Chronic animal health assessment during axial ventricular assistance: importance of hemorheologic parameters

Shear-dependency of the predicted ideal hematocrit

 The ideal hematocrit is the hematocrit (Hct) value resulting in the highest value of Hct/viscosity (h/η) ratio and can thus be predicted from viscometric measurements with the use of equations such as Quemada's one which yield the determination of the bell-shaped curve of h/η as a function of Hct. In a series of recent papers we applied this approach to various populations, using viscometry at high shear rate (1000 s-1). However the shape of this curve has been reported to be dependent on the shear rate, resulting in a right-shift in this top value when Hct increase. We present here in 11 young recreative athletes the evolution of the predicted top of the h/η curve and optimal theoretical Hct and the discrepancy between theoretical and optimal values over the range of shear rates 1 to 6000 s-1. Results show that the predicted optimal value of both h/η and Hct increases when shear rate increases and that the discrepancy between predicted laquooptimalraquo and actual values decreases and becomes almost asymptotic at very high shear (500 s-1). It is minimal at 2720 s-1. The correlation between predicted laquooptimalraquo and actual values of both parameters describes the same evolution. Therefore, it is better for assessing h/η and its agreement with theoretical values, and for determining the theoretical ideal hematocrit, to measure blood viscosity at shear rates equal or superior to 500 s-1.

Mechanical blood trauma in assisted circulation: sublethal RBC damage preceding hemolysis

After many decades of improvements in mechanical circulatory assist devices (CADs), blood damage remains a serious problem during support contributing to variety of adverse events, and consequently affecting patient survival and quality of life. The mechanisms of cumulative cell damage in continuous-flow blood pumps are still not fully understood despite numerous in vitro, in vivo, and in silico studies of blood trauma. Previous investigations have almost exclusively focused on lethal blood damage, namely hemolysis, which is typically negligible during normal operation of current generation CADs. The measurement of plasma free hemoglobin (plfHb) concentration to characterize hemolysis is straightforward, however sublethal trauma is more difficult to detect and quantify since no simple direct test exists. Similarly, while multiple studies have focused on thrombosis within blood pumps and accessories, sublethal blood trauma and its sequelae have yet to be adequately documented or characterized. This review summarizes the current understanding of sublethal trauma to red blood cells (RBCs) produced by exposure of blood to flow parameters and conditions similar to those within CADs. It also suggests potential strategies to reduce and/or prevent RBC sublethal damage in a clinically-relevant context, and encourages new research into this relatively uncharted territory.

In Vitro and In Vivo Performance Evaluation of the Second Developmental Version of the PediaFlow Pediatric Ventricular Assist Device

Ventricular assist devices (VADs) have significantly impacted the treatment of adult cardiac failure, but few options exist for pediatric patients. This has motivated our group to develop an implantable magnetically levitated rotodynamic VAD (PediaFlow®) for 3-20 kg patients. The second prototype design of the PediaFlow (PF2) is 56% smaller than earlier prototypes, and achieves 0.5-1.5 L/min blood flow rates. In vitro hemodynamic performance and hemolysis testing were performed with analog blood and whole ovine blood, respectively. In vivo evaluation was performed in an ovine model to evaluate hemocompatibility and end-organ function. The in vitro normalized index of hemolysis was 0.05-0.14 g/L over the specified operating range. In vivo performance was satisfactory for two of the three implanted animals. A mechanical defect caused early termination at 17 days of the first in vivo study, but two subsequent implants proceeded without complication and electively terminated at 30 and 70 days. Serum chemistries and plasma free hemoglobin were within normal limits. Gross necropsy revealed small, subclinical infarctions in the kidneys of the 30 and 70 day animals (confirmed by histopathology). The results of these experiments, particularly the biocompatibility demonstrated in vivo encourage further development of a miniature magnetically levitated VAD for the pediatric population. Ongoing work including further reduction of size will lead to a design freeze in preparation for of clinical trials.

