Outlet Cannula Research Articles

Abstract 12777: The Lingering Snag in Era of Mechanical Pumps: A Case of Acute Pump Thrombosis Immediately After Heartmate 3 Implantation

Background: Pump thrombosis (PT) remains a dreaded complication of left ventricle assist device (LVAD) implantation. PT immediately after HeartMate 3 (HM3) implantation is rare but should be kelp in mind if early low flow alarms are noted. Case: A 57-year-old male with ischemic cardiomyopathy, Stage D heart failure (LVEF ~20%), INTERMACS category 3 on 5mcg/kg/min Dobutamine drip underwent advanced heart failure therapy evaluation and was deemed suitable candidate for LVAD implant. His ECHO showed LVIDD 6.04 cm and LV Diastolic Volume Index 122 mL. He underwent HM3 implantation via left anterior thoracotomy and upper-hemi sternotomy on 5/24/2022. Surgery was successful and the patient was taken off bypass. Pre-implant some LV trabeculations and debris were removed. However, less than 1 hour from being off bypass, the low flow alarms began which did not resolve after volume and blood pressure optimization. No RV failure was noted on TEE. Decision was made to re-open the sternum. Upon exploration, no compression of the outflow graft was noted, and inlet cannula was not mispositioned. Inspection of LVAD inlet and outlet cannula showed significant amount of white thrombus (Figure 1) and device was ultimately exchanged. Thrombus was confirmed on histopathological analysis. Post-operatively no low flow alarms were noted after pump exchange. Factor V Leiden screen and protein C activity were normal and Antithrombin 3 was falsely low (due to recent thrombus) on post-op day 1. Discussion: This is the second case of HM 3 device thrombosis within 1 hour of implant in the literature. Most of the earlier reported cases were on post-Op Day 3. The etiology of PT is unknown and further investigation is required to elucidate the same. This case will create awareness among physicians to keep PT in differentials when low flow alarms are encountered immediately post-op despite volume and BP optimization.

A Valveless Pulsatile Pump for the Treatment of Heart Failure with Preserved Ejection Fraction: A Simulation Study.

Effective treatment of patients with terminal heart failure and preserved ejection fraction (HFpEF) is an unmet medical need. The aim of this study was to investigate a novel valveless pulsatile pump as a therapeutic option for the HFpEF population through comprehensive in silico investigations. The pump was simulated in a numerical model of the cardiovascular system of four HFpEF phenotypes and compared to a typical case of heart failure with reduced ejection fraction (HFrEF). The proposed pump, which was modeled as being directly connected to the left ventricle, features a single valveless inlet and outlet cannula and is driven in co-pulsation with the left ventricle. We collected hemodynamics for two different pump volumes (30 and 60mL). In all HFpEF conditions, the 30mL pump improved the cardiac output by approximately 1L/min, increased the mean arterial pressure by > 11% and lowered the mean left atrial pressure by > 30%. With the larger (60mL) stroke volume, these hemodynamic improvements were more pronounced. In the HFrEF condition however, these effects were three times less in magnitude. In this simulation study, the valveless pulsatile device improves hemodynamics in HFpEF patients by increasing the total stroke volume. The hemodynamic benefits are achieved with a small device volume comparable to implantable rotary blood pumps.

Characterizing the HeartMate II Left Ventricular Assist Device Outflow Using Particle Image Velocimetry.

Ventricular assist devices (VADs) are implanted in patients with a diseased ventricle to maintain peripheral perfusion as a bridge-to-transplant or as destination therapy. However, some patients with continuous flow VADs (e.g., HeartMate II (HMII)) have experienced gastrointestinal (GI) bleeding, in part caused by the proteolytic cleavage or mechanical destruction of von Willebrand factor (vWF), a clotting glycoprotein. in vitro studies were performed to measure the flow located within the HMII outlet cannula under both steady and physiological conditions using particle image velocimetry (PIV). Under steady flow, a mock flow loop was used with the HMII producing a flow rate of 3.2 L/min. The physiological experiment included a pulsatile pump operated at 105 BPM with a ventricle filling volume of 50 mL and in conjunction with the HMII producing a total flow rate of 5.0 L/min. Velocity fields, Reynolds normal stresses (RNSs), and Reynolds shear stresses (RSSs) were analyzed to quantify the outlet flow's potential contribution to vWF degradation. Under both flow conditions, the HMII generated principal Reynolds stresses that are, at times, orders of magnitude higher than those needed to unfurl vWF, potentially impacting its physiological function. Under steady flow, principal RNSs were calculated to be approximately 500 Pa in the outlet cannula. Elevated Reynolds stresses were observed throughout every phase of the cardiac cycle under physiological flow with principal RNSs approaching 1500 Pa during peak systole. Prolonged exposure to these conditions may lead to acquired von Willebrand syndrome (AvWS), which is accompanied by uncontrollable bleeding episodes.

