Background
Left Ventricular Assist Device (LVAD) therapy has become a well-established treatment
option for endstage heart-failure either as a bridge to transplant (BTT) or destination
therapy (DT). Currently, continuous flow LVADs are operated at a constant rotational
speed, which results in a fixed relation between the difference between pump inlet and
outlet pressure (=head pressure) and pump flowrate. This results in a cardiac support,
which is not very adaptable to physiologic demand or preload changes due to reduced
venous return and subsequent preload reduction e.g. due to hypovolemia. Nonetheless it
could be shown, that optimal setting of LVAD support is essential to optimize patient
outcomes.
Proper measurement for cardiovascular function under LVAD support is therefore of key
importance, especially as unwanted pumping conditions such as "overpumping", causing a
collapse of structures of the left ventricle (LV), also referred to as suction. This
could lead to suction induced arrhythmia (or tachycardia) as well as impairments to the
right ventricle (e.g. tricuspid valve insufficiency) due to a ventricular septum shift.
Ultrasound and cardiac catheterization are the standard clinical methods for the
evaluation of morphological and functional properties of the heart, and also eventually
changing properties of the aortic valve. Further diagnostics include conventional ECG
analysis but also heart rate variability to obtain information of the neurohumoral
status. Exercise stress tests and recently developed activity diagnostics give further
important information about overall recovery and quality of life.
All these methods however require time-consuming procedures, being unacceptably
cumbersome for frequent monitoring. On the other hand, a frequent and effective
noninvasive evaluation of the cardiac status provided by the rotary blood pump (RBP)
itself would have a remarkable impact on LVAD patients and their therapy, especially for
the evaluation of their overall and cardiac recovery.
Previously, pump signals from another centrifugal blood flow device (HVAD Medtronic
Inc., Minneapolis, USA) have been used to estimate hemodynamic variables noninvasively.
These were used to track patient progress, or to detect unwanted pumping states, such as
ventricular suction. Variables such as contractility, relaxation, suction, Aortic Valve
Opening and cardiac rhythms, can be extracted. These may aid clinicians in their
decisions on the level and strategy of pump support. While this is not currently widely
implemented in clinical practice, this type of monitoring holds the promise of deeper
insights into patient physiology. As demonstrated in a recent study, continuous
high-resolution HVAD monitoring sheds light on suction occurrences. Interindividual and
intraindividual characteristics of longitudinal suction rates were observed. Longer
suction clusters have higher probabilities of tachycardia within the cluster and more
severe types of suction waveforms. This work showed the necessity of improved LVAD data
monitoring.
The Abbott Inc. HeartMate 3 LVAD is a centrifugal continuous flow pump with a fully
magnetically levitated rotor enabling frictionless movement, textured blood-contacting
surfaces to establish a tissue interface with blood and wide blood flow gaps between the
rotor and its housing to lower shear stress and consequently blood trauma.
Monitoring of the pump and with this the cardiac status with the HeartMate 3 is
currently very limited to infrequent log-files with one data entry every 15 minutes and
only limited amount of entries. Due to the low resolution data, the standard HM3
monitoring is not feasible for the evaluation of suction events or in depth analysis of
the interaction between LVAD and the remaining native heart function.
Study protocol The developed new methods are based on pump flow, pump motor and magnetic
levitation data only. These methods include also the evaluation of heart rate
variability and arrhythmias and its influence to HeartMate 3 suction events.
In this study non-invasive recordings of pump data will only be performed at rest, with
speed changes under guidance of echocardiography on occasion. Beat-to-beat/High
resolution pump data recordings for the HeartMate 3 have not been performed before, thus
the recorded pump data will be analyzed and compared to clinical diagnostics in this
pilot study. Previous feasibility and safety tests for the pump data recording have been
performed in the laboratory setting. The pump data recording is used for research only
as observational study and not for diagnostic purposes.
