Tài liệu Types of pacemakers and the hemodynamics of pacing pptx

An early pacemaker was designed with a lead to the atrium that would sense the P wave and then, after an appropriate AV delay to allow ventricular filling, the pacemaker triggered a spik

Authors: Moses, H Weston; Mullin, James C.

Title: A Practical Guide to Cardiac Pacing, 6th Edition

Copyright ©2007 Lippincott Williams & Wilkins

> Table of Contents > 5 - Types of Pacemakers and the Hemodynamics of Pacing

5 Types of Pacemakers and the Hemodynamics of Pacing Permanent Pacemaker Types

Pacemaker Code

The pacemaker code is a mechanism to briefly describe types of pacemakers (Table 5-1) The code has evolved over several years to accommodate changes in pacing systems and there have been recommendations for up to a five-position code with multiple letters From a practical point of view, the four-position code described here represents the general usage Position I refers to the chamber(s) being paced V stands for ventricle, A stands for atrium, and D stands for dual (atrium and ventricle) There is really no O in this setting (an older implantable cardioverter defibrillator that did not have pacing backup could be designated O, but this is of historic interest only) Manufacturers will often designate S for single This means it can be used to pace either the atrium or the ventricle They are simply describing it more accurately and not designating it as one or the other since it can be used for either Position II refers to the chamber(s) being sensed Again, V is for ventricle, A is for atrium, and D is for dual (atrium and ventricle) Again, the designation of S is often used by the manufacturer in a generic manner because of its potential application for either atrial or ventricular placement In position II, the designation O refers to absent sensing (and thus refers to fixed, asynchronous pacing) When a magnet is placed over most pacemakers, the sensing is disabled; for instance, a VVI pacemaker would become VOO

Position III refers to the device's response to sensing I represents the inhibited mode,

meaning that when the pacemaker senses an event, it will be inhibited from further pacing; this is the most common form of sensing T indicates a triggered response When the

pacemaker senses an event, it will trigger the device to deliver a pacing stimulus In single-chamber situations, the sensed event and triggered impulse occur within the same single-chamber This

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is rarely used An example of its use would be to program a VVI pacemaker to a VVT In that case, a sensed event would also trigger a pacer spike This would,

of course, be useless if a normal QRS were sensed On the other hand, VVT can

be used as a diagnostic device to see if a T wave was being sensed (a spike would appear on the T wave) Modern devices allow evaluation of such problems

in other ways and VVT is really of historic interest.

TABLE 5-1 Pacemaker code

Category Chamber(s)

paced:

Chamber(s) sensed:

Response to sensing:

Programmability, rate modulation:

O = None O = None O = None R = Rate modulation

A = Atrium A = Atrium T = Triggered

V = Ventricle V = Ventricle I = Inhibited

D Dual (A + V)D Dual (A + V)D Dual (T + I) Manufacturer's

designation only S = Single (A or V) S = Single (A or V)

S stands for single and is a manufacturer's designation This addresses the fact that a single VVI pacemaker for instance can also be used as AAI pacemaker (i.e it is a single chamber

pacemaker capable for pacing in the atrium or ventricle and sensing in the atrium or

ventricle)

When dual-chamber terminology is introduced, many new to pacing are confused by the third

D The third D refers to the ability to trigger a spike or to inhibit In particular, a sensed P wave in the atrium allows a triggered ventricular response in the ventricle

An easier way to explain this is to discuss a type of pacemaker that is no longer available The VAT pacemaker would sense in the atrium and then after an appropriate AV delay, trigger a spike in the ventricle The original ideal patient for this would have been an active person with significant heart block Simply putting in a VVI pacemaker would cause pacing with no relation to the P waves and lose all the potential advantages of AV synchrony An early pacemaker was designed with a lead to the atrium that would sense the P wave and then, after

an appropriate AV delay to allow ventricular filling, the pacemaker triggered a spike in the ventricle This would lead to AV synchrony It would have the advantage, in an active person,

of tracking the P waves at an increased, appropriate, physiologic heart rate with an

appropriate AV interval

The rest of the VAT pacemaker discussion is simply to describe the term trigger Even the novice can envision numerous potential problems with this device Atrial flutter could cause the heart to race inappropriately If the patient developed sinus node incompetence, there would be no way to pace the atrium If frequent PVCs occurred or if intermittent normal AV conduction

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occurred, the device would be pacing unnecessarily in the ventricle This is of historic interest only and is basically to describe the concept of the triggered response in a dual-chamber pacemaker

