To the Editor
Use of the Pulse oximeter has been suggested as the standard of care for general anesthetics 1. It is well recognized that continuous assessment of arterial oxygenation is important in the clinical management of anesthetized patients. The use of Pulse oximetry has become widespread as evidence has accumulated that this non-invasive monitor of oxygen saturation is accurate over a wide clinical ranse2.
Vasopressor use, peripheral vascular disease hypothermia, hypotension, intravascular injection of dyes and hemoglobinophathies can all contribute to inaccurate arterial saturation readings by pulse oximetry 3,4, most of which are interpreted as falsely low readings.
Factitously high pulse oximeter readings have been reported as a consequence of interference of the probe by ambient light or infrared fight, and as the result of “default” readings from the instrument which occur when the pulse signal is low or contains interferences. Recently, we obtained a falsely high pulse oximeter reading, despite a normal signal, in a patient who was hypoxemic.
The patient was a 70 year-old male undergoing elective repair of a thoracoabdominal aortic aneurysm under one-lung anesthesia. After intubation with a double lumen endobronchial W the patient was positioned in the right lateral position. Monitors included EKG, ETC02, temperature, esophageal stethoscope left radial arterial line and Pulmonary artery catheter A pulse oximeter probe was placed on the right index finger and shielded from ambient fight by a towel wrapped around the hand. Approximately 15 minutes after deflation of the patient’s left (nondependent) lung, arterial blood gases were obtained to assess the adequacy of ventilation. Prior to the blood gas being drawn, the oximeter had intermittently indicated poor signal quality. However, at the time that the blood gas v-s drawn, the pulse oximeter indicated a normal pulse wave form and arterial oxygen saturation of I 00%. Arterial blood gas results were pH 7.4 1, pCO2 40, paO2 56. A second arterial blood gas reading obtained with the oximeter indicating 100% saturation and a normal wave form was pH 7.41, paCO2 41, PaO2 72. A new probe was obtained and the oximeter read I 00% saturation. A reading from the same instrument on an O.R. colleague whose usual room air saturation is 95%, was also falsely elevated at 99%. A new oximeter was obtained and determined our colleague’s arterial saturation as 96%. Throughout the remainder of the case, the new oximeter was utilized. The original pulse oximeter was returned to the manufacturer for inspection and was reportedly free of defects.
We report this case as an episode of unexplained factitiously elevated arterial oxygen saturation readings by pulse oximetry. This was seen in a patient undergoing one-lung anesthesia, when noninvasive monitoring of arterial oxygen saturation is highly desirable because of the potential for changes in intra-pulmonary shunt (6). We found this incident particularly disturbing because while most other reported instances of inaccurate pulse oximetry data involves saturation readings which are inaccuratel low, this case represents an instance in which inaccurate readings from a pulse oximeter were potentially falsely reassuring.
Gail Van Norman, Fellow in Cardiothoracic Anesthesia, F.W. Cheney, Acting Director, Cardiothoracic Anesthesia, University of Washington Department of Anesthesiology
1. American Society of Anesthesiologist: Standards for basic intro-operative monitoring Anesthesia Patent Safety Foundation Newsletter, March, 1987.
2. Yelderman, M and New, W. Evaluation of pulse oximetry. Anesthesiology 59:349-352.
3. Kessler, MR et al.: Spurious pulse oximeter desaturation with methylene blue injection. Anesthesiology 65:435-436. 1986.
4. Tremper, KK and Barker, SI: Pulse oximetry. Anesthesiology 70:98-108. 1989.
5. Cosarino, AT et al.: Falsely normal saturation reading ,with the pulse oximeter. Anesthesiology 67: 830-83 1. 1987.
6. Brodsky, IB et al.: Pulse oximetry during one-lung ventilation. Anesthesiology 63:212-214. 1985.