VectorExpress LV Automated Test Feature
VectorExpress™ LV Automated Test is a unique in-office programmer based algorithm that allows automated testing of clinician-selected pacing polarities. The clinician can test up to 16 LV pacing vectors, and then choose the LV pacing vector with the appropriate capture threshold and impedance to ensure capture and maximize device longevity while avoiding phrenic nerve stimulation (PNS). It is available with devices that support a quadripolar (4-electrode) LV pacing lead.
This feature can be found in some Medtronic CRT-D devices. Please go to manuals.medtronic.com or consult with your local Medtronic representative regarding device models available in your geography.
VectorExpress is an in-office only test. It is accessible from the Quick Look™ II screen -> under the Tests icon.
VectorExpress LV Automated Test conducts a pacing threshold search to determine the LV pacing amplitude threshold at the selected pulse width for each selected LV pacing polarity. As a baseline, the VectorExpress test measures the AP to RVS conduction time to find the intrinsic AV interval. As long as the AP to RVS conduction time is >50 ms longer than the LVP to RVS conduction time, the test can run in a DDD/R mode. It then uses the LV Pace to RV sense time to determine capture vs. loss.
- When an LV-only pace is delivered and an RV sense is detected shortly after the LV pace (V-V conduction), capture is confirmed.
- When an LV-only pace is delivered and a RV sense is not detected at the measured V-V conduction interval time, non-capture is confirmed.
A pacing threshold search is conducted for each selected LV pacing vector at an initial test amplitude of 2.5 V.
- If the 2.5 V test amplitude is above the pacing threshold, the device decreases the test amplitude in steps of 0.25 V until a test amplitude is classified as being below threshold, or until the minimum test amplitude of 0.25 V is reached.
- If the 2.5 V test amplitude is below the pacing threshold, the test amplitude is increased to 6.0 V. If the 6.0 V test amplitude is below the pacing threshold, the test reports that the threshold is above 6.0 V for the LV pacing polarity. Otherwise, the device decreases the test amplitude in steps of 0.5 V until a test amplitude is classified as being below threshold or until 3 V is reached.
A support cycle is delivered according to the programmed ventricular pacing configuration and outputs, to maintain pacing support during the threshold search. The support cycle is delivered after every test pace that does not result in capture. The last test amplitude that captures the left ventricle is the LV pacing threshold for that pacing polarity. If the results are inconclusive or too many intrinsic events occur, VectorExpress aborts testing on the current LV pacing polarity and proceeds to the next LV pacing vector.
VectorExpress 2.0, with LV electrical delay, available in Claria™ Quad CRT-D and Percepta™ Quad CRT-P devices measures the RV-LV electrical delay to each LV cathode from either a paced or sensed RV event. A prolonged RV-LV electrical delay has been associated with improved CRT response rates in multiple prospective, multicenter studies.1,2,3
Devices with VectorExpress 2.0
LVS timing is calculated based on the EGM signal between the LV cathode to RV coil corresponding to each measured electrode
Considerations for automated LV capture threshold and lead impedance measurement Pacing mode – VectorExpress LV Automated Test is available if the permanently programmed pacing mode is DDD, DDDR, or VVIR. Testing requires overdrive pacing of the patient’s heart rate, and the maximum test rate is 120 bpm. If the heart rate is too high, the test will not run. If the patient is programmed to either DDD or DDDR mode, and is not in an atrial arrhythmia at the test start time, VectorExpress will operate in DDD. Otherwise, it will operate in VVI mode. When the test operates in DDD mode, the maximum heart rate allowed is 105 bpm. When the test operates in VVI mode, the maximum heart rate allowed is 88 bpm.
Default values – VectorExpress LV Automated Test defaults to the LV output programmed pulse width and will test all 16 vectors by default. Claria™ and Amplia™ Quad CRT-D devices with dual cathode pacing (multiple point pacing) display an additional 5 pacing vectors. Pulse width and number of vectors tested can be adjusted.
Pacing threshold and safety margin – After performing a VectorExpress LV Automated Test and selecting the LV vector to be permanently programmed, make sure that the permanently programmed pulse width and amplitude parameters provide an adequate safety margin above the pacing threshold.
