March 3, 2000

An implantable cardioverter defibrillator (ICD) is a medical device used to correct tachycardia arrhythmia, a medical condition related to rapid cardiac rhythm disorders. The first implantation occurred in 1980 and its popularity has grown to over 35,000 implants per year [10]. An ICD is a very complex product because people depend on the device for their health. The design of the ICD, as with any medical device, has additional constraints beyond those of conventional electronic devices.

There are four different chambers in the heart: the right atrium receives deoxygenated blood, the right ventricle pumps deoxygenated blood to the lungs, the left atrium receives oxygenated blood, and the left ventricle pumps oxygenated blood to the rest of the body. The heart's electrical system controls the contraction of the ventricles, and it is when this electrical system malfunctions that patients may need to have an ICD. Ventricular Tachycardia (VT) is a condition of rapid beating in which the heart does not pump blood as efficiently as it needs. VT may cause dizziness, weakness, and fainting. Ventricular Fibrillation (VF or V Fib) is defined as rapid uncontrollable quivering. This condition is much more serious than VT; it can cause sudden cardiac death.

The ICD evolved from the idea of the heart monitor and the external paddles similar to those seen in hospitals. If a device were located within the body, then an electric shock could be sent directly from the power source to a specific location within the heart. This configuration maximizes the efficiency of the device.

The design of the ICD may be broken down into three main areas: pulse generator, external programmer, and leads. The pulse generator monitors the heart, detects if the heart needs assistance, and sends a pulse from the battery to the heart through the leads. The pulse generator also contains the battery. The external programmer is how the physician communicates with the device to extract data and monitor the device. The leads connect the ICD to the location within the heart where the rhythm is monitored and where the pulse is delivered.

An electrocardiogram (ECG or EKG) is the electrical signal of the heart. It is this signal that is monitored by the ICD to determine if intervention is needed by the device. Depending upon the rhythm of the heart there are three different types of treatments that the ICD may instigate: pacing, cardioversion, and defibrillation [6]. A pacing pulse is delivered when the ventricular tachycardia is not too fast. The pacing pulse is a short series of small pulses that resets the rhythm of the heart, much like a temporary pacemaker. When the ICD stops the small pulses, the heart returns to a normal rhythm. Cardioversion involves one larger shock sent to the heart to stop beating that is too rapid. It is used when the pacing pulse does not have the desired effects. The third type of therapy is defibrillation. This is a very strong shock sent to the heart to stop ventricular fibrillation. This is the same therapy as the large paddles used externally in a hospital.

The first main design objective of the pulse generator was to be able to identify a cardiac event. The analog EKG signal is measured to detect abnormalities and then identify the type of arrhythmia. Depending on the arrhythmia, the correcting pulse is sent to the heart from the pulse generator. A new method of identifying a cardiac event is being investigated. Digital signal processing is used to take a sample of the signal every 4 milliseconds. The slew rate is defined at the change in amplitude of the signal over three samples. An elevated slew rate, one that exceeds a limit set by the patient's physician, signals the ICD to measure the width of the cardiac pulse. Each pulse is then measured and defined as narrow or wide. The ICD compares this pulse with the next seven pulses. Only when six of the eight recorded beats are wide, does the ICD decide that the heart is in VT, or else the ICD wide pulse counter is reset and the event is determined as a non-event [4].

The major problem with the event detection is when it results in inappropriate shock. Patients have received shocks during exercise when the heart is supposed to beat faster and stronger than usual. These erroneous shocks were determined to be the result of incorrect slew rate measurements that determined when the ICD should begin recording pulse width. In a clinical trial by Medtronic, it was determined that the algorithm that used digital signal processing to identify a cardiac event worked better for patients with slow VTs or with other narrow complex supraventricular tachyarrhythmias which originate in the atria and continue into the ventricle. This means that different event detection algorithms should be used for different patients to get the best results from the ICD.

The battery is a component of the pulse generator. When a patient enters into a cardiac event, the battery of an ICD is called upon to create a large surge of power and a high energy density. This need creates a design problem because the battery must be very dependable and have a lifetime of several years, depending of course on the amount of shock delivered to the patient. The battery must power the pulse generator's constant heart monitoring, provide high voltage pulses immediately on demand, and monitor the functionality of the ICD itself. One type of battery produced is a hybrid cell battery such as a lithium/iodine or lithium manganese dioxide [3].

The microchips used in Medtronic devices are unique to Medtronic. All of the chips are designed for the specific function. Then they are manufactured in Medtronic's semiconductor fabrication plant. Keeping such activities in house may be more expensive, but it also allows for a more precise design since the systems engineers can be very specific in the functions and applications of the chips. This increased specialization gives the Medtronic more flexibility in its design.

ICDs have a special problem similar to many medical devices. It must be taken into consideration, that once the ICD is implanted the physician does not have physical contact with the device. The external programmer is a device in the physician's office and is used to monitor the functionality of the ICD and to reprogram the software. Software changes may involve many things, including the selection of which algorithm to use for event detection or the setting individual parameters for different algorithms. Radio frequency (RF) signals are used to communicate with the device. Software error may also me corrected by reprogramming the device while the device is still within the patient [2].

For better medical treatment, the ICD also has the ability to store data. When a cardiac event is identified, the signals from the heart are stored and the data may be extracted via the external programmer, which then erases the memory from the ICD. The Medtronic ICDs store the number and types of therapies produced by the device, how successful each therapy was in correcting the cardiac event, and the activity of the heart during and after a therapy. The ICD can also relay the remaining battery capacity and software settings to the external programmer [10].

