Spaying, as I understand it now, amounts to a hysterectomy and Mimi's womb clearly needed removal. The infection had swollen her womb to several times its normal size, weighing in at 600g. That contributes to quite a sizeable percentage of her total body weight, considering she's quite a small dog to begin with.
Mimi's doing fine as of our visit tonight. She's no longer in an anaesthetized stupor but was very happy to see us. She wolfed down the chix leg we brought for her and paced her cage, barking weakly the whole time, clearly wanting us to take her home. But she needs one more night of observation before she can be discharged. That should be tomorrow night.
Dr Kasey also showed us some other areas of concern including her weakened heart and worrisome-looking liver, and informed us that he was putting her on long-term medication for those problems. But going by her reception of us tonight, there's still a lot of fight in the old girl yet.