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Ann Thorac Surg 1997;64:1604-1605
© 1997 The Society of Thoracic Surgeons
DR ROBERT J. KEENAN (Pittsburgh, PA): Doctor Zwischenberger, I congratulate you; that is a lovely study.
I have several questions related more to some of the things that we have noted in our experience with ARDS, much of which has related to reperfusion injury after transplantation. The first question is, we have used venovenous ECMO as opposed to a venoarterial oxygenation circuit and found that to be relatively simple. Aside from the roller pump, what do you see as potential advantages of your AVCO2R over a straightforward venovenous ECMO circuit?
The second question is, we have found in reperfusion injury and in other models and other reasons for ARDS that oxygenation is a bigger problem than CO2 retention. Clearly you have a circuit that deals with CO2 but not oxygenation and you are compensating for that in your injury model by having a very high FiO2, which in itself may cause injury down the line. How do you deal with that?
The third question is, is your circuit heparinized in any way? You are obviously dealing with a pulsatile flow based on the sheep's own cardiac output. Do you see problems in extending this apparatus for longer-term support based on the cardiac output, because clearly you are going to have periods in which the output may in fact be low as a result of the hemodynamic instability of the injury?
DR ZWISCHENBERGER:Thank you, Dr Keenan. Your questions are insightful and need to be addressed.
First of all, in comparing AVCO2R with venovenous ECMO, venovenous ECMO still requires a servoregulated venous drainage system, a pump generating positive pressure at a relatively high flow, and, because of the ever-present possibility of air embolization, minute-to-minute bedside monitoring. As you can see from our illustration of the sheep standing in the cage with an AVCO2R circuit, AVCO2R is a simple arteriovenous shunt that can be managed without specialized bedside management, except a watchful eye for any bleeding complications. So, I think AVCO2R is definitely less labor intensive than venovenous ECMO.
Second, this particular study was designed to assess the performance capabilities of our AVCO2R circuit. If a patient has severe respiratory failure with both CO2 retention and hypoxia unresponsive to pressure-controlled ventilation, AVCO2R is probably an inadequate support technique. However, for patients with CO2 retention syndromes or early ARDS, AVCO2R can provide total CO2 removal to allow lung rest and potentially prevent the progression of respiratory failure resulting from barotrauma-volutrauma. In addition, a high FiO2 in the absence of concomitant barotrauma may not be injurious, and its effects on the lungs must be studied.
Finally, we systemically heparinize the animals, because we were not studying the dose-response of anticoagulation in an AVCO2R circuit.
DR JAMES R. MAULT (Durham, NC): Doctor Zwischenberger, I congratulate you for advancing the art of extracorporeal circulation. Having had quite an experience years ago with continuous hemofiltration for renal failure, my colleagues and I learned the lesson that an arteriovenous circuit without a blood pump is a significant simplification of the technique, and I think there are some real attractive advantages to approaching gas exchange in the same manner.
The question I have is somewhat of a big picture question. Five years ago we experienced similar excitement about the IVOX device, which offered the potential for simplicity regarding the insertion and the ability to rest the lungs and avoid the injurious trauma of mechanical ventilation. And, as we all know, its downfall was the intermediate gas exchange capabilities of the technique. Do you think that this technique would be able to avoid this problem given the earlier question about oxygenation and actual measurement of oxygen consumption capabilities?
DR ZWISCHENBERGER:Thank you, Dr Mault. After the initial excitement about the potential application of the intravenacaval oxygenator and CO2 removal device (IVOX), clinical trials showed that only approximately 30% of CO2 was removed and oxygen transfer was insignificant. In large animal studies in which the management technique of permissive hypercapnia was used, one was able to achieve up to 60% to 70% of CO2 removal. Our current AVCO2R circuit design takes advantage of the simplicity of an arteriovenous circuit and accomplishes near-total CO2 removal at normocapnia to achieve the goal of decreased barotrauma and lung rest. So, we think AVCO2R represents an important advancement over the intravenacaval gas exchanger. To understand the advantages and simplicity of AVCO2R, one has to uncouple the concepts of CO2 and oxygen exchange, that is, to utilize the remaining lung function to achieve apneic oxygenation and let the AVCO2R accomplish CO2 removal. The AVCO2R technique achieves total CO2 removal with resultant lung rest and potentially reduces the mortality associated with respiratory failure. We plan to define the safety and efficacy of AVCO2R in future clinical trials.
Because of the design of this pumpless circuit, we believe there will be less resistance, less turbulence, less shear stress, and improved laminar flow, giving the possibility that the heparinization requirements can be lowered.
Related Article
Ann. Thorac. Surg. 1997 64: 1599-1604.
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