Pharmacodynamics and Pharmacokinetics of Milrinone Administration to Increase Oxygen Delivery in Critically III Patients

Objectives: The positive inotropic and vasodilator actions of phosphodiesterase (PDE) inhibitor drugs may offer therapeutic alternatives to β-agonists in critically ill patients. We hypothesized that milrinone administration would increase cardiac index (CI) and oxygen delivery (Do₂) in ICU patients, and that a pharmacokinetic model previously developed in cardiac surgery patients may be used to predict milrinone plasma concentrations in a medical-surgical ICU population. Setting: ICU in two tertiary-care, university medical centers. Design and interventions: A prospective, open-label, multicenter, dose-escalating study in three successive groups of eight ICU patients who received a 10-min loading dose of milrinone (25 μg/kg [LOW], 50 μg/kg [MED], and 75 μg/kg [HIGH]). In addition, all patients then received a milrinone infusion of 0.5 μg/kg/min for 1 h. Measurements: Hemodynamic measurements included heart rate (HR); mean arterial, pulmonary artery, central venous, and pulmonary artery occlusion pressures; and thermodilution cardiac output. Oxygen transport indexes included arterial and venous blood oxygen tensions to determine Do₂ and oxygen consumption (V̈o₂). Data were analyzed by univariate repeated measures analysis of covariance, with baseline values utilized as covariate regressors. Results: Twenty-four adult ICU patients 20 to 84 years of age completed the study. The three groups did not differ, except that the patients in the MED group were significantly older (67±4 years, mean±SEM) compared with either the patients in the LOW (48±7 years) or HIGH (47±6 years) group. While HR did not change in the LOW group (90±4 to 93±3 beats/min), HR increased significantly in the HIGH group (94±5 to 112±8 beats/min) (baseline to 60 min infusion time points). All milrinone doses increased both CI and Do₂. At the end of the 10-min loading dose, CI increased 0.3 L/min/m² in the LOW group, 1.1 L/min/m² in the MED group, and 0.9 L/min/m² in the HIGH group. DO₂ increased 8% in the LOW group, 33% in the MED group, and 23% in the HIGH group, similar to the changes in CI. Mixed venous oxygen saturation increased 3 to 5% during the 10-min loading dose of milrinone. During this same time period, mean arterial pressure decreased 6 to 16% and pulmonary artery pressures decreased 9 to 15%. Peak plasma milrinone concentrations increased as a function of the loading dose (159±9 ng/mL in the LOW group, 302±33 ng/mL in the MED group, and 411±45 ng/mL in the HIGH group). However, milrinone concentrations were similar in all three groups after the 1-h infusion; 113±14 ng/mL (LOW), 147±22 ng/mL (MED), and 119±14 ng/mL (HIGH). In all patients with final plasma milrinone concentrations greater than 100 ng/mL (15/23), the CI increased by at least 0.4 L/min/m² (range, 0.4 to 1.8 L/min/m²). Conclusions: Our study confirms that a milrinone loading dose of 50 μg/kg/min followed by an infusion of 0.5 μg/kg/min achieves adequate plasma concentrations of 100 ng/mL or greater, which significantly increases both CI and Do₂. In addition, a previously established pharmacokinetic model of milrinone disposition is confirmed in this mixed ICU population.