Pacemakers output pulse

The cardiac pacemaker lead is a relatively fragile cable of insulated conductor wire implanted into the hostile environment of the human body. Its function is to interface the power source and sophisticated electronics of the pulse generator with the heart. The pacemaker lead plays a critical role in delivering both the output pulse from the pulse generator to the myocardium and the intracardiac electrogram from the myocardium to the sensing circuit of the pulse generator. 

The processor also receives inputs from the sensor(s) for rate-response and telemetry/programming commands. All these inputs are used by the microprocessor to determine whether when and to which chamber to deliver an output pulse. Some inputs, such as from the crystal oscillator, occur as fast as millions of times per second. Other inputs, such as from the reed switch, might not occur for months or years. In the case of a totally pacemaker-dependent patient, there may be no sensed inputs for the life of the patient. 

Hysteresis Rate When the parameter called hysteresis is programmed ON, the maximum interval from the last intrinsic event to the first paced event may exceed the interval from the last paced event to the next paced event (Fig. 3.9). Therefore, the pause following any intrinsic R wave is greater than the interval from one paced beat (V) to the next paced beat (V). This results in the pacemaker being inhibited from delivering an output pulse if the intrinsic rate exceeds or is equal to the hysteresis rate. The word hysteresis is derived from Latin, meaning to lag behind. ... 

A cardiac pulse generator is a device having a power source and electronic circuitry that produce output stimuli. Functionally, at its simplest, current sourced by the device s battery travels through a connecting pathway to stimulate the heart and then flows back into the pacemaker to complete the circuit. 

Lower Rate Limit The most basic interval in the VVI mode is the LRL (Fig. 3.2). The LRL determines the maximum length of time the pacemaker circuitry will wait for intrinsic ventricular activity (R wave) to occur before initiating a ventricular output stimulus. When there is no hysteresis (see the following), this interval applies both to the period from the last intrinsic event to the first-paced event or the period from the last-paced event to the nextpaced event (Fig. 3.2). Therefore, the LRL determines the longest interval permitted between any two ventricular events. The LRL may also be called the minimum rate, lower rate timer, and basic (or base) pacing rate. The LRL is usually expressed in pulses per minute. 

If the patient is not pacemaker dependent, program the pulse generator to the 000 pacing mode (if available) or to subthreshold stimulus output. Pacemaker runaway, observed in one experimental study, is probably due to radiofrequency energy entering the output circuits, as described above for pacemaker-MRl interactions. 

Fig. 19.4 Strength-duration curve created based on a pulse duration threshold at 2.5 V amplitude and a pulse amplitude threshold at a 1.0 ms pulse duration. This curve is automatically constructed by the Medtronic programmer based on testing with the Medtronic Kappa 400 pacemaker. If the output were to fall within the shaded area, it would be subthreshold. The two times and three times safety margin curves are also provided along with the location of the current programmed parameters (X). The curve, based on two data points, is generated from a library of templates which resides within the 9790 programmer.

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