Introduction To Implantable Pacing And Defibrillation Systems

Implantable pacing and defibrillation systems require multiple components, as well as external instruments, for proper function and programming. The implantable portion of the system is composed of the implantable pulse generator (IPG, or pacemaker) or an implantable cardioverter-defibrillator (ICD, or defibrillator) and the pacing and/or defibrillation leads. The IPG and ICD are most commonly implanted in a subcutaneous location in the left pectoral region. Typically, depending on the patient's handedness, the condition of the upper venous system, the presence of other devices, or physician preference, the device may also be placed in the right pectoral region. It may be placed in a submuscular location when the physician is concerned about erosion of either the IPG or ICD through the skin (most common in thin, elderly, or very young patients) or for cosmetic reasons (to reduce the obvious nature of the device). Another variation is to place the device in an abdominal location. This is commonly done in small children to avoid discomfort or interference with the motion of the arm.

In support of the implanted hardware, an external programmer is used to telemeter information to and from the programmable IPG. This allows the physician to set/reset parameters within the device and enables the physician to download information relating to the status of the patient and the device. A complete defibrillation system is shown schematically in Fig. 1 (pacing systems use a similar configuration).

Pacing and defibrillation systems can be implanted using one of several methods. Early systems used leads attached to the epicardial surface of the heart, with the IPG or ICD placed in the abdomen of the patient (because of its large size). Although this technique is still used in certain clinical situations (i.e., for neonates), a transvenous approach for attaching the leads to the heart and a pectoral placement of the IPG or ICD are far more common. Typical procedural techniques for implantable pacing and defibrillation are described to provide a more thorough understanding of the system requirements.

Following anesthesia and sterile preparation of the incision site, one of two techniques is used to access the venous system for the implantation of transvenous leads. Venous access is achieved through either a surgical "cutdown" to the cephalic

Defibrillation Heart Failure

Fig. 2. Chest x-rays of an endocardial, dual-chamber pacing system in a young patient (left, anterior view; right, lateral view; Sainte-Justine Hospital, Montreal, Quebec, Canada; used with permission). The implantable pulse generator (IPG, pacemaker) is implanted in the left pectoral region. The superior lead is implanted in the right atrial appendage, and the inferior lead is in the right ventricular apex.

Fig. 2. Chest x-rays of an endocardial, dual-chamber pacing system in a young patient (left, anterior view; right, lateral view; Sainte-Justine Hospital, Montreal, Quebec, Canada; used with permission). The implantable pulse generator (IPG, pacemaker) is implanted in the left pectoral region. The superior lead is implanted in the right atrial appendage, and the inferior lead is in the right ventricular apex.

vein (the jugular vein is also used, but this is rare) or a transcutaneous needle puncture to the subclavian vein. The cut-down involves a careful surgical dissection to the vessel, placement of a cut through the vessel wall, and direct insertion of the lead into the vessel lumen. The subclavian puncture uses a needle to puncture the vessel, followed by passage of a guidewire through the needle. Subsequently, an introduction catheter (percutaneous lead introducer) with an internal dilator is forced over the wire and into the vein. The dilator is removed, leaving the catheter behind. The lead is then inserted through the catheter. (This technique can be viewed in styletNew.mpg on the Companion CD.)

Following insertion into the vein, the leads are advanced through the superior vena cava and into the right atrium for final placement in the right atrium, right ventricle, or coronary sinus/ cardiac veins (providing access to the left atrium and ventricle). The leads are secured in the desired location within the heart using either a passive or active means of fixation with electrical performance verified (see Section 5.10). Next, an anchoring sleeve is used at the venous entry site to secure each lead to the tissue. This isolates the lead from mechanical forces outside the vein, ensuring adequate lead length remains within the heart to accommodate motion caused by activity, respiration, and heart motion. Following lead implantation, the proximal terminal ends are connected to the IPG or ICD, which is then placed in a subcutaneous or submuscular pocket formed in the tissue. The site is then sutured closed, thus completing the implantation. (Chest x-rays of a dual-chamber endocardial pacing system are shown in Fig. 2. Additional radiographic images of several pacing configurations are found in xray1.jpg, xray2.jpg, xray3.jpg, xray4.jpg, xray5.jpg, andxray6.jpg on the Companion CD.)

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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