Biventricular Pacemakers Sensing

Biventricular Pacemakers Sensing
Biventricular Pacemakers Sensing
Sergio Dubner
Director Arrhythmias and Electrophisiology Service,
Clinica y Maternidad Suizo -Argentina,
Buenos Aires, Argentina
Left ventricular pacemaker lead technology was evolving and different types of pacemaker lead were
successively used. Sensing was modified according with this evolution, starting with initial experience that
utilized epicardical left ventricular pacing through leads positioned at limited thoracotomy [1] . In other
experiences a lead specifically designed for left atrial pacing via CS was used. This tine-free lead has a
bipolar, coaxial polyurethane-coating with a 5.8 mm non-steroid eluting canted electrode tip [2-3]
Another step was a tine-free unipolar polyurethane coated coaxial lead designed specifically for left ventricular
pacing via the coronary sinus (figure 1). To aid with CS cannulation, a specifically designed guiding sheath
was employed. This sheath is pre-shaped to facilitate CS entry, thus allowing pacemaker lead placement
through its lumen. After pacemaker lead positioning the sheath can be pulled back and split externally along
its entire length to allow separation from the underlying pacing lead. [4,5] . In addition the lead used was of a
novel design with a terminal adaptation allowing lead passage over a pre-positioned guidewire (side-wire
pacing leads) [6] .
Leclerck et al used a modified transeptal technique more complex and without long term follow-up
[7,8]
Right atrial bipolar lead
RA connector (IS-1)
Right ventricular bipolar
lead
RV connector (IS -1, DF-1)
LV unipolar (IS -1)
CS connector
(programmable)
Figure 1 shows the different position and polarities of
atrial and left and right ventricular leads
Although at the present time, most of the pacemakers have a three lumen head (with different type of sensing
functions) during initially experiences standard DDDR devices were employed where the left ventricular lead
was connected to the atrial port of the device and the right ventricular lead was connected to the ventricular
port. Setting the AV delay to its minimum near simultaneous biventricular capture could be achieved. Unipolar
or bipolar pacing and sensing could be used dependent on the pacemaker lead utilized. [9]
In the PATH-CHF study [10,11] 2 DDDR pacemakers were implanted in all patients. The first pacemaker was
implanted and connected to a right atrial bipolar lead and to a unipolar right ventricular lead. The second
pacemaker was implanted and connected to a second bipolar right atrial lead and a unipolar ventricular
epicardial lead. Atrial sequential biventricular pacing could be initiated by setting one pulse generator to DDD
mode while the other was programmed in VVT mode. Sensing could be adjusted according to each individual
lead.
Another step was the use of a Y connection for right and left ventricular leads in a single DDDR pacemaker,
with simultaneous pacing and sensing in both ventricles.
Actually, a three chamber pacemakers with three lumen head are used and sensing vary from different
models: all of them have bipolar (programmable to unipolar if required) sensing in right atrium; but some of
them have unipolar sensing at the left ventricle independent of the RV sensing (usually bipolar); some have
bipolar sensing between the RV and the LV tip lead and some of them RV-only sensing, as shown in table I.
Sense configurations are shown in table II
SENSITIVITY
Overview
If a cardiac signal of sufficient amplitude and morphology occurs during the sensing period, the pacemaker
output will be inhibited or triggered depending upon the mode selected. The sensing circuit is specially
designed to reject extraneous signals while sensing P waves or R waves.
Sensitivity determines the minimum intracardiac signal that the device can detect when intrinsic atrial or
ventricular events occur. The higher the mV value, the lower the sensitivity. When senstivity is programmed to
a very sensitive setting (a low value) the device may detect signals unrelated to cardiac depolarization
(oversensing: eg. sensing of myopotencials). When sensitivity is programmed to a less sensitive setting (a
higher value) the device may not detect the cardiac depolarization signal (undersensing). Sensitivity must be
programmed to a value that prevents sensing of extraneous signals, but ensures accurate sensing of intrinsic
cardiac signals. Intrinsic atrial signals are typically smaller than ventricular signals, so lower sensitivity settings
are typically programmed for the atrium.
Left ventricular signals are typically smaller than right ventricular signals. In some devices this is an
independent value but in others, the measurement is a combination of signals from the left and right
ventricles and ventricular signal may be attenuated (reduced). So, sensitivity settings should be
programmed according this situation.
Whether selecting sensing parameters at implant or verifying sensing at follow-up, the same considerations
apply:
a. Select sensing polarity for leads
b. Determine sensing thresholds
c. Select appropriate sensitivity settings
A. Selecting Sensing Polarity
Atrial and ventricular sensing polarities can be programmed for each chamber when used with bipolar leads.
