What Is The Clear Liquid Injected Through A Drain Port After Brain Aneurysm Surgery?

After surgery for a ruptured aneurysm (brain bleed) and a drain installed, 2 large syringes of a clear liquid were injected through a port in the drain tube. What is it and what purpose does it serve? Ps. It did not come out of thr drain until the next day after a second was putn¹