Measuring pacing thresholds – The LV thresholds may be measured automatically with the VectorExpress LV Automated Test or manually with the Pacing Threshold Test. Adequate safety margin should be applied to the threshold for the selected pacing polarity.
Tachyarrhythmia detection suspended – Tachyarrhythmia detection is suspended during a VectorExpress LV Automated Test. However, the test monitors for suspected tachyarrhythmias. If a ventricular tachyarrhythmia is suspected or if an atrial tachyarrhythmia is suspected and the test is operating in DDD mode, the test is aborted and tachyarrhythmia detection is resumed.
VectorExpress LV Automated Test - Example
Select Tests > VectorExpress.
Select the test value for Pulse Width, or accept the displayed value.
Select one or more sets of LV pacing polarities to test, or accept the displayed and testing default (16 vectors) selections. Testing all 16 vectors is expected to take 2-3 min. It is recommended that all 16 vectors be tested at implant. If one group of vectors does not provide any acceptable values or is known to cause phrenic nerve stimulation, then this group can be deselected prior to initiating the test. This would result in shorter test duration.
Select [START Test]. During testing, the amplitude, pulse width, and amplitude decrement being tested are annotated on the live rhythm displayed on the programmer screen and recorded on the strip chart. In addition, a test progress bar and the pacing polarity being tested are displayed on the test screen.
If necessary, abort the test by selecting [STOP Test].
When VectorExpress LV Automated Test is completed, the LV Test Results are displayed automatically.
LV Test Results - Example
Test results, including relative longevity, capture threshold, and the most recent successful impedance measurement for each LV pacing polarity are reported in a single row.
All devices have the standard programmable vectors.
Claria™ and Amplia™ Quad CRT-D devices have 5 additional LV pacing vectors available in addition to the standard 16 LV pacing vectors to provide more options for CRT non-responders.
Calculated relative longevity impact if the vector was used.
Capture threshold test results.
Impedance test results.
Phrenic nerve stimulation test results as determined by additional manual testing. The results are displayed as Yes (along with the PNS threshold as measured), No, or Not Tested.
Edit the test results of the selected (highlighted) pacing polarity. Capture threshold test results can be edited at any time after they have been conducted. Phrenic nerve stimulation test results can be edited whether tests have been conducted or not. Relative longevity and impedance test results cannot be edited. However, if you edit capture threshold results, the relative longevity results may change. Permanently programmed parameters are not affected by editing test results.
Test results can be sorted by LV Pace Polarity or Relative Longevity.
Filter the test results by LV electrode or best LV Electrical Delay, if available. This filter option is only available in devices with VectorExpress 2.0.
This is the programmed LV Pace Polarity. Opening this field allows selection and permanent programming of the desired LV Pace Polarity.
This is the programmed LV output for LV Amplitude and LV Pulse Width. Opening this field allows selection and permanent programming of the desired LV amplitude and pulse width.
Print the LV Test Results Report before ending the patient session if you need to retain the information in the patient's file as they are deleted when the patient session is ended. The report prints the results in the order in which they are sorted in the LV Test Results window. If an information symbol [i] is displayed under Capture Threshold results, then "- - -" is displayed on the printed report. Explanations for incomplete capture threshold measurements are not included on the printed report.
Open this information button to display explanations for tests that are unsuccessful and marked with the [i] Information symbol.
Select a LV Pacing Polarity to manually test for phrenic nerve stimulation. PNS testing is not a part of the automated VectorExpress test. The preferred vector(s) can be selected from the LV Test Results screen and then the PNS test can be run manually on the selected vector(s). The LV Test Results will be updated with the PNS testing even if it the PNS test is accessed via the Pacing Threshold Test screen.
Phrenic Nerve Stim Test
The Phrenic Nerve Stim Test can be accessed by opening the Tests icon then selecting Pacing Threshold or from the Test button on the bottom of the LV Test Results page.
The Phrenic Nerve Stim Test is only available if you select LV as the Chamber. The default Test Type is Amplitude with a decrement after 3 pulses, with the default output set to the permanently programmed amplitude and pulse width.