Abstract 12486: An Early Survey of the Incidence and Distribution of LVAD Thrombus Using a High-avidity Tc-99m Fibrin Probe

Background: 99mTc-F4A is a tetrameric, fibrin-specific probe characterized in vitro, in mice and in left ventricular assist devices (LVAD) explanted from patients. Objective: To survey the incidence and spatial distribution of thrombus by 99mTc-F4A signal intensity in LVADs removed for pump exchange (n=9) or heart transplant (n=11) in an early cohort of patients. Methods: LVADs excised from patients were rewired and operated (~9,400 RPM) in a mock circulatory loop (1/2 tygon, 200ml, 50:50, PBS: plasma; heparin). 99mTc-F4A (500μCi) was injected into the loop, circulated for 30 min. Activity was localized within LVADs by NanoSPECT/CT. Results: The median time of LVAD support prior to explant was 7.6 ± 7.2 months. Thrombus was detected in 19/20 LVADs by high 99mTc-F4A signal and corroborated by gross pathology in 4 LVADs by the manufacturer. 99mTc-F4A signal was scored as strong, weak, or none, and was assessed for the inlet cannula (metallic and Gor-Tex tubing segments), the inlet stator/bearing, turbine, outlet bearing, and outlet cannula regions. Inlet cannulas were available for 8/20 LVADs with 7/8 having thrombus. Strong signal was noted in 4/7 of the metallic segment and in 5/7 of the Gor-Tex segments. 99mTc-F4A signal occurred around the inlet bearing in 8/20 LVADs and the outlet bearing region in 5/20 LVADs, but the incidence of thrombus within the axial turbine was negligible. Thrombus was observed in the outlet cannula in 10/20 pumps. (Fig) Strong 99mTc-F4A signal was noted most often at the inlet bearing 75% (6/8) and in outlet cannulas 50% (5/10). Strong signal was noted in 13/20 LVADs in at least one location. Of 5/20 LVADs with multiple deposits, 4/5 of these pumps had a strong 99mTc-F4A signal. Conclusion: These initial ex vivo survey data reveal a high incidence of LVAD thrombus in exchanged and routinely transplanted LVADs. 99mTc-F4A imaging may offer a sensitive and specific diagnostic tool to noninvasively assess and better manage LVAD thrombus.

Abstract 16514: Screening for Outflow Cannula Malfunction of Left Ventricular Assist Devices Using Doppler Echocardiography: Establishing New Reference Values

Background: Echocardiographic assessment of left ventricular assist devices (LVAD) is used as a screening tool to evaluate the integrity and mechanics of the pump and circuit. We aimed to 1) establish the normal range and upper reference limit of peak velocity of the outflow cannula for the modern era of LVADs and 2) assess the performance of the current reference limit and our new reference limit in patients with continuous flow LVADs as a screening tool for cannula malfunction. Methods: LVAD outflow peak velocities were measured with Doppler echocardiography (TTE) in 57 patients with LVADs (HeartMate II (HMII) = 44, HeartWare (HW) = 13). The average velocity and the upper and lower normal reference limits (defined as +/- two standard deviations from the mean) for each LVAD type was then calculated. The upper reference limit was then used as a screening threshold for cannula malfunction. Results: The average outflow cannula peak velocity for the HMII cohort was 1.94 m/sec +/- 0.42 with an upper and lower reference limit of 2.78 m/sec and 1.10 m/sec, respectively. The average outflow cannula peak velocity for the HW cohort was 2.36 +/- 0.53 with an upper and lower reference limit of 3.42 m/sec and 1.3 m/sec, respectively, which was significantly higher than the HMII cohort (p=0.004). Velocities from both cohorts were dramatically higher than the commonly citied upper reference limit velocities of 2 m/sec. Use of the previously cited reference limit poorly discriminated between normal and malfunctioning LVAD cannulas whereas our new reference limit identified all four patients with outlet cannula malfunction (Figure1). Conclusions: In both HeartMate II and HeartWare LVADs, the average peak outflow velocity and reference limit for the normal population as measured by Doppler-TTE was markedly higher than currently cited LVAD reference limits. Patients with peak outflow velocities above our upper reference limit should be evaluated for LVAD outflow cannula malfunction.

