LVAD Pump Flow Does Not Adequately Increase With Exercise

Hemodynamic parameters at rest predicting exercise capacity in patients supported with left ventricular assist device

Left ventricular assist devices improve prognosis and quality of life, but exercise capacity remains limited in most patients after device implantation. Left ventricular assist device optimization through right heart catheterization reduces device-related complications. However, hemodynamic parameters associated with exercise capacity under optimized conditions. The aim of this study was to elucidate the predictors of exercise capacity from hemodynamic parameters at rest after left ventricular assist device optimization. We retrospectively reviewed 24 patients who underwent a ramp test with right heart catheterization, echocardiography and cardiopulmonary exercise testing more than 6 months after left ventricular assist device implantation. Pump speed was optimized to a lower setting that achieved right atrial pressure < 12 mmHg, pulmonary capillary wedge pressure < 18 mmHg, and cardiac index > 2.2 L/min/m2, then exercise capacity was assessed by cardiopulmonary exercise testing. After left ventricular assist device optimization, the mean right atrial pressure, pulmonary capillary wedge pressure, cardiac index, and peak oxygen consumption were 7 ± 5 mmHg, 10 ± 7 mmHg, 2.7 ± 0.5 L/min/m2, and 13.2 ± 3.0 mL/min/kg, respectively. Pulse pressure, stroke volume, right atrial pressure, mean pulmonary artery pressure, and pulmonary capillary wedge pressure were significantly associated with peak oxygen consumption. Multivariate linear regression analysis of factors predicting peak oxygen consumption revealed that pulse pressure, right atrial pressure, and aortic insufficiency remained independent predictors (β = 0.401, p = 0.007; β = - 0.558, p < 0.001; β = - 0.369, p = 0.010, respectively). Our findings suggests that cardiac reserve, volume status, right ventricular function, and aortic insufficiency predict exercise capacity in patients with a left ventricular assist device.

Exercise Performance and Quality of Life of Left Ventricular Assist Device Patients After Long-Term Outpatient Cardiac Rehabilitation

PURPOSE

Exercise performance and quality of life (QoL) of left ventricular assist device (LVAD) patients improve after early cardiac rehabilitation (CR). The purpose of this study was to examine the efficacy of multiprofessional long term phase 3 outpatient CR, and whether cardiopulmonary exercise testing (CPX) and 6-min walk testing (6MWT) post-LVAD implantation predict hospital readmission.

METHODS

This retrospective observational cohort study included 29 LVAD patients (58.6 ± 7.7 yr, female: 13.8%, body mass index: 29.4 ± 3.3 kg/m 2 ). Functional performance tests (CPX, 6MWT, sit-to-stand test), QoL, and psychological surveys (Kansas City Cardiomyopathy Questionnaire, hospital anxiety and depression scale, and Control Convictions about Disease and Health [KKG]) were performed at baseline and at the end of CR.

RESULTS

The CR was initiated at a median (IQR) of 159 (130-260) d after LVAD implantation for a duration of 340 (180-363) d with 46.8 ± 23.2 trainings. The 6MWT (408.4 ± 113.3 vs 455.4 ± 115.5 m, P = .003) and sit-to-stand test (16.7 ± 6.9 vs 19.0 ± 5.3 repetitions, P = .033) improved, but relative peak oxygen uptake (V˙ o2peak : 9.4 [8.2-14.4] vs 9.3 [7.8-13.4] mL/min/kg, P = .57) did not change. Using receiver operating characteristic curve analysis, baseline V˙ o2peak values were associated with readmission 1-yr after CR onset (C-statistic = 0.88) with a cutoff value of V˙ o2peak < 9.15 mL/min/kg (100% sensitivity, 78% specificity, P < .001). The Kansas City Cardiomyopathy Questionnaire self-efficacy and knowledge (+6.3 points), QoL (+5.0 points), and social limitation (+7.1 points) demonstrated clinically important changes. In addition, the hospital anxiety and depression scale showed a significant reduction in anxiety (4.6 ± 3.2 vs 2.6 ± 2.4, P = .03).

CONCLUSIONS

Long-term CR is safe and LVAD outpatients showed improvement of QoL, anxiety, and submaximal exercise performance. In addition, V˙ o2peak and 6MWT have prognostic value for readmission.

