| Summary: | The pulse CO-Oximetry allows continuous, noninvasive monitoring of hemoglobin (SpHb). We assessed the impact of increased end-tidal carbon dioxide (EtCO<sub>2</sub>) on the accuracy and trending ability of SpHb in laparoscopic surgery. Participants (<i>n</i> = 64) were randomly allocated to the low carbon dioxide (CO<sub>2</sub>) group (EtCO<sub>2</sub>: 30–35 mmHg) or the high CO<sub>2</sub> group (EtCO<sub>2</sub>: 40–45 mmHg). The SpHb and laboratory hemoglobin (tHb) were obtained during surgery. The correlation coefficient (r) between SpHb and tHb showed greater tendency in the low CO<sub>2</sub> group (r = 0.68) than in the high CO<sub>2</sub> group (r = 0.43). The bias (precision) was −1.18 (1.09) with a limit of agreement (LOA) of −3.31 to 0.95 in low CO<sub>2</sub> group and −1.02 (1.24) with a LOA of −3.45 to 1.42 in high CO<sub>2</sub> group; they did not differ significantly between the groups (<i>p</i> = 0.246). The low CO<sub>2</sub> group showed a high concordance rate of 95.9% and a moderate correlation between ΔSpHb and ΔtHb (r = 0.53). However, the high CO<sub>2</sub> group showed a concordance rate of 77.8% and no correlation between ΔSpHb and ΔtHb (r = 0.11). In conclusion, increased EtCO<sub>2</sub> significantly reduced the trending ability of SpHb during laparoscopic surgery. Caution should be executed when interpreting SpHb values during laparoscopic surgery in patients with hypercapnia.
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