Ruth Kam MBBS (Singapore), FRCP (Edin),
M Med Int Med (Singapore) FAMS, Testamur NASPExAM. Senior Consultant Cardiologist,
National Heart Centre, Singapore.
Address
for correspondence: Ruth Kam, Senior Consultant Cardiologist, National
Heart Centre, Singapore.
Email:
Ruth_KAM@nhc.com.sg
Introduction:
The concept of
a closed – loop feedback system, that would automatically assess pacing
threshold and self -adjust pacing output to ensure consistent myocardial
capture, has many appeals. Enhancing patient safety in cases of an unexpected
rise in threshold, reduced current drain, hence prolonging battery longevity
and reducing the amount of physician intervention required are just some
of the advantages.
Autocapture (AC)
is a proprietary algorithm developed by St Jude Medical CRMD, Sylmar, CA,
USA, (SJM) that was the first to commercially provide these automatic functions
in a single chamber pacemaker (Microny and Regency), and subsequently in
a dual chamber pacemaker (Affinity, Entity and Identity family of pacemakers).
This article
reviews the conditions necessary for AC verification and performance and
the problems encountered in clinical practice.
What is Autocapture?
The AC algorithm
comprises four fully automatic pacemaker functions:
1. Capture confirmation
2. Back-up high voltage pulse in case of loss of capture
3. Threshold search and documentation
4. Output regulation
Capture Confirmation
This is achieved
by the ability to detect the presence or absence of an evoked response (ER)
potential by the pacemaker circuitry. There exists 2 sense amplifiers in
AC pacemakers – a PR sense amplifier to sense spontaneous R waves and an ER
sense amplifier to sense evoked response. After the delivery of a pacing pulse,
the ER sense amplifier is blanked for 14 ms (so as to disregard the residual
polarization effects of the pacing pulse) and then open from 15 to 62.5 ms
in order to detect the ER. Accurate discrimination between the polarization
signal (PS), which is the result of the electrical charge that remains on
the lead tip after the pacing pulse has been delivered and the ER, which
is produced by local myocardial capture, is crucial for its determination.
A high lead polarization signal can produce so much “background noise” relative
to the ER signal that the pacemaker is unable to distinguish capture from
non-capture.
Back-up pulse after loss of capture
Patient safety
is enhanced by delivery of a 4.5 V/0.49 ms back-up pulse when no ER signal
is detected within the 15 – 62.5 ms detection window after the pacing pulse.
Threshold search
An automatic
threshold search is initiated under one of three conditions:
a. Whenever the pacemaker detects loss of capture in two
consecutive beats, which it interprets as a rise in prevailing threshold.
b. Every eight hours in the absence of (a).
c. Manually using a programmer or a magnet.
In the Microny
and Regency, the pacing output is reduced by 0.3V till two consecutive losses
of capture and back-up pulses are emitted. The output is then increased by
0.3V until there are two consecutive capture confirmations. This is taken
as the capture threshold.
For the dual chamber pacemakers like Affinity and Identity, during a threshold
search, the AV interval is first shortened to ensure there is no fusion
with intrinsic conduction (25 ms for sensed AV and 50 ms for paced AV delay).The
pacing output is reduced stepwise by 0.25 V until loss of capture and a back-up
pulse is emitted. After two consecutive losses of capture at the same output,
the output is increased by 0.125 volts until there are two consecutive capture
confirmations at the same output. This value is then taken as the stimulation
threshold. The pacemaker also documents each threshold result in the form
of a graph over time.
Output regulation
After determination
of the threshold, the pacemaker adjusts its output to stimulate at 0.3V (for
single chamber) and 0.25 V (for dual chamber) above the prevailing threshold.
It is important to emphasise that capture confirmation is occurring on a
beat-to beat basis and back –up pulse, threshold determination and output
regulation then follow accordingly.
What can impact proper operation of the AC function?
These variables
may be hardware-, implant-, patient-, or programming-specific.
Hardware- and implant-related factors
The use of a
bipolar lead is mandatory for AC function. The AC pacemaker paces in unipolar
mode, from tip to case,(to increase the signal to noise ratio) and senses
in bipolar mode, from tip to ring, (to minimize interference from muscle
activity).
Choice of a pacing
electrode with low lead polarization characteristics is essential. The most
important factor influencing polarization appears to be the type of coating
at the lead tip rather than the tip electrode size or the type of fixation.
Titanium nitride (TiN) coated electrodes have the lowest and platinum helix
(PH) electrodes give the highest polarization signals
1.
As a result TiN electrodes had a higher functional rate of AC (91.7%) versus
PH (0%).
