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Interact CardioVasc Thorac Surg 2009;9:780-783. doi:10.1510/icvts.2009.209783 © 2009 European Association of Cardio-Thoracic Surgery
Temporary cardiac support with a mini-circuit system consisting of a centrifugal pump and a membrane ventilatorDepartment of Cardiothoracic, Transplantation and Vascular Surgery, Hannover Medical School, Carl-Neuberg-Str. 1, 30625 Hannover, Germany Received 15 April 2009; received in revised form 23 July 2009; accepted 24 July 2009
*Corresponding author. Tel.: +49-511-532-3455; fax: +49-511-532-5404.
Commonly used extracorporeal membrane oxygenation (ECMO) systems for cardiac support are limited by bleeding complications, especially after surgery in the adult patient. Recently, we have switched from the use of a conventional ECMO system to a miniature-circuit including a centrifugal pump and the Novalung® membrane ventilator (iLA). This system allows us to administer less heparin compared to the conventional system. Between January and August 2007, 1469 patients underwent cardiac surgery at our center, of which 18 patients (1.2%) required temporary postoperative ECMO system support. Surgical procedures in these patients included coronary artery bypass grafting (CABG) surgery (n=5), valvular replacement (n=2), aortic surgery (n=2), cardiac transplantation (n=5), and other procedures (n=3). The mean age of the 18 patients was 50±15 years (n=13 male) with a mean duration of ECMO system support of 4.3 days (range: <1 to 14 days). Twelve patients (67%) were successfully weaned from ECMO system. The 30-day survival was 44% with a hospital mortality of 61%. Re-thoracotomy for bleeding was necessary in six patients (33%) under ECMO system support. In summary, the miniature ECMO system circuit seems to be suitable for middle-term cardiac support and is associated with a low rate of bleeding complications.
Key Words: Extracorporeal membrane oxygenation; Levitronix®; Centrifugal pump; Novalung®; Postcardiotomy; Temporary cardiac support
Temporary extracorporeal membrane oxygenation (ECMO) is utilized in  1% of patients with acute myocardial failure following cardiac surgery [1]. Conventionally, ECMO systems consist of a centrifugal or peristaltic pump coupled to an oxygenator. In the literature, ECMO support is associated with an increased complication rate which seems to correlate with the duration of support. Doll et al. described a high bleeding complication rate of up to 65% [2]. In addition, frequently observed negative events with ECMO include infections, renal failure, sepsis, hemolysis and neurologic complications [3]. In the past, we used an ECMO system consisting of a Biomedicus pump (Eden Prairie, MN, USA) in combination with an Affinity oxygenator (Medtronic, MN, USA). This system was limited to a short runtime due to hemolysis and bleeding complications. Assessment of the activated clotting time (ACT) is routinely used to monitor anticoagulation with heparin with a target of 200 to 220 s. Anticipating an extension of the system's runtime and a potential reduction of associated complications, a switch to a miniature-circuit ECMO system consisting of a centrifugal pump (Levitronix® Centrimag® Blood pumping system, Pharos LLC, Waltham, MA, USA) and a membrane ventilator (Novalung, Hechingen, Germany) was performed in Hannover in 2007. A key characteristic of this system is the shorter tubing system and the ability to forgo anticoagulation use for 24 h (Fig. 1). The results of this modified approach for temporary extracorporeal support in patients with myocardial failure after cardiac surgery have been analyzed in this study.
