I

ICHD Pacemaker Code. The original code to describe a pulse generator's operating mode, devised by the Intersociety Commission for Heart Disease Resources. Responsibility for updating this code has since been taken over by a combined committee for NASPE and the BPEG. See also NBG Code.

Idioventricular rhythm. A rhythm originating in the ventricles, characteristically at a rate of 20 to 40 bpm and appearing on an ECG with a very wide QRS complex. This is an escape rhythm usually seen when the normal pacemaker control of the heart rate fails, for example, during AV block.

IEGM. Abbreviation for intracardiac electrogram.

Impedance. The total resistance to the flow of current through a conductor, including resistance produced by tissue and electronic components including capacitors. In pacing, impedance and resistance are sometimes used interchangeably, but strictly speaking, impedance involves a fluctuation in resistivity, whereas resistance is a fixed number (an instantaneous point in the impedance). Impedance is usually stated in ohms, abbreviated R. In engineering, impedance is abbreviated Z. See also resistance.

Implantable cardioverter defibrillator. A device, intended to be permanently implanted into the body, capable of recognizing ventricular tachycardia or ventricular fibrillation and delivering electrical therapy to terminate such an arrhythmia. Sometimes called automatic implantable cardioverter defibrillator. Abbreviated ICD.

Implantable pulse generator. A device, intended to be permanently implanted into the body, capable of sensing intrinsic cardiac electrical activity and also delivering electrical impulses to stimulate the heart when its own rhythm is inadequate.

Inline lead. A bipolar pacing lead in which one helically shaped conductor is wrapped around and insulated from the other helically shaped conductor. Coaxial leads are thinner than side-by-side leads. Also known as a coaxial lead.

Inhibition. A type of pacemaker response in which the output pulse is suppressed (or inhibited) when an intrinsic event is sensed during the alert interval .

Insulation. Any electrically resistive material which prevents current flow. In pacing leads, insulation is the flexible outer sheath which prevents the conductor coil from making direct contact with body tissue. Typical lead insulation materials include silicone rubber elastomer and polyurethane.

Insulation break. A crack, tear or other compromise to the integrity of the pacing lead insulation. An insulation fracture can allow current to leak out into the body tissue and bypass the heart. An insulation break may be indicated by a drop in impedance of the pacing system and can cause a loss of capture and/or sensing problems (undersensing and/or oversensing). Sometimes called insulation deterioration or insulation fracture.

Interval. The period of time, measured in milliseconds, between any two designated cardiac events. In pacing, intervals are more useful measurements than rate, because pacemaker timing is based on intervals.

Intracardiac electrogram. The graphic depiction of the electrical signal emitted by active cardiac tissue and recorded through electrodes placed on or within the heart. Abbreviated IEGM. Sometimes called electrogram.

Intrinsic. Inherent or belonging to the heart itself. An intrinsic beat is a naturally occurring heartbeat. Intrinsic rate is the patient's own heart rate. Sometimes called native.

Isoelectric line. The flat line on an ECG indicating the period of time when there is no electrical activity registered.