Anatomy and Physiology of Left Ventricular Suction Induced by Rotary Blood Pumps.

This study in five large greyhound dogs implanted with a VentrAssist left ventricular assist device focused on identification of the precise site and physiological changes induced by or underlying the complication of left ventricular suction. Pressure sensors were placed in left and right atria, proximal and distal left ventricle, and proximal aorta while dual perivascular and tubing ultrasonic flow meters measured blood flow in the aortic root and pump outlet cannula. When suction occurred, end-systolic pressure gradients between proximal and distal regions of the left ventricle on the order of 40-160 mm Hg indicated an occlusive process of variable intensity in the distal ventricle. A variable negative flow difference between end systole and end diastole (0.5-3.4 L/min) was observed. This was presumably mediated by variable apposition of the free and septal walls of the ventricle at the pump inlet cannula orifice which lasted approximately 100 ms. This apposition, by inducing an end-systolic flow deficit, terminated the suction process by relieving the imbalance between pump requirement and delivery from the right ventricle. Immediately preceding this event, however, unnaturally low end-systolic pressures occurred in the left atrium and proximal left ventricle which in four dogs lasted for 80-120 ms. In one dog, however, this collapse progressed to a new level and remained at approximately -5 mm Hg across four heart beats at which point suction was relieved by manual reduction in pump speed. Because these pressures were associated with a pulmonary capillary wedge pressure of -5 mm Hg as well, they indicate total collapse of the entire pulmonary venous system, left atrium, and left ventricle which persisted until pump flow requirement was relieved by reducing pump speed. We suggest that this collapse caused the whole vascular region from pulmonary capillaries to distal left ventricle to behave as a Starling resistance which further reduced right ventricular output thus contributing to a major reduction in pump flow. We contend that similar complications of manual speed control also occur in the human subject and remain a major unsolved problem in the clinical management of patients implanted with rotary blood pumps.

Analysis of Left Ventricular Impedance in Comparison With Ultrasound Images

Cardiac monitoring of ventricular assist devices (VADs) is important for detecting heart failure risks, such as critical arrhythmia and ventricular fibrillation, and for supplying data that are useful for hemodynamic control. Specifically, impedance cardiograms (ICGs) are especially beneficial because they have no effect on the tissue or organs and can monitor various parameters simultaneously, including the heart rate and heart contractions. In this article, we measured impedance changes in porcine left ventricles using electrodes placed around the inlet and outlet cannulae of the VAD. The measured left ventricular impedance (LVI) waveform changes are caused by heart movements, such as cardiac muscle contraction and changes in blood volume as a result of heart filling and emptying. In contrast to other impedance measurements, LVI is less affected by the movement of other organs. Using a porcine model, LVIs were measured and compared with blood flow data measured with an ultrasound blood flowmeter. The ICG showed the same frequency as the animal's heart rate, and their amplitudes were closely related to cardiac output (CO). However, the waveform differed from other vital signs, such as CO, electrocardiogram, and blood pressure. Ultrasound images were used to explain the impedance waveform. In the ultrasound images, we obtained the shape and size of the animal's heart and calculated the predicted impedance data. We then compared these to the actual measured data. These results show that the impedance signal contains detailed information on heart rate and CO; these results were unaffected by the cannulae or VAD perfusion.

Analysis of Pulsatile and Nonpulsatile Blood Flow Effects in Different Degrees of Stenotic Vasculature

