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

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