The verticle axis of an EKG measures voltage in milli volts or .001 of a volt. It can go as high as .008 mv.
Voltage is the potentional for an electrical current that provides the activating energy (current, impulse) that causes the shock. Volts divided by resistance (ohms) equals current (amps). So if resistance is decreased given stable volts there will be an increase in current. A decrease in resistance (problably due to sweating, etc) can increase current that may be shunted away from the heart muscles. I'm not acquainted with how a pacemaker works...I'm providing
theory for insight.