Figure 35-3 A threshold margin test (TMT), demonstrating inadequate safety margin for pacing. Application of a magnet to some pacemakers produces asynchronous pacing in which one or more pacing stimuli are emitted with a reduced ventricular pacing voltage, pulse width, or both. This sequence of the TMT is used to determine, without a formal pacemaker interrogation, the adequacy of the pacing energy settings. In this electrocardiographic strip from an Intermedics device, the patient was being paced in the VVI mode (ventricular pacing in the inhibited mode for a single chamber) at a rate of 70 beats/min, which is equal to 857-msec intervals. On application of the magnet, this pacemaker produced four intervals (i.e., five pacing stimuli) of asynchronous pacing at a rate of 90 beats/min (667-msec intervals), demonstrating adequate battery voltage for this device. At the fifth pacing stimulus after magnet application, the pacemaker performs a TMT by reducing the stimulus pulse width to 50% of the programmed value (equal to 50% of programmed pacing energy). The failure of this stimulus to produce a ventricular systole (i.e., failure to capture) demonstrates a dangerously low safety margin for ventricular pacing, because pacing pulse width should be at least three and usually four times the threshold for capture. After five initial stimuli, Intermedics pacemakers then pace asynchronously at the programmed lower rate (70 beats/min in this case) for 60 additional cycles. On completion of these 64 cycles (65 stimuli), Intermedics pacemakers return to programmed values and ignore the magnet.