title: The accuracy of pulse oximetry in emergency department patients with severe sepsis and septic shock: a retrospective cohort study.
abstract: BACKGROUND
  Pulse oximetry is routinely used to continuously and noninvasively monitor arterial oxygen saturation (SaO2) in critically ill patients. Although pulse oximeter oxygen saturation (SpO2) has been studied in several patient populations, including the critically ill, its accuracy has never been studied in emergency department (ED) patients with severe sepsis and septic shock. Sepsis results in characteristic microcirculatory derangements that could theoretically affect pulse oximeter accuracy. The purposes of the present study were twofold: 1) to determine the accuracy of pulse oximetry relative to SaO2 obtained from ABG in ED patients with severe sepsis and septic shock, and 2) to assess the impact of specific physiologic factors on this accuracy.
  
  
  METHODS
  This analysis consisted of a retrospective cohort of 88 consecutive ED patients with severe sepsis who had a simultaneous arterial blood gas and an SpO2 value recorded. Adult ICU patients that were admitted from any Calgary Health Region adult ED with a pre-specified, sepsis-related admission diagnosis between October 1, 2005 and September 30, 2006, were identified. Accuracy (SpO2 - SaO2) was analyzed by the method of Bland and Altman. The effects of hypoxemia, acidosis, hyperlactatemia, anemia, and the use of vasoactive drugs on bias were determined.
  
  
  RESULTS
  The cohort consisted of 88 subjects, with a mean age of 57 years (19 - 89). The mean difference (SpO2 - SaO2) was 2.75% and the standard deviation of the differences was 3.1%. Subgroup analysis demonstrated that hypoxemia (SaO2 < 90) significantly affected pulse oximeter accuracy. The mean difference was 4.9% in hypoxemic patients and 1.89% in non-hypoxemic patients (p < 0.004). In 50% (11/22) of cases in which SpO2 was in the 90-93% range the SaO2 was <90%. Though pulse oximeter accuracy was not affected by acidoisis, hyperlactatementa, anemia or vasoactive drugs, these factors worsened precision.
  
  
  CONCLUSIONS
  Pulse oximetry overestimates ABG-determined SaO2 by a mean of 2.75% in emergency department patients with severe sepsis and septic shock. This overestimation is exacerbated by the presence of hypoxemia. When SaO2 needs to be determined with a high degree of accuracy arterial blood gases are recommended.
authors.affiliation: Department of Medicine, University of Calgary, (1403 - 29th Street NW), Calgary, (T2N 2T9), Canada. ben.wilson@albertahealthservices.ca
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Wilson
authors.forename: Hamish J
authors.initials: HJ
authors.lastname: Cowan
authors.forename: Jason A
authors.initials: JA
authors.lastname: Lord
authors.forename: Dan J
authors.initials: DJ
authors.lastname: Zuege
authors.forename: David A
authors.initials: DA
authors.lastname: Zygun
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D004636:Emergency Service, Hospital; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010092:Oximetry / Q000592:standards*; D010100:Oxygen / Q000097:blood*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012720:Severity of Illness Index; D012772:Shock, Septic / Q000503:physiopathology*; D055815:Young Adult
chemical_list: D010100:Oxygen
references.citation: Lancet. 1986 Feb 8;1(8476):307-10
references.pmid: 2868172
references.citation: Crit Care Med. 2004 Sep;32(9):1825-31
references.pmid: 15343008
references.citation: Intensive Care Med. 2004 Nov;30(11):2017-20
references.pmid: 15278272
references.citation: Anesthesiology. 1985 Jan;62(1):85-7
references.pmid: 3966675
references.citation: Crit Care. 2003 Aug;7(4):R67
references.pmid: 12930558
references.citation: Intensive Care Med. 1991;17(8):484-6
references.pmid: 1797894
references.citation: Intensive Care Med. 2004 Apr;30(4):709-13
references.pmid: 14722632
references.citation: South Med J. 1994 Apr;87(4):518-21
references.pmid: 8153783
references.citation: Anesthesiology. 1988 Oct;69(4):602-3
references.pmid: 3177921
references.citation: N Engl J Med. 2003 Jan 9;348(2):138-50
references.pmid: 12519925
references.citation: Heart Lung. 1989 Sep;18(5):444-53
references.pmid: 2777564
references.citation: J Clin Monit. 1990 Apr;6(2):85-8
references.pmid: 2352007
references.citation: Chest. 1993 Nov;104(5):1529-36
references.pmid: 8222819
references.citation: Crit Care. 2005;9 Suppl 4:S13-9
references.pmid: 16168069
references.citation: J Clin Monit. 1989 Apr;5(2):72-81
references.pmid: 2723709
references.citation: Crit Care. 2004 Dec;8(6):462-8
references.pmid: 15566617
references.citation: Chest. 1992 Jun;101(6):1644-55
references.pmid: 1303622
references.citation: Am J Emerg Med. 2000 Jul;18(4):427-31
references.pmid: 10919532
references.citation: Crit Care Med. 2002 Nov;30(11):2501-8
references.pmid: 12441761
references.citation: Anaesthesia. 1991 Mar;46(3):202-6
references.pmid: 2014898
references.citation: Chest. 1990 Jun;97(6):1420-5
references.pmid: 2347228
references.citation: Intensive Care Med. 2001 Oct;27(10):1606-13
references.pmid: 11685301
references.citation: Crit Care Med. 2000 Mar;28(3):703-6
references.pmid: 10752818
references.citation: JAMA. 2001 Oct 10;286(14):1754-8
references.pmid: 11594901
references.citation: Anaesthesia. 1993 May;48(5):415-6
references.pmid: 8317652
references.citation: Resuscitation. 2007 Jan;72(1):82-91
references.pmid: 17098347
references.citation: Anaesthesia. 1997 Feb;52(2):127-30
references.pmid: 9059094