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.
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).
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).
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).
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).
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).
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.
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.
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.
This research was supported by the Children’s Miracle Network Funds (AÜ).
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Copyright © 2009 by the American Society for Artificial Internal Organs
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