A COMPARISON OF TRANSCUTANEOUS, END-TIDAL AND ARTERIAL MEASUREMENTS OF CARBON-DIOXIDE DURING GENERAL-ANESTHESIA

A randomized, prospective study was performed to evaluate the accuracy of a new transcutaneous carbon dioxide (CO2) monitor (Fastrac(TM)) during general anaesthesia. Twenty-two adult patients undergoing elective surgery were subjected to three different levels of minute ventilation by varying their respiratory rates in a randomized cross-over design. Simultaneous measurements of transcutaneous CO2 (PTCCO2) and arterial CO2 (PaCO2) were obtained at three levels of minute ventilation (low, medium and high). End-tidal CO2 (PETCO2) values were also recorded from a mass spectrometer (SARA(TM)) at each time period. A total of 66 data sets with PaCO2 ranging from 28-62 mmHg were analyzed. The PTCCO2 values demonstrated a high degree of correlation with PaCO2 over the range of minute ventilation (y = 0.904x + 6.36, r = 0.92, P < 0.001). The PETCO2 measurement also demonstrated a generally good correlation with PaCO2 (y = 0.62x + 9.21, r = 0.89, and P < 0.01). However, the PETCO2-PaCO2 gradients (mean 7.0 +/- 3.1 mmHg) were greater than the PTCCO2-PaCO2 gradients (mean 2.3 +/- 2.4 mmHg) at all three levels of minute ventilation (P < 0.05). These differences were greatest when PaCO2 was in the high range (48-60 mmHg). We conclude that the new Fastrac(TM) CO2 monitor is accurate for monitoring carbon dioxide levels during general anaesthesia. The new transcutaneous devices provide an effective method for non-invasive monitoring of CO2 in situations where continuous, precise control of CO2 levels is desired.