“…They have a similar upper and lower safeguard escape rate, but utilize a single ventricular electrode for sensing and stimulation, have a lower trigger sensitivity, an almost immediate response to a trigger stimulus, logic to differentiate the QRS from the P and T waves, a refractory period of about 400 msec, and a magnetic-switch asynchronous mode. They are noncompetitive with ventricular activity except in very rare instances and insensitive to nontransmitted atrial activity.39 40 Special problems or deficits associated with their use include the following.…”