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Excerpt
While the National Institutes of Health adult ECMO trial was running to its dismal results, Kolobow et al. (7) proposed that ARDS management should include the prevention of further lung damage (barotrauma) and that extracorporeal CO2 removal rather than extracorporeal oxygenation was the key to achieving total control over ventilation. Tidal breathing is necessary to excrete CO2 rather than to oxygenate blood. Hence, extracorporeal CO2 removal could eliminate the need for tidal ventilation, opening new possibilities ranging from apneic oxygenation to high-frequency ventilation to low-frequency positive pressure ventilation.
Gattinoni et al. (8) reported on the successful use of a ventilatory strategy that included low-frequency positive pressure ventilation (three to five pressure-limited breaths per minute) and extracorporeal CO 2 removal to maintain normocapnia. Extracorporeal blood flow was generally limited to <2 L/min in venovenous bypass, whereas oxygenation was achieved by the natural lung through apneic oxygenation at relatively high distending pressures (15–25 cm H2O).
This approach was based on the experimental work by Kolobow et al. (9) on the deleterious effect mechanical ventilation exerts even on normal lungs, soon confirmed by the evolving concepts of barotrauma and volotrauma (10, 11). From then on, the concept that avoiding ventilator-associated lung injury (VILI) is of the utmost importance in ARDS management gained more ground. The huge amount of experimental and clinical evidence on barotrauma and VILI led to the ARDS network trial. In this study, patients treated with tidal volumes of 6 mL/kg showed better survival than those ventilated with 12 mL/kg (12). This important result will set the “gold standard” for ventilatory therapy for years to come; however, many patients still die of or with ARDS, and the clinician will still have to face the problem of extremely compromised gas exchange.
It may be important to remember that oxygenation and CO2 removal are largely independent processes, and they can be supported by independent approaches. Oxygenation is limited by the oxygen-carrying capacity of blood and by the need to maintain a sufficiently high venous oxygen saturation. These two reasons combined limit the amount of oxygen we can add to venous blood to approximately 50–60 mL/L (when hemoglobin is approximately 10%). Oxygenation, therefore, requires high blood flows (4–6 L/min). When the inlet blood of the oxygenator is already arterialized, the conditions are even more unfavorable, as with an arteriovenous shunt. On the other hand, CO2 removal only requires <1 L/min blood flow (the extreme value is approximately 0.5 L/min) and is almost equally effective on arterial as well as venous inlet blood.
