right ventricular assist system feedback flow control parameter for a rotary blood pump

Right Ventricular Assist System Feedback Flow ControlParameter for a Rotary Blood Pump

*Masaharu Yoshikawa, *Kin-ichi Nakata, *Kenji Nonaka, *Joerg Linneweber,*Shinji Kawahito, *Tamaki Takano, *Sebastian Shulte-Eistrup, *Tomohiro Maeda,

*Julia Glueck, †Heinrich Schima, †Ernst Wolner, and *Yukihiko Nose

*Baylor College of Medicine, Houston, Texas, U.S.A.; and †University of Vienna, Vienna, Austria

Abstract: At least 25–30% of patients with a permanentimplantable left ventricular assist device (LVAD) experi-ence right ventricular failure; therefore, an implantablebiventricular assist system (BiVAS) with small centrifugalpumps is being developed. Many institutions are focusingand developing a control system for a left ventricular assistsystem (LVAS) with rotary blood pumps. These authorsfeel that the right ventricular assist system (RVAS) withrotary blood pumps should be developed simultaneously.A literature search indicated no recent reports on the ef-fect of hemodynamics and exercise with this type of non-pulsatile implantable RVAS. In this study, a calf with animplantable right ventricular assist system (RVAS) wassubjected to 30 min of exercise on a treadmill at 1.5 mph,resulting in excellent hemodynamics. The input voltageremained unchanged. Hemodynamic recordings weretaken every 5 min throughout the testing period, andblood gas analysis was done every 10 min. Oxygen uptake(VO2), oxygen delivery (DO2), and oxygen extraction(O2ER) were calculated and analyzed. Two different

pump flows were investigated: Group 1 low assist (<3.5L/min) and Group 2 high assist (>3.5 L/min). In bothgroups, the RVAS flow rates were unchanged while thepulmonary artery (PA) flow increased during exercise;also, the heart rate and right atrial pressure (RAP) in-creased during exercise. There were no significant differ-ences in the 2 groups. The PA flow correlates to the heartrate during exercise. In all of the tests, the VO2 and DO2increased during exercise. Regarding VO2, no changeswere observed during the different flow conditions; how-ever, the DO2 of Group 2 was higher than that of Group1. Because the implantable RVAS did not have pump flowchanges during the test conditions, it was necessary to in-corporate a flow control system for the implantableRVAS. During exercise with an implantable RVAS rotaryblood pump, incorporating the heart rate and VO2 as feed-back parameters is feasible for controlling the flow rate.

Key Words: Right ventricular assist system—Implantable right ventricular assist device—Gyro pump—Flow control—Feedback parameter—Exercise test.

Right ventricular failure is one of the major com-plications in patients assisted with a left ventricularassist system (LVAS) (1). At least 25–30% of thepatients with a permanent implantable left ventricu-lar assist device (LVAD) experience right ventricu-lar failure. Some reports suggested that the percent-age of these patients needing a biventricular assistsystem (BiVAS) was 50% to 60% of the patientpopulation having a ventricular assist system (VAS)

(2). Currently, 2 implantable pulsatile LVAS are be-ing used clinically with satisfactory results; however,they cannot be used as a right ventricular assist sys-tem (RVAS) or BiVAS because of their structureand device size (3,4). Based on these clinical de-mands, it is necessary to develop not only a smallimplantable LVAS but also small implantableRVAS rotary pumps. In addition, it is necessary todevelop a control system. However, many institu-tions are focusing and developing only a control sys-tem for an LVAS. These authors feel that the controlsystem for the RVAS with rotary blood pumpsshould be developed simultaneously. The literaturesearch indicated no recent reports on the effect ofhemodynamics and exercise with this type of non-pulsatile implantable RVAS.

Received December 1999.Presented in part at the 7th Congress of the International So-

ciety for Rotary Blood Pumps, held August 26–27, 1999, in Tokyo,Japan.

Address correspondence and reprint requests to Dr. MasaharuYoshikawa, Nagoya University, School of Medicine, Departmentof Cardio-thoracic Surgery, 65 Tsurumai-cho, Showa-ku, Nagoya,466–0065, Japan.

Artificial Organs24(8):659–666, Blackwell Science, Inc.© 2000 International Society for Artificial Organs

659

In this study, the effect of exercise on the animalimplanted with an RVAS was investigated, and thehemodynamic parameters were assessed as feedbackfactors for the RVAS control system.

