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BACKGROUND: Widespread use of transcutaneous PCO2 (PtcCO2) monitoring is currently limited by concerns many practitioners have regarding accuracy. We compared the accuracy of PtcCO2 with that of PaCO2 measurements in critically ill children, and we investigated whether clinical conditions associated with low cardiac output or increased subcutaneous tissue affect this accuracy. METHODS: We performed a single-center prospective study of critically ill children placed on transcutaneous monitoring. RESULTS: There were 184 children enrolled with paired PaCO2 and PtcCO2 values. Subjects had a median age of 31.8 mo (interquartile range 3.5–123.3 mo). Most children were mechanically ventilated (n = 161, 87.5%), and many had cardiac disease (n = 76, 41.3%). The median PaCO2 was 44 mm Hg (interquartile range 39–51 mm Hg). The mean bias between PaCO2 and PtcCO2 was 0.6 mm Hg with 95% limits of agreement from −13.6 to 14.7 mm Hg. The PtcCO2 and PaCO2 were within ±5 mm Hg in 126 (68.5%) measurements. In multivariable modeling, cyanotic heart disease (odds ratio 3.5, 95% CI 1.2–10, P = .02) and monitor number 2 (odds ratio 3.8 95% CI 1.3–10.5, P = .01) remained associated with PtcCO2 ≥ 5 mm Hg higher than PaCO2. Serum lactate, fluid balance, renal failure, obesity, vasoactive-inotrope score, and acyanotic heart disease were not associated with high or low PtcCO2 values. In 130 children with a second paired PtcCO2 and PaCO2 measurement, predicting the second measured PaCO2 by subtracting the initial observed difference between the PtcCO2 and PaCO2 from the subsequent measured PtcCO2 decreased the mean bias between observed and predicted PaCO2 to 0.2 mm Hg and the 95% limits of agreement to −9.4 to 9.7 mm Hg. CONCLUSIONS: PtcCO2 provides an acceptable estimate of PaCO2 in many critically ill children, including those with clinical conditions that may be associated with low cardiac output or increased subcutaneous tissue, although it does not perform as well in children with cyanotic heart disease. PtcCO2 may be a useful adjunct monitoring method, but it cannot reliably replace PaCO2 measurement.

Volumetric capnography dead-space measurements (physiologic dead-space-to-tidal-volume ratio [VD/VT] and alveolar VD/VT) are considered more accurate than the more readily available time-based capnography dead-space measurement (end-tidal alveolar dead-space fraction [AVDSF]). We sought to investigate the correlation between volumetric capnography and time-based capnography dead-space measurements.This was a single-center prospective cohort study of 65 mechanically ventilated children with arterial lines. Physiologic VD/VT, alveolar VD/VT, and AVDSF were calculated with each arterial blood gas using capnography data.We analyzed 534 arterial blood gases from 65 children (median age 4.9 y, interquartile range 1.7-12.8). The correlation between physiologic VD/VT and AVDSF (r = 0.66, 95% CI 0.59-0.72) was weaker than the correlation between alveolar VD/VT and AVDSF (r = 0.8, 95% CI 0.76-0.85). The correlation between physiologic VD/VT and AVDSF was weaker in children with low PaO2 /FIO2 (200 mm Hg), low exhaled VT (100 mL), a pulmonary reason for mechanical ventilation, or large airway VD (3 mL/kg). All 3 dead-space measurements were highly correlated (r0.7) in children without hypoxemia (PaO2 /FIO2300 mm Hg), mechanically ventilated for a neurologic or cardiac reason, or on significant inotropes or vasopressors.In mechanically ventilated children without significant hypoxemia or with cardiac output-related dead-space changes, physiologic VD/VT was highly correlated with AVDSF and alveolar VD/VT. In children with significant hypoxemia, physiologic VD/VT was poorly correlated with AVDSF. Alveolar VD/VT and AVDSF correlated well in most tested circumstances. Therefore, AVDSF may be useful in most children for alveolar dead-space monitoring.