By the mid-1950s, responding to post-war advances in cardiac surgery--and increasing public demand for procedures such as mitral valve commissurotomy--many hospitals were eager to hire experienced surgeons to expand their cardiac care programs. Maimonides Hospital, a community hospital in Brooklyn, chose Adrian Kantrowitz as their new chief of cardiac surgery in 1955. During the next 15 years, Kantrowitz's wide ranging surgical research would make substantial contributions to the promising new field of bioelectronic technology.

At Maimonides, in addition to his surgical duties, Kantrowitz continued to study diastolic augmentation in dogs, with the goal of developing a permanent internal mechanism for boosting coronary and systemic circulation. His first idea was to use the motor power of the diaphragm muscle to share the work load of the heart muscle. A portion of the diaphragm was wrapped around the thoracic aorta and stimulated during diastole (the resting phase between beats). This produced small but significant increases in blood flow. More than this, however, this "auxiliary heart" promoted a larger circulation volume without requiring a corresponding effort from the ailing myocardium. Because the phrenic nerve (which controls the diaphragm) eventually lost its responsiveness to external stimulation, Kantrowitz and his team searched for ways to maintain a response. Although they were not able to perfect a reliable phrenic nerve stimulator, their research led them to several other projects involving neuromuscular stimulation, including bladder and bowel stimulators and cardiac pacemakers.

Patients with spinal cord injuries often lose the ability to empty their bladders voluntarily, and can suffer serious (sometimes fatal) infections from urinary tract obstruction or long-term catheter use. Kantrowitz developed an implantable, radio-controlled stimulator to remedy such problems. Initial clinical results were promising, and many colleagues wanted to try the device with their patients. However, the urinary sphincter muscle, like the phrenic nerve, became resistant to stimulation over time, so the device was abandoned. Kantrowitz had better luck with a device to alleviate a common side-effect of abdominal surgery: temporary paralysis of the bowel. For a time, his team also worked on carefully coordinated skeletal muscle stimulation that would enable paraplegics to use their legs. They were able to make one young patient stand up, but realized that walking entailed much more sophisticated patterns of stimulation than they could provide at the time. Kantrowitz, though mechanically adept, was aided in these projects by electrical engineers at General Electric (GE), who were able to translate his physiological specifications into the right materials and hardware. Their most important collaboration was the development of a cardiac pacemaker.

In normal hearts, each heartbeat is initiated by an electrical impulse from the sinus node, a small bundle of cells in the right atrium. The impulse moves across several other conductive bundles of tissue, stimulating the contraction of the atria and ventricles. When this electrical conduction fails (through innate defects, or surgical damage to the tissue) the heart rhythm can become irregular, slow, or even stop briefly. Patients suffer from unpredictable dizziness, fainting, or even fatal convulsions from this "heart block." During the 1950s, several researchers experimented with stimulating the heartbeat with external or internal electrodes. The first working pacemaker with internal electrodes (ca. 1957) was a large vacuum-tube device that sat on a cart beside the patient, and drew power from a wall outlet. By 1958, engineer Earl Bakken, working with surgeon C. Walton Lillehei at the University of Minnesota, had developed a handheld external pacer that used batteries and transistors, which was a great improvement. However, it was still cumbersome, and there was a high risk of infection at the site where the electrode wires ran through the skin into the heart. Kantrowitz and others made use of ongoing advances in electronics--smaller battery cells, transistors, and resistors--to develop self-contained, implantable pacemakers.

Kantrowitz implanted his first pacemaker in 1961, and had implanted successive models of the device in 42 more patients by 1963. (An additional 350 of these devices were implanted by other surgeons eager to try them during this period.) The basic unit consisted of five batteries, two transistors, three resistors, and a capacitor, hermetically sealed in a silicone rubber case. It measured 1¼ x 4 x 6 inches and weighed four ounces. With the patient under general anesthesia, this was implanted under the skin of the lower abdomen, and the two wire electrodes were then run under the skin to the chest, where the surgeon worked through a separate thoracotomy incision to implant the leads in the left ventricular wall. The pacemaker could be set for any fixed rate with an external controller unit, which was a unique feature of the Kantrowitz-GE device. (Current pacemaker models are about one-quarter of this size, and are implanted under the skin of the upper chest.)

Pacemakers extended and improved the quality of most patients' lives, but, as with any new technology, the early years included technical and surgical challenges. Kantrowitz, like other pacemaker developers, repeatedly had problems with broken electrode wires, dislodged leads, and leakage of the unit cases, among other things. Pacemaker batteries, too, often ran low earlier than expected, and patients didn't always notice this immediately. As more people were fitted with the devices, hospitals developed pacemaker follow-up clinics to assess pacemaker function and do routine maintenance. Kantrowitz discovered a quick way to tell if the pacemaker was still functioning, or whether a malfunction was in the generator or the wires: a portable AM radio, tuned to the lowest frequency and placed close to the unit, would pick up a ticking sound with each pacemaker pulse. By counting the ticks, physicians could also check the pacing rate, to see if it was lower due to a failing battery.

Kantrowitz's technical innovations and clinical experience contributed substantially to the rapid improvement of pacemakers during the 1960s. The pacemaker industry grew enormously, as pacing became a standard treatment and Medicare began funding the implants after 1966. Although the Kantrowitz device was an excellent product, GE (like several other companies) did not succeed in capturing a significant share of the pacemaker market in the end. By 1966, however, several concurrent projects, the left ventricular assist device (LVAD) and preparation for a human heart transplant, would occupy most of his time.