Milrinone (Primacor) is a phosphodiesterase 3 (PDE3) enzyme inhibitor I routinely use as an inodilator. That is, a drug which enhances cardiac contractility (inotropy) while decreasing vascular resistance (vasodilation).
PDE3 degrades cyclic AMP (cAMP), so through its inhibition with milrinone, cAMP can accumulate to promote calcium influx through voltage-gated channels. This results in increased inotropy! cAMP is also responsible for vascular smooth muscle relaxation; however the tone of BOTH systemic and pulmonary vessels is affected. This is beneficial for patients with pulmonary hypertension or acute right heart failure but usually results in systemic hypotension. This is why I almost always run milrinone in tandem with afterload enhancers like vasopressin or norepinephrine. The net effect: increase contractility, decrease pulmonary vascular resistance, and maintain/increase systemic vascular resistance.
Although milrinone is traditionally an intravenous drug, I’m starting to use it more through an inhalational route (off label) to enhance pulmonary vascular selectivity and minimize systemic side effects. I’ll start the aerosolizer once I restart mechanical ventilation following cardiopulmonary bypass (CPB) in patients who I feel are higher risk for acute right heart failure. Typically this is a bridge to IV milrinone (“Vitamin M”) which I’ll continue in the ICU. As an intensivist, if I’m still worried about right heart failure, I’ll add on agents like epinephrine or inhaled Flolan/Veletri in tandem with judicious fluid management (lots of bedside ultrasounds!) 🙂
Drop me a comment below with questions!