Tài liệu Programmability and specialized circuits pdf

An example of widening the pulse width to allow capture of the heart rate is shown in Figure 4-1.. At the time of pacemaker implantation, the patient had a good acute threshold of 0.6 V

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

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

Copyright ©2007 Lippincott Williams & Wilkins

> Table of Contents > 4 - Programmability and Specialized Circuits

4 Programmability and Specialized Circuits

Permanent Pacemaker Programmability

Programmability often allows for noninvasive correction of pacemaker malfunction,

optimizing battery life and adjustment of the pacemaker to the patient's physiologic needs, which may change over time Before external programming is performed, any problems must

be appropriately diagnosed and the clinical status of the patient, especially the degree of the patient's pacemaker dependency and the safety margin left after programming, must be

considered

Methods of External Programming

Early in the development of permanent pacemakers, the advantage of being able to change the rate of firing of the generator became obvious One of the earliest efforts involved a screw on the generator that, when turned, would change the pacing rate This method required making a skin incision and using a sterile screwdriver Advances in technology now allow noninvasive programming of several pacemaker functions, including changing the pacing rate To program

a pacemaker externally, a signal, usually pulsed magnetic fields or a radiofrequency signal, is sent through the patient's skin and received by the generator If a pulsed magnetic field is transmitted, it can influence a piece of metal called a reed switch to complete an electrical circuit The pacemaker must be protected against accidental or phantom programming

resulting from bombardment of the pacemaker by environmental electromagnetic waves Usually this protection requires that a very specific code be sensed by the pacemaker before it will respond to programming

Rate Programmability

The pacemaker function most commonly programmed is rate A typical programmable range

is 30 to 120 beats per minute (bpm) and most pacemakers are

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preset by the manufacturer at approximately 60 to 70 bpm Many studies have attempted to determine the pacemaker rate that optimizes cardiac output and, in general, it has been found that cardiac output varies little over a fairly wide range of paced rates, even when an atrial pacemaker is used The increase in cardiac output that occurs normally with exercise involves not only an increase in heart rate but also peripheral vasodilatation in areas of increased metabolic activity and an increase in cardiac contractility Simply increasing the rate of the pacemaker does not duplicate these complex physiologic events In some individual patients and in some clinical situations, however, an increase in pacemaker rate does increase cardiac output For example, a pacemaker-dependent patient who is metabolically unstable after surgery may benefit from a faster-paced rate If the clinical situation is critical, this can even

be determined by use of thermodilution cardiac outputs Rate programmability allows for flexibility in these instances Another reason for increasing rate is to overdrive cardiac

arrhythmias

Pacemaker rate is decreased most commonly to allow the emergence of the patient's

spontaneous rhythm Many patients with sinus bradycardia feel better with the atrial

“kick” that increases cardiac output, rather than with a ventricularly paced rhythm at a faster rate In addition, this slowing of the paced rate preserves battery life Decreasing the paced rate is used also to determine the patient's underlying rhythm or to observe the patient's

intrinsic electrocardiogram (ECG) to evaluate a possible myocardial infarction or assess drug effects on ECG intervals

Recent evidence raises the possibility of a deleterious effect of chronic right ventricular pacing Since many patients have a pacemaker only for intermittent bradyarrhythmias, then reducing the rate may allow contraction of the left ventricle through the normal His-Purkinje system This potentially beneficial effect can also, sometimes, be accomplished by

lengthening the AV interval in a dual-chamber pacemaker This information comes from studies that do not necessarily apply to all patients and clearly right ventricular pacing is appropriately necessary in patients with complete heart block, for example One theory is that the disordered contraction through the myocardium from the RV apex to the base of the heart, bypassing the His-Purkinje system, leads to a chronic myocardial dysfunction Investigations are under way as to whether selective lead placement, for instance in the right ventricular outflow tract or direct pacing of the His-Purkinje system would eliminate this effect

Table 4-1 summarizes the uses of rate programmability

Pulse-Width Programmability

As discussed in Chapter 3, the width or duration of the pacemaker spike is of considerable clinical importance, even though it cannot be measured on a standard 12-lead ECG The exact time duration of the pacemaker impulse is measured on the standard pacing system analyzer Widening the pulse width improves the ability of the pacemaker to stimulate the heart, but as predicted by the strength–duration curve, this

