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Early Experience with a Polymethyl Pentene Oxygenator for Adult Extracorporeal Life Support

Peek, Giles J.*; Killer, Hilliary M.; Reeves, Richard; Sosnowski, Andrezj W.; Firmin, Richard K.

SCHOLARLY REVIEW
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Silicone oxygenators are the standard devices used for Extracorporeal Life Support (ECLS), but they have some limitations. Microporous polypropylene hollow fiber oxygenators overcome many of these problems but, unfortunately, develop plasma leak. Polymethyl-pentene (PMP) is a novel oxygenator material. We report our initial experience with the Medos Hilite 7000LT, a PMP hollow fiber oxygenator, in six adult respiratory ECLS patients with these characteristics: age, mean 32.2 (±13) years; weight, mean 81.2 (±17) kg; PaO2/FIO2, mean 62.8 [±33] mm Hg; Murray Score, mean 3.4 [±0.3]; and sepsis related organ failure assessment score, mean 9.6 [±2.3]. One patient was cannulated within 10 hours of multiple trauma and 1 hour after thoracolaparotomy; another patient was cannulated 12 hours after a thoracotomy. All six patients survived. Heparin was infused (7.8–32.5 u/kg/hr) to maintain activated clotting time at 162 to 238 seconds; international normalized ratio was 0.9 to 3.4. Two of the six patients required transfusions of fresh frozen plasma, receiving one and five units, respectively. Fibrinogen was 1.4 to 6 g/dl; no cryoprecipitate was needed. Platelet counts were between 65 and 306, and very little platelet transfusion (mean 2.33; ±3.03 units per patient) was required to maintain these levels. Two patients did not require any platelet transfusion. Maximum blood flow was 5.3 L/min, sweep was 3 to 10 L/min, and resistance was 11 to 43 Paul Wood Units. There were no oxygenator failures. Mean duration of ECLS was 151.7 hours (±75.6). Our initial experience with PMP oxygenators in adults was satisfactory, and platelet consumption and resistance to blood flow seem to be greatly reduced with PMP.

From the *Division of Cardiac Surgery, University of Leicester, Glenfield Hospital, and the †Heartlink ECMO Centre, Glenfield Hospital, University Hospitals of Leicester NHS Trust, UK.

Submitted for consideration December 2001; accepted for publication in revised form April 2002.

Correspondence: Dr. Giles J. Peek, Division of Cardiac Surgery, University of Leicester, Glenfield Hospital, University Hospitals of Leicester NHS Trust, Groby Road, Leicester, LE3 9QP, UK.

Extracorporeal Life Support (ECLS) uses modified cardiopulmonary bypass (CPB) technology to provide prolonged support of the lungs or heart in the intensive care unit. One of the most important differences between an ECLS circuit and a CPB circuit is the necessity to completely separate the gas and blood phases by use of a membrane oxygenator, hence the synonym Extracorporeal Membrane Oxygenation (ECMO). Complete separation of blood and gas is necessary to reduce the inherent inflammatory activation 1 associated with any type of extracorporeal circulation. Indeed it was not until the advent of the solid silicone membrane oxygenator 2 that ECLS became possible. 3 Silicone oxygenators are currently the standard devices used for ECLS in patients of all ages worldwide. In the year 2000, 76% of adult ECMO runs were done using silicone devices (Karl Hultquist, personal communication, 2000). The remainder used microporous hollow fiber oxygenators. However, despite the excellent biocompatibility of silicone oxygenators, they have a number of limitations: they are bulky, they have a large priming volume and a high resistance, they are difficult to coat with heparin, and they are relatively inefficient in terms of gas exchange. Attempts have been made to overcome these difficulties by using microporous polypropylene hollow fiber oxygenators for ECLS. They are more efficient than the silicone devices and have a low priming volume, low resistance, and can easily be heparin coated. Unfortunately, the micropores, which are essential for gas exchange, become fluid permeable after a relatively short time and allow passage of plasma into the gas phase. It seems, therefore, that a novel oxygenator material is needed which, like silicone, is gas permeable, making micropores unnecessary, and, like polypropylene, can be extruded into hollow fibers, allowing the design of low resistance, low priming volume oxygenators. Polymethyl-pentene (PMP) is just such a material. Here we have reported our initial experience with the Medos Hilite 7000LT, a PMP hollow fiber oxygenator, for adult respiratory ECLS use.

