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A Hemodynamic Evaluation of the Levitronix Pedivas Centrifugal Pump and Jostra Hl-20 Roller Pump Under Pulsatile and Nonpulsatile Perfusion in an Infant CPB Model

Ressler, Noel*; Rider, Alan R.*; Kunselman, Allen R.; Richardson, J Scott; Dasse, Kurt A.; Wang, Shigang; Ündar, Akif*§¶

doi: 10.1097/MAT.0b013e3181904232
Pediatric Circulatory Support
Free

The hemodynamic comparison of the Jostra HL-20 and the Levitronix PediVAS blood pumps is the focus this study, where pressure-flow waveforms and hemodynamic energy values are analyzed in the confines of a pediatric cardiopulmonary bypass circuit.

The pseudo pediatric patient was perfused with flow rates between 500 and 900 ml/min (100 ml/min increments) under pulsatile and nonpulsatile mode. The Levitronix continuous flow pump utilized a customized controller to engage in pulsatile perfusion with equivalent pulse settings to the Jostra HL-20 roller pump. Hemodynamic measurements and waveforms were recorded at the precannula location, while the mean arterial pressure was maintained at 40 mm Hg for each test. Glycerin water was used as the blood analog circuit perfusate. At each flow rate 24 trials were conducted yielding a total of 120 experiments (n = 60 pulsatile and n = 60 nonpulsatile).

Under nonpulsatile perfusion the Jostra roller pump produced small values for surplus hemodynamic energy (SHE) due to its inherent pulsatility, while the Levitronix produced values of essentially zero for SHE. When switching to pulsatile perfusion, the SHE levels for both the Jostra and Levitronix pump made considerable increases. In comparing the two pumps under pulsatile perfusion, the Levitronix PediVAS produced significantly more surplus and total hemodynamic energy than did the Jostra roller pump each pump flow rate.

The study suggests that the Levitronix PediVAS centrifugal pump has the capability of achieving quality pulsatile waveforms and delivering more SHE to the pseudo patient than the Jostra HL-20 roller pump. Further studies are warranted to investigate the Levitronix under bovine blood studies and with various pulsatile settings.

From the *Pediatric Cardiac Research Laboratories, †Department of Health Evaluation Sciences, ‡Pediatrics, and §Surgery and Bioengineering, Penn State Milton S. Hershey Medical Center, Penn State College Medicine, Penn State Children’s Hospital, Hershey, Pennsylvania; and ¶Levitronix LLC, Waltham, Massachusetts.

Submitted for consideration August 2008; accepted for publication in revised form September 2008.

Reprint Requests: Akif Ündar, PhD, Department of Pediatrics, Surgery, and Bioengineering, Penn State College of Medicine, Department of Pediatrics—H085, 500 University Drive, PO Box 850; Hershey, PA 17033-0850. Email: aundar@psu.edu.

Evidence for the advantages of pulsatile over nonpulsatile perfusion in several clinical circumstances continues to grow.1–3 Understanding the role that individual components and settings of the cardiopulmonary bypass (CPB) circuit may play in the delivery of hemodynamic energy to the patient is an important next step in providing the best outcomes for patients after CPB procedures. The centrifugal blood pump is one such component that has been developing rapidly, yet few have the option to perfuse in both nonpulsatile and pulsatile modes. As the future turns to new ways to enhance the CPB procedure, innovative centrifugal pumps may hold the key to this challenge by generating quality physiologic pulsatility and improved patient outcomes.

The Levitronix PediVAS (Levitronix, LLC, Waltham, MA) is a magnetically levitated centrifugal blood pump that employs a bearingless motor. Pumps without mechanical bearings were proposed to avoid the limited reliability of previous pumps due to contact bearing wear.4 The PediVAS is currently used for applications in extracorporeal circulatory support such as ECMO, uni and biventricular assistance under nonpulsatile perfusion. Its self-bearing design eliminates the potential for problems related to mechanical bearings, and some of its appeal lies with its low hemolysis and minimal thrombus formation. The objective of this investigation was to test the Levitronix PediVAS pump and the Jostra HL-20 roller pump to ascertain and compare their hemodynamic outputs during nonpulsatile and pulsatile perfusion modes in a simulated pediatric CPB circuit.

