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We studied the inferior vena cava (IVC) as an index of right-heart function in 111 patients. A two-dimensional echocardiographic sector was used to visualize the IVC, and its M-mode cursor was used to generate a time-motion record of the IVC size and pulsation. Normal subjects had a small presystolic A wave (less than 125% of the end-diastolic IVC dimension), a small systolic V wave (less than 140% of the enddiastolic IVC dimension), and a 50% inspiratory decrease in IVC dimension. The A wave was absent in patients with atrial fibrillation. When normalized for body surface area, mean end-diastolic IVC dimension correlated with mean right atrial pressure (r = 0.72, p < 0.001). An A wave > 125% of end-diastolic IVC dimension was recorded in 71% of patients with sinus rhythm and an elevated right ventricular end-diastolic pressure of 10 mm Hg or greater, but in no patient with right ventricular end-diastolic pressure of less than 10 mm Hg (p < 0.001). A V wave 140% of end-diastolic IVC dimension was recorded in 75% of patients with severe tricuspid insufficiency, but in no patient with mild or no tricuspid insufficiency (p < 0.001). The inspiratory decrease in IVC dimension correlated with radionuclide right ventricular ejection fraction (r = 0.75, p < 0.001); no respiratory variation in end-diastolic IVC dimension occurred in patients with significant right ventricular dysfunction (right ventricular ejection fraction less than 25%) or in patients with constrictive pericarditis.