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

Concentrations of 64% to 70% nitrous oxide (NO) provide intra-operative analgesia. Clinically, pulse oximeter estimation (SpO) of oxygen (O) hemoglobin saturation (SaO) was observed to decrease with NO. Absorption atelectasis from breathing O was thought to decrease arterial partial pressure of O (PaO) below 70 mmHg and reduce SaO and SpO when NO was used. Administering NO from the beginning of the anesthesia might prevent atelectasis development and low PaO. The study was done in 2 parts ( < 0.05). In Part 1, isoflurane-anesthetized dogs undergoing ovariohysterectomy ( = 15 each group) breathed NO from anesthesia start (NO) or 1 hour later (NO). SpO, CO-oximetry values, and PaO were compared to dogs breathing O throughout anesthesia (control). Timing of NO introduction did not affect PaO (lowest = 94 mmHg), SaO, or SpO. With NO, the lowest SpO value was 91% and corresponded to a PaO of 151 mmHg. Carboxyhemoglobin increased (highest = 2.7%) and SaO decreased with NO (lowest = 96.7%). In Part 2, to replicate findings, 10 isoflurane-anesthetized dogs breathed NO, then O. With NO, SaO did not decrease, but carboxyhemoglobin increased and returned to baseline once NO was discontinued. The dog with the highest carboxyhemoglobin (2%) had an SaO of 96.8% (PaO = 93 mmHg). Carboxyhemoglobin and SaO changes were not clinically significant. Pulse oximetry did not reliably estimate SaO but NO was not always a factor.