The pickup of high resolution data is done with a coil mounted outside on the driveline
with only one contact (common ground) to the exposed metal housing of the driveline
connector. Therefore, the only metal connection will be this equipotential bonding
conductor connection to the outside of the metal housing of the driveline connector. The
pickup coil is only listening to the magnet field of the wires transferring data
packages between pump and peripheral controller. During the study there is no need to
exchange the existing peripheral LVAD HeartMate 3 controller, therefore at no point the
pump will be stopped.
The data of patients on the intensive care unit (ICU) or normal ward are stored in a
continuous data acquisition system (CDAS) on a notebook which is operated in battery
operation, and analyzed afterwards with a mat-lab (MathWorks Inc., USA) based software.
For outpatients a mobile data-recorder (size of approximately 8cm x 5cm x 2cm, weight
approx. 110g) powered with a 9V battery will allow a continuous data acquisition for a
period up to 2 months on a SD card.
Routinely, all pump parameters are stored in a so-called log file, in the form of
delimiter-separated character strings containing decimal and hexadecimal values. The log
file data is provided from the pump's controller and the pump itself, which are both
storing a periodical and an event log file. The periodical log files, as the name
implies, are created continuously and periodically during pump operation, while the
event log files provide additional information which is stored only when the HeartMate 3
detects a potential adverse event or abnormal pump behavior (e.g. arrhythmias, suction).
Therefore, the data of every patient is contained in multiple sets of four different log
files (periodical and event log files from the pump's controller, plus periodical and
event log files from the pump itself) with defined timestamps for each set of
parameters.
The maximal storage capacity of logfiles is only 256 entries with a minimal periodic
logging interval of 10min. Therefore the ability to detect e.g. suction events and
abnormal pump operation due to hemodynamically changes should be evaluated with high
resolution continuous data acquisition system (CDAS) data and the routinely available
logfiles and compared to each other.
Pump data will be collected from the routinely available logfiles via the device monitor
and from high resolution HeartMate 3 CDAS data, which can be picked up contactless every
second.
The data of patients on the ICU or normal ward are stored in a continuous data
acquisition system (CDAS) on a notebook which is operated in battery operation, and
analyzed afterwards with a mat-lab based software. For outpatients a mobile
data-recorder will allow a continuous data acquisition for a period up to 2 months on a
SD card.
During the post-operative patient stay at the ICU and normal ward, the pump data will be
continuously recorded together with all routinely measured hemodynamic variables.
Contextually with the pump data, during the post-operative patient stay at the ICU and
normal ward, ECG will be continuously acquired from the clinical monitors. Holter ECG
data are recorded once for 24 hours during the normal ward stay and once at home after
the outpatient visit 12 months after enrollment. The application of the Holter ECG
device and the pump data measurement will be performed during the hospital stay or at
home thus requiring no additional patient waiting time.
Additionally, for patients with implanted pacemakers/ICDs or loop recorders, the heart
rate and variability parameters will be also retrieved from the data delivered by these
devices (of course, only if the setting of the pacemakers/ICD allows changes in the
patient individual heart rate).
ECG will be recorded using the clinical monitors during the stay on the ICU and using
Holter ECG devices for ambulant patients and in out of hospital patients. SPSS and
descriptive statistics will be applied. The standard heart rate variability parameters
and more sophisticated parameters for arrhythmia pattern detection which are derived
from the pump flow and the same parameters derived from the ECG signals or if applicable
and available from implanted pacemakers/ICDs and loop recorders will be described
explorative based on mean ± standard deviation or median (interquartile range) and
visually represented by histograms and box-plots. However, these only have a purely
exploratory character in this pilot study to check the feasibility and/or practicability
of this non-invasive methods and for a later, larger study to find possible
correlations, which should also be checked in a follow-up study. Following this pilot
study, a follow-up study should test the hypothesis that the developed methods for easy
assessment of cardiac function during LVAD support, can provide an additional tool for
optimization of cardiac protection, thereby reducing adverse events and optimizing
clinical outcomes.