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Materials and Methods

Simulated Infant Circuit Design

The circuit was designed to simulate an infant undergoing a CPB procedure, and the components used have been described in our previous study.5 In an effort to produce the most clinically relevant data, components of the circuit were used and arranged in accordance with our operating room standards and specifications. The system was driven by either the Jostra HL-20 roller pump or the Levitronix PediVAS centrifugal pump with a customized controller capable of engaging either pulsatile or nonpulsatile modes. The pseudo patient was perfused by a 10 French Terumo aortic cannula (Terumo Corporation, Tokyo, Japan) with aortic compliance, systemic resistance, and venous compliance built into the circuit. Mean arterial pressure of the pseudo patient was maintained at 40 mm Hg via a Hoffman clamp. The simulated circuit was primed with a glycerin-based blood analog solution of 40% glycerin and 60% distilled water, and was maintained at room temperature. The circuit volume was comprised of the extracorporeal circuit with venous reservoir (550 ml), the simulated neonatal patient circuit (200 ml), and the Levitronix pump (30 ml), for a total priming volume of approximately 780 ml (Figure 1).

Figure 1.

Figure 1.

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Pulsatile Perfusion Pump Settings

The pulsatile pump setting of the Jostra roller pump was set at 10% base flow with a 20/80% pump start and stop point ratio. The frequency of the roller pump on pulsatile perfusion was 100 beats per minute (bpm). Pump start and stop times are dictated by the period between consecutive R waves of an ECG, and represent one complete pulse within a pump cycle. Start and stop times for pulsatile waveforms are represented as percentages of one cycle rather than degrees of revolution of the pump head.

In utilizing a customized controller, the pulsatile settings of the Levitronix centrifugal pump were made to be comparable with the Jostra roller pumps pulsatile settings. The PediVAS was set at 100 bpm with a pulse width of 60% (comparable to the 20/80% start/stop ratio of Jostra). The difference between the baseline and amplitude rpm for pulsing was set at 50% of the nonpulsatile rpm. To maintain the same flow rate when switching to pulsatile mode, the amplitude was set at the nonpulsatile rpm plus 40% of the rpm difference and the baseline was set at the nonpulsatile rpm minus 60% of the rpm difference. The pump gain between baseline and amplitude rpm’s was set at 90°.

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Experimental Design

Each trial consisted of initiating the predetermined flow rate and allowing the circuit to stabilize at those settings with the controlled 40 mm Hg pseudo patient mean arterial pressure. The arterial filter purge line remained open for each experiment. Pressure-flow data was digitized and collected via a USB module using the National Instruments (NI, Austin, TX) Data Acquisition System with a customized NI SCC module. (Figures 2 and 3) Pressure transducers were placed at preoxygenator, precannula, and postcannula sites. Flow was measured at precannula and prearterial filter sites by a TS410 dual flowmeter (Transonic Inc., Ithaca, NY). Twenty seconds of data was captured and displayed in real-time using NI LabVIEW 7.1 (National Instruments, Austin, TX) by the applicable monitors within the circuit at a rate of 1,000 pressure-flow measurements per second. Tests were performed at room temperature under flow rates of 500–900 ml/min (100 ml/min increments). Within each flow rate, trials were conducted by testing the Jostra roller pump under pulsatile and nonpulsatile perfusion, and then switching to the Levitronix centrifugal pump and repeating. The study yielded 24 trials within each flow rate, resulting in a total of 120 experiments.

Figure 2.

Figure 2.

Figure 3.

Figure 3.

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Surplus Hemodynamic Energy Equation

Surplus hemodynamic energy (SHE) is calculated as: SHE (ergs/cm3) = 1332 [EEP − MAP],

where EEP is energy equivalent pressure [(∫fpdt)/(∫fdt), f = flow and p = pressure] and MAP is mean arterial pressure.6 The difference between EEP and MAP is multiplied by 1332 to convert pressure measures to hemodynamic energy.

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Total Hemodynamic Energy Equation

The total hemodynamic energy (THE) is EEP converted to units of energy from units of pressure6:

THE (ergs/cm3) = 1332 (EEP).

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Statistical Methods

Analysis of variance (ANOVA) models were fit to the data to assess differences in the precannula mean MAP, EEP, SHE, THE between pumps (Jostra HL-20 and Levitronix PediVAS) and perfusion methods (pulsatile vs. nonpulsatile) at each flow rate. p-values and 95% confidence intervals were adjusted for multiple comparisons testing using Bonferroni’s method. All analyses were performed using the SAS software package (SAS Institute Inc., Cary, NC).

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Results

Mean Arterial Pressure

The mean arterial pressure (MAP) readings under nonpulsatile perfusion remained rather similar between the Jostra HL-20 and the Levitronix PediVAS. Under pulsatile flow settings the differences in MAP became more apparent as the Levitronix produced slightly higher MAP values than did the Jostra roller pump at each flow rate. Regardless of the pump used, MAP values increased along with increasing flow rates (Table 1).

