O

Ohm. The basic unit of electrical resistance and/or impedance. Abbreviated R.

Ohm's law. The formula established by Georg Simon Ohm (1787-1854) relating voltage, current and resistance in any electrical system and usually expressed as E = IR where E is voltage, I is current and R is resistance.

One-to-one capture. The stimulation, depolarization and contraction of a cardiac chamber caused by a pacemaker output pulse in a relationship of one contraction for each output.

Output. The electrical stimulus delivered by the pulse generator and usually defined in terms of pulse amplitude (V) and pulse width (ms). (In pacing, output used alone usually refers to electrical output of the device, while the term cardiac output is used for blood throughput of the heart.)

Output pulse. An electrical impulse originating from a pulse generator. Also known as pacing stimulus.

Overdrive pacing. A general term for pacemaker behavior in which the base rate is programmed higher than the patient's intrinsic rhythm, so that the pacemaker paces all the time. Overdrive pacing allows the pacemaker to gain electrical control of the heart and has been shown to be effective in terminating or preventing certain tachycardias and other arrhythmias.

Oversensing. Inhibition of a pacemaker by events other than those which the pacemaker was designed to sense, e.g., myopotentials, EMI, crosstalk, etc. Detection by the pulse generator's sense amplifier of inappropriate electrical signals (such as myopotentials, electromagnetic interference, T waves or crosstalk between atrial and ventricular channels in dual-chamber pacemakers). The oversensed signal may or may not be visible on a surface EGG. Oversensing can often be corrected by making the pacemaker less sensitive (increasing the mV value), programming to a triggered mode or by the judicious programming of the refractory period. See also undersensing sensing.