An in vitro model to study suction events by a ventricular assist device: validation with clinical data

Introduction: Ventricular assist devices (LVADs) are a valuable therapy for end-stage heart failure patients. However, some adverse events still persist, such as suction that can trigger thrombus formation and cardiac rhythm disorders. The aim of this study is to validate a suction module (SM) as a test bench for LVAD suction detection and speed control algorithms. Methods: The SM consists of a latex tube, mimicking the ventricular apex, connected to a LVAD. The SM was implemented into a hybrid in vitro-in silico cardiovascular simulator. Suction was induced simulating hypovolemia in a profile of a dilated cardiomyopathy and of a restrictive cardiomyopathy for pump speeds ranging between 2,500 and 3,200 rpm. Clinical data collected in 38 LVAD patients were used for the validation. Clinical and simulated LVAD flow waveforms were visually compared. For a more quantitative validation, a binary classifier was used to classify simulated suction and non-suction beats. The obtained classification was then compared to that generated by the simulator to evaluate the specificity and sensitivity of the simulator. Finally, a statistical analysis was run on specific suction features (e.g., minimum impeller speed pulsatility, minimum slope of the estimated flow, and timing of the maximum slope of the estimated flow). Results: The simulator could reproduce most of the pump waveforms observed in vivo. The simulator showed a sensitivity and specificity and of 90.0% and 97.5%, respectively. Simulated suction features were in the interquartile range of clinical ones. Conclusions: The SM can be used to investigate suction in different pathophysiological conditions and to support the development of LVAD physiological controllers.

Increased Dead Space Ventilation as a Contributing Factor to Persistent Exercise Limitation in Patients with a Left Ventricular Assist Device

(1) Background: The exercise capacity of patients with a left ventricular assist device (LVAD) remains limited despite mechanical support. Higher dead space ventilation (VD/VT) may be a surrogate for right ventricular to pulmonary artery uncoupling (RV-PA) during cardiopulmonary exercise testing (CPET) to explain persistent exercise limitations. (2) Methods: We investigated 197 patients with heart failure and reduced ejection fraction with (n = 89) and without (HFrEF, n = 108) LVAD. As a primary outcome NTproBNP, CPET, and echocardiographic variables were analyzed for their potential to discriminate between HFrEF and LVAD. As a secondary outcome CPET variables were evaluated for a composite of hospitalization due to worsening heart failure and overall mortality over 22 months. (3) Results: NTproBNP (OR 0.6315, 0.5037-0.7647) and RV function (OR 0.45, 0.34-0.56) discriminated between LVAD and HFrEF. The rise of endtidal CO2 (OR 4.25, 1.31-15.81) and VD/VT (OR 1.23, 1.10-1.40) were higher in LVAD patients. Group (OR 2.01, 1.07-3.85), VE/VCO2 (OR 1.04, 1.00-1.08), and ventilatory power (OR 0.74, 0.55-0.98) were best associated with rehospitalization and mortality. (4) Conclusions: LVAD patients displayed higher VD/VT compared to HFrEF. Higher VD/VT as a surrogate for RV-PA uncoupling could be another marker of persistent exercise limitations in LVAD patients.

Exercise oscillatory ventilation in patients with advanced heart failure with and without left ventricular assist device

BACKGROUND

Exercise oscillatory ventilation (EOV), indicating pathological fluctuations on pulmonary arterial pressure, is associated with mortality in patients with heart failure (HF). Whether left ventricular assist device (LVAD)-induced ventricular unloading can reverse EOV and may prevent short-term rehospitalization has not been investigated.

METHODS

We performed a retrospective single-center in- and outpatient analysis of patients with (n = 20, LVAD) and without (n = 27, HF) circulatory support and reduced ejection fraction (EF, 22.8 ± 7.9%). The association of cardiopulmonary exercise testing (CPET) variables and 3 months-rehospitalization (3MR) as a primary outcome was analyzed. Furthermore, CPET variables were compared regarding the presence of EOV (+/-).

