At Hillcrest this morning Janna Raphelson, one of our stellar residents, presented a case of a 75-year-old man with history of steroid-dependent interstitial lung disease, chronic kidney disease, and serious TMP-SMX allergy who presented to the VA with increased dyspnea, fatigue, and headache one month after starting dapsone for Pneumocystis jiroveci pneumonia prophylaxis. He was found to have an acute anemia that was stabilized by transfusion of pRBCs and subsequently characterized as hemolytic. On exam, the patient was cyanotic with an oxygen saturation in the mid-80% that was unresponsive to up titration of supplemental oxygen.
We were joined by Dr. Mark Hepokoski from our PCCM division as we unpacked the causes of hypoxemia and why we need to calculate an alveolar-arterial (A-a) gradient. While the patient’s A-a gradient was increased (from baseline IPF and mild interstitial edema), there was a clear discrepancy between his SpO2 (pulse oximeter) and PaO2 (blood gas). This reminded us that impaired oxygen delivery to tissues (hypoxia) has multiple potential etiologies: hypoxemic, anemic, circulatory, histotoxic. The next step in the evaluation of our patient involved obtaining co-oximetry which revealed increased levels of methemoglobin!
The pulse oximeter is only able to distinguish between saturated and unsaturated hemoglobin. If significant levels to methemoglobin build up, the machine interprets this as a saturation of approximately 85%. In addition to causing a functional anemia, methemoglobinemia shifts the hemoglobin-oxygen dissociation curve to the left. This effect causes increased affinity of hemoglobin for oxygen and decreases oxygen unloading in tissues. We compared our patient’s case to the other acquired dyshemoglobinemia that we sometimes encounter, carbon monoxide poisoning.
Despite the patient’s normal G6PD screen prior to starting dapsone, he still developed known complications of this medication’s oxidative effect: hemolytic anemia and methemoglobinemia. Our patient’s methemoglobinemia was partially treated by blood transfusions and given concern for toxicity in renal failure, he did not receive the standard treatment of methylene blue and vitamin C. The patient slowly recovered following discontinuation of dapsone (switched for inhaled pentamidine for ongoing PJP ppx) and is now back to his baseline.
Thank you to Dr. Hepokoski for sharing his wisdom and clinical expertise and thank you to all of our applicants for joining us for another great morning report!
