George H. Myers

Characteristics of Intracardiac Electrograms

Cardiac electrograms were measured on endocardial leads on patients undergoing pacemaker implantation. The data show that small area electrodes reduce R wave amplitude, decrease width of the complex, and increase slopes. The bandwidth of the electrogram increases as the electrode area decreases. The loading of the sensing amplifier changes these characteristics. Distortion is most pronounced for small area electrodes and low lead resistances.

Characteristics of Intracardiac Electrograms II: Atrial Endocardial Electrograms

The intra‐alriai electrograms (P‐wave and QRS complex) of newly implanted electrodes were recorded, and the time and frequency domain characteristics compared. The sensing impedance properties of the atrial electrode were studied in relation to the characteristics of both atrial and ventricular electrograms, and specific differences between the P and R‐waves were identified. These values, particularly the amplitude and frequency components, were sufficiently distinct to make differentiation by pacemaker sensing circuits selective and reliable.

Prediction of impending pacemaker failure in a pacemaker clinic

A pacemaker clinic has been established for the detection of impending pacemaker failure. Over a two year period, there were 121 operations on 87 patients, comprising a third of our living patients. Of the 93 pacemaker replacements evaluated, 77 (83 percent) were for failure of the pulse-forming circuit and 16 (17 percent) for wire dislogment, heart perforation on lead fracture. Sixty-four percent of the pacemakers were replaced electively as a result of changes detected in the electrical impulses or the electrocardiogram. Only 10 percent of replacements were elective in patients not attending the clinic. On the basis of observed defects in the pacemakers, a maximal yield of 83 percent elective pacemaker replacements could have been anticipated from the test procedures of the clinic. The most significant changes observed were decreases in the artifact amplitude and in rate, especially if found together or in conjunction with alteration in pulse width or impulse configuration. Absolute indications were uncommon but included rounding of the square wave of Electrodyne units and loss of synchronization of any unit with a sensing circuit (standby and synchronous pacemakers). A computer simplified the clinic procedure by providing real-time rapid analysis of the data and a neat, concise report for the records. However, the computer was not necessary for preparing and evaluating the tests although it will be used for analog to digital conversion in further development of the clinic. The clinic provides close follow-up of all patients and gives them and their physicians the assurance that trouble will usually be spotted before it occurs. We believe that premature and emergency replacement of pacemakers can be avoided with this semiquantitative method.

THE POTENTIALITY OF THE USE OF BIOLOGIC ENERGY AS A POWER SOURCE FOR IMPLANTABLE PACEMAKERS

The fact that “permanent” implantable pacemakers, containing their own power packs, require replacement every few years prompted a search for biologic energy sources that might serve as a permanent power source. The results of our work have shown, in fact, that such energy is available in sufficient amounts to produce stimulation of the dog’s heart. A number of techniques were considered before ultimately settling on piezoelectric energy sources. Intravascular turbines, thermocouples, interstitial o r intragastric batteries, and magnetostriction were discarded because they were impractical or too bulky, or because there was potential destruction of body tissues. It appeared most logical to use the movement of some part of the body that under most daily circumstances would continue to move. This would include the heart and major arteries and the diaphragm and ribs. After consideration of these, the pulsations of the aorta and the movements of the diaphragm seemed most practical. It was planned to use this mechanical energy to bend or strike ceramic piezoelectric crystals that in turn would produce electric energy by the piezoelectric effect. The electricity thus developed would have to be altered in some way to produce a properly timed and shaped impulse for stimulation of the heart. It was hoped that an electric impulse of 1.6 milliseconds duration and at least 1 volt amplitude could be obtained. Efforts to devise self-energizing pacemakers are pertinent, both because favorable experience with battery-powered implantable units attest to the feasibility of long-term implantation of foreign materials and because many patients with complete A-V block have hearts that are apparently healthy enough to sustain life for many years as long as adequate heart rates are maintained.

Is Reuse Financially Worthwhile

The issue of pacemaker reuse is a complicated one. While reuse of other medical devices is feasible, the reuse of implanted pacemakers is dependent upon multiple variables. Among these are battery life, availability of units, and costs of refurbishment When considered using current cost codes, the savings associated with pacemaker reuse are generally negligible and generally do not encourage widespread adoption of this practice.