RESULTS

Lower VO_2peak (11.6 ± 4.9 ml/kg/min vs. 14.4 ± 4.3 ml/kg/min, p = 0.039), lower increase of PETCO₂ (CI = 0.049-1.127; p = 0.068), and higher VE/VCO₂ (43.8 ± 9.5 vs. 38.3 ± 10.6; p = 0.069) were associated with 3MR. Flattening of O₂ pulse (CI = 0.139-2.379; p = 0.487) had no impact on 3MR. EOV was present in 59.5% (n = 28/47) of patients, without a significant difference between LVAD and HF patients (p = 0.959). Patients with HF/EOV+ demonstrated significantly lower VO_2peak compared with HF/EOV- (p = 0.039). LVAD/EOV+ displayed significantly lower EF (p = 0.004) and fewer aortic valve opening than LVAD/EOV- (p = 0.027).

CONCLUSIONS

Lower VO_2peak , but not EOV, was associated with 3MR. EOV occurred at a similar rate in LVAD and HF patients, which may illustrate insufficient unloading during exercise in chronic LVAD therapy and may contribute to the limited exercise capacity following LVAD implantation. Simultaneous CPET and right heart catheterization studies are needed to elucidate whether EOV may serve as a non-invasive predictor of insufficient LV unloading necessitating LVAD reprograming.

Initial clinical validation of a hybrid in silico—in vitro cardiorespiratory simulator for comprehensive testing of mechanical circulatory support systems

Simulators are expected to assume a prominent role in the process of design—development and testing of cardiovascular medical devices. For this purpose, simulators should capture the complexity of human cardiorespiratory physiology in a realistic way. High fidelity simulations of pathophysiology do not only allow to test the medical device itself, but also to advance practically relevant monitoring and control features while the device acts under realistic conditions. We propose a physiologically controlled cardiorespiratory simulator developed in a mixed in silico-in vitro simulation environment. As inherent to this approach, most of the physiological model complexity is implemented in silico while the in vitro system acts as an interface to connect a medical device. As case scenarios, severe heart failure was modeled, at rest and at exercise and as medical device a left ventricular assist device (LVAD) was connected to the simulator. As initial validation, the simulator output was compared against clinical data from chronic heart failure patients supported by an LVAD, that underwent different levels of exercise tests with concomitant increase in LVAD speed. Simulations were conducted reproducing the same protocol as applied in patients, in terms of exercise intensity and related LVAD speed titration. Results show that the simulator allows to capture the principal parameters of the main adaptative cardiovascular and respiratory processes within the human body occurring from rest to exercise. The simulated functional interaction with the LVAD is comparable to the one clinically observed concerning ventricular unloading, cardiac output, and pump flow. Overall, the proposed simulation system offers a high fidelity in silico-in vitro representation of the human cardiorespiratory pathophysiology. It can be used as a test bench to comprehensively analyze the performance of physically connected medical devices simulating clinically realistic, critical scenarios, thus aiding in the future the development of physiologically responding, patient-adjustable medical devices. Further validation studies will be conducted to assess the performance of the simulator in other pathophysiological conditions.

Left ventricular assist devices: A historical perspective at the intersection of medicine and engineering

Over the last half-century, left ventricular assist device (LVAD) technology has progressed from conceptual therapy for failed cardiopulmonary bypass weaning to an accepted destination therapy for advanced heart failure. The history of LVAD engineering is defined by an initial development phase, which demonstrated the feasibility of such an approach, to the more recent three major generations of commercial devices. In this review, we explore the engineering challenges of LVADs, how they were addressed over time, and the clinical outcomes that resulted from each major technological development. The first generation of commercial LVADs were pulsatile devices, which lacked the appropriate durability due to their number of moving components and hemocompatibility. The second generation of LVADs was defined by replacement of complex, pulsatile pumps with primarily axial, continuous-flow systems with an impeller in the blood passageway. These devices experienced significant commercial success, but the presence of excessive trauma to the blood and in-situ bearing resulted in an unacceptable burden of adverse events. Third generation centrifugal-flow pumps use magnetically suspended rotors within the pump chamber. Superior outcomes with this newest generation of devices have been observed, particularly with respect to hemocompatibility-related adverse events including pump thrombosis, with fully magnetically levitated devices. The future of LVAD engineering includes wireless charging foregoing percutaneous drivelines and more advanced pump control mechanisms, including synchronization of the pump flow with the native cardiac cycle, and varying pump output based on degree of physical exertion using sensor or advanced device-level data triggers.

