Arterial Partial Pressure of Oxygen As a Marker of Airway Closure Does Not Correlate with the Efficacy of Pre-Oxygenation

BACKGROUND The prerequisites for the early formation of anaesthesia-related atelectasis are pre-oxygenation with its resulting high alveolar oxygen content, and airway closure. Airway closure increases with age, so it seems counterintuitive that atelectasis formation during anaesthesia does not. One proposed explanation is that pre-oxygenation is impaired in the elderly by airway closure present in the waking state. The extent of airway closure cannot be assessed at the bedside, but arterial partial pressure of oxygen (PaO2) as a surrogate variable of the resulting ventilation to perfusion mismatch can. OBJECTIVE The primary aim was to test the hypothesis that a decreased efficacy of pre-oxygenation, measured as the fraction of end-tidal oxygen (FE’O2) after 3 min of pre-oxygenation, correlates with decreased PaO2 on room air. We also re-investigated the influence on FE’O2 by age. DESIGN Prospective observational study. SETTING Two regional hospitals, Västerås and Köping County Hospitals, Västmanland, Sweden, between 30 October 2018 and 17 September 2021. PARTICIPANTS We included 120 adults aged 40 to 79 years presenting for elective noncardiac surgery. INTERVENTION An arterial blood gas was sampled before commencing pre-oxygenation. RESULTS No linear correlation was found between FE’O2 at 3 min and PaO2 or age (Pearson's r = −0.038, P = 0.684; and Pearson's r = −0.113, P = 0.223, respectively). The mean ± SD FE’O2 at 3 min for the population studied was 0.87 ± 0.05. CONCLUSION The lack of correlation between FE’O2 at 3 min and PaO2 or age during pre-oxygenation has implications for further studies concerning the interaction between airway closure and atelectasis. After 3 min of pre-oxygenation, FE’O2, even in the elderly, indicated a high enough alveolar oxygen concentration to promote atelectasis after induction, therefore, it is still unclear why atelectasis formation diminishes after middle age. TRIAL REGISTRATION ClinicalTrials.gov NCT03395782