In this preliminary, crossover investigation the investigators will examine the effect of oxygen supplementation on the recovery of breathing in the immediate post-anesthesia period.

In a pilot randomized-controlled trial (NCT04723433) the investigators found that, compared with standard O2 supplementation, hyperoxia enhanced ventilation, as estimated by the fraction of time at a transcutaneous PCO2 (TcPCO2) > 45 mmHg.

More specifically, patients treated with hyperoxia (O2 titrated to: SpO2 > 96%, N=10; Liberal O2) for 90 minutes post-anesthesia, spent 61.2% of the time at TcPCO2 > 45 mmHg, compared with 80.6% of the time in those receiving standard O2 supplementation (O2 titrated to: SpO2 between 90-94%, N=9; Conservative O2 - between-group difference of 19.4% (95% Confidence Intervals: -18.7% to 57.6%), ANCOVA adjusted P = 0.140]. Results were consistent across the 90-min monitoring period. With an observed effect size of 0.73, it was estimated that 30 participants per group are required, to demonstrate this difference with a power of 80% at a two-sided alpha of 5%.

In a follow-up RCT (IRB-63878, NCT05379673) in 18 patients the investigators found that over a 90-minute window, the average percentage of time spent at TcPCO2 > 45mmHg for patients in the Conservative O2 group (N=9) was 57.8% (52 ± 44 minutes), compared with a 45.6% (41 ± 46 minutes) for those in the Liberal O2 group (N=9), (P=0.6134; two-sided two sample t-test). In addition, during the same period, respiratory disturbance index (RDI; events per hour) was higher in the Conservative O2 [median (range): 23.9 (11.0-78.6)], compared with the Liberal O2 group [19.6 (2.8-40.4), Mann-Whitney test, P=0.1615].

Due to the high variability observed in the evaluated respiratory parameters (i.e., TcPCO2 and upper airway obstruction expressed by Respiratory Disturbance Index), the researchers believe that a crossover, rather than a randomized controlled trial, design would improve the power and efficiency of this investigation and also provide an opportunity to characterize better those patients who respond, and separate them from those who do not respond to hyperoxia treatment.

This preliminary investigation will assess how ventilation during recovery from general anesthesia is affected by conservative (Conservative O2: 90% ≤ SpO2 ≤ 94%), compared with liberal (Liberal O2: SpO2 > 96%) O2 supplementation. Using a crossover study design, the investigators aim to: a) assess the breathing pattern and estimate and compare the number of apnea/hypopnea episodes between the two interventions, and b) estimate and compare the cumulative segment of time during which the transcutaneous partial pressure of carbon dioxide (TcPCO2) will exceed 45 mmHg, between the two interventions.

Hypothesis: Conservative O2 will be associated with more unstable and obstructed breathing and less time spent with TcPCO2 > 45 mmHg, compared with Liberal O2 supplementation, during recovery from anesthesia.

Inclusion Criteria:

• American Society of Anesthesiologists (ASA) physical status I-III
• Body mass index (BMI) less than 35 kg/m2
• Scheduled to undergo robotic-assisted radical laparoscopic nephrectomy.

Exclusion Criteria:

• Patients with a diagnosis of chronic obstructive pulmonary disorder (COPD), severe neurological, cardiopulmonary, psychiatric, or untreated thyroid disorder
• Chronic pain condition that is being treated with opioids
• Patients with a hematocrit lower than 30% at the end of surgery, or those with an excessive blood loss, requiring transfusion of blood products during surgery.

1. Study participants will not be aware of the inspired O2 concentration in their breathing gas mixture. All participants will be breathing through a non-rebreather mask, which will be connected via a Y -piece to both the O2 and air wall outlets.
2. The investigator who will regulating the O2 inspired concentration-related intervention, will be blinded to the continuous TcPCO2 measurement. In addition, the sleep medicine specialist who will assess the breathing pattern offline and score the number and nature of apnea/hypopnea episodes, will also be blinded to the treatment arm.