Transcutaneous measurement of carbon dioxide tension during extended monitoring: Evaluation of accuracy and stability, and an algorithm for correcting calibration drift

BACKGROUND: When polysomnography is indicated in a patient with a presumed sleep disorder, continuous monitoring of arterial carbon dioxide tension (P_aCO2) is desirable, especially if nocturnal hypoventilation is suspected. Transcutaneous CO₂ monitors (P_tcCO2) provide a noninvasive correlate of P_{aCO 2}, but their accuracy and stability over extended monitoring have been considered inadequate for the diagnosis of hypoventilation. We examined the stability and accuracy of P_tcCO measurements and the performance of a previously described linear interpolation technique designed to correct for calibration drift. METHODS: We compared the P_tcCO values from 2 TINA TCM-3 monitors to P_aCO values from arterial blood samples obtained at the beginning, every 15 min of the first hour, and then hourly over 8 hours of monitoring in 6 hemodynamically stable, male, intensive care patients (mean age 46 ± 17 y). RESULTS: Time had a significant (P =.002) linear effect on the P_tcCO -P_aCO difference, suggesting calibration drift over the monitoring period. We found no differences between monitor type or interaction between time and monitor type. For the 2 monitors the uncorrected bias was 3.6 mm Hg and the limits of agreement were -5.1 to 12.3 mm Hg. Our linear interpolation algorithm improved the bias and limits of agreement to 0.4 and -5.5 to 6.4 mm Hg, respectively. CONCLUSIONS: Following stabilization and correction for both offset and drift, P_tcCO tracks P_aCO with minimal residual bias over 8 hours of monitoring. Should future research confirm these findings, then interpolated P_tcCO may have an increased role in detecting sleep hypoventilation and assessing the efficacy of treatment.