INTRODUCTION:Sentinel lymph node biopsy represents a major milestone in the treatment of breast cancer. It requires the use of methylene blue that interferes with light absorption and gives a false estimate of the arterial oxygen saturation (SaO2).Pulse oximetry is the standard non invasive monitor. However, several clinical conditions interfere with the accuracy of the oximeter including peripheral vasoconstriction, hypothermia, hypotension, dyshemoglobinemias, as well as the placement of a sphygmomanometer cuff on the ipsilateral extremity and the application of high-intensity heat lamps.
CASE PRESENTATION:A 70-year-old woman was scheduled for bilateral breast lumpectomy with sentinel lymph node sampling and left axillary node dissection. She had a history of hypertension, COPD, and bilateral breast needle biopsies done without complications. The patient was a smoker with no known drug allergies and she was taking aspirin, olmesartan , zolpidem and albuterol.Physical examination revealed a blood pressure of 160/80 mmHg, a pulse of 65 beats/min with a height of 154 cm, and a weight of 68 kg. The heart exam was normal and the lung auscultation revealed decreased breath sounds bilaterally. The CXR showed findings consistent with COPD. Other laboratory data, including hemoglobin concentration, serum electrolytes, and electrocardiogram, were normal.The patient received intraoperative monitoring including electrocardiogram, end-tidal CO2 tension, right brachial indirect blood pressure, and left index finger pulse oximetry.While in the supine position, the patient was well prepped and draped. Three cc of methylene blue were injected on the right nipple areolar complex and the procedure on the right side was uneventful with normal SaO2 by oximetry. Thirty minutes later, another 3 cc of methylene blue were injected on the left nipple areolar complex. Within 20 seconds the SaO2 alarm sounded and the monitor indicated that the SaO2 is 80% with a satisfactory pulse search signal. All other monitoring signs were stable, and the patient turned blue from her waist upwards. She was placed on 100% oxygen and the SaO2 gradually improved. Arterial blood gas showed a PaO2 of 280 and 100% SaO2 . The surgical procedure was completed and the patient, while in the recovery room, was well perfused. Co-oxymetry on the blood gas demonstrated a methemoglobin level of 2.1% and the SaO2 was 97% on room air.
DISCUSSIONS:Although rare under anesthesia, oxygen desaturation is a potentially lethal condition that requires prompt recognition. Although rare, one must initially consider a methylene-blue-induced methemoglobinemia when using a pulse oximeter during general anesthesia, as illustrated by this case. Since oxygen desaturation and methemoglobinemia resolved spontaneously without treatment, it was very unlikely that methylene blue caused methemoglobin to form. Instead, a more reasonable explanation would be the interference by methylene blue on light absorbance.It has been reported that methylene blue has a peak light absorption of 668 nm , which very closely corresponds to the light absorbance peak of 660 nm for deoxygenated hemoglobin. Since the pulse oximeter operates on the principle of differences in light absorption between oxyhemoglobin and deoxyhemoglobin, methylene blue absorbs most of the 660-nm light emission and gives a false estimate of the percentage of oxyhemoglobin and SaO2. Once the methylene blue is rapidly distributed and is cleared by the kidney, the SaO2 by oximetry returns to normal.
CONCLUSION:The injection of methylene blue can cause a false indication of SaO2 by interference with changes in light absorption when pulse oximetry is used as an indicator of SaO2. Should any doubt exist, an arterial blood sample should be obtained to confirm PaO2 and SaO2.
DISCLOSURE:Rabih Touma, No Financial Disclosure Information; No Product/Research Disclosure Information