Select the starting Test Values for Pace Polarity, Mode, Lower Rate, AV Delay, Amplitude, and Pulse Width, or accept the displayed values. After each manual test is completed, a check mark is displayed beside the pacing polarity to help you manage the testing process.
Press and hold [TEST Press and Hold]. Observe the patient for the presence of phrenic nerve stimulation. When phrenic nerve stimulation is lost or not detected at all, immediately release [TEST Press and Hold]. The device resumes its original pacing values and displays the Test - Results window for the selected Test Type.
To view the LV Test Results for this patient session, select the LV Test Results [>>] button displayed on the Pacing Threshold screen after you complete the first test.
LV Phrenic Nerve Stim Threshold Test – Results
To view a test strip from the Phrenic Nerve Stim Threshold Test, select the Test Strip icon in the lower-left corner of the Test - Results window.
Accept the Phrenic Nerve Stim Present and Threshold test results displayed or select the fields and modify the values as appropriate for the patient.
To print an LV Phrenic Nerve Stim Threshold Test report, select [Print…].
Select [Close] to exit the Test - Results window. You will return to the Pacing Threshold test window. You can choose to test additional vectors for PNS or select LV Test Results to return to the LV Test Results screen for your up to date test results. All manual LV threshold or LV PNS testing is automatically updated into the LV Test Results.
Information provided by LV Test Results
For devices that support a quadripolar (4-electrode) LV pacing lead, the LV Test Results window reports pacing threshold and phrenic nerve stimulation threshold test results, the most recent successful impedance measurement, and the relative impact of test results on device longevity for each LV pacing polarity tested. Results for polarities not tested are not displayed. If you repeat a test, the new test result replaces the previously reported result. Polarities tested are sorted in LV Pace Polarity order, LV1 through LV4, followed by multiple point pacing (MPP) when MPP is programmed On. You can also sort polarities tested based on the order of Relative Longevity, from maximum to most reduced longevity. The test result order is updated automatically each time you test or retest with either a manual Pacing Threshold Test or VectorExpress LV Automated Test.
Calculated Relative Longevity indicates the impact of the pacing threshold and impedance test results on device longevity for the selected LV pacing polarity. The calculation of Relative Longevity is based on current battery status and on typical (not actual) parameter settings and device events. It includes a 1.5V ventricular safety margin.
- "Maximum" is displayed if the calculated device longevity is the maximum for the pacing polarities tested.
- Reduced device longevity, relative to the pacing polarity with the maximum calculated device longevity, is displayed in months less or years less than the Maximum.
- "- - -" is displayed if relative longevity could not be calculated because capture threshold measurement or lead impedance measurement are not available, lead impedance is <200 Ω or >3000 Ω, or battery status is reinitializing. RRT is displayed if the device has reached Recommended Replacement Time.
Remaining vs. Relative Longevity
- If a negative delay such as (-8 ms) is reported, the LV sense preceded the RV sense by the amount of time indicated.
- If the patient has prolonged AV intervals such that an RV pace occurs, the RV pace to LV sense is the measured value.
LV Electrical Delay measurements are taken at the start of the VectorExpress LV Automated Test execution. Larger LV electrical delays indicate greater electrical separation of the RV and LV electrodes. The LV pacing vector selection can be simplified by filtering the test results to eliminate vectors with shorter electrical delays from the LV Test Results. LV Electrical Delay, Relative Longevity, and phrenic nerve stimulation test results are all factors to consider when selecting an appropriate LV vector.
Pacing Programming Access
LV Pace Polarity, LV Amplitude, and LV Pulse Width can all be programmed directly from the LV Test Results window. The permanently programmed values of each are displayed.
The Viva™ Quad XT CRT-D example below displays a rhythm strip recording of an in-office VectorExpress test. The test begins with an AV conduction test and then proceeds to testing the LV pacing vectors. The first 2 vectors tested are shown in this example.
- The test starts with an AV conduction test. A long PAV (320ms) is used to observe the time it takes an A pace to conduct to the RV.
- Support cycles are delivered using normal pacing. During these cycles the pacing impedance is measured. This example starts with LV1-RVcoil vector.
