Design of a Continuous Flow Centrifugal Pediatric Ventricular Assist Device

Thousands of pediatric patients suffering from cardiomyopathy or single ventricular physiologies secondary to debilitating heart defects may benefit from long-term mechanical circulatory support due to the limited number of donor hearts available. This article presents the initial design of a fully implantable centrifugal pediatric ventricular assist device (PVAD) for 2 to 12 year olds. Conventional pump design equations, including a nondimensional scaling approach, enabled performance estimations of smaller scale versions (25 mm and 35 mm impeller diameters) of our adult support VAD. Based on this estimated performance, a computational model of the PVAD with a 35 mm impeller diameter was generated. Employing computational fluid dynamics (CFD) software, the flow paths through the PVAD and overall performance were analyzed for steady state flow conditions. The numerical simulations involved flow rates of 2 to 5 LPM for rotational speeds of 2750 to 3250 RPM and incorporated a k-epsilon fluid turbulence model with a logarithmic wall function to characterize near-wall flow conditions. The CFD results indicated best efficiency points ranging from 25% to 28%, which correlate well with typical values of blood pumps. The results further demonstrated that the pump could deliver 2 to 5 LPM at 70 to 95 mmHg for desired physiologic conditions in resting 2 to 12 year olds. Scalar stress levels remained below 300 Pa, thereby signifying potentially low levels of hemolysis. Several flow regions in the pump exhibited signs of vortices, retrograde flow, and stagnation points, which require optimization and further study. This CFD model represents a reasonable starting point for future model enhancements, leading to prototype manufacturing and experimental validation.

Diamagnetic Effects of Blood in a Magnetically Levitated Blood Pump

Magnetic bearings are becoming increasingly popular in ventricular assist devices. In most cases, blood fills a portion of the gap between the magnetic actuators and rotor. Understanding the effects of the operating fluid on magnetic suspension is necessary, particularly when the device geometry features a relatively large gap (typically defined as greater than 150 um and as large as 2 mm in some pumps). A large gap reduces cell damage and allows unrestricted flow, it but increases the distance and amount of material through which magnetic fields must pass. These net effects of the operating fluid on the magnetic suspension can be characterized in terms of a contribution to the overall damping and stiffness of the system. This contribution may be caused by traditional tribological properties (density and viscosity) as well as the magnetic properties of the medium. This research isolates the effects of fluid diamagnetism on a magnetically levitated blood pump. Experimental transient and frequency responses of the system operating at 3000 to 6000 rpm are presented while pumping different liquid media: Blood, water, and fluids with similar densities and viscosities to each, but different magnetic susceptibilities. In order to calculate the stiffness and damping coefficients, a mathematical model of the system is iteratively updated to match experimental transient response. The resulting coefficients are validated in the frequency domain by means of simulations, which agree with experimental frequency response data. The relationship between diamagnetism, damping and stiffness coefficients of the test fluids with known magnetic susceptibility is also used to obtain a quantitative estimate of the magnetic susceptibility of blood.

The application of quantitative oil streaking to the HeartQuest left ventricular assist device

Methods of flow visualization using oil streaking are established techniques for investigating surface shear and near wall flow patterns. Recent studies have used an array of oil dots on a surface which form streaks when exposed to shear forces. This method is generally qualitative, but it is possible to make quantitative measurements of the shear if the oil streaks have been calibrated. This paper presents the application of a quantitative oil streak method to the HeartQuest left ventricular assist device (LVAD). An array of dots was applied to the top housing of the pump, yielding quantitative values for the shear and qualitative patterns of the near wall flow in that region. The results were used to locate regions likely to promote thrombosis, such as stagnation points or recirculation regions. Regions of high shear, where hemolysis might occur, also can be identified with this method. In addition to being an important design technique, quantitative oil streaking assisted in the verification of computational fluid dynamics results within the HeartQuest LVAD.

