title: Carboxyhemoglobin levels in medical intensive care patients: a retrospective, observational study.
abstract: BACKGROUND
  Critical illness leads to increased endogenous production of carbon monoxide (CO) due to the induction of the stress-response enzyme, heme oxygenase-1 (HO-1). There is evidence for the cytoprotective and anti-inflammatory effects of CO based on animal studies. In critically ill patients after cardiothoracic surgery, low minimum and high maximum carboxyhemoglobin (COHb) levels were shown to be associated with increased mortality, which suggests that there is an 'optimal range' for HO-1 activity. Our study aimed to test whether this relationship between COHb and outcome exists in non-surgical ICU patients.
  
  
  METHODS
  We conducted a retrospective, observational study in a medical ICU at a university hospital in Vienna, Austria involving 868 critically ill patients. No interventions were undertaken. Arterial COHb was measured on admission and during the course of treatment in the ICU. The association between arterial COHb levels and ICU mortality was evaluated using bivariate tests and a logistic regression model.
  
  
  RESULTS
  Minimum COHb levels were slightly lower in non-survivors compared to survivors (0.9%, 0.7% to 1.2% versus 1.2%, 0.9% to 1.5%; P=0.0001), and the average COHb levels were marginally lower in non-survivors compared to survivors (1.5%, 1.2% to 1.8% versus 1.6%, 1.4% to 1.9%, P=0.003). The multivariate logistic regression analysis revealed that the association between a low minimum COHb level and increased mortality was independent of the severity of illness and the type of organ failure.
  
  
  CONCLUSIONS
  Critically ill patients surviving the admission to a medical ICU had slightly higher minimum and marginally higher average COHb levels when compared to non-survivors. Even though the observed differences are statistically significant, the minute margins would not qualify COHb as a predictive marker for ICU mortality.
authors.affiliation: Department of Respiratory and Critical Care Medicine, Otto Wagner Hospital, Sanatoriumstrasse 2, A-1140 Vienna, Austria.
authors.forename: Andreas S
authors.initials: AS
authors.lastname: Fazekas
authors.forename: Marlene
authors.initials: M
authors.lastname: Wewalka
authors.forename: Christian
authors.initials: C
authors.lastname: Zauner
authors.forename: Georg-Christian
authors.initials: GC
authors.lastname: Funk
mesh_terms: D000368:Aged; D015415:Biomarkers / Q000097:blood; D002263:Carboxyhemoglobin / Q000378:metabolism*; D015331:Cohort Studies; D003422:Critical Care / Q000639:trends*; D016638:Critical Illness / Q000628:therapy*; D005260:Female; D017052:Hospital Mortality / Q000639:trends; D006801:Humans; D007362:Intensive Care Units / Q000639:trends; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D012720:Severity of Illness Index
chemical_list: D015415:Biomarkers; D002263:Carboxyhemoglobin