Great article here: http://ems12lead.blogspot.com/2008/11/transcutaneous-pacing-tcp-problem-of_15.html

Some highlights from the end:

Here are some clinical pearls to get you through the procedure.
• The most common cause of failure with transcutaneous pacing (TCP) is poor pad placement combined with insufficient milliamperes! Remember, the pacer goes up to 200 mA! If you lose your nerve at between 70-90 mA, there’s a good chance you’re not going to achieve capture. Consider anterior/posterior pad placement to "sandwich" the left ventricle between the pads and reduce transthoracic resistance.
• Look for a tall, broad T wave that is the telltale sign of true electrical capture.
• Perform, but do not rely solely on a manual pulse check. Consider using an instrument like an SpO2 monitor, doppler, or bedside 2D echo (for inhospital patients) to verify mechanical capture.
• Run a continuous rhythm strip that shows the transition from "false" capture to true electrical capture. Be able to document the exact milliamperes that capture is gained, and capture is lost. (Note: one of the "quirks" of the human heart is that once you gain capture it is harder to lose. In other words, you might achieve capture at 120 mA, but then you might have to dial it back down to 80 mA to lose it again). Many protocols state that you should add 10 mA as a "safety margin" once capture is achieved. In my experience this is unnecessary for the reason stated.
• Finally, you can consider placing the pacer in "non-demand" mode and examine the absolute refractory periods of the underlying rhythm and the (presumed to be) paced rhythm. If the paced rhythm and the underlying rhythm are marching through each others’ absolute refractory periods, you don’t have true electrical capture.

During a recent shift on the ICU I found the nurses relying on the HR displayed on the lifepak 12 to document capture at a rate of 72. Meanwhile, the patient’s palpable radial pulse was 36 and the Spo2 captured a heart rate of 32! This was not effective pacing at all! The mA was set at 40mA and they refused to increase it because the patient was AxOx3 and it caused her pain. I don’t know if she made it through the night at that rate or maybe they finally sedated her and increased the mA to achieve mechanical capture. I tried explaining the difference between electrical capture and mechanical capture and which is more important but she seemed pretty adamant that if she had electrical capture it was fine. The patient was in 3rd degree AVB as well.