Factors Influencing the Rate of Flow Through Continuous-Flow Left Ventricular Assist Devices at Rest and With Exercise ∗

Loss of Pulsatility: Another Cause of Syncope in Patients with Left Ventricular Assist Devices

We report a case of a 55-year-old man who presented with recurrent syncope 15 months after HeartWare left ventricular assist device (LVAD) implantation and was found to have diminished LVAD flow and pulsatility on tilt table testing leading to severe orthostatic hypotension (OH). The prevalence of OH is common, but autonomic dysfunction leading to OH has not been well described in patients with chronic LVAD support. The diagnosis of OH in this setting is challenging due to the decreased pulsatility in the flow generated by LVADs, and tilt table testing can be useful in the evaluation of OH in these patients.

Physiological principles of Starling-like control of rotary ventricular assist devices

Introduction: This review explores the Starling-like physiological control method (SLC) for rotary ventricular assist devices (VADs) for severe heart failure. The SLC, based on mathematical models of the circulation, has two functions modeling each ventricle. The first function controls the output of the VAD to the arterial pool according to Starling's law, while the second function accounts for how the blood returns to the heart from the veins. The article aims to expose clinicians to SLC in an accessible and clinically relevant discussion. Areas Covered: The article explores the physiology underlying the controller, its development and how that physiology can be adapted to SLC. Examples of controller performance are demonstrated and discussed using a benchtop model of the cardiovascular system. A discussion of the limitations and criticisms of SLC is presented, followed by a future outlook on the clinical adoption of SLC. Expert Opinion: Due to its simplicity and emulation of the natural cardiac autoregulation, SLC is the superior physiological control method for rotary VADs. However, current technical and regulatory challenges prevent the clinical translation of SLC of VADs. Further technical and regulatory development will enable the clinical translation of SLCs of VADs in the coming years.

Restoring pulsatility and peakVO2 in the era of continuous flow, fixed pump speed, left ventricular assist devices: 'A hypothesis of pump's or patient's speed?'

Despite significant improvement in survival and functional capacity after continuous flow left ventricular assist device implantation, the patient's quality of life may remain limited by complications such as aortic valve insufficiency, thromboembolic episodes and gastrointestinal bleeding attributed to high shear stress continuous flow with attenuated or absence of pulsatile flow and by a reduced peak oxygen consumption (peakVO₂) primarily associated with a fixed pump speed operation. Revision of current evidence suggests that high technology pump speed algorithms, a 'hypothesis of decreasing pump's speed' to promote pulsatile flow and a 'hypothesis of increasing pump's speed' to increase peakVO₂, may only partially reverse these barriers. A 'hypothesis of increasing patient's speed' is introduced, suggesting that exercise training may further contribute to the patient's recovery, enhancing peakVO₂ and pulsatile flow by improving skeletal muscle oxidative capacity and strength, peripheral vasodilatory and ventilatory responses, favour changes in preload/afterload and facilitate native flow, formulating the rationale for further studies in the field.

Debate: creating adequate pulse with a continuous flow ventricular assist device: can it be done and should it be done? Probably not, it may cause more problems than benefits!

PURPOSE OF REVIEW

The feasibility and benefits of creating adequate pulsatility with continuous flow left ventricular assist devices (LVADs) have long been debated. This review discusses recent technical and clinical findings to answer whether such intervention should be implemented in the standard patient management.

RECENT FINDINGS

Only a limited amount of pulsatility can be generated by periodic speed steps, both considerably smaller in flow increase and in pace rate than the natural circulation. Organ systems are not impeded in their normal function and even not in recovery by a continuous flow. Known problems such as gastrointestinal bleeding are not necessarily due to pulsatility per se, or not important for therapeutic progress, such as minor modifications of the arterial walls.

SUMMARY

The speculative benefits of augmented pulsatility with continuous flow LVADs could be overrated and are still incompletely evaluated. Potential risks that might arise from this strategy should be carefully weighed before implementing extensive pulsatility as standard patient management.

Physical Capacity in LVAD Patients: Hemodynamic Principles, Diagnostic Tools and Training Control

Over time left ventricular assist devices (LVAD) have become an alternative to heart transplantation because of enormous technical development and miniaturization. Most patients present a significant improvement in clinical conditions and exercise capacity. Nevertheless, exercise tolerance remains markedly limited even after LVAD implantation compared to a control group. The complex physiological and hemodynamic changes in LVAD patients, both at rest and during exercise, are not yet understood, or at least not completely.

It is the aim of the present paper to describe the current state of scientific knowledge. Furthermore, the spectrum of diagnostic tools, including the noninvasive inert gas rebreathing method for measurement of cardiac output and associate parameters, are discussed. Options for training control in this special patient group are presented.

Investigation of the Characteristics of HeartWare HVAD and Thoratec HeartMate II Under Steady and Pulsatile Flow Conditions

The aim of this study was to elucidate the dynamic characteristics of the Thoratec HeartMate II (HMII) and the HeartWare HVAD (HVAD) left ventricular assist devices (LVADs) under clinically representative in vitro operating conditions. The performance of the two LVADs were compared in a normothermic, human blood-filled mock circulation model under conditions of steady (nonpulsatile) flow and under simulated physiologic conditions. These experiments were repeated using 5% dextrose in order to determine its suitability as a blood analog. Under steady flow conditions, for the HMII, approximately linear inverse LVAD differential pressure (H) versus flow (Q) relationships were observed with good correspondence between the results of blood and 5% dextrose under all conditions except at a pump speed of 9000 rpm. For the HVAD, the corresponding relationships were inverse curvilinear and with good correspondence between the blood-derived and 5% dextrose-derived relationships in the flow rate range of 2-6 L/min and at pump speeds up to 3000 rpm. Under pulsatile operating conditions, for each LVAD operating at a particular pump speed, an counterclockwise loop was inscribed in the HQ domain during a simulated cardiac cycle (HQ loop); this showed that there was a variable phase relationship between LVAD differential pressure and LVAD flow. For both the HMII and HVAD, increasing pump speed was associated with a right-hand and upward shift of the HQ loop and simulation of impairment of left ventricular function was associated with a decrease in loop area. During clinical use, not only does the pressure differential across the LVAD and its flow rate vary continuously, but their phase relationship is variable. This behavior is inadequately described by the widely accepted representation of a plot of pressure differential versus flow derived under steady conditions. We conclude that the dynamic HQ loop is a more meaningful representation of clinical operating conditions than the widely accepted steady flow HQ curve.