forced expiratory volume in 1 second (FEV1) less than 70 percent of predicted forced vital capacity (FVC) less than 70 percent of predicted FEV1/FVC less than 0.7 inability to perform pulmonary function tests (for the same reasons, it is important to document normal underlying lung function) echocardiographic estimated right ventricular systolic pressure 50 mm Hg or right heart catheterization mean pulmonary artery pressure greater than 25 mm Hg (pulmonary hypertension may result in progressive hypoxemia due to intracardiac shunt or right ventricular failure) inadequate echocardiographic window to allow for accurate transthoracic contrast (bubble) echocardiogram (CE) (this is the test used to identify IPVDs) antibiotic use within the last 1 month (this is the intervention being tested) (note that all subjects will be under the concurrent care of a gastroenterologist or hepatologist, and some patients may accordingly be on prophylactic antibiotic therapy for prior SBP or variceal hemorrhage; these patients will be excluded) (20 percent expected rate of exclusion due to this criterion) current use of exogenous nitrates (may increase exhaled NO levels) norfloxacin intolerance (norfloxacin administration is the study intervention): allergy or intolerance to norfloxacin or other fluoroquinolones history of tendon rupture associated with norfloxacin or other fluoroquinolones glucose 6-phosphate dehydrogenase deficiency (possibility of hemolytic reactions with norfloxacin) known prolongation of the QTc interval to a duration that is > 50% of the R-R interval, subjects taking QTc-interval prolonging drugs, subjects with uncorrected hypokalemia, clinically significant bradyarrhythmias or acute myocardial ischemia (norfloxacin may worsen this) pregnancy (norfloxacin contraindicated) age less than 18 or greater than 70 expected death/transplantation within 3 mo (treating physician's discretion) lactose intolerance (placebo contains lactose) Smoking within the last 1 month