First aid

I'm not a first aid instructor (although I am Combat Medic, Master First Aid, CPR, O2, Defibrillator qualified). This is not simply to collect certificates - I like to be informed and trained, and I need them for work. All I will advise is - do recognised First Aid courses such as St Johns, Red Cross, Divers Alert Network, and refresh them frequently (1-3 years). If any readers out there have done really good outdoor-oriented course let me know and I can link to them. However there are some basics to bear in mind, and expedient measures I've found useful.
First aid can be broken down to three factors -
1. Common sense
2. Knowledge
3. Technique
Common sense is the most important. You need to orient yourself to the situation; triage the factors; then use your training and ability to solve the problems.

Basics

First aid courses are full of acronyms eg. DRABC Danger, Response, Airway, Breathing and Circulation; which I find kind of annoying, basically because I go through my whole OODA loop thing (another acronym, but it covers everything).

First, protect yourself. This is the D in the DRABC. If a member of your team falls and breaks something, well you are close enough that it is conceivable you could be next. The world is full of First Aiders who got hit by other cars on the road when attending accidents. Observe and Orient - then Decide and Act.

The actions you carry out next depend on your First Aid training and common sense. Check someone is responsive (R), and if not - onto the ABC as you have been taught and trained in. Rather than re-writing the flow chart, I've pillaged one from the Western Australian Police that covers these initial steps:

First aid flowchart for unconscious victim

If they are responsive, then you progress further through the OODA loop, and triage the situation. Another of my bugbears about basic first aid courses is that they make it a bit too easy. If someone is injured, there are often multiple injuries. These need to be prioritised. Once you orient yourself to the situation, then bring your training into play.

Aside from not breathing, there are some sure things that can kill someone quickly.

Shock

Treatment for shock is, from the many courses I've been on, not emphasised enough. Shock is the body's way of dealing with stress by pretty much shutting down non-essential functions - primarily blood flow to parts of the body that need it. Consequently, the symptoms of shock reflect this: Pale, cold and clammy, rapid feeble pulse, rapid shallow breathing, thirst, weakness, anxiety, restlessness, inability to speak, and nausea. Basically the patient will look and feel like crap. From experience, it is scary for both the victim and the first aider.
Causes of shock are basically any trauma to the body: blood loss, pain (eg. if you fell down a cliff and your foot now serves as a pillow), hypothermia, heat stroke and so on. Pretty much anything that has physically hammered your body function induces a set of physiological conditions under which even sudden movement can kill. Reassure the patient, don't move them unnecessarily, keep them warm, don't give them anything to drink, but wetting the lips is usually OK. Remember, you are possibly miles away from help and the cavalry is not just over the hill. You know it, and the patient knows it. They will be scared. Reduce fluid loss - bleeding springs to mind, elevate the legs to get blood back toward the brain. Treat the cause of shock simultaneously.

Bleeding

Bleeding is controlled by direct pressure. Simple, huh? It doesn't matter if it's arterial, veinous, or capillary. Bleeding is bleeding. Direct pressure should be applied immediately, then have a wee look and ascertain the severity. A dressing secured in place by bandaging, or even insulation or duct tape will hold things in place until you can decide what to do from your wee look. A deep gash is best just left packed with a dressing, secured in place by tape ie. just pack it and leave it. If you really need to clean it out, do so with antiseptic such as Betadine or Potassium permanganate solution, then re-dress it. I've found butterfly sutures to be of limited use (the blighters won't stick) until the bleeding has stopped. Suturing can be done if so inclined - I've done it before (including to myself). As an aside, surgical staples work better. But honestly - I get better healing just by dressing and bandaging and letting the body do the rest.
There may be some folks out there who are still keen on applying tourniquets for severe bleeding. Well, if the limb is only holding on by a flap of skin and is going to fall off anyway, yes - it may be necessary. For anything less, don't do it. Stick with the direct pressure until the body's clotting system kicks in. After clotting - DON'T start cleaning and scraping around. You'll just start the bleeding again. Let the body start its own healing process.