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Methods

The study was designed as a retrospective case series, looking at the first six adult size patients (weighing more than 60 kg) to receive ECLS with the Medos 7000LT (Stolberg, Germany) in our institution, an ECLS Organization registered ECMO unit treating neonatal, pediatric, and adult patients. The Medos 7000LT is a PMP oxygenator designed for long-term use, and it has an ionic heparin coating (Rheoparin). One observer (G. J. P.) collected data from the case notes and the ECMO specialists’ record sheets using a standard ProForma. The patients were treated according to standard institutional protocols as described in previous studies, 4,5 except that the Medos 7000LT PMP oxygenator was substituted for the previously used silicone devices.

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Patients

Between March 30, 2001, and April 30, 2001, six adult sized patients were cannulated for venovenous ECLS at our institution. Their mean age was 32.2 [±13] years, and their mean weight was 81.2 [±17] kg). Four were men. Their diagnoses were as follows: acute respiratory distress syndrome caused by trauma (two patients), acute respiratory distress syndrome caused by sepsis, pneumonia caused by legionella, pneumonia caused by pneumococcus, and Mendelson’s Syndrome. Their mean PaO2/FIO2 ratio was 62.8 [±33] mm Hg, mean Murray Score was 3.4 [±0.3], and mean sepsis related organ failure assessment score was 9.6 [±2.3]. One patient was cannulated within 10 hours of multiple traumas and 1 hour after thoracolaparotomy; another patient was cannulated 12 hours after a thoracotomy (lung resection and decortication) and 1 hour after surgical tracheostomy. Three patients required extracorporeal renal support during ECLS, and one patient developed hyperbilirubinemia and hepatic impairment requiring extracorporeal liver support (Mars, Teraklin, Rostock, Germany) after decannulation from ECLS. All six patients survived to hospital discharge.

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Results

The first three patients had two oxygenators arranged in parallel. The next three patients had a single oxygenator, as it was realized that two oxygenators were not necessary. Circuits were primed as usual (the figures in parentheses are for circuits with two oxygenators) with CO2 flush, albumin wash, and Plasmalyte-A Prime, then displaced with two units packed cells, 300 (380) units Heparin, 39 (50) ml Sodium Bicarbonate 8.4%, and 6 (8) ml Calcium Chloride 10%. Both trauma patients had 1 million units Aprotinin (Bayer, Newbury, Berkshire, England) added to the prime, and received an Aprotinin infusion (1/2 million units per hour) for the first 24 hours of ECLS. Before cannulation, patients received heparin (50–100 u/kg IV). All patients were cannulated for venovenous ECLS. Three patients were cannulated with one drainage cannula (right internal jugular vein), and three required two drainage cannulae (Table 1). Of the latter, two were cannulated according to our institutional protocol, 4 and the last had drainage cannulae placed in both femoral veins and a return cannula in the left internal jugular vein. This approach was used because of staphylococcal cellulitis overlying the right internal jugular vein. Ventilation could be reduced to rest settings (30% O2, rate 10, positive end expiratory pressure 10–15, pressure control ventilation = 10) a mean of 12.8 hours after cannulation (±14.3).

Table 1

Table 1

Heparin was administered by continuos infusion (7.8–32.5 u/kg/hr) to maintain an activated clotting time (162–238 seconds) and an activated partial thromboplastin time between 1.2 and 6.6 times control; international normalized ratio (INR) varied between 0.9 and 3.4. Only two patients required transfusion of fresh frozen plasma to maintain their INRs within the normal range, receiving one and five units respectively. The thrombin time varied between 21 and 200 seconds; the average of the median thrombin times was 76.4 seconds. Fibrinogen ranged from 1.4 to 6 g/dl, and it was not necessary to give any cryoprecipitate to maintain these fibrinogen levels. Platelet counts were well maintained with a minimum of 65 and a maximum of 306. Only small amounts of platelet transfusion (mean 2.33; ±3.03 units per patient in total) were required to maintain these levels, and two patients did not require any platelet transfusion. Mean red cell requirement during the ECLS course was 11.8 units (±4.38).

