A 23 year old primiparous woman gives birth vaginally following an uncomplicated labour. Ten minutes after delivery of the placenta she suddenly develops heavy vaginal bleeding estimated at 1200 mL, becomes pale and clammy. Her pulse is 130 beats per minute and blood pressure 80 over 50 mmHg. The uterus feels boggy on abdominal palpation. What is the most appropriate immediate management step?