As I've seen a lot of people here, just as I myself, have been very confused about critical damage and healing, I took a while and read through the core book a bit more thorough. Here goes..
On the subject of Damage (p. 250), in the last section it states "As long a Character's total Damage is equal to or less than his Wounds, his body still functions normaly. If a character's Damage ever exceeds his Wounds, he begins to take Critical Damage."
On the Medicae skill (p. 102), under the First Aid section, it states first aid can be applied once and will heal your Intelligence bonus on Lightly injured characters but only ONE point on Heavily or Critically injured characters.
The Enhanced Healing special ability (p. 69) states an Apothecary may restore an additional 1d5 wounds with any successful Medicae Test for First Aid.
While the Narthecium (p. 173) states that it "raises the threshold for which the patient is considered Lightly Damaged to 3 times his Toughness Bonus and doubles the amount of damage healed by first aid", the Damage section already states that once you exceed your wounds you take critical damage and aren't working properly. Hence this is just for characters with low toughness or many wounds.
So, no matter your toughness bonus, how many wounds you have or anything else, once you exceed your wounds you take Critical Damage. You are Critically damaged and your natural healing doesn't apply anymore. Also the Apothecary must, despite any tools he is using, heal you as though Critically Damaged.
Now, how much is that? Normally 1, but with a Narthecium it's double that meaning 2. The Enhanced healing states you may restore an additional 1d5 wounds with any successful Medicae test for First Aid which means you roll Medicae, succeed = Heal 1. You're using a Narthecium so it's double that for 2 points healed. Then you add your special ability which states "an additional 1d5" and roll that for a total of 3-7 points healed.
Critical is ALWAYS critical.
With enhanced healing and narthecium you heal 2+1d5 critical damage.
On another note, remember that resting a full day while lightly damaged heals your full toughness bonus. A full week while heavily damaged also heals your full toughness bonus.
Also note that Extended Care heals Critical Injuries by 2 points PLUS one for each degree of Success each week. This really removes a lot of "downtime" on a good roll. Enjoy