R

R. (1) Abbreviation for an intrinsic ventricular event. Also known as an R wave. (2) Abbreviation for resistance.

R wave. (l) In pacing, an intrinsic ventricular event. R wave refers to the entire native QRS complex. (2) On an EGG, the first positive deflection in the QRS complex, representing ventricular depolarization.

Rapid atrial pacing. A type of overdrive pacing of the atrium at an accelerated rate to terminate supraventricular tachycardia or atrial flutter.

Rate-adaptive AV delay. See rate-responsive responsive AV delay.

Rate modulation. The ability of pacemakers to increase the pacing rate in response to physical activity or metabolic demand. Rate-modulated pacemakers utilize some type of sensor other than sensing intrinsic atrial depolarization. Sometimes called rate adaptation or rate response.

Rate-responsive AV delay. A programmable special feature of certain advanced pulse generators allowing the PV interval during atrial tracking to decrease progressively as the sensed atrial rate increases. Sometimes called rateladaptive AV delay. Abbreviated RRAVD.

Rate smoothing. A programmable pacemaker function which prevents excessive cycle-to-cycle changes in pacing rate. Rate smoothing allows tracking, but prohibits any sudden acceleration or deceleration in pacing rate.

Reaction time. The programmable value in rate-modulated pacemakers that determines the minimum time allowed for an increase in pacing rate (due to sensor-detected activity) from the programmed base rate to the programmed maximum rate. See also recovery time.

Recommended replacement time. The point in the pulse generator service life at which the manufacturer recommends pulse generator replacement because battery depletion had reached a point where system failure is likely to occur within three to six months. The elective replacement time is usually indicated by a change in rate, mode or pacing function or by an elective replacement indicator on the programmer. Abbreviated RRT. Sometimes called elective replacement time.

Recovery time. The programmable value in rate-modulated pacemakers that determines the minimum time required for a decrease in pacing rate (due to sensor-detected activity) from the programmed maximum rate to the programmed base rate. See also reaction time.

Reed switch. A type of switch within the pulse generator. The reed switch closes when a magnet is placed over it, forcing the pulse generator to pace asynchronously. The removal of the magnet opens the reed switch and restores sensing and pacing at the programmed parameters.

Reentry. An abnormal conduction pathway, either anterograde or retrograde, through which an electrical impulse repeatedly enters and exits. Reentry can cause ectopic contractions, premature atrial depolarizations, supraventricular tachycardias, ventricular tachycardias and other arrhythmias.

Reentrant tachycardia. An abnormal condition in which a cardiac impulse may reexcite a region through which it has just traveled. Some reentry loops are very large and involve entire conduction pathways. Others are microscopic within a small area of tissue. See also circus movement. Also known as circus tachycardia. refractoriness. A measure of the recovery of excitability of cardiac tissue under electrophysiologic investigation.

Refractory. (1) Inability of tissue to respond to a stimulus. (2) Inability of a pacemaker to respond to an incoming signal .

Refractory period. (1) The length of time the myocardium is incapable of responding to a stimulus. (2) In pacing, an interval or timing cycle following a sensed or paced event during which the sense amplifier will not respond to incoming signals. Dual-chamber pacemakers have separate refractory periods for each chamber (atrial and ventricular). In most modern pacemakers, the refractory periods are programmable values. See also atrial refractory period and ventricular refractory period.

Relative atrial refractory period. In some pacemakers, the second and final portion of the atrial refractory period during which the atrial sense amplifier is responsive to certain types of extraneous signals; it is preceded by the absolute atrial refractory period, during which atrial sensing circuitry is completely unresponsive to input signals. Not all devices have atrial refractory periods divided into absolute and relative periods.

Relative refractory period. (1) Physiology. In the myocardium, the time period during which the muscle can be depolarized with a higher energy level than is normally required. (2) Devices. The noise-sampling portion of the refractory period. See noise- sampling period.

Repolarization. The recovery process of excitable tissue following depolarization. Ventricular repolarization is represented on the ECG by the T wave; atrial repolarization is not usually visible on the ECG since it gets obscured by the simultaneously occurring QRS complex. Repolarization may also be described as the cell potential returning to its resting value.

Resistance. The opposition to the flow of electrical current in a conductor, usually stated in ohms. Unlike impedance, resistance relates only to electronic components, not to tissue or body fluids. However, in pacing, resistance and impedance are sometimes used interchangeably. Abbreviated R. See also impedance.

Retrograde conduction. Backward conveyance of the electrical impulse originating in the junction or ventricles and traveling backward to the atria, causing an atrial depolarization. This is abnormal conduction. Retrograde conduction can be seen on the ECG by P waves of opposite polarity to sinus P waves. It usually follows an ectopic or ventricular paced event. Sometimes called VA conduction. See also anterograde conduction.

Rheobase. The "flattened" portion of the strength-duration curve indicating the point at which increasing pulse width is no longer associated with a progressive fall in voltage. Rheobase thus indicates the voltage (pulse amplitude) at which capture will not be improved by an increase in pulse width. See also strength-duration curve, chronaxie.

Ring electrode. The proximal electrode on a bipolar pacing lead.

R-on-T phenomenon. A potentially dangerous condition induced when a premature ventricular contraction occurs during the T wave of the preceding QRST complex. R-on-T phenomenon can induce ventricular tachycardia or ventricular fibrillation.