Particle image velocimetry measurements of blood velocity in a continuous flow ventricular assist device

The third prototype of a continuous flow ventricular assist device (CFVAD3) is being developed and tested for implantation in humans. The blood in the pump flows through a fully shrouded four-bladed impeller (supported by magnetic bearings) and through small clearance regions on either side of the impeller. Measurements of velocities using particle image velocimetry of a fluid with the same viscosity as blood have been made in one of these clearance regions. Particle image velocimetry is a technique that measures the instantaneous velocity field within an illuminated plane of the fluid field by scattering light from particles added to the fluid. These measurements have been used to improve understanding of the fluid dynamics within these critical regions, which are possible locations of both high shear and stagnation, both of which are to be avoided in a blood pump. Computational models of the pump exist and these models are currently being used to aid in the design of future prototypes. Among other things, these models are used to predict the potential for hemolysis and thrombosis. Measurements of steady flow at two operating speeds and flow rates are presented. The measurements are compared with the computed solutions to validate and refine, where necessary, the existing computational models.

Successful newborn sickle cell trait counseling program using health department nurses

Every year more than 50,000 infants with sickle cell trait are identified through newborn screening programs. An infant with trait provides a "genetic window" into a family that may be at risk for having a child with sickle cell disease. No national standards have been established for newborn trait follow-up and family counseling. In the United States, counseling to families having an infant with trait is usually very limited in scope and, in many cases, nonexistent. In West Tennessee, the Mid-South Sickle Cell Center (MSSCC) and the Department of Health have jointly developed an effective, practical, and affordable trait counseling program. A five-step counseling protocol, an accompanying counseling manual, a documentation checklist, and a fact sheet for parents were developed to support trait counselling. By training and using health department nurses, this program has provided counseling to a large percentage of families in which an infant tests positive for sickle cell trait.

Recurrent Streptococcus pneumoniae sepsis in children with sickle cell disease

Streptococcus pneumoniae sepsis is the most common invasive infection among patients with sickle cell disease. The risk of a recurrent episode of sepsis and subsequent death in those patients who have had a previous septic event is much higher. Patients with sickle disease who have had pneumococcal sepsis should continue penicillin prophylaxis indefinitely and should not be candidates for out-patient management of febrile episodes.

Outpatient therapy with ceftriaxone and oral cefixime for selected febrile children with sickle cell disease

PURPOSE

Children with sickle cell disease are at increased risk for bacterial sepsis and, when febrile, are usually hospitalized for intravenous antibiotic therapy pending results of blood cultures. In this study, we prospectively identified a group of febrile patients with sickle cell disease who were at low risk for sepsis and treated them with outpatient therapy.

PATIENTS AND METHODS

Children identified as low risk for sepsis were treated with an initial dose of intravenous ceftriaxone, followed by outpatient therapy with oral cefixime, and were monitored for 14 days after the initial visit. Compliance was assessed by phone calls to parents and by analysis of urine samples.

RESULTS

In 107 eligible febrile episodes (80 patients) over a 21-month period, no patient developed sepsis. One child developed bacteremia 3 days after completing the course of cefixime, and one had splenic sequestration on the fourth study day. Both patients did well. Side effects of cefixime were modest, and overall compliance was excellent (approximately 95%), although urine samples were returned by only 56% of parents.

CONCLUSION

We conclude that outpatient therapy is safe and effective in febrile patients with sickle cell disease who meet the criteria for a low risk of sepsis.

Penicillin- and cephalosporin-resistant strains of Streptococcus pneumoniae causing sepsis and meningitis in children with sickle cell disease

OBJECTIVE

We investigated the possibility that antimicrobial-resistant pneumococci were causing invasive disease in children with sickle-cell disease (SCD).

STUDY DESIGN

Records of all children with SCD observed at the Mid-South Sickle Cell Center (MSSCC) at LeBonheur Children's Medical Center were reviewed from January 1990 to June 1994. Children with SCD and pneumococcal sepsis were identified. The Streptococcus pneumoniae isolates from these children were examined for serotype and antimicrobial susceptibilities. Two additional children not observed in the MSSCC had pneumococcal sepsis caused by penicillin-resistant isolates and were also included.

RESULTS

Antimicrobial susceptibility testing of the six penicillin-resistant isolates revealed that four were resistant to trimethoprim-sulfamethoxazole, two to erythromycin, and one to clindamycin. The two isolates that were resistant to ceftriaxone also were multiply resistant. From the MSSCC, 26 children had pneumococcal sepsis during the 4 1/2-year period studied. Five of these children (19%) died. Four (15%), including one who died, were infected with penicillin-resistant strains.

