To start things off, I am bound to get some things wrong, I am still fairly new to this game and some of this is guesswork, if I do make mistakes correct me.

From watching people play Barotrauma and playing the game myself I've seen many people playing Medical Officer incorrectly in the way they treat their fellow crew, and in some cases are worsening the chances of the wounded's chance of survival. So I thought I'd share my knowledge to others who may want to know of it.

Basic Medical Items in BTE

Health Scanner HUD:

This is a great tool in the medics arsenal but I often see medics never use it or use it all the time. Using it all the time is bad as it reduces communication with the crew as you don't have your headset on. However I also see medics never use it, and this could partly be because diagnosing a patient is mostly common sense (I will come back to this later) but the HUD has other uses too. One being that the HUD displays the actual name of the player you are looking at, so if they are disguised you will be aware of their true name. It also displays whether the person is infected with the Calyx parasite, and how severe their infection is. This makes the HUD a very situational tool, but not a useless one.

DO: Scan players in the event of an outbreak and if you suspect someone of being disguised. DON'T: Wear your HUD 24/7.

Defibrillator:

I thought I would start off with this as I see many people use this wrong. To my knowledge, the defibrillator is only used to treat oxygen deprivation and (It also reduces stun duration and is effective at helping treat overdoses, thanks Chiko) It's benefits last 12 seconds and it's best used in conjunction with CPR (note its effects don't stack so unless using it as a weapon, don't spam it). It deals minor damage to those it shocks but this is generally negligible. However what I often see is medics arriving to the scene of someone bleeding out, and their first instinct is to start shocking them with the defibrillator. This is, without a doubt, completely useless because the chances are they are bleeding and the defibrillator does not increase health or reduce bleeding. This means using a defibrillator like this reduces the dying crewman's survival chance because you are just damaging them more.

DO: Use defibrillator as well as CPR when the patient is deprived of oxygen. DON'T: Use defibrillator as the be all and end all solution to every ailment.

Bandages:

They are much more abundant around ships compared to to fibrinozine but are much less effective. Because of them being abundant around the ship it is recommended that the crew, especially security, has some ready as they are likely to be the first at the scene of an injury and them stopping bleeding before a medic arrives may save the life of a crew-member. It doesn't offer any actual healing though, so save it for when someone's bleeding. Each use of a bandage removes half a bleeding icon over 5 seconds.

DO: Use on players who are bleeding out ASAP. DON'T: Use it to heal people.

Fibrinozine:

Just like bandages, they are useful for stopping bleeding. Unlike bandages, they offer an increase in health. It is incredibly negligible, however: the increase in health is only 1 HP per injection. Fibrinozine is also much more effective than bandages. Over the course of 10 seconds, 1 injection removes 2.5 bleeding icons

DO: Use on players who are bleeding out ASAP. DON'T: Use it to heal people.

Chloromydride:

This drug is amazing but I barely see anyone use it, probably because only a small fraction of medics use the chemical fabricator and an even smaller fraction of medics use it to craft medicine and not poison. To craft it you need stablilozine, sugar and erythrozine. All of this can be found in a medical room except for sugar, which can be crafted from biomass in the chemical fabricator or stolen from the kitchen. It only works on people with less than 1hp. It rapidly removes bleeding and also gives 1 health per second and some oxygen. The effects last for 30 seconds but don't stack. This drug on its own isn't enough to revive someone but it means that for 30 seconds they aren't suffering damage from their injuries and all their bleeding is gone. This means they have a guaranteed 30 seconds where they won't succumb from their injuries unless they give up or an external source of damage finishes them off. This gives a medic a much larger window to save dying crew and also means they can treat more patients at once with much more ease.

DO: Use it on a dying crew member as the first drug injected to prevent their demise. DON'T: Inject it into patients more than once every 30 seconds.

Corrigodone:

This is a favourite of medics who don't use the chemical fabricator and aren't aware of superior options available healing-wise. It offers 3 health a second for 5 seconds, resulting in each injection giving 15 HP. It is good for healing damage and can be injected fully injected into dying people to revive them AFTER all bleeding has stopped. If you are unsure if bleeding has been stopped, use your HUD and check.

DO: Use it when auxiliorizine isn't available or use it for crafting auxiliorizine. DON'T: Waste it all on minor injuries or when auxiliorizine can be crafted and don't use it to revive comatose players who are still bleeding.

