If you read closely the text in Soraka's W - Astral Infusion, it doesn't make sense at all.
It costs **10% of MAX HEALTH** (+some mana)
and at the end of that text is written, that she cannot play it when she is under **5% of MAX HEALTH**
She should die when she play it at 7% of health but she never dies by healing.
I guess that it's supposed to be "under 10%" instead of ~~"under 5%"~~, because Soraka cannot play it when she doesn't have enough health for that.
It's just minor bug in text, but it can be missleading.