Pacing and defibrillation systems monitor and treat inappropriate cardiac rhythms. These rhythms result in cardiac outputs (COs) that are inadequate to meet metabolic demands and are often life threatening. Currently, over 600,000 Americans have pacemakers, and 150,000 have implantable cardioverter-defibrillators (ICDs) (1).

To understand the function of pacing and defibrillation systems best, the underlying physiological situations indicated for their use must also be defined and understood. As with the design of any biomedical device or system, a "first principles" understanding of the appropriate physiological behavior is a prerequisite to the definition of the performance characteristics of the device. The information in this chapter is not totally comprehensive and thus should not be used to make decisions related to patient care. This chapter aims to provide a basic understanding of the physiological conditions that require intervention with pacing or defibrillation systems as well as technical information on these systems to provide a foundation for future research and reading on this topic.

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

2. CARDIAC RHYTHMS AND ARRHYTHMIAS 2.1. Cardiac Function and Rhythm

CO (liters/minute) is defined as the heart rate (HR, beats per minute) multiplied by the stroke volume (SV, liters), or CO = HR x SV. Normally, the HR is determined by the rate at which the sinoatrial node (the "biological pacemaker") depolarizes. In healthy individuals, the sinoatrial node provides the appropriate HR to meet variable metabolic demands. More specifically, the sinoatrial nodal rate is modulated by (1) sympathetic and parasympathetic innervation; (2) local tissue metabolites and other molecules; (3) neurohormonal factors; or (4) the perfusion of the nodal tissues. SV is the quantity of blood ejected from the heart during each ventricular contraction. The instantaneous stroke volume is governed by a number of factors, including HR, degree of ventricular filling/ atrial performance, atrial-ventricular synchrony, and myo-cardial contractility.

Multiple physiological and pathological conditions exist that may result in an inappropriate CO. These conditions need 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.

Fig. 1. Schematic of a typical implantable defibrillation system and the associated programmer. ICD, implantable defibrillation device.

2.2. Conditions of the Sinoatrial Node

• Normal sinus rhythm: Sinoatrial nodal rate is appropriate for the current metabolic demand (see normal.mpg on the Companion CD).

• Sinus bradycardia: A slow sinoatrial nodal rate, resulting in a slow HR. This may or may not be functionally appropriate. HRI ^ COI.

• Sinus tachycardia: A fast sinoatrial nodal rate, resulting in a higher HR. This may or may not be functionally appropriate. HRt ^ COt (for excessive HRs, COI because of reduced filling time).

• Sick sinus syndrome (SSS): Unpredictable sinoatrial nodal rate. The rate is not appropriately coordinated with physiological demand. COt or COI

• Chronotropic incompetence: Inappropriate response of the sinoatrial node to exercise. CO is too low for metabolic demands.

• Block: No sinoatrial nodal rhythm. The patient will have either no HR (asystole) or a rate defined by other regions within the heart. A rescue rhythm from the atrioventricular node normally occurs (40-60 beats/min, a so-called junctional rhythm). HR = 0 ^ CO = 0 or HRI ^ COI

2.3. Conditions of the Atrioventricular Node

• First-degree heart block: Defined as an atrioventricular interval longer than 200 ms. (The normal atrioventricular interval is about 120 ms.) SVI ^ COI

• Second-degree heart block: Atrial and ventricular activity are not 1:1. Two types of second-degree block are defined:

♦ Mobitz type I: "Wenckebach phenomenon." A ventricular beat is dropped after a progressive elongation of the atrioventricular interval. HRI (missed beat) ^ COI

♦ Mobitz type II: A ventricular beat is dropped without a progressive elongation of the atrioventricular interval. This is often an early indication of progressive disease of the conduction system. HRI (missed beat) ^ COI

• Third-degree heart block: No atrioventricular nodal conduction (conduction from the atrium to the ventricles). The atria contract at the sinoatrial nodal rate, and the ventricles are either asystolic or contract at a ventricular rescue rate (25-55 beats/ min). HRI and SVI ^ COI

2.4. Arrhythmias

• Atrial tachycardia/flutter: High atrial rate of nonsinoatrial nodal origin. Not a physiological rate and therefore is decoupled from metabolic demand. HRt SVI ^ COt or COI (see AT.mpg on the Companion CD).

• 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 demand. Stasis of blood in the atria can result in clot formation and stroke. HRt SVI ^ COt or COI (see AF.mpg on the Companion CD).

• Ventricular tachycardia: High ventricular rate decoupled from sinoatrial nodal and atrial activity. This commonly results from a reentrant conduction loop or an ectopic foci (spontaneously beating region of myocardium). Ventricular rate is nonphysiological and therefore decoupled from metabolic demand. HRt SVI ^ COt or COI (see VT.mpg on the Companion CD).

• Ventricular fibrillation: Chaotic depolarization of the ventricles. No organized HR. CO = 0 (see VF.mpg on the Companion CD).

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