Biocompatibility assessment of the first generation PediaFlow pediatric ventricular assist device

The PediaFlow pediatric ventricular assist device is a miniature magnetically levitated mixed flow pump under development for circulatory support of newborns and infants (3-15 kg) with a targeted flow range of 0.3-1.5 L/min. The first generation design of the PediaFlow (PF1) was manufactured with a weight of approximately 100 g, priming volume less than 2 mL, length of 51 mm, outer diameter of 28 mm, and with 5-mm blood ports. PF1 was evaluated in an in vitro flow loop for 6 h and implanted in ovines for three chronic experiments of 6, 17, and 10 days. In the in vitro test, normalized index of hemolysis was 0.0087 ± 0.0024 g/100L. Hemodynamic performance and blood biocompatibility of PF1 were characterized in vivo by measurements of plasma free hemoglobin, plasma fibrinogen, total plasma protein, and with novel flow cytometric assays to quantify circulating activated ovine platelets. The mean plasma free hemoglobin values for the three chronic studies were 4.6 ± 2.7, 13.3 ± 7.9, and 8.8 ± 3.3 mg/dL, respectively. Platelet activation was low for portions of several studies but consistently rose along with observed animal and pump complications. The PF1 prototype generated promising results in terms of low hemolysis and platelet activation in the absence of complications. Hemodynamic results validated the magnetic bearing design and provided the platform for design iterations to meet the objective of providing circulatory support for young children with exceptional biocompatibility.

Leukocyte-platelet aggregates and monocyte tissue factor expression in bovines implanted with ventricular assist devices

Infection and thromboembolism remain significant complications associated with ventricular assist device (VAD) support, including the newer rotary VADs, limiting wider adoption of this promising technology. These complications persist in spite of extensive preclinical testing in large animal models. The amount of biocompatibility information collected during preclinical trials is limited due to a lack of available assays. We thus developed three flow cytometric assays to measure leukocyte-platelet aggregates and monocyte tissue factor expression and applied them to 26 bovines implanted with two types of rotary VADs. All animals displayed low levels of circulating aggregates and monocytes expressing tissue factor prior to device implant. The assay values significantly increased following VAD implant, then usually declined to a lower, yet significantly elevated versus baseline, level indicative of ongoing inflammation. The implementation of more robust biocompatibility assays for the evaluation of cardiovascular device performance and modification might ultimately contribute to the development of safer artificial organs.

Effects of Turbulent Stresses upon Mechanical Hemolysis: Experimental and Computational Analysis

Experimental and computational studies were performed to elucidate the role of turbulent stresses in mechanical blood damage (hemolysis). A suspension of bovine red blood cells (RBC) was driven through a closed circulating loop by a centrifugal pump. A small capillary tube (inner diameter 1 mm and length 70 mm) was incorporated into the circulating loop via tapered connectors. The suspension of RBCs was diluted with saline to achieve an asymptotic apparent viscosity of 2.0 +/- 0.1 cP at 23 degrees C to produce turbulent flow at nominal flow rate and pressure. To study laminar flow at the identical wall shear stresses in the same capillary tube, the apparent viscosity of the RBC suspension was increased to 6.3 +/- 0.1 cP (at 23 degrees C) by addition of Dextran-40. Using various combinations of driving pressure and Dextran mediated adjustments in dynamic viscosity Reynolds numbers ranging from 300-5,000 were generated, and rates of hemolysis were measured. Pilot studies were performed to verify that the suspension media did not affect mechanical fragility of the RBCs. The results of these bench studies demonstrated that, at the same wall shear stress in a capillary tube, the level of hemolysis was significantly greater (p < 0.05) for turbulent flow as compared with laminar flow. This confirmed that turbulent stresses contribute strongly to blood mechanical trauma. Numerical predictions of hemolysis obtained by computational fluid dynamic modeling were in good agreement with these experimental data.