Vessel lumens that have been chronically narrowed by atherosclerosis should be increased in flow velocity and intrastenotic area pressure to maintain an equal flow. This might be followed by a decrease in hemodynamic energy, leading to a reduction of tissue perfusion. In this study, we compared hemodynamic energies according to degrees of stenotic vasculature between pulsatile flow and nonpulsatile flow. Cannuale with 25, 50, and 75% diameter stenosis (DS) were located at the outlet cannula. Using the Korea Hybrid ventricular assist device (KH-VAD) (pulsatile pump: group A) and Biopump (nonpulsatile pump: group B), constant flow of 2 L/min was maintained then real-time flow and velocity in the proximal and distal part of the stenotic cannula were measured. The hemodynamic energies of two groups were compared. At 75% DS, proximal energy equivalent pressure (EEP) delivered to the distal end was only 41.9% (group A) and 42.5% (group B). As the percent EEP fell below 10%, pulsatility disappeared from the 50% stenosis in group A. The surplus hemodynamic energy (SHE) of group B at all degrees of stenosis must have been 0, which was also the case of group A at 75% stenosis. This research evaluated the hemodynamic energy on various degrees of DS in both pulsatile and nonpulsatile flow with mock system. Using a pulsatile pump, pulsatility disappeared above 50% DS while hemodynamic energy was maintained. Therefore, our results suggest that pulsatile flow has a better effect than nonpulsatile flow in reserving hemodynamic energy after stenotic lesion.

The Effect of Fluid Viscosity on the Hemodynamic Energy Changes During Operation of the Pulsatile Ventricular Assist Device

Blood viscosity during operation of ventricular assist device (VAD) can be changed by various conditions such as anemia. It is known generally that the blood viscosity can affect vascular resistance and lead to change of blood flow. In this study, the effect of fluid viscosity variation on hemodynamic energy was evaluated with a pulsatile blood pump in a mock system. Six solutions were used for experiments, which were composed of water and glycerin and had different viscosities of 2, 2.5, 3, 3.5, 4, and 4.5 cP. The hemodynamic energy at the outlet cannula was measured. Experimental results showed that mean pressure was increased in accordance with the viscosity increase. When the viscosity increased, the mean pressure was also increased. However, the flow was decreased according to the viscosity increase. Energy equivalent pressure value was increased according to the viscosity-induced pressure rise; however, surplus hemodynamic energy value did not show any apparent changing trend. The hemodynamic energy made by the pulsatile VAD was affected by the viscosity of the circulating fluid.

Non-Invasive Flow Measurement of a Rotary Pump Ventricular Assist Device Using Quantitative Contrast Echocardiography

Many implantable ventricular assist devices (VADs) have no direct measurement of pump output. The aim of this study was to test the hypothesis that quantitative contrast echocardiography can be used to measure VAD output. Contrast-enhanced Doppler velocity-time integral (VTI) was measured in the VAD inlet and outlet cannulae. Doppler flow (Doppler Q=Doppler VTIxcannula area) was compared with measured flow (Q). A total of 130 flow measurements were made (at 6400 and 12,000 rpm). Doppler Q in the outflow and inflow cannulae showed an excellent correlation with measured Q (outlet Doppler Q=1.0052 xQ+0.048, R2=0.9865; inlet Doppler Q=1.5043 xQ+0.003, R2=0.9904), but inlet Doppler Q was 50% higher. Correcting for the flow profile of the conical inlet tube yielded excellent correlation (inlet Doppler Q=1.0029 xQ+0.002, R2=0.9904). Noninvasive Doppler flow techniques can be used to accurately measure VAD flow, but the flow profile in the cannula needs to be taken into account.

Novel Transaortic Double Barrel Ventricular Cannula

A novel transaortic ventricular cannula, known as the ‘double barrel’ cannula (DBC), is designed to minimize the invasiveness of Ventricular Assist Device (VAD) implantation by combining the inlet and outlet cannulae into a single dual lumen cannula. Both flows will pass through a single opening in the apex of the Left Ventricle with the outflow then continuing past the aortic valve, into the aortic arch. This design offers several potential advantages over the current state-of-the-art. These include less invasive surgery and providing mechanical support to the septum. By routing the outflow through the aortic valve, the need to access the external structure of the ascending aorta is eliminated thereby eliminating the need for open heart surgery. In determining the DBC's design, close attention has been paid to the outflow portion of the cannula, which passes through the aortic valve. It was anticipated that this portion of the DBC could have the largest impact on the device's usability in practice. The object of this study was to test the performance of the valve with the cannula passing through it. Three different geometries are circular, equilateral triangular, and one-third semicircular. Experiments measuring aortic insufficiency during the diastolic phase were conducted. The experiment was designed to analyze several geometries passing through an aortic valve under diastolic flow conditions. All experiments used a simple flow loop which allows a natural porcine aortic valve to be viewed from downstream. The loop was driven with a pneumatic ventricular simulator in order to produce a cyclic flow. Three cannulae cross-sections were molded from RTV11 Silicone. During this test, High Speed Cinematography and flow rate measurement were used to quantify valve sealing and leakage. All data was collected and analyzed for the three cross-sectional geometries during diastole. The performances of the three geometries were compared using American Heart Association guidelines of aortic insufficiency (Al) diagnosis. The flow rate data was integrated in order to determine the volume of ventricle ejection and valve regurgitation. All three geometries exhibit low and acceptable levels of Al (⩽15%Al), with the circular geometry causing the least amount of Al at 7.1%. The experimental control (Porcine aortic valve with no cannulae) exhibited an Al or 2.4%, validating both the harvested aortic valve and experimental flow loop for further testing. Using the high speed cinematography, several phenomena were observed during the sealing of the porcine valve; including leaflet folding leaflet bunching, and cannula displacement due to valve closure.

