Noninvasive monitoring in emergency resuscitation

The transcutaneous oxygen (PtcO2) monitoring technique uses a Clark electrode applied noninvasively to the skin surface. To obtain PtcO2 values that respond rapidly to physiologic changes, the electrode is heated to 44 to 45 C. Since its introduction in 1972, the PtcO2 sensor has become standard for monitoring oxygenation of neonates in respiratory distress. However, when applied to critically ill or injured adult patients, PtcO2 values often are much lower than arterial oxygen (PaO2) values. The explanation for this is that PtcO2 reflects not only arterial oxygen tension, but also cardiac output and oxygen delivery. Thus, during low-cardiac-output shock states, PtcO2, values are low, even when PaO2 is normal or high. PtcO2 monitoring in adults thus is useful in assessing oxygen delivery. Comparison to an arterial blood gas can easily differentiate whether a low PtcO2 value might be due to hypoxia or to low cardiac output. Other noninvasive monitors (conjunctival oxygen, pulse oximeter, transcutaneous CO2, end-tidal CO2) also show promise. In the emergency department, PtcO2 monitoring is useful in assessing the presence and severity of shock and hypoxia and as a physiologic monitor for titrating resuscitation.