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, there is an accumulation of cAMP which promotes calcium influx through voltage-gated channels resulting 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.
Although milrinone is traditionally an intravenous drug, I’m starting to use it more through an inhalational route to enhance pulmonary selectivity and minimize systemic side effects. I’ll start the aerosolizer once we restart mechanical ventilation following cardiopulmonary bypass (CPB) in patients who I feel are higher risk for acute right heart failure.