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Ladder diagram. A schematic representation of the conduction sequence of the heart. Ladder diagrams are frequently used to illustrate pacemaker timing cycles and their interactions with the heart.

Lead. The insulated wire plus electrode(s) and terminal pin used to connect the pulse generator to the cardiac tissue. The lead carries the stimulus from the pulse generator to the heart and, in demand modes, relays intrinsic cardiac signals back to the sense amplifier of the pulse generator. A single-chamber pulse generator requires one lead, while a dual-chamber pulse generator usually requires two (one for the atrium, another for the ventricle).

Lead dislodgement. The detachment of the pacing lead from the intracardiac location to which it had been positioned. Lead dislodgement, if it occurs, usually happens in the first weeks immediately following implantation.

Lead placement. The way in which pacing leads are inserted and fixated in the heart. Endocardial leads are always placed in the right side of the heart (right atrium or right ventricle). Usually the lead is advanced transvenously to enter the heart by way of the superior vena cava. Once the lead is inside the heart, the physician advances it to the desired endocardial location under continuous fluoroscopic observation. Correct lead placement is verified by a combination of fluoroscopic observation, mechanical stability and appropriate electrical measurements for capture and sensing.

Lower rate. In dual chamber pacemakers, the programmed rate at which the pacemaker will pace the heart in the absence of intrinsic atrial events, It is the slowest the pacemaker will ever pace.