I read with interest the editorial in a recent issue of CHEST (March 2008)1 regarding mean pulmonary arterial pressure measurements made from systolic pulmonary artery pressure. This interesting concept that mean pulmonary pressure is predictable from the systolic pulmonary arterial pressure seems to fail in at least one situation. An infant with an unrestrictive ventricular septal defect with low pulmonary vascular resistance and high pulmonary blood flow, and another infant with high pulmonary vascular resistance and low blood flow both could have same systolic pressure, but different diastolic pressures and different mean pulmonary arterial pressures, as hitherto calculated. In other words, the mean recorded pressures of 90/20 and 90/50 mm Hg are different. Perhaps, the compliance in a hyperkinetic, volume-loaded, fully recruited vasculature system and an obliterative pulmonary vasculature with fixed vascular resistance behave differently from the lung with normal physiology under varying circumstances. Of course, within the given system, the mean pressure may be largely driven by systolic pressure, as documented by the authors.