Heart Block And The Development Of The Pacemaker

An unexpected clinical benefit from the development of open heart surgery was the discovery of a revolutionary new concept for treatment of complete heart block. Heart block is caused when the electrical impulse that begins high in the right atrium fails to reach the pumping chambers—the ventricles. Deprived of their normal signal, the ventricles may beat slowly on their own, but at a rate that limits activity and typically results in heart failure. In the early intracardiac procedures for more difficult surgeries, it was subsequently determined that complete heart block occurred because of injury of the heart's conduction system, induced by stitches in about 10% of the operations. With the existing treatment for complete block, the application of positive chronotropic drugs or electrodes to the surface of the chest, there were no 30-day survivors.

In 1952, Paul Zoll, a cardiologist in Boston, utilized the first human pacemaker unit on a patient—a large tabletop external unit with a chest electrode. It was successfully used to resuscitate patients in the hospital, but delivered 50-150 V, which was incredibly painful for children and typically left scarring and/or blisters. In addition, it used an alternating current electrical source, limiting the mobility of the patient to the length of the cord. Spurred by such adversity, Lillehei and his research team found, in 1956, that an electrode directly connected to the ventricular muscle from a pulse generator producing repetitive electrical stimuli of small magnitude (5-10 mA) provided very effective control of the heart rate and an 89% survival rate for patients with prior heart block.

On January 30, 1957, a pacemaker lead made of a multistrand, braided stainless steel wire in a Teflon sleeve was implanted into a patient's ventricle myocardium, with the other end brought through the surgical wound and attached to external stimulation. The utilized pacemaker (pulse generator) was a Grass physiological stimulator borrowed from the university's Physiology Department. This procedure was designed for short-term pacing, with removal of the wires 1-2 weeks after the patient's heart had regained a consistent rhythm.

Following near disaster for a pacemaker-dependent patient when the electrical service failed in the University hospital because of a storm, Lillehei asked his medical equipment repairman, Earl Bakken, to design a battery-powered, wearable pacemaker to improve the patient's mobility (Fig. 9). This Bakken did, using a circuit modified from a diagram for a transistorized metronome in Popular Electronics magazine as a model (Fig. 10); a few months later, such as device was used clinically on April 14, 1958.

Bakken's transistor pulse generator made a miraculous "overnight" transition from bench testing to clinical use; this invention then set the stage for creation of the cardiac pacing industry. For the next decade or so, it would become common practice to put new devices or prototypes (even fully implant-able ones) into clinical use immediately and then iron out the imperfections based on the accumulation of clinical experiences. This humanitarian practice developed because most of the early patients were close to death, and no other treatments existed (5). Eventually, Medtronic, Inc. (Minneapolis, MN),

Fig. 9. C. Walton Lillehei and young patient with battery-powered, wearable pacemaker.
Fig. 10. Earl Bakken's original design for the battery-operated pacemaker.

under Bakken's leadership, became the world's leading manufacturer of cardiac pacemakers, beginning with the Model 5800.

The first generation of the 5800 pacemaker was black, but was quickly changed to white to look cleaner and more sanitary (Fig. 11). Between 1959 and 1964, only a few hundred pacemakers were sold because of the reusability of this system and the short-term postsurgery focus; orders soared once the pacemaker became implantable and redefined for long-term pacing use. Nevertheless, the 5800 pacemaker became the symbol for Medtronic's shared belief in medical progress through technology; this was celebrated during an unveiling, at his retirement celebration in 1994, of a bronze statue of Earl Bakken holding the 5800 pacemaker. Years later, the 5800 was viewed by Lillehei as a technological watershed: "It was the fruit of interdisciplinary collaboration, exemplifying the marriage of medicine and technology, long-lasting friendships, and mutual respect."

Fig. 11. A page from the Medtronic catalog advertising the 5800 pacemaker.
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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