HYDRODYNAMIC AND HEMODYNAMIC DESIGN IMPROVEMENT OF THE MAGNETICALLY LEVITATED HEARTQUEST LVAD

The HeartQuest LVAD is a magnetically levitated blood pump featuring an unshrouded centrifugal impeller and open blood flow journal within the magnetic gap. Both in-vitro and CFD analysis have shown that flow demonstrates increased residence time, flow separation, and reversal at off-design conditions. This study investigated design improvements, specifically to the impeller and volute, that would both shift and widen the optimal operating conditions of this pump. All other features of the device, including the magnetic subsystems, impeller diameter, inlet volute, and outlet cannula dimensions were not changed. The design improvement was performed with our CFD-based design optimization tool. The impeller, volute, and diffuser were optimized with a high-performance hydrofoil profile replacing the inner profile of the current design and with gradually changed sectional areas of the diffuser. Extensive CFD analyses show that the best efficiency point of the new design was shifted to approximately 5.5lpm from original 9.0lpm and the magnitude of maximal efficiency was increased significantly with a lower level of predicted hemolysis. Within a wider flow rage, CFD results revealed that there are no unfavorable reverse flow and separation in the diffuser between 2.0 and 8.0lpm. These results demonstrate the value of hemodynamic optimization to improve the favorable operating range of a turbodynamic blood pump.

ALGORITHM OF CO-PULSATION ESTIMATION FOR THE COUNTERPULSATION USING THE VAD OUTLET CANNULA PRESSURE

The ventricular assist device(VAD) helps to reduce the load against the patient’s native heart (NH). The pulsatile VAD generates pulsatile flow. If the pulsatile VAD pumps the blood simultaneously with the NH, the NH experiences elevated afterload and this could deteriorate the NH rather than help to recover. Thus, counterpulsation algorithms to avoid co-pulsation have been adopted by many VADs, however these utilize electrocardiography and arterial pressure signals which may have difficulties to acquire consistently for a long period. In this study, the co-pulsation estimation algorithm for the counterpulsation is developed using the VAD outlet cannula pressure signal. This signal is good to maintain for a long time. The pulse pressure by the NH is reflected to the VAD outlet pressure signal. Thus, the VAD outlet pressure signal was separated into pulse peaks by the VAD and the NH using Fast Fourier Transform, and the pulse rate and pulse peak time of each the NH and the VAD were calculated. From these results, the next co-pulsation time was estimated, and these are calculated using the MATLAB software. The experiments were performed for various pump rate conditions with mock circulation system and the performance was evaluated. For all conditions, the co-pulsation time was estimated well. This co-pulsation estimation algorithm could be used in the counterpulsation algorithm of the implantable VAD or the extracorporeal life support system.

Energy Equivalent Pressure and Total Hemodynamic Energy Associated with the Pressure-Flow Waveforms of a Pediatric Pulsatile Ventricular Assist Device

A pulsatile pediatric ventricular assist device (VAD) with a dynamic stroke volume of approximately 12 ml was tested to quantify the effect of flowrate and systolic duration on pulsatility as quantified by the energy equivalent pressure (EEP), defined as the hemodynamic energy per unit volume of fluid pumped. The VAD was tested on a mock circulatory loop, adjusted to maintain a systemic arterial pressure of approximately 90/60 mm Hg (systolic/diastolic) and a mean of 75 mm Hg. The EEP was calculated for each beat for 1 minute at both the proximal end of the pump outlet cannula and at the distal end (arterial EEP). Nominal mean flowrates were 0.50, 0.75, 1.00, and 1.25 l/min. Systolic duration was set at either 230 or 400 milliseconds. With a rapid systolic ejection (230 milliseconds), the arterial EEP ranged from 5.58% to 8.41% relative to the mean arterial pressure. The highest EEP occurred at the lowest flowrate. With a slower (400 milliseconds) systolic ejection, the arterial EEP ranged from 2.33% to 4.20%. Hemodynamic energy loss in the outlet cannula was also quantified by the differential EEP and shown to increase markedly as systolic duration was decreased, but was relatively insensitive to mean flowrate.

