Inodilators (Inodilatators)

Definition

• Morgan & Mikhail's Clinical Anesthesiology, 7e
• Harriet Lane Handbook, 23e

Classification

1. PDE-3 Inhibitors (Bipyridines)

Mechanism of Action

• Inhibit phosphodiesterase-3 (PDE-3) in cardiac and vascular smooth muscle cells
• PDE-3 normally degrades intracellular cAMP; its inhibition raises cAMP levels
• Elevated cAMP activates Protein Kinase A (PKA), which phosphorylates multiple myocardial targets, increasing Ca²+ availability to the contractile apparatus and augmenting systolic contractility
• The same rise in smooth muscle cAMP causes vasodilation (both systemic and pulmonary)
• They also have lusitropic effects - improving diastolic relaxation

Key Drugs

• Dose: Loading 50 mcg/kg IV over 15 min, then infusion 0.25-0.75 mcg/kg/min
• Half-life ~2.4 hours; renally cleared (dose-reduce in renal impairment)
• Reduces systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR)
• Particularly useful when pulmonary hypertension coexists with low cardiac output
• Action is beta-receptor independent - effective even in patients on beta-blockers
• Can be used in patients with beta-blocker therapy (unlike catecholamines)

• Similar mechanism and cardiovascular profile to milrinone
• Less commonly used due to risk of thrombocytopenia with prolonged use
• Fuster & Hurst's The Heart, 15e; Harrison's Principles of Internal Medicine, 22e (2025)

2. Calcium Sensitizer - Levosimendan

Mechanism of Action

• Binds to calcium-saturated troponin C in the myocardial thin filament
• This prolongs actin-myosin coupling and increases the sensitivity of the contractile proteins to existing intracellular Ca²+ - WITHOUT increasing intracellular Ca²+ levels
• Also has PDE-3 inhibitory properties contributing to vasodilation
• Additionally exhibits anti-inflammatory and anti-apoptotic properties

Key Advantage over PDE-3 inhibitors

• Does NOT increase myocardial oxygen demand (unlike agents that raise intracellular Ca²+)
• This is especially relevant in ischemic heart failure

Clinical Note

• Not currently FDA-approved in the United States
• REVIVE-II and SURVIVE trials did NOT show overall mortality benefit vs. placebo or dobutamine
• SURVIVE trial showed more tachycardia and atrial fibrillation vs. dobutamine
• Fuster & Hurst's The Heart, 15e; Morgan & Mikhail's Clinical Anesthesiology, 7e

Clinical Uses

• Acute decompensated heart failure with low cardiac output and/or cardiogenic shock
• Pulmonary hypertension with right/left ventricular failure (milrinone reduces PVR)
• Perioperative management of poor ventricular function (e.g., severe mitral regurgitation)
• Cardiogenic shock - often combined with a vasopressor (e.g., norepinephrine) to maintain MAP while improving CO
• Pediatric cardiac surgery (milrinone reduces SVR and PVR post-bypass)

Adverse Effects

Contraindications / Cautions

• Avoid in acute coronary syndrome (ACS) without shock - experimental data show they can cause myocardial necrosis
• Avoid in patients with normal or elevated BP hospitalized for heart failure (ADHERE registry: increased mortality)
• Use only in monitored settings at the lowest effective dose for the shortest duration

Summary

Inodilators increase cardiac output by improving contractility AND reducing cardiac workload through vasodilation. The PDE-3 inhibitors (milrinone prototype) work via cAMP elevation, while levosimendan works via calcium sensitization - a mechanism that does not increase myocardial oxygen demand. They are reserved for acute low-output states, guided by hemodynamic monitoring, and carry risks of arrhythmias and hypotension.