advances in rewarming have improved the prognosis for patients with hypothermia, especially those with cardiac arrest treated with extracorporeal rewarming. The latest review in our current concepts series covers prehospital care, transport, resuscitation fluids, and extracorporeal membrane oxygenation. An involuntary drop in core body temperature to lt 35 degreesc 95 degreesf is a condition read more november 17, 2011: daniel brodie, md, and matthew bacchetta, md. Co directors of the center for respiratory failure at newyork presbyterian/columbia, have published an important review article about ecmo in the new england journal of medicine. The article details how extracorporeal membrane oxygenation ecmo can take over the function of the lungs in adults with acute respiratory distress syndrome ards to give severely damaged lungs time to rest and heal. The h1n1 flu pandemic led to a wider use of extracorporeal membrane oxygenation ecmo , proving its power in hypoxemic emergencies. The results obtained during this pandemic, more than any randomized trial, led to the worldwide acceptance of the use of membrane lungs.

Moreover, as centers that applied this technique as rescue therapy for refractory hypoxemia recognized its strength and limited technical challenges, the indications for ecmo have recently been extended. Indications for veno venous ecmo currently include respiratory support as a bridge to lung transplantation, correction of lung hyperinflation during chronic obstructive pulmonary disease exacerbation and respiratory support in patients with the acute respiratory distress syndrome, possibly also without mechanical ventilation. The current enthusiasm for ecmo in its various aspects should not, however, obscure the consideration of the potential complications associated with this life saving technique, primarily brain hemorrhage extracorporeal membrane oxygenation ecmo is a temporary artificial extracorporeal support of the respiratory system and/or cardiac system used for the treatment of cardiopulmonary failure refractory to conventional therapies. More than 1,0 papers on ecmo published between january 2009 and may 2011 pubmed search clearly indicate a renewed interest in the technique, initially triggered by the cesar trial publication 1 and the recent h1n1 flu pandemic. Indeed, during the pandemic the number of patients with acute respiratory distress syndrome ards who were not safely treatable with current clinical settings of mechanical ventilation 6 to 8 ml/kg tidal volume normalized on ideal body weight and plateau pressure lower than 30 to 35 cmh₂ o and who therefore received extracorporeal respiratory support appeared to be greatly increased. After providing a brief background and some technical notes, we will review the most important and recent findings related to the technique, its application and future applications. Long term ecmo, as support for severe respiratory failure, was first successfully used in 1972 in an adult patient with post traumatic respiratory failure 2 .

A few years later, at the university of california, bartlett and colleagues successfully treated a newborn with ecmo 3 . The enthusiasm for this new technique led to a randomized trial sponsored by the national institutes of health that compared venous arterial ecmo with conventional mechanical respiratory support in severe ards 4 . After randomization of 90 patients the trial was stopped for futility because the mortality in both groups was around 90%.

However, one should note that the greatest concern for mechanical ventilation, at that time, was the high fraction of inspired oxygen and not the high ventilator pressure/volume that is, nonphysiological stress and strain. In this ecmo trial, therefore, the only difference in mechanical ventilation settings between treatment and control patients was a lower fio₂ in the group that received the extracorporeal support. Moreover, the ecmo technology at these times was very primitive, with consistent risks for the patients due to high priming volumes of the extracorporeal circuits and elevated bleeding risks associated with systemic anti coagulation. The limitations of this trial passed mostly unrecognized, however, and the discouraging results therefore led to the ecmo technique being abandoned worldwide. During the same years, kolobow was developing a new membrane lung optimized for carbon dioxide co₂ removal as a possible application in patients with chronic obstructive pulmonary disease. While testing this new device in spontaneously breathing animals, however, we observed when part of the metabolically produced co₂ was removed by the membrane lung that the ventilation of the animals proportionally decreased to maintain a constant blood partial pressure of co₂ 5 . Artificial co₂ removal could therefore be a powerful and valuable tool to control the respiratory drive up to complete apnea 6 .

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Indeed, the focus of extracorporeal support was shifted from oxygenation to co₂ removal, aiming to provide lung rest to allow better healing. The hypothesis of limiting potential harmful stimuli was based uniquely on common sense, however, since concepts such as baby lung 7 , atelectrauma 8 , and so forth, were still unknown. The first applications in humans of the concept of lung rest were reported in 1980 9 . Soon after this experience a larger series of patients treated with extracorporeal co₂ removal and low frequency positive pressure ventilation was described by the same authors 10 . However, a small randomized trial conducted in the united states at the beginning of the 1990s failed to show an outcome advantage of additional extracorporeal support as com pared with conventional mechanical ventilatory support 11 .

Despite this lack of evidence, a few centers around europe continued to provide veno venous extracorporeal support in selected series of patients, usually as a last resort 12 . In contrast, in the united states bartlett and colleagues continued to provide extracorporeal support with standard mechanical ventilation, with more liberal indications and with encouraging results 13. A renewed interest in this technique arose after the publication of the cesar trial, which clearly showed an improvement in the death rate and severe disability 6 months after randomization, when patients with severe respiratory failure were treated with extracorporeal support in an expert high case volume center compared with nonspecialized hospital care 1 . Moreover, although not the primary outcome, an actual difference in survival of around 25% was observed for patients considered for ecmo treatment at 28 days, the primary outcome of most ards literature. However, the final explosion of the application of this extracorporeal support was due to the use of ecmo as a rescue therapy in australia and new zealand during the h1n1 influenza pandemic 15 . This increased use is also probably due to several technical improvements, which allowed a simpler and safer use of the technique.

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Among these innovations we can mention the introduction of nonporous hollow fiber devices, characterized by low resistance to blood flow, and poly methylpentene fibers that, combined with nonthrombogenic coatings, decrease the need for platelet transfusion and continuous heparin infusion. Moreover, recently introduced wire reinforced walls of vascular access devices allow very thin cannula walls, reducing resistance to blood flow. As the acronym ecmo includes several techniques with different aims, a brief reminder of the artificial lung's physiology is indicated to fully understand the different approaches, before discussing the latest reports on the topic. The amount of oxygen provided via artificial lung is a direct function of the blood flow.

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The blood flow required during veno venous bypass to achieve acceptable arterial oxygenation is usually between 3 and 6 l/minute, partially depending on the cardiac output of the patient, on the hemoglobin concentration and on saturation. Of note, the gas flow required to fully oxygenate the incoming blood through the artificial lung may be quite low. For example, if we consider an entering hemoglobin saturation of 60% with 3 l/minute extracorporeal blood flow, 10 g/dl hemoglobin concentration, 40 mmhg partial pressure of oxygen in venous blood and approximately 85 ml/l oxygen content , to reach 100% saturation we would need 200 ml/minute oxygen.