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Pacing and Defibrillation 23 Pacing and Defibrillation TIMOTHY G. LASKEt PhD, ANNA M. LEGREID, PharmD, AND PAUL A. IAIZZO, PhD CONTENTS INTRODUCTION CARDIAC RHYTHMS AND ARRHYTHMIAS INTRODUCTION TO IMPLANTABLEPACING AND DEFIBRILLATIONSYSTEMS CARDIAC PACING CARDIAC DEFIBRILLATION SUMMARY ACKNOWLEDGMENTS COMPANION CO MATERIAL REFERENCES SOURCES 1. INTRODUCTION 2. CARDIAC RHYTHMS AND ARRHYTHMIAS Pacing and defibrillation systems monitor and treat inappro- 2.1. Cardiac Function and Rhythm priate cardiac rhythms. These rhythms result in cardiac outputs CO (liters/minute) is defined as the heart rate (HR, beats (COs) that are inadequate to meet metabolic demands and are per minute) multiplied by the stroke volume (SV, liters), or often life threatening. Currently, over 600,000 Americans have CO = HR x SV. Normally, the HR is determined by the rate at pacemakers, and 150,000 have implantable cardioverter- which the sinoatrial node (the "biological pacemaker") depo- defibrillators (ICDs) (1). larizes. In healthy individuals, the sinoatrial node provides the To understand the function of pacing and defibrillation sys- appropriate HR to meet variable metabolic demands. More tems best, the underlying physiological situations indicated for specifically, the sinoatrial nodal rate is modulated by (1) sym- their use must also be defined and understood. As with the pathetic and parasympathetic innervation; (2) local tissue design of any biomedical device or system, a "first principles" metabolites and other molecules; (3) neurohormonal factors; understanding of the appropriate physiological behavior is a or (4) the perfusion of the nodal tissues. SV is the quantity of prerequisite to the definition of the performance characteristics blood ejected from the heart during each ventricular contrac- of the device. The information in this chapter is not totally tion. The instantaneous stroke volume is governed by a num- comprehensive and thus should not be used to make decisions ber of factors, including HR, degree of ventricular filling/ related to patient care. This chapter aims to provide a basic atrial performance, atrial-ventricular synchrony, and myo- understanding of the physiological conditions that require cardial contractility. intervention with pacing or defibrillation systems as well as Multiple physiological and pathological conditions exist technical information on these systems to provide a foundation that may result in an inappropriate CO. These conditions need for future research and reading on this topic. to be defined to understand the functional requirements of pacing and defibrillation systems and to motivate the logic behind the system features and performance characteristics. From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ 323 324 PART IV: DEVICES AND THERAPIES / LASKE ET AL. Programmer • Atrial fibrillation: Chaotic depolarization of the atrium. No atrial hemodynamic input to the ventricles, and a nonphysi- ological rate is conducted through the atrioventricular node to the ventricles. Ventricular output is decoupled from metabolic /~ Pe°agdammi ng ]1('~ .... demand. Stasis of blood in the atria can result in clot formation and stroke. HR? SVS --> CO? or COS (see AF.mpg on the Companion CD). • Ventricular tachycardia: High ventricular rate decoupled from sinoatrial nodal and atrial activity. This commonly re- Heart sults from a reentrant conduction loop or an ectopic foci (spontaneously beating region of myocardium). Ventricular Fig. 1. Schematic of a typical implantable defibrillation system and the associated programmer. ICD, implantable defibrillation device. rate is nonphysiological and therefore decoupled from meta- bolic demand. HR? SVS -4 CO? or COS (see VT.mpg on the Companion CD). • Ventricular fibrillation: Chaotic depolarization of the ven- 2.2. Conditions of the Sinoatrial Node tricles. No organized HR. CO = 0 (see VF.mpg on the Compan- • Normal sinus rhythm: Sinoatrial nodal rate is appropriate for ion CD). the current metabolic demand (see normal.mpg on the Com- 3. INTRODUCTION TO IMPLANTABLE panion CD). PACING AND DEFIBRILLATION SYSTEMS • Sinus bradycardia: A slow sinoatrial nodal rate, resulting in a slow HR. This may or may not be functionally appropriate. Implantable pacing and defibrillation systems require mul- HRS -+ COS. tiple components, as well as external instruments, for proper • Sinus tachycardia: A fast sinoatrial nodal rate, resulting in a function and programming. The implantable portion of the sys- higher HR. This may or may not be functionally