The Bicircuit System: Innovative Perfusional Options for Surgical Treatment of the Thoracic Aorta

Purpose Surgical treatment of the thoracic aorta may become challenging when a rapid switch from left heart bypass (LHB) to cardiopulmonary bypass (CPB) is required. Description We designed a BICIRCUIT system using a centrifugal pump, a heparinized CPB circuit with a hollow fiber oxygenator, two 3/8 × 3/8 × 3/8 connectors (one placed at the bell inlet draining blood from the left atrium or the venous reservoir and the second placed at the bell outlet directing blood to the oxygenator or femoral artery). Our priming volume was 1100 mL; when switching from LHB to CPB, no additional priming volume was required. The inlet cannula was inserted in the left atrium (for LHB) or femoral vein (for CPB); the outlet cannula was placed in the femoral artery. Evaluation We used the BICIRCUIT in 18 patients: 7 patients with a thoracoabdominal aneurysm, 7 patients with a traumatic rupture of isthmic aorta, and 4 patients with a Stanford type B aortic dissection. Conversion to CPB was required in 3 patients because of hemodynamic deterioration and in 1 patient because of hypothermic circulatory arrest. As we observed progressive worsening of blood gases in another patient during LHB, we also used the blood oxygenator without changing the position of the cannulas. No system failures were observed. Postoperative spinal disturbances did not develop in any patients. All patients were successfully discharged. Conclusions Our BICIRCUIT system offers three different options: LHB, LHB along with blood oxygenation, and CPB. Each option can be safely achieved and adds to the armamentarium of surgeons and perfusionists in caring for patients with pathology of the thoracic aorta.

LEVITRONIX CENTRIMAG® SHORT-TERM VENTRICULAR ASSIST SYSTEM IN THE BOVINE MODEL

The Levitronix CentriMag® Blood Pump is the first disposable centrifugal pump designed for short-term ventricular assistance (LVAD). The device, based on advanced magnetic levitation technology, allows for operation in blood without mechanical bearings and seals. Preclinical studies were conducted in vivo to demonstrate safety of the CentriMag®. The device was implanted (n=5) surgically via left thorocotomy. The inlet and outlet cannulae were inserted into the left ventricular apex and the descending aorta, respectively. All calves were anticoagulated with heparin and coumadin. The pump was run continuously for 1 month (2X safety factor for humans). Pump speed was adjusted to maintain a target flow of 5.0L/min. Plasma free hemoglobin (PFH), complete blood count, chemistry, and coagulation profiles were determined on a daily basis. At study term, full necropsy and histology were performed. Pump data were collected for an average of 28±2 days. The average pump speed was 3300±300 rpm; corresponding average flow was 5.0±2.0 L/min. All calves reached the scheduled end point without exhibiting any hemolysis, end-organ dysfunction, or adverse effects related to the device. Renal and hepatic function, hematological parameters, and PFH remained normal. Our data indicate that the Levitronix CentriMag® pump is an easily implantable, thrombus-free, and nonhemolytic centrifugal LVAD that provides adequate flow. This suggests that the device is ready for clinical use as a short-term LVAD.

Flow through an outlet cannula of a rotary ventricular assist device.

Rotary ventricular assist devices (VADs) are primarily used during surgery and short-term postoperative assistance. There are issues involving the use of rotary VADs including the behavior of flow emanating from the assist device which might induce flow irregularities that could potentially lead to stroke or cerebrovascular accident. An in vitro flow visualization study using particle image velocimetry was conducted to evaluate flow fields at the bend of outlet cannulae with angles ranging from 30 degrees to 75 degrees. A multiple disk centrifugal blood pump produced steady, laminar flow fields at the outlet. Flow fields at the bend of each cannula demonstrated the development of a boundary layer along the inner wall and shift of fluid momentum to the outer wall. Stagnant zones along the inner wall were prevalent for each cannula and increased in severity with increasing angle. The outlet flow fields of rotary VADs have the potential to produce thrombus, feasibly leading to thromboembolisms should the bend angle of the outlet cannula be acute enough. Therefore, the placement of the outlet cannula or conduit should be taken into consideration during implantation.