CONCLUSION

Pneumococcal sepsis, meningitis, and infections of other foci in children with SCD may be caused by S. pneumoniae that is resistant to one or more antimicrobial agents, including penicillin. The addition of vancomycin to the antibiotics currently used for initial management should be considered in areas where the antibiotic resistance of S. pneumoniae is prevalent.

A randomized study of outpatient treatment with ceftriaxone for selected febrile children with sickle cell disease

BACKGROUND

Because of their susceptibility to pneumococcal sepsis, children with sickle cell disease and fever are usually hospitalized for antibiotic therapy. Outpatient treatment may be a safe and less expensive alternative for selected patients.

METHODS

After evaluation in the emergency room, children ranging from 6 months to 12 years of age who had sickle hemoglobinopathies and temperatures exceeding 38.5 degrees C were randomly assigned to treatment as either inpatients or outpatients. We excluded from randomization children at higher risk of sepsis (as defined by specific criteria, including temperature above 40 degrees C, white-cell count below 5000 per cubic millimeter or above 30,000 per cubic millimeter, and the presence of pulmonary infiltrates) or with complications of sickle cell disease (such as a hemoglobin level below 5 g per deciliter, dehydration, or severe pain); these children were treated as inpatients. All patients received an initial intravenous dose of ceftriaxone (50 mg per kilogram of body weight). Those treated as outpatients returned 24 hours later for a second dose of ceftriaxone, whereas the in patients were treated as directed by their physicians.

RESULTS

None of the 86 patients (with a total of 98 febrile episodes) in the randomized groups had sepsis, as compared with 6 of the 70 patients (7 of 86 episodes) excluded because of higher risk (P = 0.004). Among the 44 children (50 episodes) assigned to outpatient treatment, there were 11 hospitalizations (22 percent of episodes) within two weeks after treatment (95 percent confidence interval, 12 to 36 percent), whereas after inpatient care only a single patient (2 percent of episodes) was rehospitalized. When the randomized groups were compared, outpatient treatment saved a mean of $1,195 per febrile episode. The median hospital stay was 3 days (range, 1 to 6) for the children randomly assigned to inpatient care and 4 days (range, 1 to 18) for the higher-risk children treated as inpatients (P < 0.001).

CONCLUSIONS

With the use of conservative eligibility criteria, at least half the febrile episodes in children with sickle cell disease can be treated safely on an outpatient basis, with substantial reductions in cost.

High risk of recurrent stroke after discontinuance of five to twelve years of transfusion therapy in patients with sickle cell disease

Although long-term transfusion therapy is at least 90% effective in preventing recurrent strokes after an initial cerebrovascular accident in patients with sickle cell disease, it is unknown how long transfusion therapy should be continued. To address this question, we prospectively discontinued transfusions in 10 patients with sickle cell disease whose median duration of transfusion therapy after an initial stroke was 9 1/2 years (range 5 to 12 years). Before the transfusions were discontinued, patients were examined by cerebral angiography, magnetic resonance imaging of the head, neuropsychologic testing, electroencephalography, and a complete neurologic examination. Within 12 months after transfusion therapy was stopped, 5 of 10 patients had had an ischemic event. Three events caused relatively mild deficits in the same areas as those originally affected. Two were associated with massive intracranial hemorrhage, including one on the contralateral side of original involvement. An additional patient died suddenly of unknown causes. Of the four remaining patients, three declined to resume transfusion and are relatively well at greater than or equal to 18 months after therapy was stopped. The studies performed before transfusions were stopped were not predictive of recurrent stroke. The risk of recurrent cerebrovascular accident in this group was significantly greater than the estimated risk of 10% in patients who are receiving long-term transfusion therapy (p = 0.002). This adverse outcome suggests that patients with sickle cell disease who have had a stroke must receive long-term transfusion indefinitely or a suitable therapeutic alternative must be devised.

An alternative to routine operative cholangiography

The use of the Fogarty biliary balloon catheter has added to the safety of common duct exploration. Review of our own series seems to support this concept. With this adjunct adn a careful anatomic dissection of the biliary system, one can decide to do cholangiography or common duct exploration based on carefully considered clinical factors when stones are suspected. Perhaps neither need to be done in many instances. Without trying to question the virtures of operative cholangiography, we believe that it can be used selectively rather than routinely.