Adult respiratory distress syndrome (ARDS) often occurs in response to sepsis, shock, or ischemia/reperfusion (I/R) of a remote organ and is a frequent cause of mortality in the ICU patient. Pulmonary vascular resistance (PVR) increases during ARDS, yet direct observations of the pulmonary microcirculation are needed to characterize the vascular response. The purpose of this study was to quantitate the changes in hemodynamic variables, subpleural arteriolar diameters (AD), and alveolar cross-sectional areas (ACSA) during intestinal I/R-induced lung injury in rats, using a new method of in vivo videomicroscopy. Sprague-Dawley rats were anesthetized and cannulated, and superior mesenteric arteries were looped. A thoracotomy was performed with animals ventilated with air with 1 cm PEEP. Hemodynamic and videomicroscopic data were obtained before and during 45 min of SMA occlusion and after reperfusion, up to 120 min. Maximal vessel dilation was measured using topical 10(-5) M nitroprusside. The ability of vessels to constrict was confirmed by applying topical 10(-6) M endothelin-1. Intestinal I/R produced decreases in arterial pH, mean arterial pressure, and cardiac output. Despite these alterations, subpleural AD remained maximally dilated. Arterioles maintained the ability to constrict as demonstrated by the response to topical endothelin-1. ACSA did not change, indicating a uniform inflation of the lung. Using a unique method of in vivo pulmonary videomicroscopy, we have shown that AD do not change following 120 min of intestinal I/R, despite systemic hemodynamic instability. It appears that pulmonary arteriolar vasoconstriction does not contribute to increased PVR during the early phase of lung injury.