A Sensorless Modular Multiobjective Control Algorithm for Left Ventricular Assist Devices: A Clinical Pilot Study

Background

Contemporary Left Ventricular Assist Devices (LVADs) mainly operate at a constant speed, only insufficiently adapting to changes in patient demand. Automatic physiological speed control promises tighter integration of the LVAD into patient physiology, increasing the level of support during activity and decreasing support when it is excessive.

Methods

A sensorless modular control algorithm was developed for a centrifugal LVAD (HVAD, Medtronic plc, MN, USA). It consists of a heart rate-, a pulsatility-, a suction reaction—and a supervisor module. These modules were embedded into a safe testing environment and investigated in a single-center, blinded, crossover, clinical pilot trial (clinicaltrials.gov, NCT04786236). Patients completed a protocol consisting of orthostatic changes, Valsalva maneuver and submaximal bicycle ergometry in constant speed and physiological control mode in randomized sequence. Endpoints for the study were reduction of suction burden, adequate pump speed and flowrate adaptations of the control algorithm for each protocol item and no necessity for intervention via the hardware safety systems.

Results

A total of six patients (median age 53.5, 100% male) completed 13 tests in the intermediate care unit or in an outpatient setting, without necessity for intervention during control mode operation. Physiological control reduced speed and flowrate during patient rest, in sitting by a median of −75 [Interquartile Range (IQR): −137, 65] rpm and in supine position by −130 [−150, 30] rpm, thereby reducing suction burden in scenarios prone to overpumping in most tests [0 [−10, 2] Suction events/minute] in orthostatic upwards transitions and by −2 [−6, 0] Suction events/min in Valsalva maneuver. During submaximal ergometry speed was increased by 86 [31, 193] rpm compared to constant speed for a median flow increase of 0.2 [0.1, 0.8] L/min. In 3 tests speed could not be increased above constant set speed due to recurring suction and in 3 tests speed could be increased by up to 500 rpm with a pump flowrate increase of up to 0.9 L/min.

Conclusion

In this pilot study, safety, short-term efficacy, and physiological responsiveness of a sensorless automated speed control system for a centrifugal LVAD was established. Long term studies are needed to show improved clinical outcomes.

Clinical Trial Registration

ClinicalTrials.gov, identifier: NCT04786236.

Echo-guided left ventricular assist device speed optimisation for exercise maximisation

Objective

Current generation left ventricular assist devices (LVADs) operate with a fixed rotation speed and no automated speed adjustment function. This study evaluates the concept of physiological pump speed optimisation based on aortic valve opening (AVO) imaging during a cardiopulmonary exercise test (CPET).

Methods

This prospective crossover study (NCT05063006) enrolled patients with implanted third-generation LVADs with hydrodynamic bearing. After resting speed optimisation, patients were randomised to a fixed-modified speed or modified-fixed speed CPET sequence. Fixed speed CPET maintained baseline pump settings. During the modified speed CPET, the LVAD speed was continuously altered to preserve periodic AVO.

Results

We included 22 patients, the mean age was 58.4±7 years, 4.5% were women and 54.5% had ischaemic cardiomyopathy. Exertional AVO assessment was feasible in all subjects. Maintaining periodic AVO allowed to safely raise the pump speed from 2900 (IQR 2640–3000) to 3440 revolutions per minute (RPM) (IQR 3100–3700; p<0.001). As a result, peak oxygen consumption increased from 11.1±2.4 to 12.8±2.8 mL/kg/min (p<0.001) and maximum workload from 1.1 (IQR 0.9–1.5) to 1.2 W/kg (IQR 0.9–1.7; p=0.028). The Borg scale exertion level decreased from 15.2±1.5 to 13.5±1.2 (p=0.005).

Conclusions

Transthoracic AVO imaging is possible during CPETs in patients with LVAD. Dynamic echo-guided pump speed adjustment based on the AVO improves exercise tolerance and augments peak oxygen consumption and maximum workload.

