Changes in co-oximetry values and pulse oximetry in isoflurane-anesthetized dogs with and without nitrous oxide

Concentrations of 64% to 70% nitrous oxide (N₂O) provide intra-operative analgesia. Clinically, pulse oximeter estimation (SpO₂) of oxygen (O₂) hemoglobin saturation (SaO₂) was observed to decrease with N₂O. Absorption atelectasis from breathing O₂ was thought to decrease arterial partial pressure of O₂ (PaO₂) below 70 mmHg and reduce SaO₂ and SpO₂ when N₂O was used. Administering N₂O from the beginning of the anesthesia might prevent atelectasis development and low PaO₂. The study was done in 2 parts (P, 0.05). In Part 1, isoflurane-anesthetized dogs undergoing ovariohysterectomy (n = 15 each group) breathed N₂O from anesthesia start (N₂O_early) or 1 hour later (N₂O_late). SpO₂, CO-oximetry values, and PaO₂ were compared to dogs breathing O₂ throughout anesthesia (control). Timing of N₂O introduction did not affect PaO₂ (lowest = 94 mmHg), SaO₂, or SpO₂. With N₂O, the lowest SpO₂ value was 91% and corresponded to a PaO₂ of 151 mmHg. Carboxyhemoglobin increased (highest = 2.7%) and SaO₂ decreased with N₂O (lowest = 96.7%). In Part 2, to replicate findings, 10 isoflurane-anesthetized dogs breathed N₂O, then O₂. With N₂O, SaO₂ did not decrease, but carboxyhemoglobin increased and returned to baseline once N₂O was discontinued. The dog with the highest carboxyhemoglobin (2%) had an SaO₂ of 96.8% (PaO₂ = 93 mmHg). Carboxyhemoglobin and SaO₂ changes were not clinically significant. Pulse oximetry did not reliably estimate SaO₂ but N₂O was not always a factor.