Auxiliorizine:

Essentially Corrigodone's big brother, it is a very effective medicine. It gives 0.5 health a second for 60 seconds so each injection gives 30 HP. This makes it twice as effective as corrigodone healing wise. It also slightly reduces bleeding and slightly boosts oxygen levels but it is mainly used for healing. To craft it, you need flash powder, erythrozine and corrigodone but using just one of these ingredients each means you can craft auxiliorizine 6 times. This means that in reality auxiliorizine is 12 times as productive as corrigodone.

DO: Use it to heal players and revive people. DON'T: Use it to revive players who are still bleeding.

Erythrozine:

I never use this drug. It's completely overshadowed by CPR as CPR doesn't require an inventory slot to hold. Technically it can be used to revive people in rooms where no oxygen is available but if that's the case they will probably suffocate as soon as they are revived. Overall I found it better to drag people's bodies to an exosuit locker or put a diving mask on them and then applying CPR as they stand more of a chance of long term survival. Perhaps I am underestimating it but I honestly feel it is fairly useless.

DO: Use it to craft better drugs. DON'T: Use it to revive people in situations where they will die immediately after revival.

Liquid Oxygenite:

Incredibly effective at treating oxygen deprivation of all kinds. It gives 10 oxygen a second for 60 seconds. This makes it extremely effective as a counter against sufforin: sufforin results in -5 oxygen a second, plus an extra -5 oxygen due to available oxygen being set to VERY low. It's easy, for every 1 dose of sufforin give 1 dose of liquid oxygenite (IMPORTANT: and CPR to treat passive oxygen loss)

DO: Use it to treat sufforin. DON'T: Waste it on drowning people unless you can't get them a mask or exosuit.

Ethylzine:

This drug is meant to cure drunkenness/inebriation. Honestly, if someone gets drunk to the point they need medicine to survive you should withhold the medicine. Tough love, they shouldn't do that to themselves next time.

DO: Not use it. DON'T: Use it.

I believe that's all the important medicine out of the way now, so lets get down to diagnosis.

Most Common Diagnosis

Diagnosis 1: If they are in an area without oxygen and aren't moving, have no health bar above their head or have a health bar with health, chances are that they are suffocating. Bring them to an area with oxygen or grab them and put on a diving mask/exosuit containing oxygen. Then proceed to use a defibrillator (if you have one) once every 14 seconds, applying CPR when you aren't using the defibrillator. Keep this up until they wake up. NOTE: suffocating players who are at full health don't have a health bar over their head and because of this are indistinguishable from dead bodies without the use of the HUD.

Diagnosis 2: If they are conscious but their health is lowering they are likely bleeding or unlikely are poisoned. In this event one should always administer fibrinozine/bandages until they stop losing health.

Diagnosis 3: If they are not moving and have an empty health bar above their head they are dying of injuries and are probably bleeding. Inject chloromydride once and use the scanner to see when their bleeding stops (if chloromydride isn't available use fibrinozine/bandages and hope the bleeding has stopped. Then inject healing drugs. You will know if the bleeding has stopped if you are a medic and have a scanner.). Once the bleeding has stopped inject fully auxiliorizine or any other healing drug and use the scanner. They're status should go from critically injured to major injuries if the drugs are working, but if their status falls back to critically injured you have to inject more drugs. After a while their health bar should begin to fill up and if this happens they will regain consciousness shortly thereafter. If the conditions from diagnosis 1 apply too, carry out the procedure for diagnosis 1 straight after carrying out this procedure.

This should cover the most important diagnosis that players should know.

CPR Mechanics Thanks to Nil for the info!

To help understand CPR and it's uses let's first understand some numbers:
When someone is downed they passively lose 0.5% health/oxygen a second.
That is not counting bleeding damage, which adds on to that.
Giving CPR to a bleeding person has ZERO effect on anything and giving CPR to a bleeding person results in losing -2% health per chestbump.
If they reach -100% health, they die.
This means a person without meds and not bleeding can survive for 200 seconds.
Chloromydride gives 1 health a second, and removes bleeding, cancelling out their health loss.
Auxiliorizine gives 0.5 health a second which also cancels out health loss.
Chloromydride gives 0.6% oxygen a second, cancelling out any oxygen loss.
Regardless of medical skill, CPR gives 0.5% oxygen a second.
1 Injection of chloromydride is enough to stop ALL bleeding.

