A 68-year-old man in stable atrial fibrillation with rapid ventricular response undergoes transesophageal echocardiogram and successful synchronized cardioversion under conscious sedation in the medical intensive care unit (ICU). Topical 20% benzocaine spray is used for oropharyngeal anesthesia prior to transesophageal probe insertion. His starting SpO2 is 99% on 6 L/min oxygen via nasal cannula. Within a few minutes, the patient appears blue-gray and SpO2 decreases to 84%. His saturation fails to improve on 10 L/min oxygen via simple face mask. The rest of the vital signs remain stable.
Which of the following findings would be LEAST consistent with the patient’s clinical condition?A. Carboxy-Hgb 18%
Correct Answer: A
Conventional pulse oximeter determines the ratio of light absorbance at two wavelengths (660 nm-red and 940 nm-infrared) and plots this against direct measurements (arterial oxygen saturation, SaO2 ) to determine the peripheral oxygen saturation (SpO2 ). SpO2 generally correlates well with arterial SaO2 ; however, this relationship can be inaccurate when local tissue perfusion is impaired.
The given clinical context of recent benzocaine use and presence of normal vital signs except SpO2 suggest benzocaine-induced methemoglobinemia. This patient needs to be further evaluated by cooximetry. In this case, PaO2 will remain normal (answer C) since it measures the oxygen dissolved in blood and methemoglobin affects the oxygen carrying capacity of hemoglobin. However, if the PaO2 were lower, causes for an elevated A-a gradient should be explored (ie, aspiration, bronchospasm, etc.). Mild hypercarbia due to hypoventilation is likely during procedural sedation (answer D). Since this patient’s vital signs are stable (after cardioversion), it is reasonable to expect a normal SpO2 waveform, as compared to the poor waveforms seen in low perfusion states (answer B).