The lead is a crucial component of the ICD, because it must collect data from the heart and return a shock. Some ICDs may have just one transvenous lead, but this design does not always reach the needed defibrillation threshold (DFT). Other designs alter the position of the leads, lead material, shock polarity, and/or shock waveforms [12]. Another feature of some ICDs is an Active Can. An Active Can means that the outer case aids in applying a shock to the cardiac tissue [1]. Most Medtronic leads are constructed with a thin flexible wire surrounded by polymer that does not invoke an immune response within the body.

The first step of the design process was to define the need. The medical community had already established the fairly straightforward need of a better way to control abnormal heart function that would result in a longer life or better quality of life for the patients. The need to stop abnormal heart function was aided by the invention of the paddles that send a shock to the heart through the torso. This shock must be very large to correctly affect the heart through the rest of the tissue; therefore there had to be a better solution.

Currently, there are two effective internal methods for treating abnormal heart functions such as VT. Antiarrhythmic drugs can stabilize an irregular heartbeat by slowing the conduction of the electrical signal within the heart. A study was conducted at the National Institute of Health (NIH) to compare the effects of antiarrhythmic drugs to the effects of ICDs. The study found a 39% decrease in deaths of patients who received the ICDs. The conclusion from the study supported ICD use over the drug therapy. Drug therapy is still a viable alternative, but it may no longer be the first choice of physicians [8]. This evaluation of antiarrhythmic drugs and ICDs is an example of controlled convergence where ideas for narrowed down to find the most ideal solution.

The next step of the design process is to define the requirements and technical specifications of the device. One requirement was the development of software and hardware that will allow for the prediction of an event. Prevention is the ideal that medical science strives to achieve. Many medical conditions have subtle precursors that, if detected, can allow the entire event to be avoided. There are no such obvious precursors for cardiac events that an ICD is designed to relieve. Fractal mathematics and chaos theory provide one possible method of such predictions. The seemingly random patterns of fibrillation may actually turn out to be deterministic. An ICD may be able to deliver very small electrical stimulations to correct cardiac activity before ventricular fibrillation [7].

Most electronic device designers consider the installation of a device during the design process. Medical devices must also be installed, but another more appropriate term is implantation. An ICD must be designed for easy implantation. Less time it takes to implant a device means less stress is put on the patient. This results in a need for simple packaging of an ICD. The current Medtronic ICD takes just one hour to implant. It is packaged into one unit about the size of a pager with the leads extending from it [9]. As the device gets smaller, the implantation time is decreased. The size of the device also determines where it can be implanted. Early devices were to large to be placed very close to the heart, so they were placed in the lower abdomen and long lead ran up to the heart. As technology has increased and the size of the battery and pulse generator has decreased, the location of the implanted ICD had moved to a subclavical location.

Once a preliminary design has been created, evaluation of that design must begin. There are many additional steps in the evaluation process of a medical device, compared to the evaluation processes of other electronic products. The U.S. Food and Drug Administration is a regulation organization that handles the safety food, drugs, and cosmetics in the US. The FDA regulates the antiarrhythmic drug discussed previously. Medical devices on the other hand are controlled by the FDA's Center for Devices and Radiological Health. Much paperwork is required to design a medical device and to get that device to market. A patent should be filed as soon as possible to claim intellectual property. Once the design process has begun, trial may begin with animal subjects. Only after these trials have proven that the ICD is not harmful in animals, human trial may begin. A Premarket Approval must also be filed with the Center for Devices and Radiological Health. The FDA and CDHR must retest all of the submitted data in third-party trials. The process to have devices approved in the United States is long and tedious, and every nation tends to have different requirements.

In the medical industry, clinical trials are the standard method for testing well-developed products that are far along in the design process. In a way this type of testing is more difficult than testing that may be needed to design other electronic systems, but there is the added advantage of feedback from the test subjects. Each of the patients involved in a clinical trial is frequently questioned concerning their opinions of the device. The ICD is a life saving device, but human dependence to a machine frightens many people. Many patients reported that they first feared the device and that it may run out of power, generate false shocks, or fail to detect a cardiac event. Most fears subsided as the patients got used to the idea of the device and saw positive results. Patients in one Australian trial were asked what additional features would they like to see in the next generation of ICDs. Due to the severe nature of the shock, several people felt that a warning tone could be helpful before a shock is generated. The sensation of the large shock that responds to ventricular fibrillation was described as a large kick in the chest. A split second warning may or may not be able to help the patient as much as it may help the people around the patient. Another feature that participants in the clinical study suggested was an accompanying drug delivery system. Many people felt that there could be some type of drug that would lessen the pain of the electrical shock. Another issue that arose during the Australian study was the issue of the external programmer that is located at the doctor's office. Many patients seemed to like the idea of dialing in ICD clinic and using the phone, placed over the chest is a specific way, to facilitate communication between the ICD, the external programmer, and the patient's physician [11].

The ICD encompasses many different engineering areas. Electrical engineers can work on the signal processing for event detection and on generating electrical shocks. The battery system must also be designed for a long and dependable life. Mechanical engineers have to design a unit that can be implanted into a patient. Chemical engineers and materials engineers are helpful in designing leads and biologically safe materials. People from many disciplines must work together, but some of the areas are separate enough to allow for a lot of parallel design. New leads may be developed while new cardiac event detection algorithms are derived. This is a major strength of the design process. One weakness is the difficulty encountered in testing the device. Medical devices are under a lot of scrutiny. Government regulation tends to slow the design process, but since testing the ICD may put someone's life at stake, the added precautions must be taken.

The ICD is medical device that has successfully been designed taken to market. It has had a great positive impact on the medical community and on the many lives that it has saved. There are many added considerations that must be taken in the design of medical devices, such as general health risks, implantability, immune response, clinical trials, etc. The involvement of organizations such as the FDA's Center for Devices and Radiological Health help to reduce any risk, but the implantable cardioverter defibrillator has been proven to be well worth the risk.