U

Underdrive pacing. Pacing at a rate slower than a tachycardia, which can sometimes interrupt and terminate a reentrant arrhythmia.

Underlying rhythm. The basic intrinsic rhythm on which the paced rate or certain arrhythmias may be superimposed, e.g. sinus arrest, sinoatrial (SA) exit block, atrial fibrillation or AV block.

Undersensing. The failure of the pacemaker to sense P or R waves with the result that inappropriately timed, asynchronous or competitive output pulses are delivered. Undersensing can sometimes be corrected by programming the pacemaker to a more sensitive setting, i.e., decreasing the mV value. See also oversensing.

Unipolar. Having one pole. In pacing, unipolar may refer to a lead with a single electrode or a pulse generator that can accommodate such a lead. Unipolar is actually a misnomer, in that the circuit has two poles (the pulse generator case serves as the other pole) even if the lead itself does not. See also bipolar.

Unipolar lead. A pacing lead with a single electrical pole at the distal tip of the pacing lead (negative pole). The anode (positive pole) is the pulse generator case. The cathode is the electrode through which the stimulating pulse is delivered. See also bipolar lead.

Unipolar pulse generator. A pulse generator that can accommodate a unipolar lead and operate in a unipolar pacing and sensing configuration.

Upper rate. In DDD pacemakers, this is the programmed limit to which the ventricles may be paced in response to sensed atrial activity. Also see maximum tracking rate.