Table 1

Table 1

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Energy Equivalent Pressure

Higher energy equivalent pressure values under nonpulsatile flow were seen for the Jostra HL-20 than the Levitronix pump due the inherent pulsatile characteristics of the Jostra pump. When switching to pulsatile flow, the values for EEP increased significantly with the two pumps, with greater increases seen in the Levitronix pump than the Jostra pump. Smaller differences between EEP and MAP were found with the Levitronix pump, reflecting its true nonpulsatile characteristics under nonpulsatile flow. The values for EEP also increased with increasing flow rates for both pumps (Table 1).

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Surplus and Total Hemodynamic Energy

Under nonpulsatile perfusion, SHE values were higher for the Jostra pump since it produces inherent pulsatility by design, while values for the Levitronix true nonpulsatile pump were nearly zero. Pulsatile perfusion produced significantly higher levels of SHE in both pumps; however, the Levitronix PediVAS under pulsatile perfusion produced much more SHE than the Jostra pump at each flow rate. Total hemodynamic energy was also greater under pulsatile perfusion than nonpulsatile perfusion for each pump and increased with higher flow rates. The values for THE were found to be greater with the Levitronix pump than with the Jostra roller pump under pulsatile perfusion (Table 1).

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Effective Flow Rates

The arterial filter purge line remained open during each experiment, resulting in a diversion of flow at each flow rate between 500 and 900 ml/min at the post arterial filter site. The open purge line was found to divert as much as 13% of the flow during the lowest flow rate of 500 ml/min and 11% at the highest flow rate of 900 ml/min. Minor circuit backflows for the Levitronix pump were only observed under pulsatile flow at 500 ml/min flow rate in the post cannula site. However, at the post pump site there were no backflows recorded at each flow rate, thereby attributing any backflow to circuit resistance unrelated to the pumps performance.

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Discussion

The Levitronix PediVAS is already in use as a nonpulsatile pump, and has been approved in the United States under an IDE for investigational use up to 14 days as a ventricular assist device. The addition of a pulsatile perfusion mode is sensible in light of the potential benefits of pulsatility in blood pumps, especially during CPB.7–9

The data from this study demonstrate a substantial increase in the amount of SHE delivered to the patient during pulsatile mode as opposed to nonpulsatile mode with both pumps, and are found to increase along with greater flow rates. Most importantly, the pulsatile capabilities of the Levitronix PediVAS in comparison with the Jostra HL-20 prove its worth in a pediatric CPB circuit under pulsatile perfusion. The levels for SHE were found to be significantly higher with the Levitronix centrifugal pump than the Jostra pump at the precannula site under pulsatile perfusion. We also examined THE to better compare the levels of energy delivered by the pumps under nonpulsatile mode. Similar to SHE, the Levitronix pump provided much greater THE when using pulsatile perfusion. However, in nonpulsatile mode the THE values were lower in the Levitronix pump than the Jostra pump. This directly correlates with the pure nonpulsatile characteristics of the Levitronix pump, where SHE values are essentially zero. Additionally, the pressure-flow waveforms that the Levitronix pump created, better demonstrate near-physiologic pulsatility during pumping. Providing physiologic pulsatility is currently a goal in pump design for blood pumps that support pulsatile perfusion. As of now, roller pumps used for pulsatile perfusion produce a diminished pulsatility at one third of actual physiologic flow.10

A major limitation of the study was the unexplored versatility of the pulsatile mode settings on the Levitronix pump. The custom control mechanism allowed for the adjustment of several pulsing parameters including baseline rpm, amplitude rpm, beat frequency, pulse width, and gain between baseline and amplitude rpm. For our particular study, these parameters were set to compare with the Jostra HL-20 pulse settings and were not fully investigated outside those settings. Although the Levitronix pump produced more SHE than the Jostra pump under similar settings, the performance of the Levitronix pump may have been further enhanced by optimizing these parameters. Concurrent studies with the Jostra and Levitronix pumps show the effects on pulsatility and SHE delivery when altering various pulsatile parameters. (unpublished data) Another limitation to consider is that the study used a blood analog solution in place of animal blood as the circuit perfusate. Further studies should use animal blood to investigate the effects of the Levitronix higher rpm’s in relation to blood hemolysis.

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Conclusion

The results of this study provide evidence favoring the capabilities of the Levitronix pump under a customized pulsatile setting. During pulsatile CPB, the Levitronix PediVAS pump produced more SHE than the Jostra HL-20 roller pump at each flow rate. While refinements in the pump’s pulsatile operation need to be assessed, the results show that the addition of pulsatile mode to the Levitronix PediVAS holds the potential during pediatric CPB procedures.

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Acknowledgment

This research was supported by the Children’s Miracle Network Funds (AÜ).

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References

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