Nonhuman patient presenting with heart attack-like symptoms:
Step 1: Confirm diagnosis of heart attack/life-threatening arrhythmia. This usually requires ...
Step 2: Locate heart(s) and ...
Step 3: Figure out which one(s) is malfunctioning. Because ...
Step 4: Decide how to restart or supplement flow ...
... depends on the physiology.
Allow me to digress briefly here with some observations of centaur anatomy that I worked out in junior high. (Yes, I used anatomy books for humans and horses to analyze possible and optimal configurations. Yes, I also have a lot of ulterior resources to draw on from lives in phantasmagoric and spacefaring civilizations. The latter has the best emergency aid classes of all. Mine isn't the only life farmemory can save!)
* The size and shape of a centaur body means that the most efficient way to circulate its blood supply is to have an upper heart in the humanoid chest and a lower heart in the equine chest. Not all centaurs have two hearts, or in the same exact location, but this is the most common configuration so check for that first and others only if this proves wrong.
* The respective sizes of those body sections mean that in most cases, the equine organs are dominant. That is, taking out the human heart/lungs will not kill a centaur but will incapacitate them. Taking out the horse heart/lungs will kill them, but much more slowly than a human with only one set. Suspect another configuration only if clues strongly contradict this one.
* That greatly changes what you need to worry about. Specifically, you have to move blood from the undamaged section of the body to the damaged section, and keep it going until more help arrives. You have more time to work with and assistance from the functional organs, but you have more territory to cover, the bends and valves make it tricky, and if you don't know centaur anatomy pretty well they will screw you right up.
* CPR is pretty different on the two sections. The basic concept of CPR is similar to humans on the upper section, but you are hindered by the fact that all but the most advanced CPR (e.g. for entrapped or spine-compromised victims) assumes supine position. Most centaurs will not fold out straight enough along the spinal column to lie smoothly on their back. So you will usually be doing CPR with the victim lying on one side. Among the easier methods include bracing the victim's humanoid back against a sturdy object (don't forget to pad it slightly) or doing 2-person CPR pushing on front and back (naturally padded).
CPR for the equine portion most closely resembles CPR for horses or other quadrupeds. That is, you can do it by 'fanning' the front legs open and closed, because just pushing the ribs with your hands will get you nowhere without special equipment that's not very portable. (Sitting and bouncing on the chest also seems to work. I saw the leg-pumping method in a home veterinary manual for farmers, but couldn't find a version online) Preferably, team up several people so that someone else can also fan the back legs for more complete circulation. (Read about foal CPR and multispecies CPR for civilians and veterinarians. You are now better prepared for interspecies emergencies than most human-trained medics.)
You now have to contend with bends and valves. A circulatory system typically has structures to modulate flow compensating for physics. Giraffes need some weird anatomy to get that blood all the way to their brain. Centaurs have to deal with a right-angle (approximately) bend and a high-demand vital organ at altitude. This makes it tricky to push blood from one section of the body to the other, and that's if the build is good and you don't -- to pick one example not at random -- have masses of scar tissue complicating the route.
If you do not already know the anatomy and are presented with a coded centaur, your options for figuring out fast are: a healer or a scanner if you have one, external examination, and pressure feedback. You will usually be stuck with the latter two. Look for clues in the juncture, how flexible is it? Flexible centaurs often have better trans-circulation when their spine is closer to straight, while rigid centaurs usually fare better in right-angle position, due to differences in their internal anatomy. In any case, you can often feel the difference between a locked and unlocked circulatory system. If it's able to transmit pressure and flow freely, you should feel a lot of motion when you manipulate it. If it's not, there will be much resistance -- above the resistance you will always feel from trying to move a system several times your own size. It's like trying to do lung compressions when you don't have an airway, if you know what that feels like. (Remember, no airway = no patient.) So observe for signs of how effective or ineffective your manipulations are as you do them, and if it's not working, adjust victim position and try something else. They're already dead! You can't kill them any deader! So be confident and be creative.
