: Rate control by slowing AV conduction.
  • Toxicity & Adverse Effects (Narrow Therapeutic Index):

    • Cardiac: Any arrhythmia, classically scooped "hockey-stick" ST segments on ECG.
    • GI/CNS: Nausea, vomiting, confusion, xanthopsia (yellow vision).
    • Antidote: Digoxin immune Fab.

Hypokalemia is a key precipitant of digoxin toxicity because it reduces competition for binding sites on the Na⁺/K⁺ ATPase.

Beta-Adrenergic Agonists - The Beta Boosters

  • Mechanism: Stimulate β-receptors → ↑ adenylyl cyclase → ↑ cAMP → ↑ intracellular Ca²⁺ → ↑ myocardial contractility & heart rate.
  • Agents & Actions:
    • Dobutamine: Primarily β₁ agonist. ↑ Contractility >> ↑ Heart Rate. Used in acute decompensated heart failure & cardiogenic shock.
    • Dopamine: Dose-dependent effects.
      • Low: D₁ receptors → Renal vasodilation.
      • Med: β₁ receptors → ↑ Contractility, ↑ HR.
      • High: α₁ receptors → Vasoconstriction.
    • Isoproterenol: Non-selective β₁/β₂ agonist. ↑ HR & contractility, but potent vasodilation (↓ SVR).
  • Adverse Effects: Tachyarrhythmias, angina, headache.

Positive Inotropic Agents: Mechanisms of Action

⭐ In cardiogenic shock, dobutamine is a primary choice. However, if the patient is severely hypotensive (e.g., SBP < 70 mmHg), dopamine or norepinephrine are preferred for their potent vasoconstrictor effects.

PDE-3 Inhibitors - cAMP's Lifeguards

Milrinone mechanism of action on cardiac myocyte cAMP

  • Drugs: Milrinone, Inamrinone.
  • Mechanism: Selective PDE-3 inhibition prevents cAMP breakdown in cardiac and vascular smooth muscle.
    • Heart: ↑ cAMP → PKA activation → ↑ Ca²⁺ influx → ↑ contractility (inotropy) & heart rate.
    • Vessels: ↑ cAMP → smooth muscle relaxation → significant vasodilation (↓ preload & afterload).
  • Use: Short-term management of acute decompensated heart failure.
  • Adverse Effects: Ventricular arrhythmias, hypotension, thrombocytopenia.

⭐ Reserved for severe, refractory heart failure as long-term use is associated with increased mortality.

High‑Yield Points - ⚡ Biggest Takeaways

  • Digoxin inhibits the Na+/K+ ATPase, increasing intracellular Ca2+ and contractility; toxicity presents with xanthopsia and hyperkalemia.
  • Beta-agonists (Dobutamine) and PDE-3 inhibitors (Milrinone) increase cAMP, boosting contractility in acute heart failure.
  • Both are used for acute decompensated HF but increase myocardial O2 demand and risk of arrhythmias.
  • The final common pathway for inotropy is elevated intracellular calcium.
  • Treat digoxin toxicity with DigiFab (antibody fragments).

Practice Questions: Positive inotropic agents

Test your understanding with these related questions

An 82-year-old male with congestive heart failure experiences rapid decompensation of his condition, manifesting as worsening dyspnea, edema, and increased fatigue. Labs reveal an increase in his serum creatinine from baseline. As part of the management of this acute change, the patient is given IV dobutamine to alleviate his symptoms. Which of the following effects occur as a result of this therapy?

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Flashcards: Positive inotropic agents

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Cardiac glycosides (e.g. digoxin) exert their effects via inhibition of the _____ pump

TAP TO REVEAL ANSWER

Cardiac glycosides (e.g. digoxin) exert their effects via inhibition of the _____ pump

Na+-K+ ATPase

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