Can a 9 volt battery through your bloodstream kill you?
IF you get cause currents in the 10's of microamps range onto the actual surface of the heart death can occur.  

This paper makes that statement Electrical Safety in the Operating Room: Dry Versus Wet and my recolle4ction is that the writer has come to specialise in establishing best safety practices in that area. 

He says:


Microshock refers to very small currents (as little as 10–50 μA) and applies only to the electrically susceptible patient, such as an individual who has an internal conduit that is in direct contact with the heart. This conduit can be a pacing wire or a saline-filled central venous or pulmonary artery catheter. In the electrically susceptible patient, even minute amounts of current (10 μA) may cause ventricular fibrillation.


Once started ventricular fibrillation will often not stop without external action - usually very high magnitude electric  shock - whose purpose is NOT to supply a heart start signal but to FULLY STOP the fibrillation signals so that the normal heart waveforms can re-establish.