- The initial test pace cycles for the vectors are delivered at 2.5V. The test will perform up to 3 attempts to obtain 2 consistent LV-RV conduction times. In this example, a PVC intercedes but the test is able to ignore it and continue.
- Once the LV-RV time is established, the test pace amplitude begins to decrement and looks for capture. This cycle has a consistent LV-RV conduction time, indicating capture.
- This cycle has a very long LV-RV conduction time, indicating loss of capture at 2.0V. A 2.25V threshold is reported for this vector.
- The next vector (LV1-LV2) measurement starts with 2 support cycles. During these cycles, the pacing impedance is measured.
- The test paces start at 2.5V. Notice there is clear loss of capture (see the late RVS). Therefore, no second cycle at 2.5V is needed.
- A support cycle is delivered before moving on to test the higher voltages.
- Loss of capture occurred at 2.5V (#7), therefore output increases to 6V. The test will perform up to 3 test pace attempts to obtain 2 consistent LV-RV times. This example illustrates consistent capture in 2 cycles.
- Once the LV-RV time is established, the test pace amplitude decrements and looks for capture. In this example each cycle captures, therefore, the test amplitude output continues to decrement until loss of capture is identified (not shown in this example).
Accuracy of an Automated Pacing Capture Thresholds Algorithm for Quadripolar Left Ventricular Lead4
Figure 1: Differences in Pacing Capture Threshold (PCT)
The figure above presents the distribution of the difference between the manual pacing capture threshold test and VectorExpress. The Y axis represents the frequency or the number of vectors tested.
- At least one vector was measured in 62 of 65 patients.
- Thresholds were measured for 780 of 992 of vectors.
- The agreement or match between VectorExpress and manual PCT was high (95.3% accuracy).
"Match" defined as:
- Within ± 0.5V if manual threshold < 2.5V
- Within ± 1.0V if 6V ≥ manual threshold ≥ 2.5V
This confirms that VectorExpress produced accurate measurements and can be useful clinically.
VectorExpress 2.0 Clinical Evidence
VectorExpress 2.0 is based on the premise that pacing from the site of latest activation may improve patient outcomes and the CRT responder rate. A prospective, observational, multicenter study showed that patients had a greater response rate with an intrinsic RV-LV electrical delay of greater than 80 ms.1
Figure 2: Distribution of positive responses to CRT in the groups below and above the cut-off value of RV-to-LV interval (A) and RV-to-LV interval corrected for the QRS duration
Gold et. al demonstrated that the intrinsic RV-LV interventricular delay was a potent independent predictor of improved remodeling and quality of life (QOL) responses in patients receiving CRT. A subset of 419 patients enrolled in the SMART-AV trial was included in this RV-LV substudy. Figure 3 below illustrates that all 3 echocardiographic remodeling measures, LVESV, LVEDV and EF, as well as QOL were significantly improved as RV-LV time increased.2
Figure 3: Comparison of the Changes in LVESV, LVEDV, EF and QOL From Implant Baseline to 6 Months for the 4 RV-LV Quartiles
- D'Onofrio A, Botto G, Mantica M, et al. Incremental value of larger interventricular conduction time in improving cardiac resynchronization therapy outcome in patients with different QRS duration. J Cardiovasc Electrophysiol. May 2014;25(5):500-506.
- Gold MR, Singh JP, Ellenbogen KA, et al. Interventricular Electrical Delay Is Predictive of Response to Cardiac Resynchronization Therapy. JACC: Clin Electrophysiol. In press; 2016
- Stabile, G., D'Onofrio A, Pepi, P. et al. Interlead anatomic and electrical distance predict outcome in CRT patients. Heart Rhythm. November 2015; 12 (11) 2221-2229
- Johnson B, et al, Accuracy of an Automated Pacing Capture Thresholds Algorithm for Quadripolar Left Ventricular Leads. HRS 2014, Poster presentation, PO04-185.
Sources: Viva™/Viva Quad/Brav™/Brava Quad Reference Manual, Claria MRI™/Claria MRI Quad Reference Manual, Percepta™, Serena™, Solara Reference Manual.