During general anesthesia, maintaining an oxygen saturation (SpO2) of at least 90% is critical to ensure adequate tissue oxygenation and prevent hypoxia. Levels below 90% indicate significant desaturation and can lead to cellular injury and organ dysfunction. Pulse oximetry is commonly used intraoperatively to monitor oxygenation continuously. A SpO2 above 90% corresponds to a PaO2 of approximately 60 mmHg on the oxyhemoglobin dissociation curve, representing the safe lower limit before oxygen delivery to tissues becomes compromised. Thus, maintaining oxygen saturation ≥90% is a key safety parameter during anesthesia.

Reference: Miller's Anesthesia, Ronald D. Miller, 9th Edition.