FIG 4. Transcutaneous oxymetry. Measurement of transcutaneous partial pressure of oxygen (PtcO₂) and carbon dioxide (PtcCO₂) in six healthy volunteers shows markedly high transcutaneous measurement of partial pressure of oxygen (ranging from 300–500 mmHg) after inhalation of 100% oxygen at 20 min. Transcutaneous measurement of partial pressure of oxygen was substantially higher when a sealed mask was used, compared with a loose mask. Transcutaneous measurement of partial pressure of carbon dioxide remains low, and no significant difference (P = .96) was seen between the loose mask and the sealed mask.