It turns out, that if a person is not bleeding each CPR chestbump has a chance to revive, this chance scales up with medical skill. Security have something around 25% chance or more to revive per chest bump, medics have an even higher chance. Also, medics and others above 50 medical do NOT deal damage when doing CPR, unless the victim is bleeding. This means they can chestbump away and not deal direct damage to the victim, however they still passively lose health, which can be avoided via the admission of auxiliorizine, corrigodone and (preferably as it cancels bleeding) one injection of chloromydride. Therefore enforcers, when someone gets downed bandage them up and then administer some kind of healing drug as to prevent their dying, then give CPR. You can save many lives this way. And medics, 1 injection of chloromydride and then CPR after their bleeding has stopped can make you a miracle worker. Now, for classes with sub par medical (anything that isn't medic or enforcer) you have about a 5-10% chance of revival per chestbump (ZERO IF THEY ARE BLEEDING) but also deal damage to the patient. Nil gave an example where someone had about 40 seconds of life left when downed and a crew-member definitely had enough time to get drugs to aid with CPR or just to keep the downed player alive long enough for medics/enforcers to arrive. Instead he gave CPR and gave the downed player about 10 seconds left to live. The downed player died. Often as non medical professionals it is better to find medical items and use those and call for help instead of administering CPR. The only time this isn't the case is from oxygen loss. If that is the problem, chestbump away (note, medics can give a player a whole oxygen bar with CPR so they are much better at it).

My Usual Inventory

3 syringes ready in my inventory at all times. 1 with chloromydride, 1 with fibrinozine and 1 with auxiliorizine.
A diving mask for emergencies (sometimes I carry a spare if no one takes a spare mask).
A Health Scanner HUD.
Med-bag carrying spare meds, always carrying a spare fibrinozine and auxiliorizine.
A wallet, I cannot understate how useful they are. Not only are protein bars great for self healing, they can carry ID cards. When someone is injured I recommend taking their card and placing it in your wallet so others can't steal it. Don't forget to give it back though!

Husk Outbreak Protocol

I know you can use the HUD to see if people are infected if they are stage 2 (not talking phase) and stage 3 (non-curable phase) If someone was very recently infected chances are 1-4 shots of calyxanide will be enough to cure them but as the infection progresses more will be required. In the case you don't have the necessary ingredients to craft more calyxanide I recommend essentially a necessary RDM if you know that when they turn they will cause more chaos and lead to your/other crews infection. In the event they can't be saved tell them to come to medical for treatment. Lock the door to medical by pressing the red button next to the door. Pick up a bodybag, inject them with a drug to sedate them and finally put the bodybag on them while they are still alive. When they eventually die, you see their name turns red, confirming they are dead and are now an AI husk. DO NOT open the bag at this point as they will definitely infect you. As long as they are in the bag they are indefinitely stunned and can't infect people. If possible, explain it the murder you just committed to everyone on the ship and that you will dispose of the body through the airlock, or better yet ask a security to execute them while they are in the bodybag. The reason you do this is so that no one can open the bag and free a new husk to infect people. If you see someone grab the bag without permission run away to avoid being infected in the event they open the bodybag.

As a side-note, I strongly encourage going down to engineering at the beginning of the game to make sure the batteries in life support are at least turned up to 40% and the grid-load is being met. If you don't do this, you can't craft meds so you may as well bully engineers into getting the power started.

Finally, in the event of an injury if you have nothing to provide medically for the patient then just call for a medic to your position and get back to your job. Once a medic arrives make space for them and don't cluster around the patient. If you are security and the medic is doing a terrible job knock them out, cuff them and take their meds, then do the job yourself. Also act as riot control and tell crew who aren't providing help to the situation to get lost. Please feel free to add your knowledge to my guide, it will be appreciated if you are correct.

Nice. Some things i'd like to point out about the defib.

They do reduce stun damage so they are effective at waking up characters.
Since they also deal with oxygen deprivation issues, it can be used on characters that are suffering from some drugs or alcohol overdose so a shock will reduce the effects.
The effects of a defib shock don't stack so there's no point on immediate shocks unless you are using it as a weapon.

That's pretty cool thanks for the info :). I thought I saw that it reduced stun time in the xml but I wasn't sure and I didn't know about it helping with overdoses.