In vitro validation of a self-driving aortic-turbine venous-assist device for Fontan patients

Abstract

Background: Palliative repair of single ventricle defects involve a series of open-heart surgeries where a single-ventricle (Fontan) circulation is established. As the patient ages, this paradoxical circulation gradually fails, because of its high venous pressure levels. Reversal of the Fontan paradox requires an extra subpulmonic energy that can be provided through mechanical assist devices. The objective of this study was to evaluate the hemodynamic performance of a totally implantable integrated aortic-turbine venous-assist (iATVA) system, which does not need an external drive power and maintains low venous pressure chronically, for the Fontan circulation.

Methods: Blade designs of the co-rotating turbine and pump impellers were developed and 3 prototypes were manufactured. After verifying the single-ventricle physiology at a pulsatile in vitro circuit, the hemodynamic performance of the iATVA system was measured for pediatric and adult physiology, varying the aortic steal percentage and circuit configurations. The iATVA system was also tested at clinical off-design scenarios.

Results: The prototype iATVA devices operate at approximately 800 revolutions per minute and extract up to 10% systemic blood from the aorta to use this hydrodynamic energy to drive a blood turbine, which in turn drives a mixed-flow venous pump passively. By transferring part of the available energy from the single-ventricle outlet to the venous side, the iATVA system is able to generate up to approximately 5 mm Hg venous recovery while supplying the entire caval flow.

Conclusions: Our experiments show that a totally implantable iATVA system is feasible, which will eliminate the need for external power for Fontan mechanical venous assist and combat gradual postoperative venous remodeling and Fontan failure.

Keywords: Fontan; congenital heart disease; hemodynamics; mock-up circulation tests; pediatric ventricle assist device; single-ventricle physiology; venous hemodynamics.

Chronic mechanical venous support driven by an aortic turbine for Fontan patients.

Figure 1

Chronic mechanical venous support without external drive power for Fontan patients. An iATVA device connected through a common shaft that steals a minimal amount of aortic blood to drive a blood turbine and transfers this available pumping energy to the venous side where it is most needed. The iATVA device can resolve the Fontan paradox passively by reversing its gradual vascular complications. The main function of the blood turbine—the inlet/outlet of which can be reconstructed from PTFE, is to extract the available energy from the pressure potential difference that exists in typical pediatric shunts, such as the Blalock–Taussig shunt.

Figure 2

Top row: Components and assembly of the iATVA. Bottom row: Basic blade angles and impeller dimensions (in millimeters) for the turbine and pump side. The turbine and pump side of the device is designed to be enclosed in a single chamber. The completed device is made of 5 parts: turbine shroud, turbine impeller, pump shroud, pump impeller, and a connection housing in between, which is attached to both shrouds and carries the single bearing holding both impellers. All attachments are designed to be a tight fit.

Figure 3

Ideal characteristics of the iATVA system estimated from turbomachinery theory. Top row: On the left, available turbine power is plotted as a function of turbine outer diameter and outlet blade angle, for adult total cardiac output (CO) and corresponding inferior vena cava (IVC) flow split (80%). Note that the turbine power is equal to pump power divided by the total mechanical efficiency (0.50). Selected operating point of the iATVA prototype is shown as a red dot. On the right, for this operating point (shown on left) the pump side performance is plotted as a function of outlet blade angle and pump outer diameter. Characteristics for the pediatric flow conditions are similar. Bottom row: Possible venous pressure decrease ranges for the pump impeller as a function of the turbine pressure decrease (aortic minus turbine outlet) and aortic steal percent. Pediatric and adult flow conditions are shown on the left and right, respectively. LPM, Liters per minute.

Figure 4

For the 3 iATVA prototypes (P1, P2, and P3) corresponding venous pressure increase subject to a steady aortic turbine flow rate is plotted. Rotational speeds are measured at selected operating points labeled in blue. Positive pump pressures are recorded for all operating points. The 95% confidence interval for flow and pressure measurements are 0.014 liters per minute (LPM) and 0.29 mm Hg, respectively. Turbine pressure decrease varies 70 to 100 mm Hg for 1.0 to 1.3 LPM blood turbine operating flow rates. ΔP, Net venous pressure augmentation provided by the pump impeller.

Figure 5

Sample hemodynamic waveforms established for the single-ventricle (TCPC) state, with iATVA (right column) and without iATVA (left column), in the mock-up circuit. “Pump pressure” is measured at the outlet of the pump corresponding to the pulmonary artery pressure. Pediatric and adult Fontan hemodynamic conditions correspond to 3.5 and 5 liters per minute (LPM) of total cardiac output, respectively, and satisfy the corresponding SVC/IVC flow splits as stated in the text. Measurements correspond to iATVA prototype 3, which is connected to the system with a Y connector in between the IVC outlet and TCPC inlet. The pump is switched on by removing the clamps on the turbine inlet and pump inlet. Average hemodynamic values and waveforms established for the single-ventricle (TCPC) state (without iATVA) in the mock-up circuit are provided in Table 1. TCPC, Total cavopulmonary connection; iATVA, integrated aortic-turbine venous-assist; SVC/IVC, superior vena cava/inferior vena cava.