Bipolar Sensing Polarity - The lead tip and the lead ring electrode are the poles of the sensing circuit.
Because bipolar sensing is more localized, it reduces the likelihood of sensing myopotentials and
electromagnetic interference. It may also permit sensitivity to be programmed to a more sensitive setting.
Unipolar Sensing Polarity - The lead tip (occasionally the lead ring) and the noninsulated stimulator case
are the sensing electrodes. Unipolar sensing may allow sensing of smaller intrinsic signals than does bipolar
sensing and therefore, can be selected when intrinsic cardiac signals are difficult to detect with bipolar
sensing. Oversensing due to myopotentials is more common with unipolar sensing than with bipolar sensing.
Bipolar RV/LV Sensing Polarity - In the LV-1 lead, the electrical activity will be sensed between the lead tip
and the right ventricular lead ring. The distance the stimulus travels between the lead tip and the right
ventricular lead ring will be affected by the size of the heart. The greater the distance, the more prone the
device is to sensing myopotentials.
Bipolar Sensing Polarity Confirmation
Before programming from unipolar to bipolar sensing, some of the programmer verifies the presence of a
functioning bipolar lead by testing impedance for the lead. Testing is done under magnet operation for four
seconds at an Amplitude of 5.0 V and a Pulse
Width of 1.0 ms if the permanent settings are at or below this level.
If permanent settings are above these values, the measurement will be made at the permanent settings.
If bipolar lead impedance is between 200 ohms and 3000 ohms, a bipolar lead is assumed to be present.
If bipolar lead impedance is outside this range, a unipolar lead is assumed to be present. The programmer
warns that the test failed, and sensing polarity remains set to unipolar.
This interlock feature may be overridden and lead sensing polarity forced to bipolar.
Impedance for the ventricular two-lead system is measured across the parallel combination of both leads in
most devices. However, if pairing a ventricular lead with a polished platinum tip electrode with a lead with a tip
electrode of a different material may create a source impedance mismatch that could adversely affect
sensing.
B. Determining Sensing Threshold(s) at Implant
Before connecting a lead, implanting physician should measure the sensing potentials in the unipolar and the
bipolar configurations. Adequate intracardiac signal should be present in both configurations to ensure proper
sensing in either.
Verifying Sensing Threshold(s) at Follow-up
Intracardiac signal amplitudes decrease during the lead maturation process. Most programmers provide an
automatic sensitivity test that allows the follow-up clinician to verify a patient's sensitivity settings. The
automatic test provides for atrial or ventricular monitoring. The test provides the sensitivity setting just above
and below the point at which P waves or R waves are sensed.
The cardiac signal presented to the stimulator by the ventricular two-lead system is a composite
signal from the parallel combination of both ventricular leads in some devices. The sensing test treats
this signal as a single input with a measurable amplitude that can be used to determine an appropriate
setting for ventricular sensitivity. Usually this signal may be attenuated (reduced) in the biventricular
configuration. Conducting the Sensing test for the ventricular two-lead system does not require any
special considerations.
C. Selecting Sensitivity Settings
Atrial and ventricular sensitivity are independently programmable. In general, a 2:1 to 3:1 sensitivity safety
margin (threshold sensitivity value divided by 2 or 3) is adequate for newly implanted or chronic leads. For
example, an atrial sensitivity of 1.0 mV should be satisfactory for intrinsic atrial signals between 2.0 mV and
3.0 mV.
Ventricular sensitivities 1.0 mV or 1.4 mV with wide atrial pulse widths or high atrial amplitudes may result in
Ventricular Safety Pacing (if On) with some lead systems at high sensor-driven pacing rates. Reprogramming
Ventricular Sensitivity to a less sensitive setting (higher numerical value) is one option under such
circumstances. Other options include programming a longer Ventricular Blanking Period.
Excessively sensitive (low) settings can cause some or all of the following problems:
- oversensing due to electromagnetic interference (EMI), myopotentials, T waves, or crosstalk
- undersensing due to overloading of the sensing circuit
- noise reversion operation
Effects of Myopotentials During Unipolar Pacing
Myopotentials can affect device operation when sensing polarity is unipolar, especially with atrial sensitivity
settings of 0.5 through 1.0 mV and ventricular sensitivity settings of 1.0 and 1.4 mV.
Myopotentials sensed on the atrial channel outside the total atrial refractory period (SAV + PVARP) start
sensed AV intervals in the DDDR, DDD, and VDD modes.