appropriate. tem is composed of the implantable pulse generator (IPG, or HR? ~ COt (for excessive HRs, COS because of reduced pacemaker) or an implantable cardioverter-defibrillator (ICD, filling time). or defibrillator) and the pacing and/or defibrillation leads. The • Sick sinus syndrome (SSS): Unpredictable sinoatrial nodal IPG and ICD are most commonly implanted in a subcutaneous rate. The rate is not appropriately coordinated with physiologi- location in the left pectoral region. Typically, depending on the cal demand. CO? or COS patient's handedness, the condition of the upper venous system, • Chronotropic incompetence: Inappropriate response of the sine- the presence of other devices, or physician preference, the atrial node to exercise. CO is too low for metabolic demands. device may also be placed in the right pectoral region. It may be • Block: No sinoatrial nodal rhythm. The patient will have either placed in a submuscular location when the physician is con- no HR (asystole) or a rate defined by other regions within the cerned about erosion of either the IPG or ICD through the skin heart. A rescue rhythm from the atrioventricular node nor- (most common in thin, elderly, or very young patients) or for mally occurs (40-60 beats/min, a so-called junctional rhythm). cosmetic reasons (to reduce the obvious nature of the device). HR=0~CO=0orHRS~COS Another variation is to place the device in an abdominal loca- 2.3. Conditions of the Atrioventricular Node tion. This is commonly done in small children to avoid discom- fort or interference with the motion of the arm. • First-degree heart block: Defined as an atrioventricular inter- val longer than 200 ms. (The normal atrioventricular interval In support of the implanted hardware, an external program- is about 120 ms.) SV$ -+ COS mer is used to telemeter information to and from the program- mable IPG. This allows the physician to set/reset parameters • Second-degree heart block: Atrial and ventricular activity are not 1:1. Two types of second-degree block are defined: within the device and enables the physician to download infor- • Mobitz type I: "Wenckebach phenomenon." A ventricular mation relating to the status of the patient and the device. A beat is dropped after a progressive elongation of the atrio- complete defibrillation system is shown schematically in Fig. 1 ventricular interval. HRJ, (missed beat) ~ COS (pacing systems use a similar configuration). • Mobitz type II: A ventricular beat is dropped without a Pacing and defibrillation systems can be implanted using progressive elongation of the atrioventricular interval. This one of several methods. Early systems used leads attached to the is often an early indication of progressive disease of the epicardial surface of the heart, with the IPG or ICD placed in the conduction system. HR$ (missed beat) -+ COS abdomen of the patient (because of its large size). Although this • Third-degree heart block: No atrioventricular nodal conduc- technique is still used in certain clinical situations (i.e., for tion (conduction from the atrium to the ventricles). The atria neonates), a transvenous approach for attaching the leads to the contract at the sinoatrial nodal rate, and the ventricles are either heart and a pectoral placement of the IPG or ICD are far more asystolic or contract at a ventricular rescue rate (25-55 beats/ common. Typical procedural techniques for implantable pac- rain). HRS and SVS "-'>COS ing and defibrillation are described to provide a more thorough 2.4. Arrhythmlas understanding of the system requirements. Following anesthesia and sterile preparation of the incision • Atrial tachycardia/flutter: High atrial rate of nonsinoatrial nodal origin. Not a physiological rate and therefore is decoupled from site, one of two techniques is used to access the venous system metabolic demand. HR? SVS ~ COt or COS (see AT.mpg on for the implantation of transvenous leads. Venous access is the Companion CD). achieved through either a surgical "cutdown" to the cephalic CHAPTER 23 / PACING AND DEFIBRILLATION 325 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 trans- xray3.jpg, xray4.jpg, xray5.jpg, and xray6.jpg on the Compan- cutaneous needle puncture to the subclavian vein. The cut- ion CD.) down involves a careful surgical dissection to the vessel, 4. CARDIAC PACING placement of a cut through the vessel wall, and direct insertion of the lead into the vessel lumen. The subclavian puncture 4.1. History uses a needle to puncture the vessel, followed by passage of a Discoveries relating to
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