Figure 6

Performance comparison of the 3 iATVA prototypes in pulsatile hemodynamic conditions generated in the single-ventricle mock-up flow loop. Results correspond to the adult Fontan hemodynamics at 5 LPM total cardiac output. Negative ΔP values are due to bearing leakage and insufficient aortic steal. Error bars are omitted for clarity because ΔP values between the pumps are statistically significant. ΔP, Net venous pressure augmentation provided by the pump impeller.

Video 1

Assembly process of a magnetically coupled integrated aortic-turbine venous assist device (iATVA) with ceramic bearings. The iATVA prototype supporting a single-ventricle circulation, in vitro demonstrated through an instrumented mock-up flow loop. Video available at: http://www.jtcvsonline.org/article/S0022-5223(18)30717-7/fulltext.

Figure E1

Different iATVA configuration alternatives schematically illustrated. All alternatives are implanted in the single ventricle (SV) circulation. Configurations 1, 2, and 4 vary in terms of the blood turbine outlet drainage location. This study investigated configuration 1 (C1) whereby the blood turbine outlet is drained to the common atrium (ca). Configuration 2 (C2) drains to the distal of the pulmonary artery. Configuration 4 (C4) drains proximal to the axillary artery. Configuration 3 (C3) illustrates a double-inlet double-outlet venous pump configuration without a total cavopulmonary connection (TCPC). Note that there is no oxygen desaturation in iATVA configurations. UB, Upper body systemic bed; SVC, superior vena cava; RPA, right pulmonary artery; LPA, left pulmonary artery; LB, lower body systemic bed; IVC, inferior vena cava; Ao, aorta; iATVA, integrated aortic-turbine venous-assist.

Figure E2

Steady and pulsatile flow experiments. Measurement locations and iATVA connection ports are illustrated in top left illustration, corresponding to the steady flow set-up, which allows the static pressure head measurements of the venous pump at different steady aortic steal levels (top right). The iATVA was introduced in the mock-up single-ventricle circuit featuring an idealized glass TCPC replica (bottom row). The mock-up system flow rate was set by the pulse duplicator by adjusting stroke and stroke volume. Desired pressure level was maintained by changing the IVC and SVC resistances. Pulsatile pressure measurements were acquired from the aortic outlet and the iATVA pump/turbine inlets and outlets. Pulsatile flow measurements were acquired from the IVC, SVC, and turbine inlet/outlet and pump outlet. Turbine impeller speed was measured for each flow rate using a tachometer. The Y-connection established a parallel bypass circuit, which allowed for easy switching between the single-ventricle circulation with and without iATVA devices. See also Video 1, recorded during the experiments. ΔP, Net venous pressure augmentation provided by the pump impeller; iATVA, integrated aortic-turbine venous-assist; SVC, superior vena cava; TCPC, total cavopulmonary connection; IVC, inferior vena cava.

Figure E3

Venous pump flow (through the IVC) and venous pressure augmentation (ΔP) characteristics for iATVA prototype 2 (top chart) and prototype 3 (bottom chart) obtained from the pulsatile mock-up single-ventricle circulation flow loop. For both prototypes pediatric and adult operation modes corresponding to 3.5 and 5 LPM total cardiac output are tested, respectively (displayed in the red boxes). Average values of pulsatile waveforms are plotted after the iATVA is implanted in the single-ventricle circulation system. ΔP, Net venous pressure augmentation provided by the pump impeller; LPM, liters per minute.

Figure E4

For the adult Fontan hemodynamic conditions, iATVA mock-up pulsatile test results at physiological (indicated by the red box) and off-design operating conditions spanning 80 to 150 mm Hg mean aortic pressure are plotted. Only prototype 3 results are presented as trends were similar for all iATVA prototypes. Mean values for the venous flow (through the IVC pump), impeller speed, and turbine flow rates are plotted. LPM, Liters per minute; IVC, inferior vena cava; SVC, superior vena cava.

Figure E5

A close-up picture of the turbine impeller and shroud of the iATVA prototype 3 is presented. Six additional blades are produced separately using a high-accuracy 3-D micro stereolithography printer (Projet 1200; 3D Systems, Rock Hill, SC) from VisiJet FTX Green resin (3D Systems) and glued to the prototype 1 impeller. A total of 3 iATVA prototypes are tested in static as well as dynamic conditions.

Figure E6

Ideal characteristics of the iATVA system estimated from turbomachinery theory for 2 further miniaturized turbine impeller sizes of 20 and 30 mm. The total mechanical power transfer efficiency is 0.50; total cardiac output is 5 LPM; turbine aortic steal is 10% and inferior vena cava pump flow is 3.5 LPM.