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improvement is only a modest one at greater widths An example of widening the pulse width

to allow capture of the heart rate is shown in Figure 4-1 At the time of pacemaker

implantation, the patient had a good acute threshold of 0.6 V when the pulse width was 0.6 msec; however, after 4 weeks, the threshold rose to approximately 5 V with a pulse width of 0.6 msec, and the patient was intermittently losing capture Slight widening of the pulse width caused consistent capture To allow a reasonable safety factor, the pulse width in the example was left at 1.8 msec

TABLE 4-1 Uses of rate programmability

1 Optimizing cardiac output in the unusual patient who requires a specific rate for best

cardiac output and in unusual clinical situations such as in a pacemaker-dependent patient who is medically unstable with a low cardiac output

2 Overdrive suppression of arrhythmias

3 Increasing battery longevity by decreasing rate

4 Increasing the amount of time a patient is in sinus rhythm (with augmented cardiac output due to the atrial kick) by decreasing rate

5 Reduction of rate to allow the native QRS to be the source of LV contraction (this can also

be, at times, accomplished by lengthening the AV interval in a dual-chamber pacemaker) Recent evidence suggests that there may be a deleterious effect of chronically pacing the right ventricle

6 Evaluation of intrinsic ECG to assess:

a nderlying rhythm

b Myocardial infarction

c Drug effect on the ECG

d Effect of a metabolic abnormality on the ECG

ECG, electrocardiogram

Figure 4-1 Widening Pulse Width to Maintain Capture.

In this example, the patient's threshold rose to 5 V when the pacer spike was at 0.6 msec By increasing the duration of the pacer spike to 1.8 msec, capture can now occur at

approximately 4.2 V Because this particular pacemaker generates only 5 V, the safety margin may not be adequate

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We are using an example in which capture is improved, but often the strength–duration curve is so flat beyond 1.5 msec that a widened pulse width may not improve capture safely and effectively Also, we are assuming that no other evidence of pacemaker malfunction, such

as electrode displacement, is present and that the patient is not so pacemaker dependent that any further threshold rise causing transient loss of capture would be a risk to the patient Another approach to the problem would be to raise the voltage of the pacemaker above 5 V, but this is not technically possible with all pacing devices

If the clinician believes that widening the pulse width is reasonably safe in such a situation, then doing so may save the patient a surgical procedure; in fact, the threshold may improve with time The tradeoff with the widened pulse width is a shorter battery life, but with the long-lived lithium batteries, battery life often is not a major consideration The relationship between pulse width and battery depletion is not linear: Doubling the pulse width shortens battery life by less than half This happens because the first half of the pacemaker spike generates more current than the second half (see Fig 3-3 in Chapter 3)

Another use of programming pulse width is to preserve battery life In the example depicted

in Figure 4-2, a patient has a low chronic threshold of 5 V at 0.1 msec pulse width Because the battery is fixed at 5 V, energy is wasted By narrowing the pulse width from the usual 0.5 msec to 0.3 msec, energy is preserved (i.e., fewer electrons are expended per pacing spike, and therefore battery life is prolonged) and a reasonable safety margin still exists The patient still should be monitored periodically to ensure that the threshold is not rising The current approach to preserving battery life in this situation is to lower voltage (although voltage

cannot be raised easily above 5 V, it can be lowered easily in most pacemakers) Narrowing the pulse width is left in this text as a teaching point

Programming pulse width can be used to estimate threshold Figures 4-1 and 4-2 are drawn as

if we knew the shape of the strength–duration curve in both patients; this is done for the sake of explaining concepts In practice, however, the exact shape of the strength–duration curve in an individual patient often is unknown Therefore, we can obtain an estimate of the threshold as determined by pulse width at a fixed voltage (i.e., one point on the

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strength–duration curve) by narrowing pulse width until capture is lost This information is sometimes clinically useful By programming voltage, additional points on the

strength–duration curve can be obtained

TABLE 4-2 Uses of pulse width programmability

1 Increase pulse width to capture the heart in a patient with high thresholda

2 Narrow pulse width to reduce annoying electrical side effects of pacing such as pectoral muscle or diaphragmatic stimulation (although this usually is an unsuccessful maneuver and best attempted by lowering voltage)

3 Narrow pulse width to preserve battery life

4 Assess threshold (as determined by pulse width at a fixed voltage)

aBefore using pulse width programming to troubleshoot these problems, the entire pacemaker system should be evaluated as well as the clinical status of the patient to ensure that another mode of therapy such as electrode revision of generator change would not be more

appropriate Also, a reasonable safety margin must be left in the system for chronic and acute rises in threshold

