Milrinone and the pulmonary vascular system

In a multicentre study of 99 adult patients undergoing cardiac surgery, if post-operative cardiac failure was demonstrated (pulmonary capillary wedge pressure greater than 8 mmHg, cardiac index less than 2.5 litre min-1 m-2), then a bolus dose of milrinone (50 micrograms kg-1) was given, followed by an infusion at one of three rates (0.375, 0.5 or 0.75 microgram kg-1 min-1), and haemodynamic effects were assessed. Mean pulmonary artery pressures fell by 15% initially (P less than 0.001), and this significant reduction was maintained throughout the infusion period and reversed with the withdrawal of milrinone. Mean pulmonary vascular resistance fell progressively throughout the infusion period, the maximum change (30-40%) being evident at the 12 h point (P less than 0.05). Reversal of this effect after terminating the milrinone infusion was less marked than with pulmonary capillary wedge pressure or mean pulmonary artery pressure. A group of 39 of these patients from two centres were retrospectively divided into three groups: 1. Mitral valve replacement with high baseline pulmonary vascular resistance (greater than 200 dyne s cm-5) 2. Coronary revascularization with lower baseline pulmonary vascular resistance (100-200 dyne s cm-5) 3. Coronary revascularization with high baseline pulmonary vascular resistance (greater than 200 dyne s cm-5). There was a significant reduction in pulmonary vascular resistance and increase in cardiac index in all patients. At the 15 min point, there was significant between-group variation, the fall in pulmonary vascular resistance and increase in cardiac index being greater in Group 1 than in Groups 2 or 3. This difference between groups was not maintained during the infusion.