CLINICAL USE OF A NEW TRANSVENOUS ELECTRODE

Recently we described a new type of transvenous electrode that displayed remarkably low thresholds for electrical stimulation of the heart and minimal polarization effects.I3 This electrode, called a differential current density (DCD) electrode, is constructed of a helical coil of platinum-iridium or Elgiloy, widened at its tip to form a cylinder with a surface area of more than 1 cm2. It is encapsulated in a silicone-rubber housing. The cylinder is opened at the very tip through the Silastic capsule where contact with the endocardium is made (FIGURE 1). When a current is applied, it passes through the hole at the tip so that the current density is low at the metal and high at the hole. The metal cyPnder is sufficiently large so that polarization effects are minimal at current densities needed for stimulation. At the same time, the effective area of the hole at the electrodemyocardial interface is so small that there is a high current density and therefore an extremely low excitation threshold. The concept of using a hole to increase current density was originally suggested by M a ~ r o . ~ Preliminary studies with various models of the DCD electrode in dogs revealed that excitation thresholds were ten to 20 times lower than those seen with standard metal electrodes. Moreover, it appeared that the transvenous model was sufficiently stable to warrant clinical trial.

Compendium of Pacemaker Technology. II

The simplest pacemaker is one that emits pulses at a fixed interval without being influenced by the hearts activity. A pacemaker with no sensing function is called a fixed rate or asynchronous pacemaker; when spontaneous heart actions occur, competition will be produced between the electrically generated impulse and the spontaneous beat. A fixed-rate pacer may fire arbitrarily in any and every phase of the hearts cycle, occasionally producing uncomfortable irregularity of the heart beat and rarely ventricular fibrillation.* To avoid this, non-competitive pacemakers were designed which coordinate their function with that of the heart by sensing, via an electrode, the electrical activity of the heart. This ability to detect the cardiac activity is called the sensing function of a pacemaker. In order to prevent recycling of the generator by its own output or by the hearts response, the pulse generator is designed to be non-sensing [refractory period] for a short

An Appraisal of Radioisotope Fueled Pacemakers After 5 Years

Personal observation of 131 radioisotope‐powered pulse generators in 120 patients over a period of 5 years has confirmed the expectations of the developers. Actuarial survival of the pulse generator at 5 years was 96 percent. There were only two cases of component failure (1.5 percent) and no battery failures.13 To date there has been no evident toxicity from radiation effects. The total cost of pacing is less with nuclear units than with lithium or with mercury‐zinc powered models.

Compendium of Pacemaker Technology II. DEFINITIONS AND GLOSSARY (PART III)

In its simplest form a pacemaker has the task of converting the energy of its power supply into electrical impulses. The longevity of a pacemaker is, therefore, directly dependent on the storage capability of its batteries. (A battery is a unit, often comprised of several celJs.) Primary ceJJs possess two different chemical substances that produce electrons by their chemical reactions. When two different metal electrodes are immerged into an electrolyte, ions are dissolved that, if negatively charged, wander to the anode (electrode collecting the negative charge) or, if positively charged, to the cathode (electrode collecting the positive charge). The primary cell as a voltage source has the negative charge collecting anode as its negative pole and the cathode as its positive pole. Almost all primary cells used within the past 18 years were composed of zinc (Zn) as the anode and mercuric oxide (HgO) as the cathode. In modern batteries, lithium (Li) is used as the anode, but the cathode material may be a variety of different substances.

Failure modes of American pacemakers—in vitro analysis

Abstract Diagnosis of impending cardiac pacemaker failure by non-invasive techniques is widely practiced in pacemaker clinics. In these clinics various parameters of the pacemaker wave form are measured by means of external electrodes, and certain changes in these parameters are regarded as an indication of impending battery depletion. The authors have participated in such a clinic for a number of years. 1 During that time, it has become apparent that the sensitivity of the parameters to battery voltage varies considerably between different models. This paper represents an attempt to quantitate externally measurable parameters of cardiac pacemakers for 19 different models manufactured by four American companies. These pacemakers are listed in Table I; effects of changing battery voltage and load resistance were measured and are shown in the table.