The inferior vena cava diameter and its respiratory response are used to estimate right atrial pressures in spontaneously breathing patients but its value in patients receiving mechanical ventilation is unvalidated. Forty-nine patients undergoing mechanical ventilation were prospectively evaluated in the intensive or coronary care units with two-dimensional echocardiography of the inferior vena cava and simultaneous measurements of mean right atrial pressures by central venous or pulmonary artery catheter. Correlation between inferior vena cava diameter at expiration and mean right atrial pressure was only 0.58. The correlation between inspiratory change in inferior vena cava diameter and mean right atrial pressure was poor (r = 0.13). Despite these correlations, an inferior vena cava diameter of < or = 12 mm predicted a right atrial pressure of 10 mm Hg or less 100% of the time, but sensitivity was only 25%. An inferior vena cava diameter > 12 mm had no predictive value for right atrial pressure.