M

Magnet mode. A general term for the pacemaker's response when a magnet is applied over the implanted pulse generator. The magnet closes a reed switch within the pulse generator, causing the pulse generator to pace at a predetermined rate and in a preset manner. Magnet mode behavior differs by manufacturer and from model to model, but usually involves asynchronous pacing. In general, magnet mode is used to verify pacemaker function by disabling the pulse generator's sense amplifiers (i.e., to force it to pace even if native activity is present) and to assess battery status. Often the primary indicator of battery depletion warranting pulse generator replacement is a change in the magnet-induced pacing rate.

Magnet rate. The predetermined rate at which a pacemaker will pace, in an asynchronous manner, when a magnet is applied initiating magnet mode. Magnet rate varies from model to model.

Marker channel. A feature of some pacemakers which is designed to simplify EGG interpretation by identifying—via telemetry—when, and in which chamber pacing and/or sensing occurs.

Maximum sensor rate. A programmable value in rate-modulated pacing systems which sets the highest pacing rate that can be achieved in response to sensor input. When the sensor is controlling the pacing rate, the pacing rate will not exceed the programmed maximum sensor rate in a single chamber system or during AV pacing. The maximum sensor rate may be programmed lower than, equal to or greater than the maximum tracking rate. Abbreviated MSR.

Maximum tracking rate. A programmable value in dual-chamber sensing and tracking modes (such as VDD, VDDR, DDD or DDDR) which determines the highest ventricular pacing rate that can be achieved in response to atrial sensed events with 1:1 AV synchrony at the programmed AV delay. Sometimes known as upper rate limit or the ventricular tracking limit at the highest synchronous rate. Abbreviated MTR.

Maximum tracking rate interval. The interval associated with the maximum tracking rate, i.e., the minimum interval between consecutive ventricular paced events or between a sensed event followed by a ventricular paced event in response to sensed atrial activity. Abbreviated MTRI.

Metal-induced oxidation. A process of deterioration of polyurethane lead insulation. Metal-induced oxidation is caused by polyurethane's interactions with the metallic ions in the body and/or the conductor coil. Abbreviated MIG.

Microampere. The basic unit for electrical current in pacing. One microampere equals one millionth of an ampere, the standard unit for electrical current in other contexts. Most pulse generators continuously draw between 10 and 50 microamperes from the power cell. Current drain is usually stated in microamperes. Abbreviated µA.

Microjoule. The basic unit of electrical energy in pacing. One microjoule equals one millionth of a joule, the standard unit for electrical energy in other contexts. Note that defibrillator output is expressed in joules, while pacemaker output is expressed in microjoules. Abbreviated µJ. microprocessor. The central processing unit of a microcomputer, usually contained on a single chip. Microprocessors used in pulse generators have made it possible to incorporate a large number of sophisticated capabilities into a small device.

Milliampere. The basic unit of measure of the pacing output pulse current. One milliampere equals one thousandth of an ampere. Abbreviated mA.

Millisecond. The basic unit of time in pacing. One millisecond equals one thousandth of a second. Most pacemaker timing functions, e.g., pulse width and timing intervals, are expressed in milliseconds as are stimulation and coupling cycles during electrophysiologic testing. Abbreviated ms.

Millivolt. The basic unit of pulse generator sensitivity. One millivolt equals one thousandth of a volt. Note that in pacing, millivolts are used to measure sensitivity, while volts are used to measure pulse amplitude. Abbreviated mV.

Mobitz I. A type of AV block in which the PR interval progressively lengthens until a P wave appears without an associated QRS complex; the PR interval after the pause associated with the nonconducted beat is shorter than the PR interval preceding it. The QRS complex may be normal or abnormal. Sometimes called Wenckebach or Type I second-degree AV block.

Mobitz II. A type of AV block in which the PR interval remains constant, but, from time to time, a P wave occurs without an associated QRS complex. The QRS complex is usually wide. There are no generally recognized warning signals on the ECG to indicate when a beat will be dropped. Mobitz II may abruptly and without warning revert to ventricular asystole and, hence, is considered an indication of a potentially lethal condition. Sometimes called Type II second-degree degree AV block. mode. The basic function of the pacemaker. Modes are commonly stated in three-, four- or five-letter abbreviations, such as DDD or VVIR. The mode indicates in which chamber the pacemaker paces, if and where it senses, how it responds to sensed signals as well as what timing cycles are involved. See also NBG Code.

Monomorphic ventricular tachycardia. A tachycardia that originates from one ventricular focus. On an EGG, a monomorphic tachycardia has a uniform QRS morphology.

Monophasic polarity. On an ECG or an IEGM, deviation of a wave or complex from the baseline in only one direction (either only positive or only negative).

Morphology. The shape of a waveform or portion thereof.

ms. Abbreviation for millisecond.

Multifilar. Multiple strands of conductive material together in one single conductive element. A multifilar pacing lead has a conductor composed of two or more smaller-diameter wires connected or wound in parallel and contained within the insulation sheath.

Multifocal rhythm. An atrial or ventricular intrinsic rhythm originating from more than one location or focus or any arrhythmia caused by this. The individual beats arising from these different foci will have different morphologies or shapes on the surface EGG.

Multiprogrammability. The characteristic of most modern pulse generators allowing three or more parameters to be adjusted noninvasively before, during or after implantation. Also known as programmability.

Myocardial lead. A pacing lead designed to be attached to the outside of the heart or epicardium. Sometimes called epimyocardial lead or epicardial lead. myopotential. An electrical signal originating in muscle. Myopotentials may be caused by voluntary or involuntary movement. The pulse generator may be able to detect myopotentials arising from skeletal muscle in close proximity to the implanted pulse generator and interpret them as cardiac depolarizations, thus inappropriately inhibiting or triggering the pacemaker.

Myopotential inhibition. The inappropriate inhibition of the pulse generator on detection of a myopotential which is incorrectly interpreted as a sensed event. Myopotential inhibition may be remedied by decreasing the pulse generator's sensitivity (increasing the mV setting), programming to the triggered mode or by using a bipolar pacing configuration. Sometimes called myopotential sensing or myopotential oversensing.

mV. Abbreviation for millivolt.