CHAPTER1 Basicprinciplesofpacing - Wiley-Blackwell

1 CADE: CHAP01 2005/11/23 16:13 PAGE1 #1 CHAPTER 1 Basic principles of pacingMalcolm KirkThe aim of this chapter is to give sufficient background and information aboutcardiac pacemakers to allow interpretation of electrocardiograms (ECGs) andtelemetry strips of normal pacemaker behavior. For more in-depth inform-ation, such as would be necessary for programming pacemakers, a standardpacin gtext should be consulted. Several of these are listed in the biblio italicized terms are defined in the glossary at the end of the pertinent anatomy for cardiac pacin gincludes the sinoatrial (SA) node,the atrioventricular (AV) node, and the His-Purkinje system (Figure ).The SA node is located at the superior aspect of thecrista terminalis(notpictured), near the junction with the superior vena cava.

2 It is normally thedominant pacemaker in the heart, because its rate of depolarization exceedsthat of other areas that normally possess properties of automaticity, such asthe more inferior areas of thecrista terminalisand the His-Purkinje SA node can, in turn, be suppressed by an even faster rhythm, such as anatrial tachycardia, or pacin gby an implanted bundleIVCM itral valveTricuspid valveLeft bundlebranchRight bundlebranchPurkinje fibresAtrioventricular(AV) nodeRight atriumSinoatrial(SA) nodeSVCF igure of conduction system anatomy1 CADE: CHAP01 2005/11/23 16:13 PAGE2 #22 Chapter 1 The AV node is normally the only electrical connection between theatria and the ventricles. Electrical activation proceeds from the right atrium,through the AV node to the His-Purkinje system, and then to the His-Purkinje system comprises myocardial cells that are specialized forrapid conduction.

3 Its anatomic components are (in order of activation) the Hisbundle, the bundle branches (right and left) and the Purkinje fibers. The His-Purkinje system delivers the electrical impulse rapidly from the AV node towidely dispersed areas of the left and right ventricular endocardium, makingactivation nearly simultaneous throughout the ventricles. This rapid conduc-tion, and simultaneous activation of the right and left ventricles, results inthe narrow QRS complex seen on a normal ECG. If an impulse is transmit-ted throughout the ventricle without using the His-Purkinje system, it takeslonger for the ventricles to be activated, and hence the QRS complex is example would be a premature ventricular contraction (PVC). Anotherwould be ventricular pacing, because the pacemaker lead is usually not posi-tioned so as to activate initially the His-Purkinje system.

4 (Furthermore, in thiscase the ventricles are activated sequentially rather than simultaneously.)The components of the surface ECG reflect the cardiac chambers and con-ductin gsystem (Fi gure ). The activation of the atria creates theP waveon the surface ECG. Electrical conduction through the AV node to the His-Purkinje system is relatively slow, so there is normally a 120 200 millisecond(ms) delay between the start of atrial activation and the start of ventricu-lar activation. The delay between the onset of the surface P wave and theonset of the QRS complex is due mostly to conduction through the AV node,with some contribution from intraatrial conduction, and His-Purkinje systemconduction. The activation of the ventricles creates the QRS noted above, the AV node and His-Purkinje system are normally theonly electrical connection between the atria and the ventricles.

5 Failure ofelectrical conduction through the AV node and/or His-Purkinje system resultsS NH I SB BPQRSPA triumA V NodeFigure conductin gsystem is reflected in the normal QRS complex. SN=sinus node, His=His bundle, BB=bundle branches, P=Purkinje : CHAP01 2005/11/23 16:13 PAGE3 #3 Basic principles of pacing3inheart block, also known as AV block, and is one of the indications for pace-maker implantation (see Chapter 3). When assessin ga pacemaker patient,it is important to consider whether or not the patient has intactintrinsicconductionthrough the AV node and His-Purkinje system. If electrical con-duction from the atrium to the ventricles is not present (AV block), then thepatient is likely to be dependent on ventricular pacin gto maintain an adequateheart muscle cells, like other excitable cells, have a restin gelectrical gradi-ent across the cell membrane.

6 In the quiescent state (durin gdiastole), theinside of the cell (cytoplasm) is electrically negative relative to the outside ofthe cell. That is to say, the cell membrane separates positive (outside) andnegative (inside) charges. Thus the cell membrane ispolarized. It becomesdepolarizedwhen an electrical current causes openin gof sodium (Na+) andcalcium (Ca2+) channels in the cell membrane, allowin gthese positive ionsto rush into the cell. This flow of positive ions into the cell has two import-ant consequences: propagation of electrical activity (the action potential) andcontraction of the of the action potentialPropagation is the spread of depolarization in a wave across the heart. Becausethe flow of positive ions into the cell is itself a small current, it causes openingof Na+and Ca2+channels in adjacent cells.

7 The openin gof these channels, inturn, creates a current that causes openin gof channels in cells beyond, andso forth, so lon gas adjacent cells areexcitable, and notrefractory(see below).RefractorinessRefractoriness is a normal property of cardiac tissue. After depolarization, cellsneed a certain amount of time to recover before they can be stimulated the most general sense,refractorinessis the opposite ofexcitability. After acardiac muscle cell has beendepolarized(also calledexcited), it cannot be depol-arized again until the membrane has become polarized again (or repolarized).The time between an electrical stimulus that excites a certain part of the heart,and the latest repeat stimulus that cannot excite the same tissue is known astherefractory period.

8 A stimulus that fails to excite the heart because it occurstoo soon after the previous stimulus or depolarization is said to find the amount of time required for recovery of excitability ( the refractoryperiod) depends on the type of cardiac tissue (atrium, ventricle, AV node,conductin gsystem), and may be influenced by medications or by rate of stim-ulation. The refractory period of the AV node is important in pacin gand atrialarrhythmias. It will determine how frequently atrial impulses can be trans-mitted to the ventricle. For example, in atrial fibrillation, the atrial rate canCADE: CHAP01 2005/11/23 16:13 PAGE4 #44 Chapter 1exceed 400 beats per minute (bpm), but not every impulse is transmitted tothe ventricle.

9 Most of the atrial beats will be blocked at the AV node, becausethey reached the AV node at a time when it is refractory and cannot response of the AV node to rapid stimulation rates differs from other car-diac tissue, in that the refractory period of the AV node generally increaseswith increased rates of stimulation, whereas the refractory period of the atriaand ventricles decreases with increased rates of stimulation. The AV nodethus limits the maximum rate at which the ventricle can follow a rapid of a pacemaker systemA pacemaker consists of apulse generator(Figure ) and pulse generator contains the battery of the pacemaker, as well as thecircuits that deliver the pacin gstimuli. The lead input and circuitry in a pace-maker pulse generator dedicated to a particular chamber of the heart is knownas achannel.

10 For example, the ventricular channel transmits the ventricularpacin gimpulse to the ventricular leads are electrical conductors (wires), covered with transmit the electrical impulses from the pulse generator to the heart,and from the heart to the pulse leads are usually inserted into the subclavian vein or its tribu-taries, and positioned on the inner surface (endocardium) of the heart. Theyare attached to the endocardium by a small screw mechanism, or are held inplace by tines. If a screw (also known as a helix) is used to fix the lead to theheart, the lead is called an active fixation lead (Figure ). A passive fixationFigure pulse generator. Dimensions: cm cm of St. Jude : CHAP01 2005/11/23 16:13 PAGE5 #5 Basic principles of pacing51 fixation lead.