The timing sequences of implanted pacemakers are complex, and one of the visual aids for understanding pacemaker behavior is the ladder diagram. Events An event is usually either a sensed event (such as sensing a p wave or r wave) or a paced event when a chamber paced is paced or shocked. The relationship in time of events is grafted from left to right to illustrate increasing time. The time intervals are usually (as the background of the is page), small squares 40 ms. wide and large squares 200 ms. wide. The spatial relationship of events is graphed vertically, with higher being higher in the heart and lower being lower. Timers Timed events are expressed as sloping lines, the more the slope the faster the timer. Intervals Intervals refer to periods of time that elapse after an event. One example is the refractory post ventricular atrial refractory period (PVARP) which for the period of time after a ventricular event that the sensing of the pacemaker is turned off in the atrium.

Example: This is a tracing of intervals from a Telectronics DDD pacemaker. The atrial lead is the top line and the bottom line is the ventricle. The open diamonds are atrial sensed events and the stars (*) are ventricular paced events. The timers are seen as sloping lines. The downward sloping line is the AV interval (in this case about 2.5 squares or 100ms.) The upward sloping line is the timer for the next stimulated p wave that never comes due to the sensed p. There are three intervals show as boxes on the straight lines. The atrial line shows the atrial refractory period, the postventricular atrial refractory period and the bottom line shows the ventricular refractory period.