Intraoperative
measurement of the ER signal during lead positioning has been recommended
by SJM in order to achieve the biggest ER signal relative to the PS so that
at the recommended ER sensitivity setting, the PS comprises less than 60%
of it. Unfortunately these measurements cannot be made with the standard
pacing system analyzer (PSA). The lead has to be connected to the pulse generator
and the system has to be interrogated with a sterile wand connected to the
APS II or APS µ-programmer. The ER signal must be at least 4.5 mV
and the polarization signal must not be more than 1.5 mV. There is also
no correlation between the spontaneous R wave as measured with the standard
PSA and the ER signal
2,3 .
If these intraoperative measurements are not made and the final lead position
is decided by the usual criteria, approximately 5 –7% of patients were found
to have inadequate ER signals and AC could not be activated
3,4.
Patient-related factors
For AC to be
activated, the pacemaker has to be able to initiate a threshold search.
For single chamber pacemakers, this is done by temporarily increasing the
pacing rate to 100 bpm or incrementally by 10 bpm up to a maximum of 120
bpm if the patient’s intrinsic rhythm inhibits the device. If the patient’s
heart rate is above 120 bpm, e.g. atrial fibrillation with rapid ventricular
response, the threshold test is postponed until the rate slows to the point
when pacing is established. If the rate is persistently high, so that an
automatic threshold search cannot be initiated, AC may have to be programmed
OFF
4 . One solution to this would be to slow
the rate with drugs. More importantly, good rate control would benefit the
patient symptomatically in terms of palpitations and heart failure, besides
ensuring AC function. For dual chamber pacemakers, this is usually not a
problem as the AV delay is temporarily shortened during a threshold search,
so as to avoid fusion beats.
Besides true
loss of capture, the AC pacemaker may misinterpret certain conditions as
loss of capture. This could be due to too low an ER signal (or inappropriate
ER sensitivity setting relative to it), micro-dislodgement of the lead, R
wave undersensing or pseudofusion beats.
If the ER signal is too small, repositioning of the lead can easily correct
the problem, provided this is known at the time of implant. If the ER signal
is not routinely measured during implant, and if increasing the ER sensitivity
cannot compensate for the small ER signal, then AC function cannot be enabled
subsequently.
Depolarisation
of the heart in fusion beats result partly from intrinsic conduction and
partly from the pacing pulse, so that an evoked response may sometimes be
seen. In pseudofusion beats, the heart’s intrinsic conduction system depolarizes
the myocardium entirely, so that when the pacing pulse arrives, the tissue
is refractory and no ER signal is detected, causing delivery of a back-up
pulse. After two consecutive pseudofusion beats, a threshold search is initiated.
When capture is confirmed, the output is then increased to 0.25 V above the
threshold. However if the output climbs to more than 4.2 volts, the pacemaker
will stimulate at high output mode (HOM) or 4.5 V at 0.49 ms. It will then
stay there until the next time a threshold search is initiated. The presence
of frequent pseudofusion beats will result in many threshold searches being
initiated. Some will give normal threshold values if pseudofusion beats
were not present during the search and some will end up in HOM if capture
losses were due to pseudofusion beats. This may not necessarily lead to
excessive energy wastage if the pacemaker is predominantly inhibited, but
it can cause confusion during follow-up especially if there are frequent
oscillations of threshold due to pseudofusion beats.
A fusion avoidance
algorithm, which is inherent in the AC pacemakers, resets the timing circuit
from the back-up pulse to slow the pacing rate and allow intrinsic beats
to come through. For dual chamber versions, this fusion avoidance algorithm
consists of extending the AV delay in the next beat following a back-up pulse,
by as much as 100 ms. An understanding of these algorithms is essential when
interpreting Holter recordings of patients with these types of pacemakers,
as these are often misinterpreted as pacemaker malfunction.
Programming related factors
R wave undersensing,
similarly, may cause the pacemaker to stimulate in the refractory period
of the ventricle and fail to capture, thus leading to many threshold
searches and possibly HOM. This problem can be easily corrected by increasing
the PR sensitivity appropriately.
In the case
of pseudofusion beats causing mistaken “loss of capture”, programming a
hysteresis rate at least 10 beats per minute below the lower rate will prevent
competition with the patient’s intrinsic rhythm and the presence of pseudofusion
beats. In the case of a dual chamber pacemaker with intrinsically conducted
beats, the “Autointrinsic Conduction Search” option can be turned on to
automatically prolong the AV interval when the pacemaker detects an intrinsically
conducted beat and thus avoid pseudofusion.
However, interactions
between these algorithms and the inherent fusion avoidance algorithms occasionally
lead to complex pacing behaviour that make ECG interpretation difficult for
the clinician
5.
Clinical Experience
Extensive clinical
experience has been obtained with the AC algorithm in several multicentre
studies. In both the European
2 as well as the
North American
6 studies, which followed patients
up to one year, the algorithm was found to be safe and effective and did
not result in exit block or other adverse events. If anything, the algorithm
tended to err on the side of safety, providing back-up pulses in all cases
where there was presumed loss of capture.
