F

Fallback. A programmable option in some pacemakers designed to handle rapid atrial rates.

Far-field signal. On an intracardiac electrogram, the intrinsic signal originating from the opposite chamber of the heart from the one in which the lead electrode is located. For example, in an atrial electrogram, the QRS wave is the far-field signal, but in a ventricular electrogram, the P wave is the far-field signal.

Feedthrough. The insulated electrical pole(s) in the pulse generator connector that serves to connect the pulse generator's internal circuitry to the pacing lead. Feedthroughs are hermetically sealed to prevent leakage of fluids into the pulse generator and are typically made of glass or ceramic material.

Fibrillation. A type of cardiac arrhythmia characterized by rapid, unsynchronized quivering of atria or ventricles. Atrial fibrillation may be asymptomatic, but ventricular fibrillation is typically fatal if not corrected within minutes.

Fibrillatory wave. The erratic baseline waves on an ECG during fibrillation. Abbreviated f wave.

Fibrosis. The buildup of scar tissue.

Fibrotic capsule. Scar tissue which forms around the pacing electrode tip or the pulse generator. This is the body's normal reaction to any foreign material. first-degree AV block. A type of AV block characterized by an increase in the time required for a P wave to be conducted to the ventricle. The PR interval is greater than 200 ms. Sometimes called first degree heart block.

Fixation. The means by which a pacing lead is attached to the heart, usually subdivided into active fixation (using a mechanism such as a screw-in tip) and passive fixation (using tines or fins).

Fixed rate pacing. A type of pacing in which output pulses are delivered at a preset rate with no modification from sensing or other input. Also known as asynchronous pacing.

Fluoroscope. The device used routinely during lead placement which provides real-time X-ray images on a monitor, allowing the physician to observe the transvenous passage and placement of the lead into the heart. Fluoroscopes may be further defined by the angle at which they take images: AP fluoroscopy (anterior-posterior) and/or lateral fluoroscopy are used for implantation procedures.

Flutter. A rapid but regular rhythm (250 to 350 bpm in the atria or 200 to 300 bpm in the ventricles) often seen on the ECG as a saw-toothed pattern. Ventricular flutter may result in death unless corrected in minutes.

Flutter wave. The rapid, regular, uniform saw-toothed waves on an ECG during atrial flutter. Abbreviated F wave.

Frequency. The number of times a signal occurs in a specific period of time. Filters in the pulse generator's sense amplifiers selectively screen out signals at frequency ranges not associated with signals of cardiac origin.

Fusion beat. A cardiac depolarization (atrial or ventricular) resulting from two foci. In pacing, it typically refers to the ECG waveform which results when an intrinsic depolarization and a pacemaker output pulse occur simultaneously and both contribute to the electrical activation of that chamber. See also pseudofusion beat.