Transcutaneous CO2 monitoring : effect of electrode position and gestation on arterial partial pressure of CO2 for the first 3 days of neonatal life

Transcutaneous measurement (TCM) yielding transcutaneous –partial pressure of carbon dioxide (TcPCO2) provides a non-invasive method of continuously monitoring CO2 tension in critically ill mature and premature neonates. This study evaluated the relationship between TcPCO2 and the direct measurement of partial pressure of carbon dioxide (PaCO2). The aims were to identify probe sites associated with the highest correlation and least discrepancy between the TCM and the direct PaCO2 measurement by comparing factorial combinations of TCM probe placements and gestational categories of neonates ≤3 days old and to identify safe operating limits for TcPCO2. Over a period of 10 months, PcCO2 and TcPCO2 readings were collected from eligible neonates ≤3 days of age who were admitted to a neonatal intensive care unit (NICU) and required an arterial line and TCM. Readings from four thorasic and four abdominal probe placement sites were analysed. Neonates (n=95) were stratified into three gestational groups for analysis (<28 weeks’, 28-32 weeks’ and >32 weeks’ gestation). TcPCO2 readings >35mmHg (>4.6kPa) tends to underestimate PaCO2. Large discrepancies between PcCO2 and TcPCO2 show that substantial error of prediction can occur. Regression analysis identified TcPCO2 ranges that predict PaCO2 levels within 35-45mmHg (4.6-6.0kPa) with 90% confidence. In conclusion, although TcPCO2 predicts PaCO2 with well below perfect accuracy, operating ranges of TcPCO2 between 41-45mmHg (5.4-6.0kPa) are likely to maintain PaCO2 within the safe levels of 35-45mmHg (4.6-6.0kPa) on most occasions (90%) during the early neonatal period.