Oxygenator performance during ECLS is summarized in Tables 1 and 2. There were no instances of oxygenator failure in these patients. The sweep gas used to ventilate the oxygenators was 100% O2. During ECLS, extracorporeal flow was adjusted to maintain the patients’ arterial PO2 (PaO2) above 45 mm Hg, and the sweep gas was adjusted to maintain the PaCO2 below 45 mm Hg. Red cells were transfused to maintain the hemoglobin concentration at 14 g/dl. The water bath was adjusted to maintain a patient temperature of 37°C. Mean duration of ECLS was 151.7 hours (±75.6).

Table 2

Table 2

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Discussion

Our initial experience of PMP oxygenators for adult respiratory ECLS seemed to indicate satisfactory performance. The most notable difference between PMP and silicone is the great reduction in platelet consumption seen with PMP. In our previously reported series of 50 adult ECLS patients, 5 a mean of 47.1 units (±62.4) of platelets were transfused to each patient during a mean ECLS run of 207.4 hours. Even allowing for the fact that the platelet pools now supplied by our blood bank are equivalent to approximately five of the units used during the former study, it is apparent that platelet consumption is markedly lower with the new material. It is also important to note that two of the patients reported here had suffered recent trauma and surgery, one being placed on ECLS within 1 hour of an extended thoracolaparotomy only 10 hours after a major injury. These patients received two and eight platelet pools respectively (equivalent to 10 and 40 old platelet units), which is similar to the number of units of platelets used in the majority of the previously reported patients using the silicone oxygenators. It had been our previous experience that patients who required surgery during ECLS or those who had been placed on ECLS after cardiopulmonary bypass using silicone oxygenators would require multiple platelet transfusions greatly exceeding the amounts used in the patients with the PMP oxygenators.

The reduction in the amount of cryoprecipitate used was also noticeable. Patients with silicone oxygenators required a mean of 1.8 units (±7.7) versus none in the patients using PMP. Fresh frozen plasma use was also reduced, with a total of six units being given to the patients using PMP, compared with more than 130 to the 50 silicone patients. Once again, even allowing for the change in presentation of fresh frozen plasma from our blood bank in four unit pools, this represents a huge reduction.

The resistance to blood flow is lower than in the silicone device, and the oxygenators are more compact. The increased efficiency allows the use of one oxygenator rather than two in parallel, which was the configuration used with the silicone oxygenators. However, it is undoubtedly safer to use two PMP oxygenators in parallel rather than a single device, especially in the event of oxygenator failure. We have subsequently used these PMP oxygenators for adult venoarterial ECMO support. We have also used the analogous Hi-Lite 800 LT for neonatal venoarterial and venovenous ECMO.

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Conclusion

In this small series, the PMP oxygenators performed satisfactorily. There seems to be a marked decrease in the transfusion requirements with the PMP versus the silicone devices. This is probably related to improved biocompatibility, which will require further investigation. We have now adopted the Medos 7000LT as our standard adult ECLS oxygenator.

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Acknowledgment

The authors thank Ms. J. Garner, Mr. R. Scott, The Glenfield Department of Perfusion, the Glenfield ECMO Coordinators, and the ECMO specialists.

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References

1. Peek GJ, Firmin RK: The inflammatory and coagulative response to prolonged extracorporeal membrane oxygenation (Review). ASAIO J 45: 250–263, 1999.
2. Bramson ML, Osborn JJ, Main FB: A new disposable membrane oxygenator with integral heat exchanger. J Thorac Cardiovasc Surg 50: 391, 1965.
3. Hill JD, O’Brien TG, Murray JJ, et al: Prolonged extracorporeal oxygenation for acute post-traumatic respiratory failure (shock-lung syndrome). Use of the Bramson membrane lung. N Engl J Med 286: 629–634, 1972.
4. Ichiba S, Peek GJ, Sosnowski AW, Brennan KJ, Firmin RK: Modifying a venovenous extracorporeal membrane oxygenation circuit to reduce recirculation. Ann Thorac Surg 69: 298–299, 2000.
5. Peek GJ, Moore HM, Moore N, Sosnowski AW, Firmin RK: Extracorporeal membrane oxygenation for adult respiratory failure. Chest 112: 759–764, 1997.
Copyright © 2002 by the American Society for Artificial Internal Organs