For major surgeries like spay or neutering deep anesthesia is necessary not only for preventing from  pain in patients but also for doing surgery easily.Anesthesia has three steps:

1-Using pre-medication: It means your veterinarian injects some special medications to calm your pet to prepare for the second step.

2-Induction: After putting the intravenous fluid via intravenous catheter, one or a complex of medications is injected to the vein to make anesthesia for a short period.In this period veterinarians have enough time to locate tracheal tube in to the trachea.

3-Maintenance: to continue anesthesia, tracheal tube is connected to the anesthetic machine which is injected oxygen and anesthetic gas to respiratory system due to tracheal tube.

After the third stage pets are in safe situation to do surgery.There are some items which are used to control anesthetic level like blood oxygen, anesthetic gas, intravenous fluid .