Physiology of Continuous-Flow Left Ventricular Assist Device Therapy

The expanding use of continuous-flow left ventricular assist devices (CF-LVADs) for end-stage heart failure warrants familiarity with the physiologic interaction of the device with the native circulation. Contemporary devices utilize predominantly centrifugal flow and, to a lesser extent, axial flow rotors that vary with respect to their intrinsic flow characteristics. Flow can be manipulated with adjustments to preload and afterload as in the native heart, and ascertainment of the predicted effects is provided by differential pressure-flow (H-Q) curves or loops. Valvular heart disease, especially aortic regurgitation, may significantly affect adequacy of mechanical support. In contrast, atrioventricular and ventriculoventricular timing is of less certain significance. Although beneficial effects of device therapy are typically seen due to enhanced distal perfusion, unloading of the left ventricle and atrium, and amelioration of secondary pulmonary hypertension, negative effects of CF-LVAD therapy on right ventricular filling and function, through right-sided loading and septal interaction, can make optimization challenging. Additionally, a lack of pulsatile energy provided by CF-LVAD therapy has physiologic consequences for end-organ function and may be responsible for a series of adverse effects. Rheological effects of intravascular pumps, especially shear stress exposure, result in platelet activation and hemolysis, which may result in both thrombotic and hemorrhagic consequences. Development of novel solutions for untoward device-circulatory interactions will facilitate hemodynamic support while mitigating adverse events. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

Prognostic Value of Peak Oxygen Uptake in Patients Supported With Left Ventricular Assist Devices (PRO-VAD)

OBJECTIVES

The purpose of this study was to examine whether peak oxygen uptake (pVO₂) and other cardiopulmonary exercise test (CPET)-derived variables could predict intermediate-term mortality in stable continuous flow LVAD recipients.

BACKGROUND

pVO₂ is a cornerstone in the selection of patients for heart transplantation, but the prognostic power of pVO₂ obtained in patients treated with a left ventricular assist device (LVAD) is unknown.

METHODS

We collected data for pVO₂ and outcomes in adult LVAD recipients in a retrospective, multicenter study and evaluated cutoff values for pVO₂ including: 1) values above or below medians; 2) grouping patients in tertiles; and 3) pVO₂ ≤14 ml/kg/min if the patient was not treated with beta-blockers (BB) or pVO₂ ≤12 ml/kg/min if the patient was taking BB therapy.

RESULTS

Nine centers contributed data from 450 patients. Patients were 53 ± 13 years of age; 78% were male; body mass index was 25 ± 5 kg/m² with few comorbidities (stroke: 11%; diabetes: 18%; and peripheral artery disease: 4%). The cause of heart failure (HF) was most often nonischemic (66%). Devices included were the HeartMate II and 3 (Abbott); and Heartware ventricular assist devices Jarvik and Duraheart (Medtronic). The index CPET was performed at a median of 189 days (154 days-225 days) after LVAD implantation, and mean pVO₂ was 14.1 ± 5 ml/kg/min (47% ± 14% of predicted value). Lower pVO₂ values were strongly associated with poorer survival regardless of whether patients were analyzed for absolute pVO₂ in ml/kg/min, pVO₂ ≤12 BB/14 ml/kg/min, or as a percentage of predicted pVO₂ values (P ≤ 0.001 for all). For patients with pVO₂ >12 BB/14 and ventilation/carbon dioxide relationship (VE/VCO₂) slope <35, the 1-year survival was 100%.

CONCLUSIONS

Even after LVAD implantation, pVO₂ has prognostic value, similar to HF patients not supported by mechanical circulatory support devices. (PROgnostic Value of Exercise Capacity Measured as Peak Oxygen Uptake [pVO₂] in Recipients of Left Ventricular Assist Devices [PRO-VAD]; NCT04423562).

Exercise Dynamic of Patients with Chronic Heart Failure and Reduced Ejection Fraction

PURPOSE OF REVIEW

Exercise causes various dynamic changes in all body parts either in healthy subject or in heart failure (HF) patients. The present review of current knowledge about HF patients with reduced ejection fraction focuses on dynamic changes along a "metabo-hemodynamic" perspective.

RECENT FINDINGS

Studies on the dynamic changes occurring during exercise span many years. Thanks to the availability of advanced methods, it is nowadays possible to properly characterize respiratory, hemodynamic, and muscular function adjustments and their mismatch with the pulmonary and systemic circulations. Exercise is a dynamic event that involves several body functions. In HF patients, it is important to know at what level the limitation takes place in order to better manage these patients and to optimize therapeutic strategies.

