End-Tidal and Arterial Carbon Dioxide Measurements Correlate Across All Levels of Physiologic Dead Space

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BACKGROUND: End-tidal carbon dioxide (PETCO2) is a surrogate, noninvasive measurement of arterial carbon dioxide (PaCO2), but the clinical applicability of PETCO2 in the intensive care unit remains unclear. Available research on the relationship between PETCO2 and PaCO2 has not taken a detailed assessment of physiologic dead space into consideration. We hypothesized that PETCO2 would reliably predict PaCO2 across all levels of physiologic dead space, provided that the expected PETCO2-PaCO2 difference is considered. METHODS: Fifty-six mechanically ventilated pediatric patients (0‐17 y old, mean weight 19.5 ± 24.5 kg) were monitored with volumetric capnography. For every arterial blood gas measurement during routine care, we measured PETCO2 and calculated the ratio of dead space to tidal volume (VD/VT). We assessed the PETCO2-PaCO2 relationship with Pearson's correlation coefficient, in 4 VD/VT ranges. RESULTS: VD/VT was ≤ 0.40 for 125 measurements (25%), 0.41‐0.55 for 160 measurements (32%), 0.56‐0.70 for 154 measurements (31%), and > 0.7 for 54 measurements (11%). The correlation coefficients between PETCO2 and PaCO2 were 0.95 (mean difference 0.3 ± 2.1 mm Hg) for VD/VT ≤ 0.40, 0.88 (mean difference 5.9 ± 4.3 mm Hg) for VD/VT 0.41‐0.55, 0.86 (mean difference 13.6 ± 5.2 mm Hg) for VD/VT 0.56‐0.70, and 0.78 (mean difference 17.8 ± 6.7 mm Hg) for VD/VT > 0.7. CONCLUSIONS: There were strong correlations between PETCO2 and PaCO2 in all the VD/VT ranges. The PETCO2-PaCO2 difference increased predictably with increasing VD/VT.

Keywords: artificial respiration; blood gas analysis; capnography; carbon dioxide; infant; mechanical ventilation; pediatric

Document Type: Research Article

Affiliations: Division of Pediatric Critical Care Medicine, Duke University Medical Center, Durham, North Carolina, USA; Department of Pediatrics, University of South Carolina, Charleston, South Carolina, USA

Publication date: March 1, 2010

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