Fractures and joint wounds

Common sense indicates that if something is broken - don't move it. Immobilise it however you can. Open fractures can cause bleeding, and the pain will cause shock. There are all sorts of formulaic bandage arrangements that are proposed for the 'ideal' fracture scenario. Let's keep it simple. First immobilise, then make a decision. If you need to reposition the limb (it's usually a limb) - do so only if you must. Then support it. The best way is to strap the limb to its friend, in the case of a leg, or to the body. If you have triangular bandages, fine. If not - good old duct tape will again prove its worth. Splinting provides additional support. Anything that reduces movement, regardless of what it looks like, will do.
Neck and spine injuries require immobilisation as best as possible. Movement should only be carried out if there is immediate danger to life. Even then, do as carefully as possible with the knowledge that bad things could happen. Splint to any rigid item - a branch, bond the limbs to the body - whatever you can find. Break out the duct tape.
Sprains and minor joint twists are best treated in cold water to reduce swelling, and rested if possible. Strapping can be considered later on, but be aware the joint will swell.

Hypothermia

Hypothermia is lowering of the body core temperature, and is dangerous. The patient will feel cold, with uncontrollable shivering, and the skin will have a bluish tinge. As hypothermia progresses, their behavior will become more irrational, they will have trouble speaking, they will be uncooperative pains in the backside. It is important for all in a hiking party to be aware of the possibility of hypothermia (ie crappy conditions) and aware of each others moods. If unconsciousness follows - this is very serious.
Treatment is shelter from the environment - get the patient dry and warm. It is important to focus on the core temperature - the limbs are of less importance. Indeed it won't hurt too much if the limbs are left a little cooler - getting the core temperature back up is critical. Body to body contact is advised - get all nice and cosy in a sleeping bag together. It is best to leave the patients clothes on. I've seen suggestions to remove the patients clothes - this seems a bit contraindicative unless the clothes are wet. Get a woolly hat on the patient! Breathe on them - warm up their immediate atmosphere. If conscious, warm drinks are OK. I hope we all know NOT to administer alcohol.

Frost bite is another cold-related condition which may or may not be associated with hypothermia. Symptoms include pain in the extremities, numbness of skin sensation, the skin does not move freely over joints, and it has a waxy look and feel to it. The reason is simple - the skin has died and frozen. Treatment is basically damage minimisation. Don't rub frozen parts. If you can thaw (ie. its not going to just refreeze), then thaw completely in warm water or with body contact such as under armpits. Protect thawed region from further exposure and don't break any blisters. Keep the body core as warm as possible, so that blood begins to flow to the extremities. Avoid making jokes about frozen fish fingers...

Heat stroke

Hyperthermia or heat stroke may be fatal. I've seen a few close calls before - it's not to be screwed with. Initially the patient will feel hot to the touch, be flushed, with a lot of sweating. This is the body trying to cool down - and an indicator that you need to slow down and get into some shade. As hyperthermia progresses, there will be tachycardia (racing pulse), the patient will feel dizzy, fatigued, lethargic - and generally become irrational. However, I've seen guys who are starting to go down start to race like lawn mowers running out of gas. They just start to lose it. You need to put your foot down and stop them.
When they stop sweating, the situation is very serious. This indicates the body's cooling mechanism is shutting down. You MUST get to a cool shaded position immerse in water, encouraging evaporation. If you have alcohol swabs rub hot spots like armpits to cool down.
Unlike hypothermia, where quick surface heating can be detrimental, rapid surface cooling is not too much of a problem. Unless you have a blast freezer lying around in your pack, which could conceivably be a bit excessive. As hyperthermia is almost invariably associated with dehydration, get the patient to drink cool fluids and keep them at rest.

Snake bite and other envenomation

Snake bite is something not to be screwed with. Yes, most snakes are not venomous (unless you're in Australia). And not all bites result in envenomation. But by the time you realise that a brown snake has indeed chewed a whole lot of venom in (rear-fanged snakes don't just strike, they chow down on you as well), there's a fair chance you've taken a good hit. Ignore advice about catching snakes for identification - in the grand scheme of things it doesn't matter. Local hospitals will have the appropriate polyvalent antivenoms for local species, if you can actually medivac in time. A broad visual identification will give an indication of whether it is a rear fanged (eg. brown snake, taipan) vs a front-fanged snake (eg. rattlesnake, death adder). This distinction is kind of important because the venoms have different mechanisms. This is when the type of antivenom is kind of important.
Ignore also the 'cut and suck' myth. If it is a front-fanged snake such as a rattlesnake, if you have any minor cuts in your mouth you could get envenomated with a muscle-destroying venom. Yuk. Venom is transported in the lymph system anyway, so sucking wouldn't do a thing. I notice suction kits are still sold in parts of the US. Don't waste your money or, more importantly, time. First aid regardless of species is the Pressure, Immobilisation, Method. Pressure is provided by roller bandages starting above the bite as far as you can on the limb (it's usually limbs), bandage down the limb past the bite, then back up again (hence the need for two bandages). Keep the bandage firm, but not overly restrictive. The venom travels in the lymph system, which is a weakly pumped transport system just below the skin. If you are bitten elsewhere such as on the torso - still apply roller bandages over and around the bite as much as you can. You will still restrict venom flow.
Immobilise the limb by splinting or simply taping to its other pair (leg) or the body (arm), and avoid moving. This is to restrict blood movement. So, how long do you keep the bandages on? Until you can get other treatment. There have been cases of a patient seeming fine, then getting envenomation symptoms after the pressure was released. I will mention spider, scorpion, and centipede bites here. With the exception of the Sydney funnel web - with a very restricted distribution - I know of few spiders or scorpions that are likely to be fatal. They might, however, hurt a lot. Treatment is the same for all envenomations. Although, a distinction has been made for Australian Red back spider bites where you are just supposed to grin and bear the pain. I personally would PIM anyway.

