S

Safety pacing. A mechanism available in certain dual-chamber pacemakers which forces a ventricular output when an event is sensed during a special interval (called the ventricular safety standby in Pacesetter devices) following the ventricular blanking period. The purpose of safety pacing is to eliminate crosstalk inhibition. See also ventricular safety standby.

Second-degree heart block. A condition characterized by the failure to conduct some, but not all, electrical impulses from the atria to the ventricles. This blockage can take different forms: Mobitz I or Wenckebach (PR interval lengthens progressively until a P wave is not conducted) and Mobitz II (constant PR interval with blocked beats).

Sense amplifier. The electronic component in the pacemaker's sensing circuit which detects and amplifies incoming signals for processing.

Sensing. The ability of the pacemaker to recognize and respond to electrical activity in the heart. How the pacemaker responds to sensed signals depends on its programmed mode and parameters.

Sensing configuration. The polarity (unipolar or bipolar) with which a pacemaker will sense. Many modern pulse generators offer a programmable sensing configuration. Some Pacesetter pacemakers offer three polarity options for sensing: unipolar tip (from the tip electrode to the pulse generator case), unipolar ring (from the ring or proximal electrode to the pulse generator case) and bipolar (from the tip electrode to the ring electrode). See also programmable polarity.

Sensing threshold. The largest intrinsic atrial or ventricular signal, expressed in mV, which can be consistently sensed by the pulse generator's sense amplifier. The sensing threshold is assessed by measuring the R wave or P wave, respectively, for ventricular or atrial sensing.

Sensitivity. A pacemaker parameter which determines the amplitude of signals to which the device's sense amplifiers will respond. Sensitivity is stated in millivolts. Note that the higher the millivolt value, the lower the sensitivity. (If sensitivity is 6 mV, a signal has to be 6 mV or larger before the signal is recognized; on the other hand, if sensitivity is 2 mV, a signal only has to be 2 mV or larger before it is recognized. Thus the lower the mV value, the more sensitive the device.)

Sensor. In rate-modulated pacing, a device or component that attempts to assess the patient's need for rate support by monitoring a condition other than intrinsic atrial activity and translating this information into a corresponding increase or decrease of programmed base rate. Typical conditions evaluated by sensors are vibrations (relating to activity), central venous blood temperature, minute ventilation or oxygen.

Sensor threshold. The programmable value for rate-modulated pacemakers from which the sensor level is derived. In general, the higher the sensor threshold, the higher the level of activity needed to bring about an initial increase in pacing rate above the base rate.

Set-screw. A type of screw in the pulse generator's epoxy connector used to tighten the lead's terminal pin in the receptacles and press it securely against the connector block.

Sick sinus syndrome. See sinus node dysfunction. Abbreviated SSS.

Silicone. An organic plastic compound used as insulation on some pacing leads.

Single-chamber pulse generator. A pulse generator that paces and senses only one cardiac chamber. Single-chamber pulse generators may be atrial or ventricular; it is often possible to use the same pulse generator for either atrial or ventricular application.

Single-pass lead. A pacing lead designed to be passed transvenously and lodge in the ventricle but in such a way that it places one or more electrodes in both the atrium and the ventricle. A single-pass lead allows pacing and sensing in the ventricle, but can sense only in the atrium.

Sinoatrial block. The failure of electrical impulses produced by a normally functioning SA node from reaching the atria. There are three degrees of sinoatrial block. Abbreviated SA block.

Sinoatrial node. A group of cells located high and posterior in the right atrium at the point where the superior vena cava joins the atrial tissue mass which acts as the natural pacemaker in the healthy heart. The healthy sinoatrial node initiates 60 to 100 beats per minute at rest. Abbreviated SA node.

Sinus node dysfunction. A collective term for any of several sinus node dysrhythmias including marked sinus bradycardia, SA block, SA arrest and bradycardia-tachycardia syndrome. AV nodal function is usually normal, thus the P wave is usually conducted. Sometimes called sick sinus syndrome or sinoatrial node dysfunction..

Slew rate. In a segment of the intracardiac electrogram, the amount of change in signal amplitude (voltage) divided by the period of time in which the change occurred. It appears graphically as the steepest slope of the waveform. Slew rate is usually expressed in mV per ms or volts per second. It is mathematically defined as dV/dt. Most pulse generator sense amplifiers require a slew rate of greater than or equal to 0.5 V/sec in order for a signal to be sensed.

Slope. The programmable parameter in rate-modulated systems which determines the increase in pacing rate above the base rate that will occur at different levels of sensor input signals. See also Auto-Set.

Spike. A small but sharply vertical deflection that appears on the surface ECG indicating that a pacemaker output was delivered. It is caused by the brief discharge of electricity produced by the pacemaker to stimulate the heart. In some situations, a pacemaker spike may not appear clearly on an EGG. See also artifact.

SSI. A manufacturer's code for single chamber inhibited pacing. Pacing and sensing occur in either atrium or ventricle with inhibited response. Once connected to a pacing lead, the mode is stated AAI (if the lead is in the atrium) or VVI (if the lead is in the ventricle).

SSIR. SSI pacing with sensor ON, providing rate-modulated pacing during physical activity.

ST segment. The portion of the ECG representing the end of ventricular depolarization (end of the R wave) and the beginning of ventricular repolarization (T wave).

Stimulation threshold. The minimum output from the pacemaker which consistently elicits a cardiac depolarization and contraction. Stimulation thresholds may be measured as voltage, current, pulse width, energy or charge. Sometimes called capture threshold or pacing threshold.

Stimulus. See output pulse.

Stokes-Adams syndrome. Paroxysmal heart block or ventricular dysrhythmia, producing low or no cardiac output resulting in such symptoms as syncope or possibly convulsions.

Strength-duration curve. The curve reflecting how much threshold voltage (pulse amplitude) and duration (pulse width) are necessary to capture the heart. This curve demonstrates the interaction of pulse amplitude and pulse width. At a point on the graph, usually about 1 ms, the curve flattens out to show the rheobase or the point where progressive increases in pulse width no longer affects the voltage threshold. See also rheobase, chronaxie.

Stroke volume. The volume of blood ejected per heart beat. Stroke volume equals end-diastolic volume minus end-systolic value.

Supraventricular tachycardia. A tachycardia that originates from the supraventricular region. Supraventricular tachycardias are usually pathologic tachycardias and include AV and AV nodal reentry tachycardia. Abbreviated SVT.

Surface electrocardiogram. A graphic depiction of the electrical signal emitted by active cardiac tissue recorded through electrodes from the body surface.

Sutureless lead. A type of epicardial pacing lead which attaches to the myocardium with an active fixation mechanism rather than a suture. An example of a sutureless lead is the "corkscrew" type active fixation pacing lead.

Sympathetic nervous system. Part of the autonomic nervous system involved in the control of involuntary bodily functions, including the control of cardiac and blood vessel activity. This system stimulates cardiac activity and produces effects opposite those of the parasympathetic nervous system, which depresses cardiac activity. Some effects of sympathetic stimulation are an increase in heart rate, cardiac output and blood pressure. See also autonomic nervous system and parasympathetic nervous system.

Synchronous. A type of pacing in which output pulses are delivered in response to a sensed event, usually after an appropriate delaying timing cycle. For example, atrial synchronous pacing provides atrial pacing in response to sensed atrial activity. Examples of synchronous modes are AAT, AATR, VVT, WTR, DDD and DDDR.

Syncope. Fainting or a black-out spell caused by insufficient blood supply to the brain.