Continuous myopotentials cause reversion to asynchronous operation when sensed in the refractory period:
- on the ventricular channel at intervals less than the ventricular refractory period in the DDDR,
DDD, DDIR, DDI, DVIR, DVI, VDD, VVIR, VDIR, VVI, VDI, and VVT modes,
- on the atrial channel at intervals less than the atrial refractory period in the AAIR, ADIR, AAI,
ADI, and AAT modes.
In the VVIR and VDIR modes, the resulting asynchronous pacing occurs at the Lower Rate,
otherwise such asynchronous pacing occurs at the sensor-indicated rate for rate response
modes or the Lower Rate for non-rate response modes.
Refractory Periods
Pacemakers with a sensing mode incorporate a programmable parameter known as the refractory period
The refractory period is the interval following a paced or sensed event during which the device is not inhibited
or triggered by detected electrical activity. The purpose of the refractory period is to prevent resetting of the
pacemaker timing cycles in response to known but physiological inappropriate electrical signals.
Pacing refractory periods prevent certain pacing timing intervals from being started by inappropriate sensed
signals such as far-field R-waves or electrical noise. Synchronization refractory periods help prevent delivery
of pacing or shock pulses during the atrial and ventricular vulnerable periods
In some cases, the programmer will automatically select refractory periods suitable to the programmed
parameters.
In the AAT, AAI and AAIR modes, the Atrial Refractory Period (ARP) is defined as the time period after an
atrial event, either paced or sensed, when activity in the atrium does not inhibit or trigger an atrial stimulus. It
prevents inappropriate atrial inhibition due to sensed far-field R-waves or electrical noise. The first portion of
the ARP is a blanking period that disables atrial sensing.
Ventricular Refractory Period (VRP): Is defined as the time period following a ventricular event, either paced
or sensed, when sensed electrical activity in the ventricles does not inhibit the device. This parameter is
available in any mode in which ventricular sensing is enabled and the interval is programmable in all
biventricular devices but not in all ICD-biventricular devices.
Use a long ventricular refractory period shortens the ventricular sensing window. Programming the ventricular
refractory period to a value greater than PVARP can lead to competitive pacing.
In dual chamber pacing modes, the Post-Ventricular Atrial Refractory Period (PVARP) controls how the device
responds to retrograde P-waves. It is defined as the time period after a ventricular event, either paced or
sensed, when activity in the atrium does not inhibit an atrial stimulus nor trigger a ventricular stimulus. It is
designed to prevent the atrial channel from sensing the ventricular pacing pulse, the far-field R-waves or
retrograde P waves.
The Post-Ventricular Atrial Blanking (PVAB) period acts as the minimum PVARP value. Sensed atrial events
that fall within the PVAB period are ignored by the Mode Switch, PVC Response, PMT intervention, and
NCAP features, but are used for dual
Chamber SVT Criteria
Blanking Periods
Blanking is the first part of the refractory period, where sense amplifiers are completely disabled. It is used to
prevent cross-chamber sensing and inhibition.
During a blanking period the device does not sense electrical signals. Blanking periods avoid sensing ICD
outputs, post-pacing polarization, T-waves, and multiple sensing of the same event.
During a blanking interval, the sensing circuit in one chamber ignores sensed electrical activity generated by a
device pulse in the other chamber (cross-talk).
The blanking periods following paced events are longer than those following sensed events to avoid sensing
the depolarization signal on the electrodes.
Some devices to enhance sensing and detection during tachyarrhythmias, do not cross-blank (blank sensing
in the opposite chamber) after a sensed event.
During the programmable "Post-Ventricular Atrial Blanking" (PVAB) period, the atrial sensing circuit is
active.
Ventricular blanking after atrial pace: In the ventricles, the atrial pace concurrently starts a retriggerable
noise rejection interval and a programmable ventricular blanking interval.
Atrial blanking after ventricular pace: In the atrium, the ventricular pace concurrently starts a retriggeable
noise rejection interval and a programmable atrial blanking interval
Noise rejection
The pulse generator´s noise response functions designed to protect the patient against inappropriate inhibition
of the pulse generator due to detected rapid electrical interference signals or "noise". Noise rejection works
through the programmable refractory period, that is composed of two segments: the absolute refractory period
and the relative refractory period. In the absolute refractory period no signals will be detected. Any electrical
signals occurring within the relative refractory period will initiate a noise-sampling window beginning at the
point where the noise signal was detected.
