In Dracula, what's stertorous breathing?

You've likely heard — or unknowingly produced — stertorous breathing. Stertor is snoring or loud, raspy, labored breathing.

In Bram Stoker's Dracula, Dr. Seward and Professor van Helsing break into Lucy's house in hopes of finding her alive:

[We] . . . found four servant women lying on the floor. There was no need to think them dead, for their stertorous breathing and the acrid smell of laudanum in the room left no doubt as to their condition.