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2:1 block rate. Determined by the total atrial refractory period (TARP) and characterized by the summation of the A-V interval and the PVARP.

TARP. Total Atrial Refractory Period; TARP is used to determine the 2:1 block rate and is the total of the A-V interval and the PVARP.

Tachycardia. Any cardiac arrhythmia characterized by a rapid rate, usually over 100 beats per minute. Tachycardias may be normal, as in the case of a sinus tachycardia induced by exercise, or may indicate a pathology.

Telemetry. The ability of a device to receive data. Unidirectional telemetry was a feature of early pulse generators which could be programmed but not interrogated. Bi-directional telemetry allows a device to both transmit and receive data. See also bi-directional telemetry.

Telemetry head. The portion of the programmer which is positioned over the implanted pacemaker for interrogating and programming the implanted device. Sometimes called telemetry module, telemetry wand or programming head.

Temporary lead. A pacing lead intended for short-term use, usually with an external pacemaker. Temporary leads may be epicardial or transvenous. A temporary lead does not have a fixation mechanism, allowing it to be easily removed when it is no longer required.

Temporary pacemaker. A nonimplantable pacemaker used temporarily outside the body to stimulate the heart and sense intrinsic cardiac activity. Also known as external pacemaker.

Terminal pin. The male plug on the pacing lead which is inserted into the pulse generator's connector receptacle.

Theoretical longevity. The calculated, projected total service life of the pulse generator from beginning of life to recommended replacement time. Theoretical longevity data are estimates based on battery capacity, programmed parameters and accelerated battery test data.

Third-degree AV block. A type of AV block in which no supraventricular impulse is conducted to the ventricles and in which there is complete AV dissociation. The QRS complex varies in appearance with the location of the block. Escape rates may be irregular and unstable. Also known as complete heart block.

Timing circuit. The components that generate accurate and precisely controlled timing intervals. timing cycle. One of the specific and predefined timing periods of various aspects of the pacing mode, such as the atrial escape interval. Timing cycles are expressed in milliseconds.

Tip electrode. The distal electrode in a bipolar pacing lead. The tip electrode is usually the negative pole.

Torsades de pointes. A form of polymorphic ventricular tachycardia characterized by QRS complexes that gradually change back and forth over a series of beats from one shape and direction to another. Torsades de pointes is associated with a long QT interval on the ECG and an absolute or relative bradycardia.

Total atrial refractory period. The timing cycle in the atrial channel during which the atrial sensing circuitry is unresponsive to signals. When the atrial refractory period is divided into an absolute atrial refractory period, during which all input signals are ignored, and the relative atrial refractory period, during which there is some monitoring for extraneous signals, both segments together form the total atrial refractory period or TARP. TARP is equal to the AV delay plus any AV delay extension and the post-ventricular atrial refractory period.

Tracking. A dual-chamber pacing function in which atrial activity is sensed and results in a paced ventricular response after a predefined delay (the AV interval).

Transthoracic pacing. (1) Cardiac pacing achieved by inserting the pacing electrode into the ventricle using a transthoracic needle and cannula. (2) A technique in temporary cardiac pacing using electrode patches placed on the chest wall. This type of pacing requires a very long pulse width (10-20 ms).

Transvenous lead. See endocardial dial lead. triggered. A pacemaker response to sensing in which a sensed event causes or "triggers" a pacemaker output. It is the opposite of inhibited response, in which a sensed event inhibits the pacemaker output. Examples of triggered modes are AAT and VVT.

Triggered. A triggered pacemaker, upon detecting a spontaneous depolarization or other signal, will deliver an electrical stimulus to the heart. In dual chamber pacemakers, the term is also used to describe the pacemaker’s response to sensed signals, e.g., the ventricular output pulse is “triggered’ by a sensed atrial event.