Exercise physiology in left ventricular assist device patients: insights from hemodynamic simulations

Left ventricular assist devices (LVADs) assure longer survival to patients, but exercise capacity is limited compared to normal values. Overall, LVAD patients show high wedge pressure and low cardiac output during maximal exercise, a phenomenon hinting at the need for increased LVAD support. Clinical studies investigating the hemodynamic benefits of an LVAD speed increase during exercise, ended in inhomogeneous and sometimes contradictory results. The native ventricle-LVAD interaction changes between rest and exercise, and this evolution is complex, multifactorial and patient-specific. The aim of this paper is to provide a comprehensive overview on the patient-LVAD interaction during exercise and to delineate possible therapeutic strategies for the future. A computational cardiorespiratory model was used to simulate the hemodynamics of peak bicycle exercise in LVAD patients. The simulator included the main cardiovascular and respiratory impairments commonly observed in LVAD patients, so as to represent an average hemodynamic response to exercise. In addition, other exercise responses were simulated, by tuning the chronotropic, inotropic and vascular functions, and implementing aortic regurgitation and stenosis in the simulator. These profiles were tested under different LVAD speeds and LVAD pressure-flow characteristics. Simulations output showed consistency with clinical data from the literature. The simulator allowed the working condition of the assisted ventricle at exercise to be investigated, clarifying the reasons behind the high wedge pressure and poor cardiac output observed in the clinics. Patients with poorer inotropic, chronotropic and vascular functions, are likely to benefit more from an LVAD speed increase during exercise. Similarly, for these patients, a flatter LVAD pressure-flow characteristic can assure better hemodynamic support under physical exertion. Overall, the study evidenced the need for a patient-specific approach on supporting exercise hemodynamics. In this frame, a complex simulator can constitute a valuable tool to define and test personalized speed control algorithms and strategies.

The left ventricular assist device as a patient monitoring system

Technological progress of left ventricular assist devices (LVADs) towards rotary blood pumps and the optimization of medical management contributed to the significant improvements in patient survival as well as LVAD support duration. Even though LVAD therapy is now well-established for end-stage heart failure patients, the long-term occurrence of adverse events (AE) such as bleeding, infection or stroke, still represent a relevant burden. An early detection of AE, before onset of major symptoms, can lead to further optimization of patient treatment and thus mitigate the burden of AE. Continuous patient monitoring facilitates identification of pathophysiological states and allows anticipation of AE to improve patient management. In this paper, methods, algorithms and possibilities for continuous patient monitoring based on LVAD data are reviewed. While experience with continuous LVAD monitoring is currently limited to a few centers worldwide, the pace of developments in this field is fast and we expect these technologies to have a global impact on the well-being of LVAD patients.

Role of Cardiac Rehabilitation After Ventricular Assist Device Implantation

Patients with heart failure suffered by a complex syndrome, where the filling of the ventricle or ejection of the blood is impaired. In this setting, the exercise capacity decreases for many reasons, one of them being the insufficient oxygen transfer due to reduced cardiac output and anemia. Ventricular assist device has emerged as a durable and safe therapy for patients with end-stage heart failure. The benefits of cardiac rehabilitation in ventricular assist device patients are enormous: the first aim is to progressively reduce the physical and functional impairments of these patients, so that they will be able to resume meaningful daily activities.

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.

Determinants of Functional Capacity and Quality of Life After Implantation of a Durable Left Ventricular Assist Device

Continuous-flow left ventricular assist devices (LVAD) are increasingly used as destination therapy in patients with end-stage heart failure and, with recent improvements in pump design, adverse event rates are decreasing. Implanted patients experience improved survival, quality of life (QoL) and functional capacity (FC). However, improvement in FC and QoL after implantation is not unequivocal, and this has implications for patient selection and preimplantation discussions with patients and relatives. This article identifies preimplantation predictors of lack of improvement in FC and QoL after continuous-flow LVAD implantation and discusses potential mechanisms, allowing for the identification of potential factors that can be modified. In particular, the pathophysiology behind insufficient improvement in peak oxygen uptake is discussed. Data are included from 40 studies, resulting in analysis of >700 exercise tests. Mean peak oxygen uptake was 13.4 ml/kg/min (equivalent to 48% of predicted value; 259 days after implantation, range 31–1,017 days) and mean 6-minute walk test distance was 370 m (182 days after implantation, range 43–543 days). Finally, the interplay between improvement in FC and QoL is discussed.