MATERIALS AND METHODS

The experiments were conducted in accordancewith the Guide for the Care and Use of LaboratoryAnimals published by the National Institutes ofHealth (NIH publication No. 86–23, Revised 1985)and the Principles of Laboratory Animal Care for-mulated by the National Society for Medical Re-search.

A half Dexter strain calf (70 kg) was subjected toan implanted ventricular assist devise (VAD) study.The cell blood counts (CBC), blood chemistries,plasma free hemoglobin, and prothrombin time wereexamined 1 week prior to implantation after the calfwas quarantined for 3 weeks.

Experimental system configuration of the GyroImplantable ventricular assist system

In this RVAS study, the pump-actuator systemwas implanted in the preperitoneal space under thediaphragm, and the percutaneous actuator cable wastunneled through the calf’s back. A custom-madeinflow cannula was inserted into the infundibulum.The outflow extension was an albumin-coated vas-cular prosthesis (Bard Albumin Coated DeBakeyVascular II, C.R. Bard, Inc., Billerica, MA, U.S.A.)that carries the blood to the main pulmonary artery.

Blood pumpThe tested blood pump was the Gyro PI 700 series,

which has been developed as an implantable VAS.The PI 700 series is a centrifugal pump with the fol-lowing dimensions: impeller diameter of 50 mm, cas-ing diameter of 65 mm, height of 45 mm, and primingvolume of 25 ml. Also, this pump has the followingdesign characteristics. First it has a magnetic cou-pling system and a pivot bearing system to obtain asealless pump casing. Next, secondary vanes are lo-cated at the bottom of the impeller to accelerateblood flow. Third, an eccentric inlet port enables thetop female pivot bearing to be embedded into thetop housing of the pump. The housing and impellerof the PI 700 series are fabricated from titanium al-loy, titanium 6 aluminum 4 vanadium (5) (Fig. 1).

Actuator and driverThe actuator-driver system used in this study was

developed and supplied by the University of Vienna.This actuator contains a DC brushless motor that isconstituted with the coil fixed in a plastic mount andhas a rotating disk with permanent magnets. The

actuator housing is made of titanium and is hermeti-cally sealed (Fig. 1).

Inlet cannulaThe prototype inlet cannula for the RVAS was

designed and fabricated. This cannula consists of 2parts: a hat-shaped silicone tip covered with Dacronfabric and an angled wire reinforced tube made ofpolyvinylchloride. If these 1 month feasibility studieswith an RVAS implantation are successful, an inflowcannula will be fabricated utilizing a polyurethanecopolymer for long-term experiments. The surface ofthe cannula tip was biolized with gelatin to obtain asuperior antithrombogenic surface (6,7) (Fig. 2).

Exercise testOnce the animals recovered from the effects of

surgery and anesthesia and reached a stable state, ex-ercise was performed on a specially devised treadmillfor 30 min. The experimental animal was put on thetreadmill, and the pump flow was set 15 min beforeexercise because it takes at least 5 min to relax the

FIG. 1. The photograph shows the Gyro PI700 series made oftitanium alloy and actuator.

FIG. 2. The photograph shows the inlet cannula for the RVAS.

M. YOSHIKAWA ET AL.660

Artif Organs, Vol. 24, No. 8, 2000

calf and attain a steady pump flow. The control dataand blood sample were taken just before exercisingthe animal. The treadmill speed was gradually in-creased to 1.3 mph at 0 slope. The pump voltage wasnever changed during the exercise period. Bloodsamples were taken every 10 min, and other perti-nent data were recorded every 5 min.

The exercise period lasted for 30 min, and then thepostexercise data were recorded every 5 min for 15min. If repeating the exercise, at least a 30 min in-terval was taken before starting the next exercisestudy.

Data collection of hemodynamics, blood gas,pump performance

The arterial pressure (AoP), the right atrial pres-sure (RAP), and the heart rates were recorded witha multichannel recorder (Gould Brush, Gould Inc.,Cleveland, OH, U.S.A.). The implantable flowprobe (Transonic System Inc., Ithaca, NY, U.S.A.)was attached on the outflow graft and connected tothe Transonic animal research flowmeter modelT206. The same implantable flow probe was at-tached on the peripheral main pulmonary arterywhere the outflow graft was anastomosed to mea-sure the pulmonary artery flow as cardiac output(CO). Information of pump performance such as ro-tational speed, voltage, current, pump flow, wave-form of current, and waveform of the pump flow wasmonitored on the computer display and recorded ev-ery 5 min in a personal computer.