Position IV, at one point had numerous options, but currently position IV will simply have an

R added if the patient has rate modulation, in which a sensor is used to modify the heart rate

of the pacemaker based on the patient's activity or metabolic need (see Chapter 7)

Position V is rarely used nowadays It refers to antitachyarrhythmia function or functions 0 = none, P = anti-tachyarrhythmia, S = shock, and D = dual (P + S) This nomenclature is really outdated with modern ICDs and from a practical point of view, only the first three letters are

in common use and an R is placed at the end if rate modulation is an option with the device Virtually all modern pacemakers contain rate modulation as an option (although it can be turned off if it's not clinically necessary)

Single-Chamber Permanent Pacemakers

There are essentially only two forms of single-chamber pacing: VVI and AAI, with the former being the most common Rate modulation is an option for either chamber AAI pacing

is selected for patients in whom the bradyarrhythmia is a sinus mechanism and AV block is not a problem and who do not have chronic or frequent atrial tachyarrhythmias They also should not have silent atria, in other words, large dilated atria which cannot be stimulated VVI pacing is most commonly used for patients with chronic atrial fibrillation and a slow ventricular response Pacing or sensing the atrium in these patients is of no value VVI pacing could be used in a patient with sick sinus syndrome as “backup pacing,†but during those  but during those times of pacing, AV synchrony would not be maintained (if the episodes of asystole are very rare and/or the patient is extremely inactive, this may not present a significant clinical

problem) The addition of rate modulation (AAIR or VVIR) is indicated when sinus node function is abnormal Chronotropic incompetence is a common form of abnormal sinus node function in which appropriate increases in sinus rate do not occur

As noted above, the AAT and VVT modes are mainly of historic interest only A patient could have a pacemaker programmed to AAT or VVT to see if any extraneous electrical activity is being sensed by the pacemaker Extraneous electrical activity would cause a

pacemaker spike at the time of sensing and could be used for diagnostic purposes Generally, other approaches are used in the more modern pacemaker era

In the past, one use of the AAT mode was in the assessment of atrial sensing with some DDD devices in patients who were not pacemaker dependent For example, first reprogram to the AAT mode with an atrial pacing rate less than sinus rate and a unipolar atrial pacing

configuration (bipolar can also be used, but as noted before, unipolar is easier to visualize on

an ECG strip or monitor) The atrial sensitivity setting is then progressively reprogrammed P.76

from the least to the most sensitive values while simultaneously observing the ECG As soon

as atrial sensing is reached, triggered atrial stimuli will begin to appear within the P wave on the ECG Until then (at less sensitive values), clinicians will observe atrial stimuli, at the programmed rate, that are dissociated from the native P waves In this way, one can determine

at what level atrial sensing occurs and program the appropriate sensitivity Most current pacemakers have built-in automatic sensing threshold determination as a standard feature New technology, such as the availability of visualizing the intracardiac atrial electrogram and the use of marker channels, has replaced the more cumbersome AAT mode technique

Dual-Chamber Permanent Pacemakers

Benefits of Maintaining AV Synchrony

Engineers and electrophysiologists have attempted to mimic sequential atrial and ventricular electrical activation of the heart with artificial pacemakers for decades A permanent AV sequential cardiac pacing became reliable only with advances in pulse generator technology, battery longevity, stable atrial lead systems, and miniaturization of circuitry The benefits of maintaining AV synchrony are discussed briefly in the next section

Physiologic Timing of Atrioventricular Valve Closure

With a patient in sinus rhythm and VVI pacing, the possibility exists of poorly timed atrial contraction in relation to ventricular contraction If the atria contract at or about the same time that the ventricles contract, low cardiac output and pulmonary congestion result Inappropriate

AV synchrony can result in the pacemaker syndrome Symptoms may include fatigue,

lightheadedness, shortness of breath, chest pain, and even syncope These symptoms can be related simply to AV dissociation or to the development of retrograde AV nodal conduction Symptoms that were supposed to be alleviated by the implantation of a pacemaker may recur

in the form of pacemaker syndrome

Improved Cardiac Output

An increase in stroke volume and cardiac output occurs when ventricular filling is augmented

by atrial contraction as a result of the Frank–Starling relationship When the atrium quickly contracts, ventricular volume at the end of diastole is greater so the ventricle begins its

contraction higher on its function curve (Fig 5-1) Augmentation of ventricular stroke volume

by atrial contraction is most important in stiff, noncompliant hearts, as may be seen in patients with congestive heart failure, chronic hypertension, and so forth The improved stroke volume with properly timed atrial contraction carries no significant additional myocardial oxygen demand and allows the heart to

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work higher on its ventricular function curve without an appreciable increase in mean pulmonary capillary pressure Thus, the improved cardiac output occurs without increased risk for pulmonary congestion.

