Troubleshooting a dialysis line: when blue runs red

We present the case of a critically ill woman whose dialysis line was noted to be circulating bright red blood. Located in the right internal jugular vein, the line had previously been working normally with the change occurring shortly after the patient was liberated from positive pressure mechanical ventilation. An arterial malposition was ruled out and subsequent investigations revealed the presence of a left-sided partial anomalous pulmonary venous connection (PAPVC) that had been previously undiagnosed. The identification of a left-sided PAPVC from blood gas measurements taken from a right internal jugular vein dialysis catheter in this case provides an informative opportunity to consider the intricate physiological relationship between the respiratory and cardiovascular systems in critically ill patients requiring invasive procedures and treatments.