Systemic vascular resistance (SVR) was calculatedaccording to this formula:

SVR 4 80 × (AoP − RAP)/CO

The arterial and mixed venous blood gas, which in-cluded oxygen saturation, pH, and partial pressuresof oxygen and carbon dioxide, were measured with ablood gas analyzer.

The relationship between oxygen transfer and im-plantable RVAS performance during exercise wasanalyzed with the blood gas data.

The oxygen transfer rate (DO2), the oxygen con-sumption (VO2), and the oxygen extraction ratio(O2ER) were derived from the following formulas:

DO2 4 Q × (1.3 × Hgb × SaO2) × 10

VO2 = Q × 13 × Hgb × (SaO2 −SvO2)

O2ER = VO2/DO2 × 100

where Q is CO. Cardiac index (CI), CO/BSA, is usu-ally used clinically; however, CO was used instead ofCI in this study because converting the formula for

the body surface area (BSA) of the calf does notexist. Hgb is hemoglobin, SaO2 is arterial oxygensaturation, and SvO2 is mixed venous oxygen satu-ration (8,9).

In this study, the initial flow rate of the implant-able RVAS was divided into 2 groups; an initial flowrate over 3.5 L/min was the high flow group and aninitial flow rate below 3.5 L/min was the low flowgroup. All data were compared in these 2 groups. Alldata was expressed as mean ± standard error of themean (SEM) and their statistical significance

RESULTS

Hemodynamic parameters and RVAS flow rateRVAS flow did not change before and during ex-

ercise and was constant throughout each exercisestudy in both groups (Fig. 3A).

The pulmonary artery (PA) flow significantly in-creased during exercise in both groups (p < 0.01)(Fig. 3B). Also, the PA flow in the high flow groupwas significantly higher than that in the low flowgroup before and during exercise (p < 0.05). Further-more, the PA flow linearly correlated with pumpflow within this RVAS flow range from 1 to 6 L/minthroughout the exercise study (Fig. 3C).

The HR also significantly increased during exer-cise in both groups (p < 0.01); however, there wasno significant difference between both groups (Fig.4A).

Arterial pressure did not show a significant differ-ence before, during, and after exercise. There was nosignificant difference between groups (Fig. 4B).

The RAP increased significantly during exercise inboth groups (p < 0.01); however, there was no sig-nificant difference between groups (Fig. 4C ).

SVR decreased significantly during exercise inboth groups (p < 0.01), and SVR in the low flowgroup was significantly higher than that in the highflow group before, during, and after exercise (p <0.05) (Fig. 5). The RVAS flow did not correlate withSVR.

Oxygen consumption and deliveryThe VO2 significantly increased during exercise in

both groups (p < 0.01); however, no significant dif-ference was shown between groups (Fig. 6A).

The DO2 significantly increased during exercise inboth groups (p < 0.01), and the DO2 in the high flowgroup was significantly higher than that in the lowflow group before, during, and after exercise (p <0.05) (Fig. 6B).

FEEDBACK PARAMETER FOR RVAS FLOW CONTROL 661


FIG. 3. The graphs show that theRVAS flow rate did not change be-fore, during, and after exercise(A); PA flow rate increased duringexercise (p < 0.01), and PA flow inthe high flow group was higherthan that in the low flow group be-fore and during exercise (p < 0.05)(B); and PA flow also was linearlycorrelated with pump flow withinthis RVAS flow range from 1 to 6L/min (C).



Regarding O2ER, this showed a significant in-crease during exercise (p < 0.01); however, a signifi-cant difference was not demonstrated in either group(Fig. 6C).

DISCUSSION

Currently, implantable LVAS with rotary bloodpumps are being developed in many institutions;however, an implantable RVAS with a rotary blood

FIG. 4. The graphs show thatheart rate increased during exer-cise (p < 0.01) (A), arterial pres-sure did not show any difference(B), and right atrial pressure in-creased during exercise (p < 0.01)(C).



pump is rarely a developmental undertaking. Ac-cording to clinical results, it is necessary to developnot only paracorporeal RVAS but also implantableRVAS. Our final goal is the development of a totallyimplantable VAS with rotary blood pumps that canbe universally LVAS, RVAS, and BiVAS (10). Thecontrol system for the implantable VAS is being de-veloped. Regarding LVAS, many institutions are inthe process of developing a control system; however,there is almost no data and control system for theimplantable RVAS using a rotary blood pump. Inthis study, the physiological influence and the neces-sity of a flow control were studied in an experimentalanimal that was implanted with an RVAS rotaryblood pump.

