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A low input impedance, intrathoracic artificial lung is being developed for use in acute respiratory failure or as a bridge to transplantation. The device uses microporous, hollow fibers in a 0.74 void fraction, 1.83 m2 surface area bundle. The bundle is placed within a thermoformed polyethylene terephthalate glucose modified housing with a gross volume of 800 cm3. The blood inlet and outlet are 18 mm inner diameter vascular grafts. Between the inlet graft and the device is a 1 inch inner diameter, thin-walled, latex tubing compliance chamber. These devices were implanted in Yorkshire pigs via median sternotomy with an end to side anastomosis to the pulmonary artery and left atrium. The distal pulmonary artery was occluded to divert the right ventricular output to the device. Pigs 1 and 2 were supported fully for 24 hrs and then killed. Pig 3 was supported partially for 20 hrs and died from bleeding complications. The first implant, in a 55 kg male pig, transferred an average of 176 ml/min +/- 42.4 of O2 and 190 ml/min +/- 39.7 of CO2 with an average blood flow rate of 2.71/min +/- 0.46. The normalized average right ventricular output power, Pn, was 0.062 W/(L/min) +/- 0.0082, and the average device resistance, R, was 3.5 mmHg/(L/min) +/- 0.62. The second implant, in a 60 kg male pig, transferred an average of 204 ml/min +/- 22.5 of O2 and 242 ml/min +/- 17.2 of CO2 with an average blood flow rate of 3.7 L/min +/- 0.45, Pn of 0.064 W/(L/min) +/- 0.0067, and R of 4.3 mmHg/(L/min) +/- 0.89. The third implant, in an 89 kg male pig, transferred an average of 156 ml/min +/- 39.6 of O2 and 187 ml/min +/- 21.4 of CO2 with an average blood flow rate of 2.5 L/min +/- 0.49, Pn of 0.052 W/(l/min) +/- 0.0067, and R of 3.4 mmHg/(L/min) +/- 0.74. These experiments suggest that such an artificial lung can temporarily support the gas transfer requirements of adult humans without over-loading the right ventricle.