Between January and August 2007, 1469 patients underwent cardiac surgery at our institution. Of these, 18 patients (1%) required temporary ECMO support due to myocardial failure and associated cardiogenic shock. The cardiogenic shock included a cardiac index of <2.2 l/min/m2 and a systolic blood pressure of <90 mmHg or a lactate value of approximately 4.0 mmol/l under positive inotropic drugs, partly an IABP was already implanted. The itemization in operations yields an occurrence in patients after coronary artery bypass grafting (CABG) surgery <1%, after heart transplantation of 12% and after heart–lung transplantation of 22%. There were no contraindications for implanting an ECMO system. All data were prospectively recorded and retrospectively analyzed. The miniature-circuit ECMO is composed of a centrifugal pump (Levitronix® Centrimag® Blood pumping system, Pharos LLC, Waltham, MA, USA) and a membrane ventilator (Novalung, Hechingen, Germany). The system consists of a replaceable polycarbonate pump with a potential flow of 9.9 l/min at 5500 rpm. The pump has a small priming volume of 30 ml and this pump has been shown to lead to a lower hemolysis rate than the Biomedicus BP-80 pump (Medtronic, Minneapolis, MN, USA) [4].The pump is connected to the Novalung® membrane ventilator (iLA) with a very short heparin-coated tubing system (Fig. 1). The iLA is originally designed as a low-resistance membrane ventilator for pulsatile blood flow without the used of a blood pump. The membrane ventilator has a surface area of 1.3 m2 with a maximal gas flow of 15 l/min, which is approximately half the surface size of the Biomedicus oxygenator. The iLA priming volume is 175 ml and with the use of a blood pump, a maximum device flow of 5.5 l/min can be achieved. The pressure gradient across the membranes is 11 mmHg at 2.5 l/min device flow. This system does not have external heat exchanging capabilities. In all studies, patients' cannulation was performed after administration of heparin (10,000 IU intravenous bolus). The baseline ACT level was thereafter measured. The femoral vein and artery of the reverse side were punctured and cannulated transcutaneously by Seldinger's technique. Additionally, a 7F single lumen catheter was inserted along the arterial cannula in the distal direction into the femoral artery for lower limb perfusion. The miniature ECMO circuit system was completely de-aired and connected to the cannulas. Pump run was started in the veno-arterial mode in the following configuration: venous line, centrifugal pump, membrane ventilator and arterial line.Continuous intravenous heparin infusion was administered for anticoagulation with an ACT target time of 160 to 180 s. Descriptive statistics are presented as mean±S.D. All statistical analyses were performed using SPSS (version 16.0 for Windows; SPSS Inc, Chicago, IL, USA).
Eighteen patients required extracorporeal cardiac support for myocardial failure after cardiac surgery. Five of these underwent CABG, three patients received valvular replacement (one aortic valve and two mitral valve replacements; one as a redo operation). Two patients after aortic surgery had a composite graft replacement performed and one received a hybrid aortic arch prosthesis. In addition, ECMO support was required in three patients following heart transplantation and in two patients after combined heart–lung transplantation. In the remaining three patients, one after pericardectomy, one after ventricular septum occlusion after myocardial infarction, and one after pulmonary thrombendarterectomy. The mean intraoperative extracorporeal circulation time was 237±115 min. With respect to the type of ventricular failure, the indication for ECMO was right ventricular failure in three patients, left ventricular failure in 10 patients and biventricular failure in five patients, which is summarized in Table 1. The cause for heart failure was postoperative myocardial infarction in nine patients. Two patients developed preoperative myocardial infarction and one suffered from dilatative cardiomyopathy. In the five transplanted patients initial graft failure occurred.
The mean age of the 13 male and 6 female patients was 50±15 years and ranged from 21 to 70 years. The ECMO mode used in all patients was veno-arterial with a mean support time of 4±4 days (range 1–14 days). Arterial blood gas analysis was performed before and after implantation as well as 24 h, 72 h and 7 days after implantation. The systemic PaO2, PaCO2, pH and PaO2/FiO2 ratio did not considerable change during cardiac support, as well as minimal changes in the mechanical ventilation parameters (Table 2).
In addition, lactate dehydrogenase (LDH) serum concentrations were measured as a marker of hemolysis. LDH levels before implantation were high, most likely due to the intraoperative pump run and further peaked after 24 h. On the day of ECMO initiation, patients required 6±5 units of red blood cells. Over the course of the ECMO, 10±12 erythrocyte concentrates per patient were administered. The analysis of complications revealed a peak in hemolysis in the early phase following ECMO initiation (first 24 h). Sixty-seven percent of patients (n=12) developed renal failure requiring hemodialysis. Leukocyte counts, C-reactive protein (CRP) serum levels, as well as verification of an infectious agent in blood cultures were considered markers for infection. In three patients, Aspergillus fumigatus, Herpes simplex or Pseudmonas aeruginosa were detected. Overall, the infection rate in patients with increased leukocyte counts, CRP levels or a verification of an infectious agent was 33% (n=6 patients). The re-thoracotomy rate due to bleeding was considerably low with 33% (n=6), given that all patients underwent major surgery before ECMO initiation. The re-thoracotomy was necessary in average after 2.3±1.7 days. On three occasions the cause of bleeding was determined (bleeding from the internal mammaria artery, from the right lung vein and from the sternum in a patient with an open chest). Sixty-seven percent were successfully weaned-off ECMO support (n=12 patients). The in-hospital mortality was 61% (n=11 patients). However, only 44% (n=8) survived the first 30 days after implantation, highlighting the high degree of morbidity of such patients. Indeed, the causes of death included multiorgan failure (n=5), non-recoverable myocardial failure (n=4), sepsis (n=3) and intracerebral bleeding (n=1).