Hence, in the presence of continuously detected electrical noise, the refractory period will be repeatedly reset
until the escape interval is complete, resulting in asynchronous pacing at the programmed mode and base
rate. The pacemaker will continue monitoring for noise. When noise is no longer detected, the pacemaker will
resume normal operation at the programmed parameters.
PACING: In both, the atrium and the ventricles, a pace concurrently starts a fixed noise rejection interval
followed by a programmable retriggerable noise rejection interval in the paced chamber.
SENSING: when an atrial depolarization is sensed, a noise rejection interval is started in the atrium. This
interval is retriggered in the continued presence of noise.
When a ventricular depolarization is sensed, a noise rejection interval is started in both the atrium and the
ventricles. This interval is retriggered in the continued presence of noise.
ADVANTAGES, PITFALLS AND SOLUTIONS
Sensing in RV only or in both ventricles, together or separately, during pacing or in sinus rhythm should
modify the response of the system. The differences of these situation are explained as well as double
counting of T waves and farfield noise.
UNDER DETECTION DUE TO SPONTANEOUS VENTRICULAR BEATS IN RV-ONLY SENSING: A system
that senses RV-only do not see any beat (ectopic) that is originated in the Left Ventricle and may stimulate it
over the refractory period. On the other hand, any intrinsic ventricular action inhibits delivery of a ventricular
pulse in the DDD mode. The ventricles are not brought into synchrony with each other.
OVER DETECTION DUE TO LV+RV SENSING: A system that senses both the RV and LV leads in the
presence of a QRS>130 msec may double sense a sinus rhythm contraction. This situation is true only when
the patient's P-R interval is shorter than the programmed AV delay and/or there are no pacing beats at the
ventricles. A similar situation could be seen if the sensing delay of both ventricles is greater than the
ventricular refractory period (programmable in ICD but not in all the pacemakers).
However, in both cases there are no RCT due to the fact that there are no pacing beats.
SOLUTIONS: Although optimal performance of biventricular devices' sensing has yet to be determined, some
companies modified the sensing system to avoid these problems and other introduced de DDT(R)/V mode
and the LV protection period.
The DDT(R)/V mode has been specially designed to ensure biventricular synchronization. Is a permanent
rate-adaptative pacing mode that represents a combination of the DDDR mode with a VVT mode. In DDT
(R)/V mode, the pacemaker triggers a ventricular pace when the AV interval completes without sensing an
intrinsic ventricular event. Furthermore, intrinsic right and left ventricular senses trigger a biventricular pace
within 10 msec. So, the pacemaker response in the ventricular channel corresponds to a VVT mode, and its
atrial or its AV-sequential behavior is analogous to a DDD mode. The clinical benefit of the DDT(R)/V mode is
based on the ability to resynchronize both ventricles even during ventricular sense events.
LEFT VENTRICULAR PROTECTION PERIOD: Sensing on the LV could avoid a pace over the refractory
period after a PVC's. The following example could help to understand the case: in the presence of a PVC's
originated in the lateral wall of the LV, if the device senses only the RV will not see the ectopic beat and will
pace the RV and then the LV (that will be during the vulnerable period). To avoid this situation, some devices
sense the LV and let you to program the refractory period during which there are no chance to stimulate the
LV if there was a sensing activity.
EXTENDED SENSING OPTIONS
Some devices has additional features to optimize the sensing functions
MANUAL OR AUTO-ADJUSTING SENSITIVITY THRESHOLDS:
Especially in cases of T-wave oversensing, the ventricular thresholds can be modified. In some devices, the
ventricular thresholds ca be manually reduced to different levels (75, 50, 37 or 25%) and a second and third
halting periods can be adjusted. Others, with auto-adjusting capacity, thresholds increase dramatically and
then gradually return to their programmed values, having been adjusted by the preceding sensed or paced
event, as shown in figure 3.
Figura 3
EARLY FARFIELD TOLERANCE: This parameter declares all atrial signals that occur just before the QRS
complex and are within the range of tolerance to be farfield signals. When necessary, this parameter allows a
1:1 rhythm to be classified as such without incorrectly assuming 2:1 atrial flutter.
ATRIAL BLANKING AFTER VENTRICULAR SENSE: If QRS farfileds are observed even though the atrial
lead used has a short electrode distance, Ablank after Vsense can be activated. The atrial blanking period
should kept as short as possible, it should be set only as long as necessary for each individual. An additional
safety measure for the scenario described is the programmable "INCREASED ATRIAL STARTING
SENSITIVITY" directly after Ablank after Vsense expires
References
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1226-28
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