Burns

Burns are treated by - a wee bit obviously - extinguishing any fire and removing the source of heat. You are going to have to use some water here - but if you can, collect and recycle it if it's in short supply. Keep rinsing until pain and burning feeling is reduced. Severe burns, as with any other major trauma, can induce shock so treat as above. Treat the cause, reassure and stabilise the patient. If the burn has removed skin, then the patient will also lose body fluids. So it is important to maintain fluid intake, notwithstanding the 'nil by mouth' as for severe shock described above. Wetting the lips and small sips is probably the best tradeoff unless you have an intravenous option. Not that common, I know, unless you're carrying a combat medic kit.
Do not apply any creams to the burnt area, despite what your granny said about rubbing burns with butter! Keep loosely covered with sterile gauze - not bandaged. Remember the wound is exposed to the environment, and the patient has effectively lost their protection from pathogens and reduced their ability to retain fluids. We need the skin to seal up (clot, basically) so fluid loss is reduced, and heal naturally. Cover as best you can, and don't move/stretch the skin. If clothing material has melted onto the skin - a likely situation among hikers as we tend to wear synthetic materials - its probably best to just leave it in place. Tearing it off will exacerbate the injury, and the melted material will help protect the area as-is. It will slough off naturally with skin regeneration. Keep the fluids up, and get the patient to rest as much as possible.

First aid kit

It is possible to fill an entire pack with a veritable hospital. Commercially available first aid kitFirst, if you don't know how to use the kit, it's as useless as mammary glands on a bull. The army combat medic kit contained all sorts of good stuff such as saline drips, intubation kits, tracheotomy kits, suture kits, and of course lots of trauma stuff. There is a limit however as to how prepared you can expect to be. A commercially available First Aid kit can provide a useful start, but you will probably populate it with far more useful stuff as you go along, and ditch the useless fluffy bits. There are some indispensable items.

