Monitoring of arterial blood gases is fundamental to respiratory critical care and oxygen therapy. Radial artery is the most commonly used site to obtain blood sample for arterial blood gas analysis. Noninvasive blood gas monitoring with pulse oximetry and capnography have emerged as important and reliable measurements for oxygenation and carbon dioxide respectively. Transcutaneous measurements of partial pressure of O2 and CO2 have been also used to reflect the arterial pressure changes.