lntravascular Volume Loading Reversibly Decreases Aitway Cross-sectional Area*

High-resolution computed tomography was used to directly determine the short-term effects of intravascular volume expansion on airway caliber. The change in airway crosssectional area caused by intravascular volume expansion (30 mVkg, Ringer's lactate) was studied in six anesthetized mini-pigs within 5 min. Twenty-five of 27 large airways (diameter, 2.01 to 5.0 mm) demonstrated decreased internal cross-sectional area (10.56f 1.26 vs 8.66f 1.03 mmx, p<0.001). Twenty of 24 small airways (diameter, 0.75 to 2.0 mm) showed decreased internal cross-sectional area (1.82 k0.16 vs 1.44 f0.16 mm2, p<0.001). These changes P aroxysmal wheezing, frequently occurring at night, requiring the patient to sit bolt upright with air hunger, forms a characteristic clinical picture known since the 19th century as "cardiac asthma."l The term, now generally reserved for cough and wheeze associated with dyspnea in patients with primary cardiac disease, reflects the recognition of increased airway resistance in heart fail~re.~ The clinical features of rapid reversibility with the assumption of an upright posture, diuresis, and sedation, as well as the response to aerosolized bronchodilators, must be taken into account when considering a potential me~hanism.l-~ Mechanisms proposed to account for the effect of impaired left ventricular function on airway function include changes in pulmonary blood volume, pulmonary and bronchial vascular pressures, pulmonary edema, airway mucosal edema, and airway hyperreactivityid Serial physiologic measurements have demonstrated increased lung resistance following intravascular volume loading, increased left atrial pressure, and heart failure, but have not demonstrated the site of increased lung resi~tance.~,'-~ The use of direct, in uiuo imaging of airway changes that occur with intravascular volume loading may provide information relevant to the anatomic basis of the airway response in heart failure. The advent of high-resolution computed tomographic (HRCT) techniques has allowed the resolution of small (approaching 200 pm) morphologic structures within the pulmonary parenchyma.l0.l' High-resolution computed tomographic techniques have demon