The estimation of cardiac function from the rotary blood pump.

The rotary blood pump is implanted as a bridge to cardiac transplantation. Mechanical, histological, and biochemical improvements have been described in patients after implantation of left ventricular assist devices (LVADs). Thus, the rotary blood pump might be used as a bridge to recovery of myocardial function. However, unlike a pulsatile pump, the rotary blood pump cannot be stopped to estimate cardiac function: if the rotary blood pump stops, backflow will occur. In this study, a new method that can estimate cardiac function without pump stop was examined. Six pigs were the subjects of this acute study. The pump was implanted as an LVAD: the inlet cannula was inserted into the left ventricle, and the outlet cannula was inserted into the ascending aorta. The motor speed was regulated at a pump flow rate of 0 L/min at diastolic phase. Then, the relationship between the dp/dt of left ventricular pressure and external stroke work of actuator was examined. This method was studied at normal, hyperdynamic, and heart-failure conditions. There was a high positive correlation between the dp/dt of left ventricular pressure and external stroke work of actuator. This method is useful and simple to estimate cardiac function without pump stop.

A miniature implantable axial flow ventricular assist device

Background. Since 1984, in collaboration with NASA engineers, we developed an axial flow pump that is 86 mm long, 22 mm wide, weighs 95 g, produces a flow of 5 to 6 L/min against a 100-mm Hg pressure at about 10,000 rpm, and requires less than 10 W of power. Methods. The pump has been implanted in 9 calves with the inlet cannula inserted into the left ventricle and the outlet cannula, consisting of an albumin-coated Dacron graft, attached by end-to-side anastomosis to the descending thoracic aorta. Results. All animals showed normal behavior until they were killed 1 to 3 months after operation. At autopsy, systemic studies of vital organs demonstrated no evidence of thromboembolism; the rpm of the pump was maintained between 9,000 and 10,000; the wattage ranged between 7 and 9; the output between 4 and 5 L/min; the hemoglobin was maintained between 32 and 35 mg/dL; the plasma-free hemoglobin ranged between 0.5 and 3 mg/dL; the BUN ranged between 8 and 14 mg/dL; the creatinine remained less than 1 mg/dL; and bilirubin studies were within normal limits. Bearing wear-tear tests up to about 5 months have been negative. Conclusions. The performance characteristics of the pump implanted in calves up to 90 days are highly gratifying, particularly in terms of pump output of 5 L/min, an index of hemolysis well within normal limits, and absence of thromboembolism.

Long Term Animal Experiments With an Intraventricular Axial Flow Blood Pump

A miniature intraventricular axial flow blood pump (IVAP) is undergoing in vivo evaluation in calves. The IVAP system consists of a miniature (phi 13.9 mm) axial flow pump that resides within the left ventricular (LV) chamber and a brushless DC motor. The pump is fabricated from titanium alloy, and the pump weight is 170 g. It produces a flow rate of over 5 L/min against 100 mmHg pressure at 9,000 rpm with an 8 W total power consumption. The maximum total efficiency exceeds 17%. A purged lip seal system is used in prototype no. 8, and a newly developed "Cool-Seal" (a low temperature mechanical seal) is used in prototype no. 9. In the Cool-Seal system, a large amount of purge flow is introduced behind the seal faces to augment convective heat transfer, keeping the seal face temperature at a low level for prevention of heat denaturation of blood proteins. The Cool-Seal system consumes < 10 cc purge fluid per day and has greatly extended seal life. The pumps were implanted in three calves (26, 30, and 168 days of support). The pump was inserted through a left thoracotomy at the fifth intercostal space. Two pursestring sutures were placed on the LV apex, and the apex was cored with a myocardial punch. The pump was inserted into the LV with the outlet cannula smoothly passing through the aortic valve without any difficulty. Only 5 min elapsed between the time of chest opening and initiation of pumping. Pump function remained stable throughout in all experiments. No cardiac arrhythmias were detected, even at treadmill exercise tests. The plasma free hemoglobin level remained in the acceptable range. Post mortem examination did not reveal any interference between the pump and the mitral apparatus. No major thromboembolism was detected in the vital organs in Cases 1 or 2, but a few small renal infarcts were detected in Case 3.