Rheologic dissimilarities in female and male blood: potential link to development of cardiovascular diseases

Oxygen Delivery Index (ODI) was introduced as the ratio of red blood cell concentration (hematocrit) to blood viscosity. The ODI can be considered an indirect characterization of oxygen transport to organs and tissues. ODI was obtained for 98 healthy donors (47 pre-menopausal women and 51 age-matched men). In this population ODI levels were found to be significantly lower (p < 0.001) in male blood (7.7 +/- 0.3 vs. 8.4 +/- 0.5 in female blood). Average ODI obtained for 15 cardiac patients (all males) was found to be significantly lower than that for healthy men. In red blood cell suspensions with the same hematocrit, ODI was found to decrease when plasma viscosity was increased via an increase in protein concentration. Additionally, it was found that ODI measured for samples of blood over a wide hematocrit range, obtained by dilution with autologous plasma, possessed the highest values at the hematocrit levels 30 to 40%. The decreased oxygen transport might contribute to the significantly higher morbidity and mortality from cardiovascular diseases for men compared to pre-menopausal women. ODI may be a useful parameter for evaluation of risk of development of cardiovascular disorders.

In vivo evaluation of a MPC polymer coated continuous flow left ventricular assist system

The aim of this study was the evaluation of the thrombogenicity and the biocompatibility of the SunMedical EVAHEART left ventricular assist system (LVAS) coated with 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer compared to a diamond-like carbon (DLC) coating. Four calves were implanted with the MPC polymer-coated LVAS. Eight calves were implanted with DLC coated LVAS. The thrombogenicity and biocompatibility of the pumps were evaluated. At explant, 60.0 +/- 37.2% (5-85%) of the pump surface area was still coated with MPC polymer after the duration of 45.0 +/- 32.0 days. In 1 out of 4 MPC and 2 out of 8 DLC coated pumps, there was a very small amount of thrombus around the seal ring; otherwise the blood contacting surfaces were free of thrombus. Major organs were normal except for a few lesions in kidneys from both groups. The MPC polymer coated EVAHEART LVAS seems to have low thrombogenicity and high biocompatibility similar to the DLC coated system. The current study demonstrated that the MPC polymer coating shows great promise for being used as an antithrombogenic substrate for the LVAS due to its ease of application, significant cost benefit, and reduction in anticoagulation therapy in acute postoperative period.

In vitro evaluation of hemolysis and sublethal blood trauma in a novel subcutaneous vascular access system for hemodialysis

Hemodialysis requires reliable frequent access to the patients' vasculature, with blood flow rates of > 300 ml/min. Currently in the U.S. market, there are three types of hemodialysis access systems: the native arteriovenous fistula, generally using 15G needles; the synthetic arteriovenous (AV) graft, also generally using 15G needles; and the percutaneous catheter. Some of the problems with current vascular access technologies include insufficient blood flow, blood trauma, thrombosis, infection, cardiac load, and venous stenosis. The LifeSite System (Vasca, Inc.) represents an alternative for vascular access, and consists of a subcutaneous valve and 12F cannula accessed by a standard 14G needle. The LifeSite valve is implanted in the upper torso with the cannula generally entering the right internal jugular vein. The purpose of this study was to compare the LifeSite System with two known vascular access systems: the 10F dialysis catheter (Tesio-Cath, MedComp) and the 15G A.V. Fistula Needle Set (JMS Co., Ltd.) with regard to blood damage produced by these devices in use. Mechanical hemolysis and sublethal blood trauma were evaluated by means of in vitro blood pumping through a circulating loop incorporating a hemodialysis vascular access system. Sublethal blood damage was examined by using a hemorheologic assay that included parameters such as erythrocyte mechanical fragility, plasma total protein and fibrinogen concentrations, and blood viscosity. The tests demonstrated that, at both studied flow rates of 300 ml/min and 450 ml/min, the LifeSite produced lower hemolysis and less sublethal damage to blood than either the Tesio-Cath catheter or the A.V. Fistula Needle Set.