The effects of aerosolized prostaglandin (PG) I2 on gas exchange and hemodynamics were investigated in patients ventilated mechanically because of severe community-acquired pneumonia. Group A were patients without preexisting lung disease (n = 6), and Group B were those with underlying chronic fibrotic interstitial lung disease (n = 6). Ventilation-perfusion distribution was assessed by the multiple inert gas elimination technique. In Group A, low doses of aerosolized PGI2 (mean, 6.6 +/- 3.0 ng/kg/min) sufficed to decrease the mean pulmonary artery pressure (Ppa) from 35.0 +/- 1.5 to 31.0 +/- 1.6 mm Hg (p < 0.05), to improve the ratio of arterial oxygen to the fraction of inspired oxygen (PaO2/FIO2 increase from 100 +/- 18 to 134 +/- 18; p < 0.05), and to decrease intrapulmonary shunt (36.9 +/- 4.7 to 27.5 +/- 4.5%; p < 0.05). Systemic arterial pressure (Psa) and cardiac output remained unchanged. In Group B, aerosolized PGI2 was ineffective in doses less than 10 ng/kg/min. A dosage of 33.6 +/- 12 ng/kg/min reduced Ppa (38.0 +/- 2.4 to 30.8 +/- 2.1 mm Hg; p < 0.05), but it also decreased Psa (80.3 +/- 3.6 to 71.3 +/- 4.7 mm Hg; NS) and PaO2/FIO2 (73.8 +/- 6.6 to 65.5 +/- 6.8 mm Hg; p < 0.05) values and increased intrapulmonary shunt (44.7 +/- 3.0 to 49.4 +/- 5.0%, NS). After withdrawal of the PGI2 aerosol, all gas exchange and hemodynamic changes returned to preaerosol baseline values within 60 min in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)