Figure 5-1 Frank–Starling Curve.

In this example, at point A the patient does not have an atrial kick, the left ventricular end-diastolic volume is relatively small, and the stroke volume poor At point B, the atrial kick has returned, the end-diastolic volume is increased, and the stroke volume is increased This occurs with no significant increase in oxygen consumption

If the patient has normal sinus node function with a normal chronotropic response to exercise, the dual-chamber pacemaker can allow the sinus node to increase normally in response to exercise and “track†with appropriate ventricular pacing to increase cardiac output and  but during those improve the sense of well-being in a patient who is exercising

A potential benefit of maintaining AV synchrony is a reduction of supraventricular

tachyarrhythmias This benefit, however, is not well established

Pacemaker Timing Cycles (Intervals)

A pacemaker's sensing and pacing behavior can be expressed in terms of timing cycles The simplest way to understand timing cycles is to examine the behavior of a VVI pacemaker at its lower rate limit, when spontaneous ventricular activity is slower than the programmed lower rate limit of the pacemaker The intervals are usually expressed in milliseconds (msec) The pacing interval is the time between two consecutive paced events without an intervening sensed event If the native QRS is sensed by the VVI pacemaker, the timing interval is reset DDD pacemakers can sense in both the atrium and ventricle, and pace in both the atrium and ventricle Sensing of a ventricular event can cause inhibition of the ventricular spike; sensing

of an atrial event can lead to pacing in the ventricle (assuming there is no AV conduction within the timed atrial escape interval) These modes are pictured in Figure 5-2 The first QRS complex shows a native P wave in QRS, the second shows atrial and ventricular pacing, the third shows atrial pacing with conduction through the AV node and a native QRS, and the fourth complex shows a native P wave with ventricular pacing

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Figure 5-2 This simplified ECG demonstrates possible variations of normal operation in a DDD pacemaker The first beat demonstrates a normal P wave with normal conduction in a native QRS The second beat represents atrial pacing followed by ventricular pacing The third beat represents atrial pacing with normal conduction to the AV node in a native QRS complex The fourth beat represents a native P wave with ventricular pacing Various

manufacturers use different terminology for identifying these events In this book, we will generally talk about the native P wave or A wave, an atrial paced beat, a native QRS complex, and a ventricular paced beat

Figure 5-3 demonstrates some additional timing cycles of the base rate of a DDD pacemaker The ECG demonstrating pacing in the atrium and pacing in the ventricle is shown; the top line labeled “atrial†describes the atrial channel It shows a very short dark area labeled  but during those “blanking.†This is a period in which the atrial sensing is totally disabled to avoid  but during those

confusing the pacemaker program with the presence of the large pacemaker spike and after potential occurring just after the spike (described in Chapter 3) This blanking period blinds the atrial sensing device during this potentially confounding and confusing area for a few milliseconds only

The area labeled AVI represents the interval between the firing of the atrial spike and the firing of the ventricular spike

After the ventricular pacing spike is a very important timing cycle called PVARP

(postventricular atrial refractory period) The concept of this PVARP is to ignore all events within this timing cycle Basically that prevents the atrial channel from responding to the ventricular spike or to the ventricular depolarization and the ventricular T wave as if those were P waves From a practical point of view, one of the most valuable uses of PVARP is to avoid responding to any retrograde conducted P wave that may occur As mentioned in the next chapter, pacemaker-mediated tachycardia is a potential problem for patients with dual-chamber pacemakers and the ability to program PVARP is valuable clinically

Following is a clear area in that row which represents the time in which atrial sensing does occur and that sensing is acted upon (for example, if a P wave is sensed, the atrial channel will inhibit) Next in that row is TARP This represents the total atrial refractory period and is

a combination of the AV interval plus the PVARP This is a clinically important measurement because it affects the maximum tracking rate of the pacemaker

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Figure 5-3 The Upper Portion Demonstrates Atrial-Based Timing.

