Occupational and Environmental Lung Diseases |

Diagnosis of Carbon Monoxide Poisoning: Which Oxygen Saturation Is Correct FREE TO VIEW

Nahreen Ahmed, MD; Roberta Goldring, MD; Kenneth Berger, MD
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New York University Langone, New York, NY

Chest. 2015;148(4_MeetingAbstracts):760A. doi:10.1378/chest.2278267
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SESSION TITLE: Occupational and Environmental Lung Diseases Case Report Posters

SESSION TYPE: Affiliate Case Report Poster

PRESENTED ON: Tuesday, October 27, 2015 at 01:30 PM - 02:30 PM

INTRODUCTION: Familiarity with the principles of pulse oximetry and its limitations as well as the method of reporting oxygen saturation on an arterial blood gas (ABG) are imperative in uncovering carbon monoxide poisoning.

CASE PRESENTATION: A 53 year male presents with 1 month of fatigue, light-headedness and dyspnea on exertion. Hemoglobin concentration was 18.1gm/dl. Primary polycythemia was excluded with normal JAK2 and EPO levels. He was referred for pulmonary testing to further evaluate the polycythemia. Spirometry demonstrated obstructive dysfunction with FEV1=60% predicted and FEV1/VC =57% suggesting a mechanism for secondary polycythemia. ABG revealed pH=7.33 and PO2=48 mmHg yielding a calculated O2-Hgb saturation=79%. However, these data were discordant with measured pulse oximeter saturation=93%. Based on these data, co-oximetry was ordered revealing a measured CO-Hgb=22% and measured O2-Hgb=65%. The patient was placed on oxygen and taken to the ED for management of carbon monoxide poisoning. The fire department was dispatched to his home where a source for CO was identified and subsequently fixed.

DISCUSSION: In this case there are three different oxygen saturations reported:calculated by the blood gas analyzer, measured by the pulse oximetry, and measured by the co-oximeter. The blood gas analyzer calculates the O2 saturation based on the measured pH and PO2 assuming a normal O2-Hgb dissociation curve. This calculation does not account for presence of dyshemoglobins (e.g. MetHb and COHb). The pulse oximeter derives O2 saturation based on spectral analysis wherein light at 2 specific wavelengths is emitted through a vascular bed while the photodiode detector on the other end measures the intensity of transmitted light. Since the absorption coefficient for COHb is similar to O2Hb, the pulse oximeter cannot distinguish these. The gold standard uses a co-oximeter where multiple wavelengths of light are used to directly measure the saturation of hemoglobin along with clinically relevant dyshemoglobins.

CONCLUSIONS: This case highlights that understanding the principles underlying the variety of tools to assess oxygen saturation may be essential to diagnosis and management of dyshemoglobinemias. Discrepancies in the calculated oxygen saturation and the pulse oximeter saturation provided a clue to the presence of a dyshemoglobin and co-oximetry was used to establish the correct diagnosis.

Reference #1: Sinex MD, James. (1999). Pulse Oximetry: Principles and Limitations. American Journal of Emergency Medicine. 17: 59-67.

DISCLOSURE: The following authors have nothing to disclose: Nahreen Ahmed, Roberta Goldring, Kenneth Berger

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