TRANSCUTANEOUS GAS TENSION MEASUREMENT

In areas of the skin where local blood flow exceeds the amount required for local O₂ consumption, capillary PO₂ may approximate PaO₂ . This may be particularly true if the local area is warmed. This principle has been exploited by manufacturers of transcutaneous PO₂ measuring instruments. These devices usually consist of a small electrode that is attached with adhesive to the skin. The skin is locally warmed to 40°C or 41°C. O₂ from capillaries diffuses through the intact skin into a Clark-type electrode that measures PO₂ directly. This value usually correlates well with PaO₂ .^[45] However, in the presence of peripheral vasoconstriction^[46] or with thick (adult) skin, the measurement may be erroneous. A reduction in cardiac output tends to result in an artifactual decrease in transcutaneous PO₂ because of the ensuing cutaneous hypoxia. Peripheral vascular disease also reduces the transcutaneous PO₂ value.^{[47] [48]}

Transcutaneous gas monitoring is particularly useful in infants, in whom local skin blood flow tends to be high and in whom repeated withdrawals of arterial blood may cause anemia. These instruments require frequent calibration. The time constant of measurement is relatively long, and a sudden decrease in PaO₂ may not be detectable quickly enough to allow a timely therapeutic response. Skin burns have sometimes resulted from prolonged application of these devices.

Transcutaneous PCO₂ monitoring devices are also available.^[49] Because changes in transcutaneous PCO₂ are slow to detect apnea or airway disconnects, measurement of these values by monitors is less widely used than monitoring of end-tidal CO₂ (see "Expired Gas Analysis").