an external pacer having a plurality of user - selectable output current levels includes a user - actuable push button switch for reducing the selected output current level by a factor of 10 . by observing the absence or presence of an r - wave while operating at the reduced current level the stimulation threshold level of the heart can be determined without the need for additional level selections , and without subjecting the heart to sustained periods of low current . in the illustrated embodiment of the invention for typical output current levels of 2 - 20 milliamperes threshold levels of 0 . 2 - 2 . 0 milliamperes are obtained .