* If you wish to restart the heart with technology, most humanoid hearts will restart with the same range of energy used to restart human hearts. But the bigger the body, the more electricity you typically need to jump-start it. Equine hearts typically use the same range as for horses. If you do not have equine emergency equipment, you may be able to adapt from more powerful electrical sources (e.g. a landline or tractor battery) but you will need a converter/controller to deliver a correct dose. This device is among the most useful interspecies emergency tools to carry as it is smallish and can cover many different body types. The fanciest ones will even let you to small household batteries for tiny victims, although in practice most ambulances carry a standard AED for "medium" victims, a small converter, and a large converter.
Regarding the option of cutting open the horse chest for direct access to the heart: meatball surgery is a last resort, because you're gambling not only on being able to restart the heart, but also on being able to reassemble your patient so as not to lose them to blood loss, infection, or other complications. As you will rarely have access to an operating room, surgeon, or equipment suited to nonhumans this is a very poor gamble. But see above -- can't kill them any deader -- if all else has failed. The catheter method used in some horse CPR is a more viable option but requires special equipment and training.
Regarding timing: in general, the bigger the organism, the slower its metabolism. This is complicated by centaurs that have a dual system, in that the humanoid heart/lungs may keep a different pattern than the equine heart/lungs. Even in a circular system they may not be in synch but may have a cooperative pattern instead. If at all possible base your compression/ventilation timing on vitals taken from a healthy centaur. Otherwise, estimate based on human standards for the humanoid body and horse standards for the equine body. But if you guess wrong regarding synchronicity, you'll probably lose the patient because the methods are incompatible -- what works on a dual system won't work on a circular one and vice versa, but either will work on a switchable system. (Some centaurs can use different modes for different circumstances, in which case either will probably keep them alive.)
DO NOT FORGET to check whether your victim has superpowers, species abilities, or other factors that contradict the application of electricity or other treatments! Most centaurs do not, but if there is anyone to ask, do so. What you don't know could kill you and/or your patient.
If you are adapting human equipment for nonhuman patients, it will not be able to tell you results as accurately. (An AED has sensors to determine whether and how to shock a patient.) For nonhumans, you will need to measure the pulse manually at the closest pulse points to the damaged heart, and then farther away, to determine whether adequate function has been restored. Humanoid pulse points are usually similar to human ones; equine ones are usually similar to horse ones. In both cases, the nearest armpit is usually your best bet. although in humans the larger carotid artery may transmit stronger signals, an option you don't have with centaur lower-heart failure.
Regarding ventilation: standard human methods typically work on the humanoid lungs, but you have to get the equipment into the right tube. Centaurs often have a dual trachea, but the respiratory system can be separate, circular, or bifocal. In any case, human equipment cannot inflate equine lungs, whereas horse equipment will rupture humanoid lungs. You must determine the number and connection of the respiratory tubes inside your patient, which is a lot harder than sussing out the circulatory features. If feasible, use human equipment to inflate the humanoid lungs and horse equipment to inflate the equine lungs. When the latter is unavailable, other types of compressor may be adapted for this purpose, but controlling the force and speed is often problematic.
Also, DO NOT FORGET to check for concussion! Centaurs are usually taller enough than humans, and have more mass, to increase the risk of concussion just by falling down. They tend to have tougher bones to protect from these falls, but that won't help if they land wrong. A 3-foot fall onto a concrete surface can kill a human, and most humans are a lot taller than that. This one is flexible in timing. If you don't see brains or blood and you're alone, skip it till the end. If you have extra hands, put another responder to searching for other complications while you address the heart failure.
And now you see why it's not enough to call a monospecies-trained human paramedic to treat a nonhuman victim. You need a veterinarian. Even if they're not trained to treat centaurs or whatever, they ARE trained in multispecies medicine -- but beware of specialties. These are often divided large/small animal or pet/livestock/exotic animal. Get a generalist if you can, or failing that, match the broad specialty to the victim as closely as possible. If calling an animal emergency line, describe the body type of the victim as best you can and what happened to them, allowing the dispatcher to select the best available personnel and equipment to send.
Do you feel sorry for the Maldives yet? I do. This is why Dr. Infanta hustled the one really good vet willing to treat animal soups, hoping to coax her from the West Coast to an island nation.
Also, shit like this is why I tend to distrust local-American medics. They are not good at solving problems they don't already know the answers to, which means for most of us with nonstandard bodies, that chaotic debate cited above is painfully familiar.