Figure 4-2 Narrowing Pulse Width to Preserve Battery Life

In this example, the patient has a low chronic threshold of 1.2 V at 0.6 msec pulse width Because the battery is fixed at 5 V, energy is wasted By narrowing the pulse width to 0.3 msec, energy is preserved and a reasonable safety margin still exists

Narrowing pulse width also can be used to reduce annoying electric complications of pacing, such as pectoral muscle twitching or diaphragmatic stimulation, that occur in an occasional patient, although this approach is often not successful; lowering voltage is a better approach Before programming is done, the pacing system should be checked to ensure that no

malfunction requiring another method of correction is causing the difficulty Table 4-2

summarizes the use of pulse width programmability

Voltage Programmability

Modern pacemakers can be programmed only to lower settings below their maximum of 5 to

7 volts A high voltage threshold suggesting a need for a higher voltage indicates a very poor threshold that needs attention such as new lead Increasing voltage with a chronically poor threshold would maintain capture in a patient, but at the expense of rapid battery depletion High-voltage units consisting of two lithium iodine batteries in series have been available in the past, but are rarely, if ever, used now

Being able to lower the voltage setting by programming allows conservation of battery life, as demonstrated in Figure 4-3 In this example, the patient has a low chronic threshold of 1.3 V

at 0.6 msec pulse width By leaving the voltage at 2.5 V and pulse width at 0.6 msec, fewer electrons are used per paced beat and battery life is preserved It should be emphasized that this approach is useful only if the chronic threshold is low and if battery life is a significant clinical consideration in the patient Another approach to this patient would be to narrow pulse width, but within a range of pulse width of about 0.4 to 0.6 msec, voltage drop would be the more appropriate approach

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Figure 4-3 Lowering Voltage to Prolong Battery Life

This patient's chronic threshold is low and stable, and not all 5 V are required to pace the patient at a pulse width of 0.6 msec The low threshold is demonstrated by programming the voltage to 1.3 V and observing that capture is still maintained By leaving the voltage at 2.5

V, a safety margin is established and battery life is conserved

The voltage programmability along with pulse width programmability also can be used to determine threshold noninvasively These uses are listed in Table 4-3

Modern pacemakers can assess threshold noninvasively and almost continuously The term automatic capture refers to a beat-by-beat analysis of pacing capture, in order to pace just above the pacing threshold to maximize battery life Another term automatic threshold refers

to periodic evaluation of the pacing threshold in order to pace at a voltage lower than the usual 2 to 1 safety margin Modern pacemakers can check threshold every few hours and save

battery life and still maintain a reasonable safety margin Also, if the patient experiences an unexpected rise in threshold above the usual programmed, the threshold would be raised automatically thus serving as a safety feature

Sensitivity Circuit and Programmability

In Chapter 3, we discussed pacemaker sensing of the intrinsic frequency of the QRS complex, but clinically we estimate whether a QRS complex will be sensed by measuring its total height (positive and negative deflection) and, less commonly, the slew rate The QRS

complex generates a weak current in the pacing system and this signal then is amplified The amplification requires

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a modest amount of battery energy but not nearly as much as is required for pacing Many signals unrelated to the QRS complex, such as current due to pectoral muscle activity or external electromagnetic interference, may cause currents in pacing systems To reduce the possibility that these currents will be inappropriately sensed as QRS complexes, two

additional components are added to the sensing circuit One is a level detector to prevent low-level electrical noise from being sensed The other is a bandpass filter to help eliminate

stronger signals that are of a different frequency than those associated with the QRS complex

A schematic diagram of the sensing circuit is shown in Figure 4-4, and the principle of the bandpass filter is shown in Figure 4-5

TABLE 4-3 Uses of voltage programmability

1 Decrease voltage to preserve battery life (if clinically indicated and if chronic voltage threshold is low and stable)

2 Increase voltage to maintain capture in a patient with a high threshold

3 Use in conjunction with pulse width programmability to determine pacing threshold

noninvasively (points on the strength–duration curve can be obtained)

4 Lower voltage to reduce annoying electrical side effects of pacing such as pectoral muscle

or diaphragmatic stimulation (sometimes a successful maneuver)

