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(aCO(2))) is desirable, especially if nocturnal hypoventilation is suspected. Transcutaneous CO(2) monitors (P(tcCO(2))) 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(2)) measurements and the performance of a previously described linear interpolation technique designed to correct for calibration drift. METHODS We compared the P(tcCO(2)) values from 2 TINA TCM-3 monitors to P(aCO(2)) 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(2))-P(aCO(2)) 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(2)) tracks P(aCO(2)) with minimal residual bias over 8 hours of monitoring. Should future research confirm these findings, then interpolated P(tcCO(2)) may have an increased role in detecting sleep hypoventilation and assessing the efficacy of treatment.