Text 15. Calibration Techniques for Pacemakers

The important concepts of pacing with pacemakers include the proper sequencing of atrial and ventricular contraction as well as physiologic rate modulation. The ideal sensor to accomplish rate modulation is a chronotropically competent sinus node unaffected by disease, drugs, or procedures. Chronotropic incompetence is not uncommon. Therefore, alternative sensors are needed to accomplish the closest thing to ideal rate modulation for pacemakers. Some types of sensors used or proposed for use in pacemakers are as follows: atrial rate, blood pH, mixed venous oxygen saturation, blood temperature, right ventricular respiratory rate, minute ventilation, evoked response, and systolic time intervals.

The proper sequencing of atrial and ventricular contraction can affect the blood pressure of the patient. The blood pressure is decreased making it more like normal blood pressure. The sequencing of atrial and ventricular contraction also affect the cardiac output. The cardiac output is increased due to a higher end diastolic volume. This higher ventricular end diastolic volume is provided by the properly timed atrial contraction by the sequencing of the atrial and ventricular contraction.

Text 16. Benefits of Pacemaker Technology

Pacemaker technology has expanded immensely over the last three decades. Each phase of development has been associated with clinical improvements Each step of progress has led to smaller, more reliable devices with greater programmability. The longevity of devices has been advanced with better generator technology and battery design. The first devices used asynchronous pacing which had a significant effect in reducing the mortality of surgically induced complete heart block.

Ventricular demand pacemakers overcame the problem of asynchronous competitive pacing, but at the same time patients exposed to pacemaker syndrome. Atrioventricular sequential pacing restored atrioventricular synchrony, resulting in hemodynamic improvement, but like the other improvements it caused the phenomenon of pacemaker mediated tachycardia.

Alternative dual chamber modes and algorithms have brought solutions to these and other problems. Adaptive-rate devices have been of benefit to patients with chronotropic incompetence and are now incorporating increasing variety of biosensors. Such devices offer improved survival and quality of life, but at the cost of increased complexity. Amazingly, the expense of the pacemaker has not increased much over the years. Cost still remains the major limitation to the use of such readily available and advantageous technology. Nearly all of the problems that pacing has presented over the years have been overcome, but the increasing complexity of pacemaker technology is now a major limitation to its proper use.

Text 17. Design Considerations of Pacemakers

During the early years of the development of the pacemaker, design considerations were plentiful. These designs helped later scientists to develop the pacemaker that is used today. An early important design was one by a physicist named Edgar H. Booth. He designed an apparatus that provided both variable voltage and variable rate. This procedure was used in the resuscitation of a stillborn infant. The infant was given sixteen volts to the ventricles via a transthoracic needle electrode. In 1932, Dr. Albert Hyman resuscitated several patients with voltage impulses. These were applied to the right atrium by a thoracic needle electrode. The apparatus, which was developed by Hyman, was similar to a magnet type device that had a spring mechanism which had to be rewound every six minutes. He gave this device a name, artificial pacemaker.

Another important design was the one that was developed by Wilfred G. Bigelow, John C. Callaghan, and Jack A. Hopps. Their apparatus applied periodic unipolar voltage pulses to the sinoatrial node. It was used in the open heart resuscitation of dogs.

By the late 1950s, researchers from around the world were using all of the past research to aid in the development of the implantable pacemaker. One important approach was the radiofrequency coupled pacemaker. It had a tuned coil of wire that was implanted under the skin and connected through a detector diode to a myocardial electrode. Another coil of wire was placed directly over the implanted wire by means of cementing. With this set up, a small radio transmitter connected to the external coil could transmit electrical energy to the myocardial electrode for stimulation.

In all these designs, the patient had to carry the external transmitter with them at all times. This led to the design of the totally implantable pacemaker.The invention of the point contact transistor in 1948 made implantable pacemakers seem possible. Ake Senning and Rune Elmqvist reported on a pacemaker that had been implanted in 1958. The apparatus was designed by Elmqvist. Two myocardial electrodes were attached to the patient’s left ventricle via a thoracotomy and were powered by a rechargeable nickel-cadmium battery. This device was implanted by Senning. The patient was Arne Larsson, who is known as the first person to have had a pacemaker implant.

Drs. William Chardack and Andrew Gage worked with Wilson Greatbatch to develop their own pacemaker. The pacer that was developed contained only eight circuit components with two transistors. It was powered by ten mercury-zinc cells. The whole pacemaker was encapsuled in an epoxy resin with an outer coating of silicone rubber. This type of cell became the principal power source, and the encapsulation became the principal method in the construction of the pacemaker for years to come.