If a character have critical damage and wounds lower than 3 times their TB due to narthecium.

Is it considered Lightly damaged or critically damaged regarding how many wounds it heal.

Also if a critically wounded character is applied first aid it only heals 1 dmg and must consider all the remaining damage as treated?

I'm not 100% certain but I've always figured critical damage was different than normal damage. If this is correct then when a character is in critical you cannot heal as you would if they only had 1-2 wounds remaining.

In a similar vein, I have also thought healing critical wounds is more akin to focusing on single areas of the body instead of all of the superficial stuff the normal wounds cover, such as resetting a bone or sealing an arterie. Therefore, I've always gone with the deal you can treat every crittical wound until you get to 0 wounds, and once there is no more life threatening injuries and you can treat the character as a whole as described in the medicae and errata listings.

Not exactly sure what the question is, but with the Narthecium:

If you have taken less than 3xTB wounds but have taken no Critical Damage: Treated as if lightly wounded (IntB Wounds healed, usually x2 with the gear)

If you have taken more than 3xTB wounds but no Critical Damage: Heavily Wounded (1 Wound healed, normally x2). Usually only matters when characters have bought lots of extra wounds and not improved Toughness to match.

If you have taken any Critical Damage: 1 Damage healed (usually x2) starting with Critical Damage. This applies regardless if they have any wounds remaining (usually only the case if they have spent Fate Points to restore wounds).

On the other hand if they have Autosanguine or Hardy they heal as if lightly wounded regardless of how much damage they have taken or if they are suffering Critical Damage (Critical Damage being healed first). Always encourage your Squad members to take Hardy or Autosanguine as soon as they can.

In all cases remaining wounds and Critical damage counts as "treated"... and yup, this sucks royally if you heal a grand total of 1 (or 2, or even 4 with Master Churururgeon or whatever) Critical Damage and they are still stuck in Critical Damage.

I'm just gonna leave this alone, the medicae skill isn't as developed as it probably should be. Thus the headaches.

A simple solution, and one I use with a small group that I run for, is to simply add the Hardy Talent to the list of starting abilities. Tada. Now all healing is done as Lightly Wounded. It's much easier and saves on the headaches of deciding how you want to treat the whole Critical Damage problem.

there are 3 types of damage, light damage, heavily damage and critically damaged. Normally if you have lost more than double your toughness bonus you are heavily wounded, less than that is lighly wounded. if using a narthicum it increases to over triple toughness bonus is heavily wounded and less than that is lighly wounded. Each of these has a different heal rate both for medicae skill and first aid, and long term care. If you have take any damage in excess of your wound total you are critically damaged, regardless of toughness bonus