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 (N2Oearly) or 1 hour later (N2Olate). 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. Copyright and/or publishing rights held by the Canadian Veterinary Medical Association.