Carbon dioxide retention and oxygen desaturation during gastrointestinal endoscopy

Background: Pulse oximetry measures arterial oxygen saturation (Spo₂), not hypoventilation, which is directly reflected by increases in carbon dioxide tension. Methods: In the present study, transcutaneous carbon dioxide tension (Ptcco₂) and Spo₂ were measured during 101 endoscopic procedures selected for long duration or comorbid illnesses, and relationships between hypercapnia and hypoxemia were evaluated. Nasal oxygen was administered only for sustained desaturation (Spo₂ < 90%). Results: Mean peak increase in Ptcco₂ was significantly higher in patients requiring oxygen for sustained desaturation (16.3 mm Hg; range, 4-52) than in patients breathing room air who had transient or no desaturation (10.2 mm Hg [range, 3-19] and 5.1 mm Hg [range, 0-15]). If nasal oxygen corrected desaturation, even transient recurrence of desaturation indicated worsening CO₂ retention, which preceded respiratory arrest in one patient. Independent predictors of hypercapnia were fentanyl and midazolam doses, oxygen requirement, and dementia. Conclusions: Severe hypoventilation may occur during endoscopy, undetected by clinical observation or pulse oximetry, but only in sedated patients who require supplemental oxygen to maintain Spo₂ above 90%. After oxygen supplementation corrects desaturation, recurrence of desaturation implies severe hypoventilation and warrants limitation of further sedation.