The programmable feature of the sensing circuit is usually the level detector By causing the level detector to block weaker signals, the sensitivity of the pacemaker is lessened Such a programming change can be used to prevent inappropriate sensing of a T wave,

after-potential, or pectoral muscle activity (one hopes without loss of the intrinsic QRS signal) Alternately, by causing the level detector to pass on weaker signals, the sensitivity of the pacemaker can be increased The latter programming change can be used to allow sensing of unusually weak QRS signals; for example, a signal that was initially weak at the time of lead placement and became weaker with the decrease in amplitude and slew rate that normally occurs over time (due to fibrosis at the tip of the electrode) The P wave is generally a lower amplitude signal than the QRS and requires a more sensitive setting

Noise-Sensing Circuit

Although a noise-sensing circuit is not programmable, we mention it here because it relates to sensing The level detector and bandpass filter of the sensing circuit prevent many

inappropriate signals from being sensed, but they are not infallible, especially when signals from the environment are strong Therefore, many pacemakers have additional protection against electromagnetic interference (EMI) because mistaking EMI for a QRS signal will cause the pacemaker to stop firing

One method of protecting against EMI is to have a noise-sampling period in each cycle that listens for several repetitions of a signal within a few milliseconds Obviously, only one QRS signal can occur in a few milliseconds; if several signals are received (for example, at a rate of about 500/sec) the pacemaker is programmed to fire in an asynchronous mode regardless of the patient's intrinsic heart rate Although this program could cause the pacemaker to fire even

if the patient's rate was faster than the paced rate, this disadvantage is outweighed by the

advantage of preventing EMI from shutting off a pacemaker in a pacemaker-dependent patient Thus, in diagnosing the cause of inappropriate sensing by a pacemaker, the presence

of EMI must be considered In another method of protecting against EMI, the pacemaker constantly samples and disregards background noise so that only the single, prominent QRS signal is detected

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Figure 4-4 Sensing Circuit.

The amplifier, which uses a small amount of current to increase the weak sensed QRS signal (or in an atrial pacer, the P wave signal), is the first portion of the circuit The bandpass filter will filter out many frequencies not normally associated with an intrinsic electrical activity The rectifier causes negative deflections to be made positive The analog signal may then be converted into a digital signal External programming allows the level detector setting to be changed so that more signals can be detected or blocked, depending on whether sensitivity is

to be increased or decreased

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Figure 4-5 Bandpass Filter

The shaded area is filtered out by the bandpass filter, and only frequencies commonly

associated with the QRS complex are allowed to pass through the sensing circuit

Refractory Period Programmability

All pacemakers' timing circuitry includes a method of turning off the sensing ability for a brief period after either a sensed QRS signal or a pacemaker spike (or at least ignoring any sensed signals during this time) This refractory period is depicted in Figure 4-6 The

advantage of a refractory period is that it does not allow the pacemaker to sense either the T wave of the preceding QRS complex or the pacemaker after-potential, which is residual electric activity occurring after a paced beat Although the bandpass filter and level detector should prevent such inappropriate sensing, they are not foolproof The only disadvantage of such a circuit is that early premature ventricular contractions (PVCs) may not

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be sensed, but the advantage of eliminating a common source of inappropriate sensing is more important The physician who sees an unsensed early PVC on a pacemaker ECG must know what the refractory period is to know whether the pacemaker is responding appropriately to the PVC Note that in pacing, the term refractory period is used differently than it is in

electrophysiology

Figure 4-6 Refractory Period After Paced Beat.

During the refractory period, the sensing circuit of the pacemaker is programmed so that it will not respond to the QRS or the T wave of any beat or the after-potential of a paced beat (sensing may occur during the refractory period, but the pacemaker does not reset its timing cycle during the refractory period) The time of the refractory period is externally

programmable In some pacemakers, the refractory period after a paced beat is longer than after a sensed beat

Figure 4-7 Refractory Period Lengthening in an Atrial Pacemaker

A: The QRS complex falls outside the refractory period of the pacemaker and is sensed by the pacemaker, which causes the timing cycle to reset and results in a rate slower than the

programmed rate B: The refractory period has been extended beyond the QRS complex, and the pacemaker now fires at the programmed rate

The refractory period can be lengthened to eliminate inappropriate sensing of a QRS signal, T wave, or after-potential An example of lengthening the refractory period is shown in Figure 4-7 A demand atrial pacemaker has been placed and is programmed to fire at a rate of 72 bpm The QRS of the paced beat falls outside the refractory period and is sensed by the pacemaker, which causes the pacemaker to reset its timing cycle and therefore fire at a rate