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The frequency of aberrant placement of central venous catheters using the internal jugular vein as the insertion site varies between 0.8 percent and 5.7 percent. The oxygen saturation of blood from the catheter tip in our patient is consistent with its position in a pulmonary vein draining a lung ventilated with 60 percent oxygen. The pulsatile nature of the waveform is explained on the basis of a catheter whose tip is wedged into a pulmonary venous branch. This converts the pressure tracing from that of the pulmonary vein to that of the upstream pulmonary artery, as has been demonstrated in animal experiments and human studies. The tracing in Figure 1 no longer demonstrates pulmonary hypertension, probably due to the patients preterminal, severe hypoperfusion state. It is interesting that, in retrospect, the high thermodilution cardiac output and elevated mixed venous oxygen saturation seen at pulmonary artery catheterization and attributed to sepsis, may have been partly due to his left-to-right shunt. During embryologic development a common pulmonary vein evaginates from the left atrium and joins the splanchnic plexus of veins in the region of the developing lung buds. Occasionally some or all of the pulmonary veins fail to establish connection with the left atrium. Pulmonary venous blood may then return to the right atrium directly or via a derivative of the primitive cardinal venous system, usually the coronary sinus, vena cava, or one of its branches.