Effect of hypercarbia and shock on transcutaneous carbon dioxide at different electrode temperatures

Transcutaneous CO2 tension (PtcCO2) was measured with heated and nonheated transcutaneous carbon dioxide electrode sensors during hypercarbia and standardized hypovolemic shock in anesthetized dogs. The 95% response times of the PtcCO2 electrode, measured during step increases in FICO2 at four electrode temperatures: 37, 39, 41, and 44 degrees C, were 15, 7.5, 5, and 3.5 min, respectively. During experiments with normal cardiac output, the PtcCO2 correlated well with PaCO2 (r = 0.96). Three PtcCO2 electrode temperatures were tested for response to hypovolemic shock. Data from all PtcCO2 electrodes failed to correlate with PaCO2 during shock. PtcCO2 values rose as cardiac output decreased; there was a good negative correlation (r = -0.95). The 37 and 42 degrees C electrodes were affected by shock at cardiac index values below 2 L/min.M2; but the 44 degrees C probe was not affected until a cardiac index of 1.5 L/min.M2 was reached. Values from the 44 degrees C electrode rapidly returned to preshock values after fluid resuscitation. The response to resuscitation lagged in the 37 and 42 degrees C electrodes and did not return the preshock values until the cardiac index had been normalized for more than 15 min. It was concluded that a PtcCO2 electrode heated to 44 degrees C would be more useful in adult patients because of its faster response to hypercarbia, shock, and resuscitaton. The PtcCO2 values are higher than the corresponding PaCO2 values but they correlate well until the cardiac index (CI) falls below 1.5 L/min.M2; then PtcCO2 values have a strong negative correlation with the corresponding CI values.