The first beat is atrial and ventricular pacing In the second beat the atrium is paced, but there

is conduction through the AV node with an AR interval of 150 msec This leads to an

effective ventricular rate of 63 bpm, although the pacemaker is programmed at a lower rate limit of 60 bpm The third beat demonstrates atrial and ventricular pacing and now, because of the atrial timing adds an AV interval of 200 msec instead of the actual 150 msec that occurs, the ventricular rate is 57 bpm instead of the 60 bpm to which it is programmed The fourth beat is atrial and ventricular pacing and thus the ventricular rate is 60 bpm In other words, the lower rate limit has been both higher and lower than the programmed 60 bpm even though the pacemaker is functioning quite appropriately with atrial-based timing

In the second example, the timing is ventricular-based The first beat demonstrates atrial and ventricular pacing The second beat demonstrates atrial pacing with conduction through the

AV node and a PR interval of 150 msec This leads to a ventricular rate of 63 bpm (950 msec interval) The third beat is atrial pacing and ventricular pacing This results in the

programmed lower rate limit of 60 bpm (1,000 msec)

An important concept to keep in mind is the difference between the blanking period and refractory period The blanking period means that the pacemaker essentially closes down its sensing circuit and does not realize that any electrical activity is taking place During the refractory period, electrical activity may in fact be recognized, but not responded to For example, if the patient were to have multiple atrial flutter waves during the PVARP, a

program can be added to use that information to decide whether there was, in fact, atrial flutter going on (the flutter waves would not have to be recognized except during the

“alert†period) This information may be used in automatic mode switching, which is  but during those discussed in the next chapter

The horizontal column labeled “ventricle†describes the ventricular portion and  but during those

demonstrates first a blanking period Basically the concept is that the ventricular lead should not recognize an atrial spike or subsequent electrical activity, such as P wave, as a ventricular event When the ventricular lead recognizes such atrial activity, it can lead to cross-talk, which can confuse the device

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After the ventricular spike is the ventricular refractory period (VRP) Its main goal is to eliminate sensing of the T wave and identifying that, incorrectly, as a ventricular event

AEI stands for atrial escape interval, which is the interval between the ventricular spike and the atrial spike

The upper rate limit of dual-chamber pacemakers represents the highest ventricular pacing rate that can be achieved in response to atrial sensed events and still maintain one-to-one AV synchrony at the programmed AV delay This is also known as the maximum tracking rate (MTR) The limit of the MTR is limited by the TARP All DDD programmers will

automatically calculate the MTR based on the selections of the AVI and the PVARP, and limit the MTR based on those selections The pacemaker will respond to atrial rates greater than the MTR with a Wenckebach pattern Continued increase in atrial rate will cause 2:1 block This is a particularly complex area of dual-chamber pacing and an area in which atrial-based pacing versus ventricular-atrial-based pacing plays a role With the addition of rate

responsive pacing, which we discuss in the next chapter, the upper rate limit response

becomes extremely complex and is generally device specific

Some Dual-Chamber Modes

DOO

When a magnet is placed on a dual-chamber pacemaker it reverts to AV pacing with no sensing This is generally used as a diagnostic tool Another occasional use of the magnet mode would be in a patient getting surgery with electrocautery involved The magnet mode would cause DOO pacing and avoid inappropriate sensing of the electromagnetic cautery device as a rapid ventricular rate “turning off†the pacemaker This can be catastrophic in but during those

a pacemaker-dependent patient (this is mentioned in Chapter 11; of course, be aware that the magnet does not protect the pacemaker from extremely strong field damage such as

cardioversion for an arrhythmia) As soon as the magnet is removed, the pacemaker will revert to its previously programmed parameters

DDI

Defined as AV sequential, non–P-synchronous pacing with dual-chamber sensing, DDI allows sensing in both chambers and inhibition in both chambers, but an atrial event will not trigger a ventricular response Occasionally a DDD pacemaker is programmed to the DDI mode because of paroxysmal atrial fibrillation If these episodes are occasional and

intermittent, reprogramming a DDD device to DDI is useful in preventing the patient from experiencing brief rapid rates The rapid atrial fibrillation or flutter waves cannot trigger beats

in the ventricle Modern pacemakers have programs called automatic mode switching, which

we discuss in the next chapter, and these algorithms have generally eliminated the need for DDI pacing On the other hand, if the atrial fibrillation is

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intermittent, the DDI mode will prevent the patient from being paced briefly at maximum tracking rate before being switched to a slower mode

Another potential use for DDI is in a patient with persistent sinus bradycardia A DDI

pacemaker with rate responsiveness will pace consistently in the atrium and the

rate-responsive device will allow a somewhat physiologic response to activity (discussed in Chapter 7)