First aid booklet. I emphasise training and practice first and foremost - your brain should kick in without needing a book. However, it gives bystanders something to do instead of racing around pulling their hair and panicking, and in case for some reason you have a brain freeze.
Roller bandages. Firm 10cm roller bandages are a must. I keep two readily accessible as my snakebite kit, and another two 5cm bandages. Of course it is useful to also have a stash of safety pins and some reasonable scissors.
Gauze dressings. A range of sterile gauze dressings should be carried for open wound dressing.
Leukoplast/micropore or similar tape. This stuff is invaluable. I carry 2 rolls of 4cm and 1 roll of 2cm tape. For large cuts, a gauze dressing secured by leukoplast provides a quick and easy method of applying pressure. I have been known to treat deep cuts on myself when I've been clumsy with a knife to just apply pressure then wrap it and skip the gauze. It heals in a few days. Leukoplast is great for blister preventative use as well. If you feel the heels and/or balls of your feet starting to rub and soften up, wrap them. I regularly did this in the army and never developed blisters.
Triangular bandages.You only need at most two for arm slings. Yes, the first aid manuals might say you need 7 or so to splint a thigh fracture, but this brings us to our next best friend...
Duct tape. Where would the world be without it? Duct tape can be used in place of leukoplast for holding dressings, and for blister prevention as above. It is not breathable, however - so it needs to be changed more often than leukoplast. However it will hold in all conditions. It is also terrific for immobilising limbs for splinting. Use in place of triangular bandages.
Resuscitation mask. These provide for a better seal for CPR, and it is easier to switch first aiders if one needs a cigarette or coffee break, or to update their Twitter account or Facebook status, Instagram a thumbs up selfie with the victim, and other important stuff. I hate to be graphic, but they are also essential when it is not obvious where the mouth is due to extensive facial damage. Unfortunately I know this from experience.
Forceps/tweezers. Humble forceps are great for extracting splinters which, while not life threatening, are bloody annoying. They are also indispensible for removing bush ticks which, in the case of paralysis ticks or the little beasties that transmit Lyme disease, can indeed be detrimental to your health.
Antiseptics. Dirt is full of lots of pathogens. Antiseptics are advisable when treating cuts. I carry tubes of Savlon and Betadine as well as betadine and alcohol wipes. As an aside, betadine wash is good for mouth and throat infections. In addition, I carry Bactroban which is an antibiotic. This is usually a prescription medicine, but if you have a friendly doctor (I do - ex-army, gives me lots of useful stuff) you can get hold of it. Keep it as cool as you can though.
Potassium permanganate. Potassium permanganate (KMnO4, permanganate of potash, Condy's crystals) has a variety of antiseptic uses, as well as sterilizing water. It can also be used as a throat wash - if you like purply-brown teeth. I stash it in my survival kit as a fire-making aid as well.
Painkillers and analgesics. I keep a stash of panadeine and ibuprofen handy. There are two main reasons. First, in combination (yes, you can take them together - my GP told me this when I had malaria) they reduce swelling in joints. So if your knees are taking a bit of a hammering, they will help prevent your leg seizing up. Second, in combination they reduce fever and, in the case of malaria or similar, bring your brain back into the land of the living.
Antihistamines. It is advisable to carry a stash of antihistamines. In this overprotected world of ours... we are kind of wimpy and not exposed to the usual allergens present in the bush. Additionally, insect bites/stings, wild foods - lots of things, basically - can induce allergic reactions. Antihistamines can reduce effects and, more importantly in severe cases, provide your only means of reducing allergic responses. If a member of your party is severely allergic to, for example, bee stings they should have epinephrin on them. Failing that, score a few epi-pens and keep them in your kit.
Foil blanket. This should be lumped in with your survival kit. Keeping the patient warm reduces shock. The blanket will also keep heat in, and in the absence of any other shelter protect the patient from the sun. It also serves as a good signalling device. Many of them also have a V mark on them to indicate to aircraft that you are having a spot of bother.
Notebook, pencil, indelible pen. This may not be the first thing that occurs to you, but keeping a record and narrative of whatever has befallen your erstwhile colleague is very important. Note the times of everything - incidents, treatments, drug administrations. If you are administering drugs to a patient who could possibly lose consciousness, it is important to put a copy of your notes into their clothing or safety pin it to them. For drug administration in particular where there is a danger of overdose write administration times and doses in texter on their forehead. OK, that's a bit of an extreme situation, but patients have been overdosed before by medics.

Other first aid kit items that may be of lesser priority are...

Band aids. They seem to have a stickiness half-life of about 2 seconds, if they stick over the blood and sweat at all. Gauze and leukoplast seems to be a quicker and more robust solution.
Butterfly sutures. Similarly, these little blighters can be tricky to get to stick. However they are light, so worth keeping around.
Suture kit. I'll admit I have sutured others, and indeed sutured myself. It's not as bad as it sounds. As an aside, surgical staples are far quicker and easier, just bulkier to carry. Gauze and tape are far easier to do and maintain.
Imodium. Anti poo-poo preparations are worth considering if you are likely to be in an area where diarrhea can hit. I would reserve its use for relatively severe cases where the patient is having trouble with water balance - ie. not keeping fluids down and losing them from both ends. Gentle coconut belly is not too much of an issue, and of course provides a great source of amusement to members of the hiking party.
Insect bite relief. Preparations such as Stingose can (marginally) help reduce irritation from sandflies and mosquitos.
Gloves and barriers. By all means carry gloves. They are light and can be used for other things. It is drilled into you in modern First Aid courses in these days of certain blood-borne disease that barriers are the first things you consider. I have been known to just clamp my hand over severe bleeds first, then glove up later. Not the best practise.
Shell dressings. I do carry 3 shell dressings simply out of army habit, and because I don't trust civilians behind firearms. Weekend hunters are kind of scary. In general, this sort of trauma is unlikely on your average bird watching stroll. However in some countries/forests hunters could be using the area. Or you could have the misfortune of being Dick Cheney's hunting 'buddy'.