I am a Connecticut EMT-I and former EMS-Instructor. EMT-A was the old designation for a basic EMT. EMT-D was developed when AED therapy became a skill for basic EMTs. At the beginning, AED training was not part of the standard EMT training; hence, those who recieved additional training to use AEDs were given the designation EMT-D. AED training is now part of the basic EMT-B curriculum and is also a module which may be taught to first responders and lay people as part the American Heart Association's CPR training.
There is a national EMT curriculum put out by the National Registry and they administer teh National Registry exam for certification. However, each state has its own certification requirements. Some accept the national standard and others do not. To confuse matters more, some states are divided into regions which set their own protocols and procedures.
Overall, the basic EMT course is about 130 hours. The EMT-Intermediate certification course can be taken after and teaches more advanced patient assessment skills, IV skills and advanced airway management skills (Combi-Tube in some states, intubation in others). Not all states or regions within a state recoginize the EMT-I level. EMT-Paramedic is the next level up and usually takes 12-14 months to complete. Paramedics are trained in advanced life support and can administer a variety of drugs under standing orders or medical control direction. They also intubate patients and can perform other specialized techniques such as needle chest decompression, etc.
With regard to the elderly patient who went from unresponsive to A+O with just O2. What was the patient's initial SaO2 and the rate and depth of his respirations? Lung sounds? It sounds like he wasn't moving enough O2. If just increasing his FiO2 was what brought him around, his brain cells must not have been getting enough on room air. By the way, its too bad you had to wait for an ALS unit. Was it not possible to intercept on the way? Did they wait on scene to try to get an IV established? That is a pet peeve of mine. IV starts should be done en route - that's why we are called mobile intensive care units!!! Unless there is a really good reason (very poor access, etc.) transport should not be delayed just to start an IV. It must have been very frustrating for you, having the training, but not the authority to do it yourself and get moving.