Limiting factors of peak and submaximal exercise capacity in LVAD patients

AIMS

Although patients supported with a Continuous-Flow Left Ventricular Assist Device (CF-LVAD) are hemodynamically stable, their exercise capacity is limited. Hence, the aim of this work was to investigate the underlying factors that lead to peak and submaximal exercise intolerance of CF-LVAD supported patients.

METHODS

Seven months after CF-LVAD implantation, eighty three patients performed a maximal cardiopulmonary exercise test and a six minute walk test. Peak oxygen uptake and the distance walked were measured and expressed as a percentage of the predicted value (%VO2p and %6MWD, respectively). Preoperative conditions, echocardiography, laboratory results and pharmacological therapy data were collected and a correlation analysis against %VO2p and %6MWD was performed.

RESULTS

CF-LVAD patients showed a relatively higher submaximal exercise capacity (%6MWD = 64±16%) compared to their peak exertion (%VO2p = 51±14%). The variables that correlated with %VO2p were CF-LVAD parameters, chronotropic response, opening of the aortic valve at rest, tricuspid insufficiency, NT-proBNP and the presence of a cardiac implantable electronic device. On the other hand, the variables that correlated with %6MWD were diabetes, creatinine, urea, ventilation efficiency and CF-LVAD pulsatility index. Additionally, both %6MWD and %VO2p were influenced by the CF-LVAD implantation timing, calculated from the occurrence of the cardiac disease.

CONCLUSION

Overall, both %6MWD and %VO2p depend on the duration of heart failure prior to CF-LVAD implantation. %6MWD is primarily determined by parameters underlying the patient's general condition, while %VO2p mostly relies on the residual function and chronotropic response of the heart. Moreover, since %VO2p was relatively lower compared to %6MWD, we might infer that CF-LVAD can support submaximal exercise but is not sufficient during peak exertion. Hence concluding that the contribution of the ventricle is crucial in sustaining hemodynamics at peak exercise.

Hemodynamic exercise responses with a continuous-flow left ventricular assist device: Comparison of patients' response and cardiorespiratory simulations

Background

Left ventricular assist devices (LVADs) are an established treatment for end stage heart failure patients. As LVADs do not currently respond to exercise demands, attention is also directed towards improvements in exercise capacity and resulting quality of life. The aim of this study was to explore hemodynamic responses observed during maximal exercise tests to infer underlying patient status and therefore investigate possible diagnostics from LVAD derived data and advance the development of physiologically adaptive LVAD controllers.

Methods

High resolution continuous LVAD flow waveforms were recorded from 14 LVAD patients and evaluated at rest and during maximum bicycle exercise tests (n = 24). Responses to exercise were analyzed in terms of an increase (↑) or decrease (↓) in minimum (Q_MIN), mean (Q_MEAN), maximum flow (Q_MAX) and flow pulsatility (Q_P2P). To interpret clinical data, a cardiorespiratory numerical simulator was used that reproduced patients’ hemodynamics at rest and exercise. Different cardiovascular scenarios including chronotropic and inotropic responses, peripheral vasodilation, and aortic valve pathologies were simulated systematically and compared to the patients’ responses.

Results

Different patients’ responses to exercise were observed. The most common response was a positive change of ΔQ_MIN↑ and ΔQ_P2P↑ from rest to exercise (70% of exercise tests). Two responses, which were never reported in patients so far, were distinguished by Q_MIN↑ and Q_P2P↓ (observed in 17%) and by Q_MIN↓ and Q_P2P↑ (observed in 13%). The simulations indicated that the Q_P2P↓ can result from a reduced left ventricular contractility and that the Q_MIN↓ can occur with a better left ventricular contractility and/or aortic insufficiency.

Conclusion

LVAD flow waveforms determine a patients’ hemodynamic “fingerprint” from rest to exercise. Different waveform responses to exercise, including previously unobserved ones, were reported. The simulations indicated the left ventricular contractility as a major determinant for the different responses, thus improving patient stratification to identify how patient groups would benefit from exercise-responsive LVAD control.