In this model, the oxygen consumption increasedduring exercise. However, the increased oxygen de-livery compensated for the oxygen demand becausethe total systemic and the pulmonary blood flow in-creased during exercise. In this study, the nativeheart could increase the systemic and pulmonaryblood flow because the RVAS flow did not show anychange throughout each study and the native heartremained almost within normal function. However,with the RV failure model, it may be necessary tocontrol the RVAS flow in order to increase its flowrate during exercise because the failing right ven-tricle could not increase pulmonary blood flow.

This study revealed that the heart rate, RAP,cardiac output, and VO2 increased and the systemicvascular resistance decreased during exercise withthe implantable RVAS model. Unfortunately, therelationship between the pulmonary vascular resis-tance and RVAS flow was not proven in this studybecause the pulmonary artery and the left atrialpressure lines were not available. However, itwas found that SVR and RAP do not influence theRVAS flow rate.

According to our previous experience with the im-plantable LVAS using rotary blood pumps, the PAflow and cardiac output do not correlate with LVASflow. The LVAS flow fluctuates depending on theSVR and left ventricular pressure. However, in thisimplantable RVAS study, the PA flow and cardiacoutput linearly correlate with the RVAS flow. This iswithin the limited RVAS flow range of 1 to 7 L/min,and that should be regulated by the preload. Thisphenomenon also supports the result that the oxygendelivery demonstrates a significant difference be-tween the high flow group and low flow group be-cause the oxygen delivery is defined by arterial oxy-gen saturation and cardiac output, and the arterialoxygen saturation was almost constant throughoutthese exercise studies.

Heart rate and VO2 are appropriate feedback pa-rameters for the flow control system during exercise.The heart rate is simple and easy to monitor. How-ever, it is necessary to combine these with other pa-rameters that can detect exercise because the heartrate is sensitive not only to exercise but also to thefactors that evoke the sympathetic nerve activationand suppress the parasympathetic nerve. A combi-nation of heart rate and VO2 may be adequate feed-back parameters for the flow control system. How-ever, a combination of these 2 parameters may notbe ample for feedback parameters because a mul-tiple combination of activity sensors already havebeen used for pacemakers. The advantage of a mul-tiple combination is to reduce a false-positive erroror false-negative error. Each sensor compensates forthe weak point of the other, but there is a disadvan-tage. An increase of the sensors may create a morecomplicated system and increase the risk of a feed-back system malfunction that may be the result fromsensor durability because each sensor has a lifetime.From the standpoint of detecting exercise, the accel-erometer used for pacemakers may be another can-didate for the feedback parameter although it wasnot used in this study. In future studies, the acceler-ometer will be evaluated in order to establish a flowcontrol system (11,12).

The future direction of this study will be one thatencompasses both intact heart model studies andchronic right ventricular failure model studies. Moreinformation and analyses such as the relation be-tween RVAS flow and pulmonary vascular resis-tance is required for the intact heart model. Thesame type of study should be performed and evalu-ated in the chronic right ventricular failure model.However, it may not be easy to create a chronic rightventricular failure model in a calf.

FIG. 5. The graph shows that systemic vascular resistance dur-ing exercise (p < 0.01) and SVR in the high flow group were lowerthan that in the low flow group (p < 0.05).



CONCLUSIONSThis study revealed that it is necessary to incorpo-

rate a flow control system for the implantableRVAD with a rotary blood pump because theRVAS flow rate remains a constant and is not influ-

enced during exercise and the oxygen delivery isdependent on the RVAS flow rate. During exercisewith an implantable rotary blood pump RVAS, in-corporating the heart rate and VO2 as feedback pa-rameters is feasible for controlling the flow.

FIG. 6. The graphs show thatoxygen consumption (VO2) in-creased during exercise (p < 0.01)(A) ; oxygen delivery (DO2) in-creased during exercise (p <0.01), and DO2 in the high flowgroup was higher than that in thelow flow group (p < 0.05) (B); andoxygen extraction (O2ER) in-creased during exercise (p < 0.01)(C).



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right ventricular assist system feedback flow control parameter for a rotary blood pump

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