Friday, July 16, 2010

Hyperoxia is worse than Hypoxia after cardiac arrest?

This blog was in part borne of an attempt to reduce the number of letters I sent to the editors of NEJM and JAMA.....but sometimes I still find it irresistable. The editors of JAMA, however, were able to resist this letter, and it was rejected, so I post it here.

To the Editor: Kilgannon et al ( report the provocative results of an observational study of the outcome of post-arrest patients as a function of the first oxygen tension measured in the ICU. Unfortunately, the definitions they chose for categorizing oxygen tension introduce confounding which complicates the interpretation of their analysis. By classifying patients with a PF ratio less than 300 but a normal oxygen tension as having hypoxia, lung injury (an organ failure which may itself be an independent predictor of poor outcomes) is confounded with hypoxia. Given the hypothesis guiding the analysis, namely that the oxygen tension to which the brain is exposed influences mortality, we find this choice curious. If patients with a normal oxygen tension but a reduced PF ratio were not classified as hypoxic, they would have been included in the normoxia group, and the results of the overall analysis may change. Such potential misclassification is important to consider given that the reasons patients were managed with hyperoxia cannot be known because of the observational nature of the study - did such patients experience less active management and titration of FiO2? Is hyperoxia a marker of a more laissez faire approach to ventilatory management? PEEP, an important determinant of oxygen tension is also not known, and this could markedly influence the classification of patients in the scheme chosen by the authors.It would be helpful to know how the results of the analysis might change if patients with lung injury (PF ratio < 300) but normal oxygen tension were reclassified as normoxic in the analysis.