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Ann Thorac Surg 1997;64:1581-1582
© 1997 The Society of Thoracic Surgeons
Centre de Pneumologie de l'Hôpital Laval, Ste-Foy, Quebec, Canada
Despite the many technical advances that have occurred in the management of assisted ventilation, the mortality of adult respiratory distress syndrome (ARDS) is still greater than 60% and the mechanical ventilation necessary to deliver adequate tidal volumes with high peak airway pressures has been shown to further aggravate the underlying pulmonary condition or even contribute to the specific lung pathology seen in ARDS [1]. Thus conventional management of these patients still presents a problem, and extracorporeal membrane oxygenation (ECMO) in which the major part of gas exchange is accomplished by the extracorporeal circulation has been promoted as a solution. Unfortunately, previous controlled studies carried out in adult patients with life-threatening ARDS have shown no improvement in survival with ECMO when compared with conventional techniques of mechanical ventilation [2]. Recent developments in the management of extracorporeal circulation have helped to increase survival in the adult population, but the results are still far below the 90% survival rate obtained in neonates who require ventilatory support through the use of ECMO.
Although most reports recommend the use of ECMO to improve oxygenation, Gattinoni and associates [3] proposed in 1986 an alternative technique where ECMO was used only for CO2 removal while the natural lungs were kept "at rest" and arterial oxygenation accomplished through apneic oxygenation. In their series, a total of 43 patients who met the ECMO criteria underwent the procedure and 21 (48.8%) survived and were eventually discharged from the hospital. In 1994, however, a randomized clinical trial involving 40 patients and comparing standard mechanical ventilation with pressure-controlled inverse ratio ventilation followed by extracorporeal CO2 removal (Gattinoni technique) failed to show a significant difference in survival between the mechanical ventilation and the extracorporeal CO2 removal groups [4]. The authors of that study concluded that extracorporeal support should not be recommended as therapy for ARDS.
Overall the Gattinoni technique as well as those of others are formidable undertakings whether they are done through venovenous or venoarterial bypass. In addition to being expensive, they also require the use of a pump system that has to be supervised almost around the clock by highly trained personnel. More recently, the use of intravascular gas exchangers has also proved to be inefficient and impractical.
Despite these negative results, the technology of using ECMO for CO2 removal could still have a significant appeal if the technique itself was simpler to use and keep operational for days, safer, and less expensive. Such a technique could be used to treat patients who have not quite reached the most severe state of ARDS but yet need to have their lungs put "to rest." It could also be used to manage with the least lung trauma such patients as those having postpneumonectomy edema or respiratory failure after a lung volume reduction operation. Ultimately, this technique could be used as a "lung dialysis unit" for patients with end-stage chronic obstructive lung disease requiring frequent hospital admissions for ventilatory assistance or for patients with bilateral diaphragmatic paralysis.
In this context, the use of a pumpless arteriovenous system of CO2 removal (AVCO2R) may represent a definite answer because it avoids the use of a pump and functions with lower flow rates, expected to produce less hemodynamic or hematologic disturbances. Indeed, the investigation by Brunston and associates in this issue of The Annals of Thoracic Surgery [5] and that of others [6] have shown that an AVCO2R system is simple to install, and effective to remove CO2 at least in the setting of animal models. One has to be cautious, however, before extrapolating that results in critically ill patients with multiorgan failure will be similar to those obtained in normal sheep or in sheep with smoke inhalation injury alone. For instance, patients with sepsis, shock, or heart failure may not have enough forward pressure to use the AVCO2R system without a pump, or the system may create hemodynamic disturbances not seen in healthy animals with normal hearts.
What can be done to improve the AVCO2R system and make it safer and more attractive? (1) Heparin coating of the oxygenator as described by Bindsley and colleagues [7] or additional heparin bonding of the circuit could be used to reduce or even eliminate the necessity of heparinizing these patients. (2) Microporous hollow-fiber membrane oxygenators, which have a greater exchange efficiency, could be used instead of solid membrane devices. One of the potential problems associated with prolonged use of microporous membranes is plasma leakage, which could be corrected by the use of an ultrathin coating of silicone not expected to affect the effectiveness of the oxygenator. (3) Rapid cannulation techniques with specially designed catheters to be introduced percutaneously should be developed. (4) Further randomized clinical trials should compare standard therapy of mechanical ventilation with AVCO2R.
Although ECMO has sometimes been used for periods of more than 1 month, the long-term impact on vital organs including the lungs of these prolonged perfusions will also have to be clarified before it can be recommended. In the meantime, one can envision the use of AVCO2R in certain clinical situations where conventional methods appear inadequate but yet the patient's pulmonary condition have not reached the point of irreversibility.
Footnotes
Address reprint requests to Dr Deslauriers, Centre de Pneumologie de l'Hôpital Laval 2725, chemin Sainte-Foy, Sainte-Foy, QC, G1V 4G5, Canada.
References
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