Continuous LVAD monitoring reveals high suction rates in clinically stable outpatients

Suction of the left ventricle can lead to potentially life‐threatening events in left ventricular assist device (LVAD) patients. With the resolution of currently available clinical LVAD monitoring healthcare professionals are unable to evaluate patients’ suction occurrences in detail. This study investigates occurrences and durations of suction events and their associations with tachycardia in stable outpatients. Continuous high‐resolution LVAD data from HVAD patients were analyzed in the early outpatient period for 15 days. A validated suction detection from LVAD signals was used. Suction events were evaluated as suction rates, bursts of consecutive suction beats, and clusters of suction beats. The occurrence of tachycardia was analyzed before, during, and after suction clusters. Furthermore, blood work, implant strategy, LVAD speed setting, inflow cannula position, left ventricular diameters, and adverse events were evaluated in these patients. LVAD data of 10 patients was analyzed starting at 78 ± 22 postoperative days. Individuals’ highest suction rates per hour resulted in a median of 11% (range 3%‐61%). Bursts categorized as consecutive suction beats with n = 2, n = 3‐5, n = 6‐15, and n > 15 beats were homogenously distributed with 10.3 ± 0.8% among all suction beats. Larger suction bursts were followed by shorter suction‐free periods. Tachycardia during suction occurred in 12% of all suction clusters. Significant differences in clinical parameters between individuals with high and low suction rates were only observed in left ventricular end‐diastolic and end‐systolic diameters (P < .02). Continuous high‐resolution LVAD monitoring sheds light on outpatient 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 shows the necessity of improved LVAD monitoring and the implementation of an LVAD speed control to reduce suction rates and their concomitant burden on the cardiovascular system.

Physiology of the Assisted Circulation in Cardiogenic Shock: A State-of-the-Art Perspective

Mechanical circulatory support (MCS) has made rapid progress over the last 3 decades. This was driven by the need to develop acute and chronic circulatory support as well as by the limited organ availability for heart transplantation. The growth of MCS was also driven by the use of extracorporeal membrane oxygenation (ECMO) after the worldwide H1N1 influenza outbreak of 2009. The majority of mechanical pumps (ECMO and left ventricular assist devices) are currently based on continuous flow pump design. It is interesting to note that in the current era, we have reverted from the mammalian pulsatile heart back to the continuous flow pumps seen in our simple multicellular ancestors. This review will highlight key physiological concepts of the assisted circulation from its effects on cardiac dynamic to principles of cardiopulmonary fitness. We will also examine the physiological principles of the ECMO-assisted circulation, anticoagulation, and the haemocompatibility challenges that arise when the blood is exposed to a foreign mechanical circuit. Finally, we conclude with a perspective on smart design for future development of devices used for MCS.

Numerical modeling of continuous-flow left ventricular assist device performance

Responses of five rotary blood pumps, namely HeartAssist 5, HeartMate II, HeartWare, Sputnik 1, and Sputnik 2, were extensively assessed in six test cases using a mathematical model of the cardiovascular system. Data for the rotary pumps were derived from pressure-flow curves reported in the literature. The test cases were chosen to attempt to cover most common clinical conditions, such as partial or full support or transitions between different levels of ventricular support. The investigated parameters are collected in a table and presented in figures, such as pressure-volume loops, H-Q curves, pump flow, and aortic pressure waveforms. HeartAssist, Sputnik 1, and Sputnik 2 pumps provide comparable level of aortic pressure, pump flow pulsatility PI(Q_P), and aortic pressure pulsatility PI(AoP) due to the similarity of pressure-flow characteristic curves of these pumps. HeartMate II provides a minimal backflow among other investigated rotary blood pumps due to the maximum pressure head at zero flow. HeartWare provides minimal pulsation of flow, which is confirmed by a flow range from -2 to 7 L/min in case 1. At the same time, the greatest degree of unloading was demonstrated by the HeartWare due to the flatness of the pressure-flow curve shape. The conclusions were made based on the obtained results, including the influence of pressure-flow curve shape on the pump performance and occurrences of adverse events, such as backflow or suction. For example, the increase of the pressure head at zero flow decreases the likelihood of backflow through the pump, and with it, increasing the flow under minimal pressure head increases the likelihood of suction.