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Invited Commentary

Automated Adjustment of Left Ventricular Assist Device Speed During Exercise

Imamura, Teruhiko*; Narang, Nikhil

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doi: 10.1097/MAT.0000000000001087
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Survival has improved in patients with advanced heart failure as new innovations in left ventricular assist device (LVAD) technology have come to market. Critical complications including stroke and pump thrombosis have significantly reduced in frequency since the introduction of both the HeartMate 3 pump (Abbott, Abbott Park, IL) and better protocols for routine LVAD care.1 With the introduction of more hemocompatible devices, another area of LVAD management needing attention is improving patient quality of life and restoring functional and exercise capacity. An area of active investigation is how the LVAD patient can meet the metabolic demands of exercise given current limitations in automatically modulating pump speed based on activity level. Furthermore, it is unknown whether device speed adjustments can stabilize hemodynamics and improve performance during exercise.

We congratulate Lai et al.2 for their study on the observed changes in hemodynamics during varying exercise loads at incremental LVAD speeds in patients with HVAD LVADs (Medtronic, Framingham, MA). The authors found that pump flow increased and pulmonary capillary wedge pressure (PCWP) decreased significantly during exercise with incremental LVAD speed changes. They also observed no significant changes in right atrial pressure and mixed venous oxygen saturation during increases in LVAD speed.

As a whole, we might be able to conclude that LVAD speed adjustment would have a favorable effect in improving exercise capacity via increasing pump flow and improving left ventricular (LV) unloading. This could be objectively evaluated by assessing peak exercise oxygen consumption in LVAD patients at baseline and higher speeds. Such findings may propel the development of a smart pump,3 which can automatically adjust device speed in response to continuous hemodynamics monitoring. Hemodynamic parameters of interest in a potential smart pump design are 1) LV afterload; 2) LV preload; 3) right ventricular (RV) afterload; and 4) RV preload.

Left Ventricular Afterload

Higher arterial blood pressure is associated with an increased risk of intracranial hemorrhage, thromboembolic events, and progressive aortic insufficiency.4 Unfortunately, the reduced arterial pulse pressure during continuous-flow LVAD support limits our ability to accurately measure arterial blood pressure with a traditional oscillometric blood pressure cuff. Doppler opening blood pressure is thus a commonly used surrogate to assess mean arterial pressure but may be subject to operator-dependent variability. The development of new wearable technologies for continuous blood pressure monitoring that can synchronize to the LVAD might be a welcomed development in the era of durable mechanical circulatory support where stroke remains as an Achilles heel.

Left Ventricular Preload

Recently, our group has demonstrated that the slope of the flow waveform at the ventricular filling phase, which is displayed on the monitor of HVAD LVAD, had a strong correlation with actually measured PCWP (Figure 1).3 Given that pump flow is dependent on the pressure gradient between LV and the aorta, elevations of LV filling pressure seen in decompensated heart failure would reduce the pressure gradient in the pump, resulting in increased flow. Based on this concept, we can estimate PCWP by measuring the slope of HVAD flow waveform at diastole. When the slope is automatically measured and PCWP is continuously estimated, a smart pump can increase LVAD speed when PCWP is elevated (e.g., during exercise) as shown in this current study.

Figure 1
Figure 1:
Concept of the relationship between the actual left ventricular filling pressure (left) and HVAD flow waveform slope (right). A: When left ventricular filling pressure remains low, the pressure gradient between the aorta and left ventricle remains constant during diastole, and pump flow remains unchanged (i.e., flow waveform slope is flat). B: When left ventricular filling pressure elevates, the pressure gradient between the aorta and left ventricle reduces in diastole, and pump flow increases (i.e., flow waveform slope becomes steep).

Right Ventricular Afterload

Right ventricular afterload can be currently monitored by CardioMEMS (Abbott, Abbott Park, IL),5 which is a recently debuted remote hemodynamic monitoring device to monitor pulmonary artery pressures, now seeing widespread use in both Europe and the United States.6 If the CardioMEMS can link with the LVAD, a smart pump can adjust its speed by using continuously measured pulmonary artery pressure. This would be a useful alternative in HeartMate 3 devices, compared with using the slope of ventricular filling phase which would be more applicable to the HVAD.

Right Ventricular Preload

Although there are several methods to noninvasively estimate right atrial pressure including echocardiography, few technologies are currently available for remote monitoring. Several novel methodologies, including assessment of intracardiac lead impedance for patients with defibrillators and implanted inferior vena cava devices to estimate 3D volume,7 might be available in the near future to remotely estimate right atrial pressure.

As observed in this study, increases in right atrial pressure during exercise might be difficult to improve by incremental LVAD speed alone. Our group consistently observed that the right atrial pressure is the most difficult parameter to normalize during hemodynamic ramp testing.8 One reason would be residual or de novo RV dysfunction due to both geometric changes and increased venous return which occurs after LVAD implantation. Oral inotropes and pulmonary vasodilators have been evaluated in previous small studies, but require more investigation to assess durable benefit for both hard outcomes and exercise capacity in patients with RV failure after LVAD implantation.

The authors have shown in this small series the potential benefits of LVAD speed increases during exercise, in augmenting device flow and improved LV unloading—this was not accompanied by improvements in mixed venous oxygen saturation with higher pump speeds despite higher pump flows.2 The complex interplay of abnormal peripheral tissue extraction of oxygen in patients with heart failure, differences in RV function and native LV contractility, and changes in oxygen consumption during device parameter adjustments make accurate measurements of cardiac output a challenge. Repeat, larger evaluations with more heterogeneous patient samples with different pump types will certainly be informative in understanding exercise potential in patients with durable mechanical circulatory support.

References

1. Mehra MR, Uriel N, Naka Y, et al; MOMENTUM 3 Investigators: A fully magnetically levitated left ventricular assist device - final report. N Engl J Med 2019.380: 1618–1627.
2. Lai JV, Muthiah K, Robson D, et al. Impact of pump speed on hemodynamics with exercise in continuous flow ventricular assist device patients. ASAIO J 2019. 66: 132–138.
3. Grinstein J, Rodgers D, Kalantari S, et al. HVAD waveform analysis as a noninvasive marker of pulmonary capillary wedge pressure: A first step toward the development of a smart left ventricular assist device pump. ASAIO J 2018.64: 10–15.
4. Saeed O, Jermyn R, Kargoli F, et al. Blood pressure and adverse events during continuous flow left ventricular assist device support. Circ Heart Fail 2015.8: 551–556.
5. Abraham WT, Adamson PB, Bourge RC, et al; CHAMPION Trial Study Group: Wireless pulmonary artery haemodynamic monitoring in chronic heart failure: A randomised controlled trial. Lancet 2011.377: 658–666.
6. Abraham WT, Stevenson LW, Bourge RC, Lindenfeld JA, Bauman JG, Adamson PB; CHAMPION Trial Study Group: Sustained efficacy of pulmonary artery pressure to guide adjustment of chronic heart failure therapy: Complete follow-up results from the CHAMPION randomised trial. Lancet 2016.387: 453–461.
7. Imamura T, Moss JD, Flatley E, et al. Estimation of central venous pressure by pacemaker lead impedances in left ventricular assist device patients. ASAIO J 2019. 66: 49–54.
8. Imamura T, Jeevanandam V, Kim G, et al. Optimal hemodynamics during left ventricular assist device support are associated with reduced readmission rates. Circ Heart Fail 2019.12: e005094.
Keywords:

heart failure; ramp; hemodynamics

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