VDD

This is not a commonly used mode DDD pacemakers require a dedicated atrial lead with good connection to the atrial myocardium A rarely used lead is available that goes only to the ventricle and therefore can pace only in the ventricle It does, however, have an additional sensing electrode further back in the right atrial area that can generally sense P waves with a reasonable degree of accuracy (but not pace the atrium) This can be used, for instance, in a patient in whom it was technically difficult to obtain two leads or in a very elderly patient in whom there is an effort to keep things simple That would be a patient with heart block who has otherwise normal sinus rhythm Therefore, it would, when working appropriately, sense a

P wave and fire the ventricular spike after the appropriate AV delay It would not, however,

be able to pace in the atrium It is of note that sensing problems are more likely to occur in this type of device than in a device with a classic atrial J lead

Pacemaker Mode Selection

The physician is faced with complex decisions regarding the type of pacemaker to place in an individual patient One of the first and easiest decisions is whether the patient is in chronic atrial fibrillation, because the atrium cannot be paced with chronic atrial fibrillation

Therefore, only a VVI pacemaker should be used in patients with chronic atrial fibrillation In fact, placing a dual-chamber pacemaker would add unnecessary costs and risks to the patient, including putting the patient at risk for inappropriately tracking atrial fibrillation rates at a very rapid rate There is no indication for dual-chamber pacing in this group of patients A complicating factor is discerning chronic atrial fibrillation In fact, it is often difficult to know

if a patient is currently in atrial fibrillation With the advent of the implantable cardioverter defibrillator (ICD), a shock to treat ventricular fibrillation occasionally ends up treating atrial fibrillation also

Intermittent atrial fibrillation is a more complex area When the patient is in sinus rhythm, a dual-chamber pacemaker can maintain the benefits of AV synchrony There has been some speculation that chronically pacing the atrium will in fact reduce episodes of atrial fibrillation This is a complex area being investigated including the use of some rather sophisticated devices, but currently there is no strong recommendation for pacing to maintain sinus rhythm with intermittent atrial fibrillation If a dual-chamber pacemaker is placed in a patient with intermittent atrial fibrillation, programming becomes more

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complicated Most modern pacemakers (discussed in Chapter 6) have an algorithm termed automatic mode switching, which will reduce the chance of tracking atrial fibrillation and rapid pacing the ventricle as a result This algorithm can switch the patient to VVI(R) or DDI(R) pacing automatically

Two general categories discussed in indications are sick sinus syndrome and AV block With sick sinus syndrome, an AAI pacemaker may be appropriate (or an AAIR pacemaker) An AAI pacemaker is used more commonly in Europe than in the United States The theoretic advantage of a DDD pacemaker is that, if AV block occurs (and it would not be rare for multiple areas of the conduction system to be affected in sick sinus syndrome), the dual-chamber aspect offers ventricular backup pacing if necessary On the other hand, recent data suggests that chronic ventricular pacing may be deleterious, so AAI would preclude that occurring If there is no indication for AV block, then AAI pacing would be appropriate With complete or intermittent AV block, a dual-chamber pacer appears to be the best option This allows the physiologic benefits of tracking the atrium and pacing the ventricle

sequentially Recent studies suggest that this is not as beneficial as initially thought On the other hand, these studies were done in elderly patients and it is very difficult to measure the benefit of AV synchrony in terms of the patient's sense of well-being Those studies do indicate that VVI pacing and heart block, with loss of AV synchrony, may be reasonable in a very elderly, inactive person

Complicated combinations arise As mentioned earlier, the patient may have both sick sinus syndrome and heart block Some patients maintain a normal increase in atrial rate in response

to exercise and a DDD pacemaker would be adequate Another patient may have

“chronotropic incompetence†and simply cannot increase his or her atrial rate A DDDR  but during those pacemaker (with rate responsiveness) would potentially benefit that type of patient

Temporary Pacemaker Application

Transvenous Temporary Pacing

The type of temporary pacemaker used is almost always a VVI pacemaker Transcutaneous pacemakers are sometimes used with variable sensing properties That is a unique situation in which essentially the entire heart is captured with a transcutaneous paced beat as described in other chapters Occasionally temporary AAI pacing can be performed This is relatively uncommon because of the potential instability of the temporary lead in the atrium

As discussed in Chapter 10, pacing after cardiac surgery is quite common and that does allow temporary transthoracic atrial and ventricular leads to be placed, and AAI pacing, VVI

pacing, and DDD pacing are all possible options in that setting Generally, this would be for a fairly brief period of time, a few days at most, because of the problem of leaving leads

through the skin and also the problem of rising thresholds and lack of capture with time P.83

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