To the Editor
We commend Applegate et al1 on their report of a novel oximetry-based system that provides an indicator of hyperoxemia, the ‘oxygen reserve index’. We are, however, intrigued to know more about the nature of the surgery that these patients underwent, as this was not reported in the article. The intraoperative Pao2 appeared to be rather high in this cohort of patients. The mean Pao2 was 206.0 (199.4-213.6) mm Hg, which is comparable with UK data (183 mm Hg),2 demonstrating the pervasiveness of intraoperative hyperoxemia. The study by Applegate et al also reports Pao2 values up to 534 mm Hg and Figure 4 in the manuscript depicts a Pao2 greater than 500 mm Hg for the first hour of surgery. Although there may be a rational explanation for such a supraphysiologic Pao2, we question its necessity during the most elective surgery.
Excessive oxygen administration promotes the generation of reactive oxygen species, which inflict cell damage and dysfunction. As was eloquently discussed by Applegate et al, the threshold at which hyperoxemia may be harmful to patients undergoing surgery is unknown but could be as low as 150 mm Hg.1 Excessive use of oxygen can create cellular hyperoxia, which tips the pro-oxidant versus antioxidant balance toward the excessive production of reactive oxygen species and promotion of oxidative stress. Paradoxically, high-risk patients, to whom oxygen frequently is administered liberally, may be the most vulnerable to this oxidative stress. Excessive oxygen use has been proposed as the common factor contributing to the earlier onset of dementia in older patients undergoing anesthesia.3 With clear parallels between many postoperative complications and diseases in which oxidative stress has been implicated, clinicians should endeavor to avoid unnecessary perioperative hyperoxemia. It is possible that oxygen reserve index may offer the ability to detect hyperoxemia, but simple measures can be used before this technology is fully assessed to minimize potential harm.
Daniel S. Martin, MBChB
Helen T. McKenna, MBBS
Clare M. Morkane, MBBCh
Royal Free Perioperative Research,
Department of Anaesthesia
University College London Division of Surgery and
Interventional Science
Royal Free Hospital
London, UK
[email protected]
REFERENCES
1. Applegate RL II, Dorotta IL, Wells B, Juma D, Applegate PMThe relationship between oxygen reserve index and arterial partial pressure of oxygen during surgery.Anesth Analg2016123626633
2. Martin DS, Grocott MPOxygen therapy and anaesthesia: too much of a good thing?Anaesthesia201570522527
3. Chen CW, Lin CC, Chen KB, Kuo YC, Li CY, Chung CJIncreased risk of dementia in people with previous exposure to general anesthesia: a nationwide population-based case-control study.Alzheimers Dement201410196204
