Final answer:
The most likely adverse cardiac effect of intravenous milrinone is tachycardia.
Step-by-step explanation:
The most likely adverse cardiac effect in a patient receiving intravenous milrinone is tachycardia. Milrinone is a medication used to treat heart failure by increasing the strength and efficiency of the heart's contractions. However, one of the potential side effects of milrinone is an increase in heart rate, leading to tachycardia.