SAN FRANCISCO-Current management strategies for patients who have adult respiratory distress syndrome (ARDS) include conventional mechanical ventilation as well as extracorporeal membrane oxygenation (ECMO). Both treatments involve delivery of oxygen and elimination of carbon dioxide. At the recent Clinical Congress of the American College of Surgeons (ACS), Joseph B. Zwischenberger, MD, who is professor of surgery, medicine, and radiology at the University of Texas Medical Branch at Galveston, reported on a much simpler technique for treating patients with ARDS: percutaneous arteriovenous carbon dioxide removal (AVCO₂R).

AVCO₂R creates a dialysis-like shunt that siphons blood from an artery through a catheter and allows blood to flow passively through a small extracorporeal loop, where nearly all of the carbon dioxide is removed. The blood is then returned to a large vein. The technique does not require an external pump or blood flow regulator, said Dr. Zwischenberger, because "the heart serves as the pump, and the size of the catheter determines blood flow." All that is needed is "to insert catheters and connect the circuit. Percutaneous AVCO₂R requires less monitoring [than other ARDS treatments] and does not require adjustment of flow or minute-to-minute care."

Percutaneous AVCO₂R is made possible by use of a low-resistance oxygenator, which is currently used to support patients during cardiopulmonary bypass. The low-resistance oxygenator performs gas exchange at average pressures of 70 to 90 mm Hg, which allows almost total removal of carbon dioxide with minimum burden on the heart.

"AVCO₂R uses the findings of researchers in the '80s and early '90s who said that you really only need to take out the CO₂ to have an impact on severe respiratory failure," Dr. Zwischenberger told Pulmonary Reviews. "The problem was, they used an ECMO circuit to accomplish it." ECMO uses a modified heart-lung machine that drains blood from the patient and forces it though an external device where gas exchange and a blood flow regulator must be monitored carefully. "I've been director of an ECMO program for 15 years, and I know what a big deal it is," he said. ECMO forces blood through extracorporeal circulation at pressures of 200 to 300 mm Hg.

AVCO₂R, in contrast, "is nothing more than a simple arteriovenous loop," Dr. Zwischenberger explained. "You insert a catheter into the femoral artery and in the femoral vein, both percutaneously, and take 10% of the cardiac output. Then you pass the blood through a low-resistance gas-exchange device, or low-resistance oxygenator, and you take out 70% to 90% of the CO₂ gas production. When you do that, you alleviate the need for a ventilator to accomplish CO₂ removal."

Dr. Zwischenberger--and his colleagues at Louisiana State University, in Shreveport--have been experimenting with the technique in large animals and in patients with ARDS. Phase I trials were recently completed at Texas Medical Branch and at Louisiana State. In phase II trials, "We've applied this technique to a series of patients with ARDS and showed that we could remove carbon dioxide without causing major adverse events," he said. Dr. Zwischenberger and colleagues are just beginning prospective randomized studies in adults and children.

The researchers described this novel form of treatment as "an intellectual leap" with respect to the overall concept of the disease process underlying ARDS. "For years, oxygenation was the sole focus," Zwischenberger said. "But we believe that carbon dioxide removal is the key to ameliorating ventilator-induced lung injury. When clinicians ventilate patients, or utilize ECMO support during respiratory failure, they think of oxygenation and CO₂ removal as simultaneous events; but they are not simultaneous at all. Oxygenation is a different physiological process than CO₂ removal."

At the ACS meeting, Dr. Zwischenberger reported on prospective randomized studies of AVCO₂R in an animal model of ARDS. All animals treated with AVCO₂R survived, and they required an average of 39% fewer days of ventilator support. "Because we were able to eliminate nearly all of the carbon dioxide production, we decreased the need for the mechanical ventilator to transfer gas, and we allowed quick and more complete recovery of all animals," he reported.

Dr. Zwischenberger has been evaluating technological improvements in treatments for ARDS. "The current studies used catheters and equipment already on the market, so the technique can be taken directly to a clinical application," he said. "We hope to develop smaller catheters and smaller equipment in order to expand the patient population to include children and infants. We're also trying to determine how carbon dioxide removal affects the development of respiratory failure in general."

-Margaret A. Inman