Vasopressors- Critical Care RN's please help!

Hi everyone. I'm a new nurse coming up on one year of experience in a Level II Trauma Center Emergency Department and I need advice. This one patient stumped me.

He was crumping the minute paramedics brought into the department. Had a BP 84/52 and a HR 125. He was breathing 40/min and our ER MD decided to intubate. We started giving him IV fluids (NS X 2 liters). He didn't have any signs of CHF- despite the SOB his lung sounds were ok and he had no JVD/ pedal edema. His initial complaint was rapid onset of left sided paralysis. He had a history of left sided weakness from a prior CVA, but this paralysis was new. Anyway, after we started fluids on him, we took him to CT to R/O bleed. When we came back, his blood pressure hovered in the 80's systolic. At this point, his 2nd liter was halfway done.

I received an order to give Cardizem 10 mg IVP, to "Stimulate his atrial kick" according to the physician. I guess she was suspecting cardiogenic shock? My question however is "Why dump fluids into somebody if you're suspecting cardiogenic shock?" I advised the MD that the BP was low, she intructed to give it slow IVP and watch his BP, which I did. I slowly gave 5 mg of Cardizem and watched his BP go down to 64, then 54. I held the rest of the medication and notified the MD. She instructed to give more fluids.

I ask "Any pressors?" I received an order to start the patient on Dopamine. I start the patient on Dopamine and titrate up. I started at 25 mcg, then titrated up to 50 mcg. This did nothing for his blood pressure and the MD wanted to then give dobutamine. At this point my patient brady'd down into the 40's and the physician stated to cancel dobutamine and start Levophed. While I'm preparing meds, the MD walks into the room and says she doesn't feel pulses and starts CPR.

Anyway, we code this guy. Initially he was in a PEA brady at 44. Then after rounds of medications and CPR, back and forth from Vfib to asystole, etc, pacing the guy, defibrillating, etc. The MD pronounces.

Not having any ICU experience besides the 13 week internship I had in school, my question is: Which vasopressor is chosen to be used at what time? Rather, which vasopressor is better for what patient? Dopamine, dobutamine, levophed? Are they the same as far as effect on BP/HR? Or are some more for HR vs. BP?

By the way, after my patient coded, we saw his labwork where his BNP was 120 and his troponin was negative. CT was negative for bleed. Do you think this guy was a PE?

ANY ADVICE/ETC WOULD BE GREATLY APPRECIATED!!