A case report
appeared in 2001, concerning a patient, in whom failure of emission of back-up
pulse after loss of capture, resulted in 8 seconds of ventricular standstill
7. This was later discovered to be due to a transient
threshold rise in the patient, which coincided with an automatic function
in the pacemaker, which measured battery current drain every 11.25 minutes
for 10 seconds, during which time the back-up pulse was not available. This
was finally rectified in that patient, using a software correction to reduce
the frequency of that measurement to once every 24 hours and provide an
automatic increase in output by 1V during the measurement. While this may
be an isolated report in the literature, it emphasizes the need for close
post-marketing surveillance of any new technology or features in any pacemaker
or defibrillator, before we embrace it fully or relax on our previous follow-up
practices.
Future Directions
Use with other leads without low polarization characteristics
Modification
of the pacing pulse has been shown in one experimental study to improve
the PS, such that pacing leads without low polarization characteristics could
have a better ER to PS ratio and hence, AC function could be enabled in a
higher proportion of these leads.
8 The pacing
pulse conventionally used is a biphasic pulse with an initial negative deflection,
followed by a rapid discharge in the positive direction to counteract the
effect of the polarization charge. Increasing the duration of the fast positive
second phase from 6 ms to 10ms significantly reduced the polarization voltage
in all leads but especially in leads with high polarization characteristics.
This has favourable implications for pacemaker replacements so that AC function
can be used with pre-existing leads from other manufacturers.
Use with epicardial leads
In paediatric
patients, epicardial pacing electrodes are frequently necessary because of
their small size and the need to preserve their veins for future use when
they reach a suitable size for transvenous implants. The higher pacing threshold,
when compared to endocardial leads, is of concern, because of higher current
drain and hence more frequent battery replacements. The use of steroid-eluting
bipolar epicardial leads from Medtronic with the Regency SR+ pacemaker was
evaluated and found to be feasible in 12 of 14 children (86%).
9,10
Use with implantable defibrillators
An acute study
11, looking at the feasibility of using the AC
algorithm in a group of patients undergoing ICD implantation, reported that
evoked response and polarization signal amplitudes using standard, single
coil, true bipolar ICD leads were sufficient to allow the AC algorithm to
function in 20 of 21 patients (95.2%). However, with dual coil ICD leads
and integrated bipolar sensing, AC could only be functional in 2 out of 9
patients (22.2%). Unfortunately, since the present AC algorithm requires
unipolar pacing, this is not recommended for use with ICD systems in case
it interferes with sensing of ventricular fibrillation.
Other threshold tracking algorithms
Although autocapture
is the first algorithm to provide automatic threshold tracking and output
regulation, other pacemaker manufacturers have also come up with competing
algorithms. These are Capture Control in Biotronik, Ventricular Capture Management
in Medtronic and Automatic Capture in Guidant pacemakers. There are important
differences between these algorithms.
With Biotronik,
ER detection also requires a bipolar lead with low polarization characteristics.
There is no back-up safety pulse during loss of capture and output is increased
in 2V steps if there is persistent loss of capture. After a programmable
length of time, the output is decreased to its original value to test if capture
can be obtained with a lower output. This kind of algorithm ensures safety
in unexpected threshold rise but will not improve battery longevity.
Other manufacturers
(Medtronic and Guidant) have devised alternative methods of capture verification,
which can operate with both unipolar and bipolar leads without low polarization
characteristics. These rely on modifications to the sense amplifiers to discriminate
between the ER and PS, but clinical experience with these algorithms is
far less extensive and has not been systematically reported to the same extent
as the original AC algorithm.
Atrial autocapture
Currently, the
AC algorithm is only available in the ventricle, but there is considerable
interest in applying it to the atrium as well. There are potentially more
difficulties with AC verification in the atrium because the atrial evoked
response is considerably smaller than the ventricle and differentiation
between the PS and ER signal is significantly more problematic. Early attempts
were made by Curtis
12, using a triphasic pulse
to minimize polarization signals. However the pacing circuitry itself would
consume so much energy as to negate any potential savings from reduction
of the pacing output. One practical approach may be to use the atrial evoked
response integral (“area under the curve”) and using bipolar sensing (tip
to ring) and unipolar pacing
13. Another approach,
using different pairs of electrodes for pacing and sensing (e.g. unipolar
pacing from atrial tip electrode and bipolar sensing from the atrial ring
electrode to the can or an indifferent electrode) has been described
14 but the disadvantage is that detection of the
atrial ER may be so delayed,due to the time needed for the impulse to propagate
to the sensing electrode, as to preclude timely delivery of a backup pulse
in the event of loss of capture.
Conclusion
Autocapture has
been shown to be safe and effective. However, to reap the full potential
of all its benefits, one must understand its limitations and pitfalls and
the ways to overcome these. “To implant and forget” is a dream not to be realized,
even with this advanced technology.
References
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