Ling S. Ong

Stimulation and sensing thresholds for cardiac pacing: Electrophysiologic and technical aspects

E LECTRICAL testing at implantation is essential to ensure long-term successful pacing. Pacing and sensing represent separate functions of a demand pulse generator and must be tested individually. The establishment of a secure pacing system requires the routine determination of stimulation and sensing thresholds. This review describes the electrophysiologic background, technical aspects, and pitfalls of these important electrical measurements.

Cephalic vein guide wire technique for implantation of permanent pacemakers.

This report describes a modified cephalic vein guide wire technique for the implantation of permanent pacemakers. The procedure was attempted in 11 patients only when pacing leads could not be passed directly into the exposed cephalic vein. Eight bipolar DDD (two leads), two unipolar DDD, and one bipolar VVI (one lead) pulse generators were implanted successfully, with only one relatively minor complication. The cephalic vein guide wire technique provides easy access to the subclavian vein and may allow implantation of most single- and dual-chamber pacemakers with only a cephalic vein cutdown.

Characterization of Pacemaker Arrhythmias Due to Normally Functioning AV Demand (DVI) Pulse Generators

Normally functioning DVI pulse generators with different electronic characteristics may cause complex cardiac arrhythmias that must not be interpreted as pacemaker malfunction. When there is no refractory period after the atrial output, a DVI pulse generator may deliver atrial pacemaker impulses at irregularly shortened intervals and produce an increase in the atrial pacemaker rate compared with the programmed free‐running AV sequential rate. Theoretically this variation of the atrial cycle length can occur only within a well‐defined range that represents the difference between the ventricular and atrial output escape intervals. In reality, the interplay of the spontaneous sinus rate, duration of AV conduction, time of sensing the ventricular electrogram in relation to the surface QRS complex, and the programmed AV sequential time all influence the atrial pacemaker rate. DVI pulse generators may also create interesting arrhythmias such as pseudopseudofusion beats (delivery of an atrial spike within the QRS complex), double pseudofusion beats, and double pacemaker impulses within the QRS complex according to the electrophysiologic circumstances and specifications of the pulse generator.

Reprogramming of implanted pacemaker following external defibrillation.

This report describes reprogramming of a pulse generator consequent to cardiac defibrillation. Analysis of the explunted pulse generator revealed normal function. We tested the electrical properties of several defibrillaiors and our studies suggest that erratic electrostatic discharges at the time of paddle application before defibrillation may create a signal sequence capable of reprogrumming the particular pulse generator by activating either the reed switch or internal electronics.

Echocardiographic features of right ventricular outflow tumor prolapsing into the pulmonary artery

Abstract The echocardiographic features of a tumor in the right ventricular outflow tract that prolapsed into the pulmonary artery during systole are described. The patient was a 19 year old woman who presented clinically with bacterial endocarditis involving a mildly stenotic pulmonary valve. An echocardiogram, obtained to evaluate the pulmonary valve for bacterial vegetations, showed abnormal echoes throughout the cardiac cycle; they suggested a tumor mass in the right ventricular outflow tract in front of the pulmonary valve with possible extension into the pulmonary artery during systole. Additional tumor echoes confined to diastole were recorded in front of the aortic root and the tricuspid valve. These features were further elucidated with a computer-generated two dimensional cineechocardiogram that clearly showed a portion of the tumor mass passing beyond the position of the pulmonary valve into the main pulmonary artery in systole and returning into the right ventricular outflow in diastole. At surgery, a large myxoma was found in the right ventricular outflow tract with a polypoid extension that projected into the pulmonary artery in systole and contained a fibrinous vegetation at its tip. The left cusp of the pulmonary valve was normal, but the other two leaflets showed evidence of endocarditis.

INTERATRIAL SEPTAL ANEURYSM, SYSTOLIC CLICK AND ATRIAL TACHYARRHYTHMIA- A NEW SYNDROME?

Abstract We are describing a case of atrial septal aneurysm (diagnosed by echocardiography), systolic click and atrial tachyarrhythmia. The implications of these findings are discussed.

Interpretation of Electrocardiograms Produced by a New Unipolar Multiprogrammable “Committed” AV Sequential Demand (DVI) Pulse Generator

This paper describes our approach to the interpretation of electrocardiograms produced by a new unipolar multiprogrammable “committed” DVI pulse generator (Intermedics) during normal function. The arrhythmias engendered by this new DVI pacemaker may be better understood by conceptualizing the recycling mechanism in terms of a simple atrial pulse generator with two important qualifica‐tions: (1) the ventricular stimulus obligatorily follows the atrial stimulus after 155 ms (AV sequential interval); (2) the pulse generator senses ventricular events (via the ventricular electrode) but recycles according to its atrial timing cycle (AA interval). These characteristics lead in turn to two important consequences: at the QA interval (from the onset of a sensed QRS complex to the succeeding atrial stimulus) must be longer than the VA interval (from a ventricular stimulus to the succeeding atrial stimulus) by a period equal to or slightly greater than the AV sequential time. This may be considered to represent a form of bysteresis, by the pacemaker refractory period always starts at the onset of an atrial cycle (AA interval) and therefore occurs after the delivery of an atrial stimulus or after a sensed ventricular event. The above characteristics may cause pacemaker stimuli to fall within the P wave, PR interval, QRS, ST segment and the ascending limb of the T wave during normal function of the pulse generator. Superficially, these peculiarities resemble malfunction and may be quite befuddling but they all occur predictably according to the electronic design of the pulse generator. (PACE, Vol. 4, November‐December, 1981)

The Noise Sampling Period: A New Cause of Apparent Sensing Malfunction of Demand Pacemakers

Two patients with Omni‐Stanicor®* pulse generators presented an apparent sensing problem characterized by intermittent reversion to fixed‐rate pacing only during atrial fibrillation with a very rapid ventricular rate, Every fixed‐rate cycle contained two unsensed beats. The first un‐sensed beat fell in the noise sampling period (the last 3/6 of the pacemaker refractory period) and, therefore, disabled the demand function of the pulse generator for a single timing cycle. The presence of two consecutively unsensed beats within one timing cycle (automatic or escape interval) during tachycardia suggests normal function of the noise sampling period of this particular pulse generator, rather than a true sensing problem. The diagnosis becomes evident if the sensing problem disappears when abbreviation of the refractory period occurs by reprogramming the pulse generator at a higher rate.

Periodic Pacemaker Spike Attenuation with Preservation of Capture: An Unusual Electrocardiographic Manifestation of Partial Pacing Electrode Fracture

This report describes the electrocardiograms of a patient with an intermittent electrode fracture presenting with periodic attenuation of the pacemaker spike without loss of ventricular capture. There was infrequent prolongation of the spike‐to‐spike interval probably because the pulse generator interpreted random false signals as interference and reverted to its fixed‐rate mode. These observations add to the spectrum of electrocardiographic abnormalities of electrode malfunction.

Sources of Error in the Determination of Output Voltage of Pulse Generators by Pacemaker System Analyzers

The use of threshold or pacemaker system analyzers with widely different characteristics has introduced potential sources of error in the determination of the output voltage of pulse generators. This is further compounded by the availability of pulse generators with diverse waveform configurations and programmability capabilities. Because of this non‐uniformity, the physician must have some rudimentary knowledge of waveform characteristics and appreciate the limitations of threshold or pacemaker system analyzers to avoid the unnecessary replacement of normally‐functioning pulse generators.