The Use of Transcutaneous Pco2 to Titrate Hypocapnia and Reduce Pulmonary Vascular Resistance During Mitral Valve Replacement in a Patient with Severe Pulmonary Hypertension

To the Editor: Pulmonary hypertension is a well-known complication of mitral stenosis. Severe mitral stenosis can be very difficult to treat in the perioperative setting. Central to the treatment is avoiding hypercarbia and the resultant increased pulmonary vascular resistance (PVR). We describe a case in which we used the TOSCA® transcutaneous Pco2 sensor (Linde Medical Sensors, Basel, Switzerland) to continuously monitor the Paco2 of a patient with severe pulmonary hypertension undergoing mitral valve replacement. A 58-yr-old woman was scheduled for a redo mitral valve replacement secondary to a stenotic St. Jude mitral valve. Her pertinent medical history included obesity, chronic obstructive pulmonary disease, cerebral vascular disease, hypertension, and type II diabetes mellitus. After induction of general anesthesia, right heart catheterization revealed a pulmonary artery (PA) pressure of 150/40 mm Hg with systemic pressure of 140/70 mm Hg. To reduce PVR, we initiated therapy with inhaled epoprostenol at 50 ng · kg−1 · min−1 through a nebulizer connected to the inspiratory limb of the breathing circuit. Baseline arterial blood gas showed a gradient of 20 mm Hg between the Paco2 and the end-tidal CO2 measured by capnography. To avoid PVR increases, we monitored transcutaneous Pco2 as an accurate and continuous surrogate for the patient’s Paco2. The patient’s minute ventilation was titrated to maintain a transcutaneous Pco2 of 30–35 mm Hg. With these interventions, we maintained pulmonary arterial pressures at approximately 90/50 mm Hg. After replacing the mitral valve and weaning the patient from cardiopulmonary bypass, we initiated therapy with inhaled epoprostenol. End-tidal capnography was again found to be inaccurate, and so minute ventilation was again titrated using transcutaneous Pco2. We maintained systemic pressures at 120/80 mm Hg with PA pressures 50/30 mm Hg, and the patient was transferred to the intensive care unit. Pco2 can be measured transcutaneously because it freely diffuses through body tissue and skin. The TOSCA® sensor consists of a Stow-Severinghaus type electrode, which includes a miniature glass pH sensor with an Ag/AgCl reference. It is typically placed on the earlobe. After the sensor is calibrated, changes in electrode pH are proportional to the logarithm of changes in the Pco2, allowing Paco2 to be continuously calculated. Mitral stenosis can lead to prolonged increases in left atrial pressure with resulting pulmonary hypertension. Inhaled epoprostenol selectively reduces PVR by stimulating adenylate cyclase, resulting in cAMP-induced pulmonary vasodilatation. In this case, therapy with inhaled epoprostenol significantly reduced the accuracy of capnography. Transcutaneous Pco2 monitoring allowed us to accurately and continuously measure Paco2, facilitating titration of mechanical ventilation. Edward C. Nemergut, MD Jay B. Sleesman, MD Keith E. Littlewood, MD Department of Anesthesiology University of Virginia Health Charlottesville, VA [email protected]