Anesthesia studies using high-flow, humidified, heated oxygen delivered via nasal cannulas at flow rates of more than 50 l · min–1 postulated a ventilatory effect because carbon dioxide increased at lower levels as reported earlier. This study investigated the increase of arterial partial pressure of carbon dioxide between different flow rates of 100% oxygen in elective anesthetized and paralyzed surgical adults before intubation.
Methods

After preoxygenation and standardized anesthesia induction with nondepolarizing neuromuscular blockade, all patients received 100% oxygen (via high-flow nasal oxygenation system or circuit of the anesthesia machine), and continuous jaw thrust/laryngoscopy was applied throughout the 15-min period. In this single-center noninferiority trial, 25 patients each, were randomized to five groups: (1) minimal flow: 0.25 l · min–1, endotracheal tube; (2) low flow: 2 l · min–1, continuous jaw thrust; (3) medium flow: 10 l · min–1, continuous jaw thrust; (4) high flow: 70 l · min–1, continuous jaw thrust; and (5) control: 70 l · min–1, continuous laryngoscopy. Immediately after anesthesia induction, the 15-min apnea period started with oxygen delivered according to the randomized flow rate. Serial arterial blood gas analyses were drawn every 2 min. The study was terminated if either oxygen saturation measured by pulse oximetry was less than 92%, transcutaneous carbon dioxide was greater than 100 mmHg, pH was less than 7.1, potassium level was greater than 6 mmol · l–1, or apnea time was 15 min. The primary outcome was the linear rate of mean increase of arterial carbon dioxide during the 15-min apnea period computed from linear regressions.

Results

In total, 125 patients completed the study. Noninferiority with a predefined noninferiority margin of 0.3 mmHg · min–1 could be declared for all treatments with the following mean and 95% CI for the mean differences in the linear rate of arterial partial pressure of carbon dioxide with associated P values regarding noninferiority: high flow versus control, –0.0 mmHg · min–1 (–0.3, 0.3 mmHg · min–1, P = 0.030); medium flow versus control, –0.1 mmHg · min–1 (–0.4, 0.2 mmHg · min–1, P = 0.002); low flow versus control, –0.1 mmHg · min–1 (–0.4, 0.2 mmHg · min–1, P = 0.003); and minimal flow versus control, –0.1 mmHg · min–1 (–0.4, 0.2 mmHg · min–1, P = 0.004).

Conclusions

Widely differing flow rates of humidified 100% oxygen during apnea resulted in comparable increases of arterial partial pressure of carbon dioxide, which does not support an additional ventilatory effect of high-flow nasal oxygenation.

Editor’s Perspective
What We Already Know about This Topic
  • Apneic oxygenation during surgery may be required to facilitate surgical interventions involving the airway or may occur during intubation or emergence
  • Controversy over the past 60 yr remains about the rate of rise of carbon dioxide during apneic oxygenation
  • Initial studies with high-flow humidified nasal oxygen therapy reported lesser than historical carbon dioxide increases during apnea, suggesting a ventilatory effect on carbon dioxide elimination
  • Subsequent randomized data in pediatric patients disputed these observations
What This Article Tells Us That Is New
  • Adults undergoing elective surgery underwent preoxygenation, standardized anesthetic induction, and randomization to 15 min of apneic oxygenation via endotracheal tube (0.25 l/min), or high-flow nasal oxygen (2 to 70 l/min) with jaw thrust or with laryngoscopy
  • The primary outcome was the linear rate of increase of arterial carbon dioxide, with a predetermined noninferiority margin of 0.3 mmHg · min–1 between groups
  • All groups met the noninferiority criteria and with comparable arterial partial pressure of carbon dioxide increases between groups, suggesting an absence of ventilatory effects for high-flow humidified nasal oxygen therapy