Temporary cardiac support in patients with cardiogenic shock after surgery with ECMO is an established treatment option. However, the survival rate is rather low in adult patients [5]. Nevertheless, it is the only therapeutic option to treat severe hemodynamic distress after cardiovascular surgery [6–8], if alternative medical interventions or the use of an intra-aortic balloon pump fail. The present study represents our initial experience with a novel miniature ECMO circuit. Until 2002, the standard for ECMO has consisted of a microporous polypropylene hollow fiber oxygenator. These silicone oxygenators are of limited use due to the high contact activation of blood coagulation factors because of a large artificial surface area and a higher blood path resistance, all of which can contribute to capillary leak syndrome. Peek et al. [9] tested the application of a polymethyl-pentene (PMP) oxygenator in 2002 and reported satisfactory initial results, including a reduction in platelet consumption and resistance to blood flow. In 2003, the PMP membrane ventilator iLA by Novalung® was successfully used in patients with hypercapnic respiratory failure as a bridge to lung transplantation in the pumpless arterio-venous mode [10]. With the pumpless iLA, we observed excellent CO2 removal, but insufficient oxygenation. This is likely due to the limited device blood flow of 1–1.5 l/min. Since oxygenation is dependant on blood flow, we consequently integrated a centrifugal pump to increase device flow [11]. As a result of the successful use of this improved system in lung transplantation, we adapted this miniature ECMO system for its use in patients with cardiogenic shock after cardiac surgery. As previously mentioned, the ECMO system is characterized by a considerable shorter tubing system, smaller artificial oxygenator surface, lower priming volume and a lower blood path resistance. Accordingly, contact activation of blood coagulation factors is reduced [12]. These are potential advantages in the prevention of hemolysis and bleeding. Indeed, here we were able to show a lower rate of re-thoracotomy in the early phase after surgery compared to the literature [2] and in three cases the bleeding could ascribe to a surgical cause. The low bleeding rate may be a result of both the low ACT target time, which we have routinely used in the past in patients under iLA support, and the 24-h delayed initiation of anticoagulation therapy. The Levitronix pump was especially designed to lower hemolysis rates, but this could not be verified in our study. However, it needs to be mentioned that LDH serum levels at the time of ECMO initiation, were already elevated. Thus, the individual impact of the intraoperative pump run, administration of erythrocyte concentrates and postoperative ECMO on hemolysis cannot be determined. We speculate that the initial hemolysis parameters were elevated at the time of ECMO initiation as a result of the intraoperative extracorporeal circulation. Later on hemolysis might have been further provoked by ECMO. Another routinely used marker of hemolysis is free hemoglobin, which, unfortunately, could not be evaluated in this retrospective study. Formica et al. [13] described a similar ECMO system consisting of a PMP oxygenator (Jostra Quadrox D oxygenator; Maquet® Cardiopulmonary AG, Hirrlingen, Germany) and a centrifugal pump (Rota flow, Maquet® Cardiopulmonary AG, Hirrlingen, Germany) with an additional heat exchanger. The maximal blood flow there was 7.0 l/min with a priming volume of 250 ml and a gas exchange surface area of 1.8 m2. In addition, therapeutic hypothermia can be applied in patients with severe cardiogenic shock or life-threatening arrhythmias [14]. This ECMO system showed acceptable results in terms of survival on ECMO and patient discharge. In our study, an excellent gas exchange could be observed longitudinally. According to Formica et al. the PMP oxygenator provided better gas exchange than the silicone membrane oxygenator. An advantage of the Quadrox oxygenator can be seen in the heat exchange abilities, which the iLA does not provide. A disadvantage of the Quadrox oxygenator over the iLA, however, is the large artificial blood contact surface.
It is important to note that the described miniature circuit ECMO is more expensive, compared to the conventional system. On the other hand, it might be a favorable alternative to biventricular assist devices or temporary artificial heart support, as these systems are extremely cost intensive and are not associated with better survival rates. A cost–benefit analysis was not a specific aim of the present study, but the actual costs of the iLA ( In conclusion, the miniature ECMO circuit system seems to be an effective and useful approach to support patients with acute myocardial failure after surgery.
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Abstract
1. Introduction
), does significantly exceed the cost of other oxygenators. However, a lower bleeding rate, associated with increased delivery of blood products, prolonged ICU stay, reoperations and a higher morbidity could justify the routine use of this system.