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

BACKGROUND: End-tidal carbon dioxide (P_ETCO2) is a surrogate, noninvasive measurement of arterial carbon dioxide (P_aCO2), but the clinical applicability of P_ETCO2 in the intensive care unit remains unclear. Available research on the relationship between P_ETCO2 and P_aCO2 has not taken a detailed assessment of physiologic dead space into consideration. We hypothesized that P_ETCO2 would reliably predict P_aCO2 across all levels of physiologic dead space, provided that the expected P_ETCO2-P_aCO2 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 P_ETCO2 and calculated the ratio of dead space to tidal volume (V_D/V_T). We assessed the P_ETCO2-P_aCO2 relationship with Pearson's correlation coefficient, in 4 V_D/V_T ranges.

RESULTS: V_D/V_T 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 P_ETCO2 and P_aCO2 were 0.95 (mean difference 0.3 ± 2.1 mm Hg) for V_D/V_T ≤ 0.40, 0.88 (mean difference 5.9 ± 4.3 mm Hg) for V_D/V_T 0.41–0.55, 0.86 (mean difference 13.6 ± 5.2 mm Hg) for V_D/V_T 0.56–0.70, and 0.78 (mean difference 17.8 ± 6.7 mm Hg) for V_D/V_T > 0.7.

CONCLUSIONS: There were strong correlations between P_ETCO2 and P_aCO2 in all the V_D/V_T ranges. The P_ETCO2-P_aCO2 difference increased predictably with increasing V_D/V_T.