provoke either of the two above characteristics at the option of the physician the amount of delay vd 57 provides the alternate pacemaker interference characteristics for example if the delay is set for about 50 to 100 milliseconds and the atrial synchronous option system is employed the pacemaker response will always be a properly sequenced a followed by a v stimulus or in the case of a continuous consecutive pvc 39 s or interference a close coupling alternating over the counter 67 cycle from leading to trailing phase of the a relative to the v stimuli this relationship in the presence of pvc 39 s offers a very desirable antiarrhythmic action and in the presence of interference provides suitable hemodynamic and superior electrophysiologic response 5 to one skilled in the art it is evident that it can be possible to detect both ventricular and atrial activity from a single atrial electrode or unipole for example almost anywhere in on or near the atrial myocardium a unipolar electrode senses both the atrial and ventricular depolarization thus a pacemaker can be made to respond to either or both cardiac chambers having only one amplifier means indeed it is possible to provide sensing means of both chamber employing a single differential or summing amplifier by this method an atrial trigger ventricular slaved pacer could provide assured dual chamber stimuli regardless of the chamber sequence this means that there will be a retrograde pre empting as well as an anterograde pre empting of any possible cardiac pathways providing the basic properties of the present invention additionally it must be pointed out that the fact that a ventricular slaved stimulus in the above pacemaker falls in the st segment of a spontaneous ventricular systole and is.