Spectrophotometry of hydroxocobalamin and hemoglobin reveals production of an unanticipated methemoglobin variant

Elevated methemoglobin levels in patients treated with high-dose hydroxocobalamin

OBJECTIVE

The aim of this study was to assess the impact of hydroxocobalamin (OHCbl) infusion on arterial blood gas and oximetry values in patients with vasoplegic syndrome.

METHODS

Blood samples collected from 95 patients receiving OHCbl infusion were assayed using the ABL90 FLEX Plus blood gas analyzer for the concentration of methemoglobin (MetHb), total hemoglobin (tHb), carboxyhemoglobin (COHb), arterial oxygen saturation (SaO2), arterial oxygen partial pressure (PaO2), and arterial carbon dioxide partial pressure (PaCO2). Interference of OHCbl on these variables was evaluated using the measured difference between the preinfusion and postinfusion samples.

RESULTS

Blood MetHb (%) measured after the infusion of OHCbl (5g) were significantly higher than the baseline levels, with a median of 4.8 (IQR, 3.0-6.5) versus 1.0 (IQR, 1.0-1.2) (P < .001). Blood COHb (%) increased from a median of 1.3 (IQR, 1.0-1.8) to 1.7 (IQR, 1.3-2.2) (P < .001) following the OHCbl infusion. No differences were seen in median levels of tHb, PaO2, PaCO2, and SaO2 between pre- and post-OHCbl treatment.

CONCLUSION

The presence of OHCbl in blood clearly interfered with the oximetry measurements of the hemoglobin component fractions by falsely increasing the levels of MetHb and COHb. Blood levels of MetHb and COHb cannot be reliably determined by the co-oximetry when OHCbl is known or suspected.

Elevated methemoglobin levels in patients treated with hydroxocobalamin: a case series and in-vitro analysis

BACKGROUND

Historically, the first step in treating cyanide (CN^-) toxicity utilized antidotes to induce methemoglobinemia. This is concerning in patients who are already hypoxemic or have elevated carboxyhemoglobin. Hydroxocobalamin (OHCbl) is now the first-line antidote for CN^- toxicity and is not known to induce methemoglobinemia. We observed elevated methemoglobin (MetHb) levels in several patients treated with OHCbl and sought to investigate the incidence of MetHb formation following administration of OHCbl.

METHODS

Chart review: A single-center, retrospective case series of patients who received 5 or 10 g of hydroxocobalamin from 01/01/2011 through 04/30/2019. Data was analyzed using descriptive statistics. In-vitro study: Discarded blood was separated into whole blood and plasma samples. OHCbl and normal saline was added to reach 0×, 1×, 2×, and 4× peak therapeutic concentrations and analyzed at times 0, 2, and 4 h after administration.

RESULTS

Chart review: Twenty-seven cases of OHCbl administration were identified. The median age was 53 years (IQR 38 - 64) and 20 (74.1%) were male. Exposure to a house fire or smoke inhalation was the reason for OHCbl administration in 21 (77.8%) patients. Five (18.5%) patients received 10 g of OHCbl while the rest received 5 g. Six (22.2%) patients developed methemoglobinemia, all after 5 g OHCbl administration; four had been exposed to fire and smoke, two received the medication for severe acidosis of unknown etiology not related to fire or smoke. The median peak level was 7.1% (IQR 2.2 - 16.4%) at a median time of 11.4 h post-administration. Two patients received methylene blue (MB), neither responded. Death occurred in 17 (63%) cases. In-vitro study: We observed a dose dependent elevation in total hemoglobin but did not detect any increase in MetHb.

CONCLUSION

We observed a noteworthy temporal association between the formation of methemoglobinemia and the administration of hydroxocobalamin. This does not appear to be an artifact of the CO-oximeters. This could have profound implications for patients who are already hypoxemic or have impaired oxygen carrying capacity from carboxyhemoglobin.

Elevated Methemoglobin Levels in a Patient Treated with Hydroxocobalamin After Suspected Cyanide Exposure

BACKGROUND

Cyanide (CN) toxicity commonly occurs during enclosed-space fires. Historically, the first step in treating CN toxicity utilized amyl nitrite and sodium nitrite to induce methemoglobinemia, which can be dangerous in this population. Hydroxocobalamin (OHCob), which binds to CN to form the nontoxic metabolite cyanocobalamin, is now the first-line antidote for CN toxicity, and has the advantage of not inducing methemoglobinemia.

CASE REPORT

A 62-year-old man presented to the Emergency Department (ED) after a house fire. He was intubated for respiratory distress and hypoxia with an initial carboxyhemoglobin of 1.3%, methemoglobin 0.3%, and anion gap 19. Eleven hours after presentation, his serum lactic acid was 9 mmol/L. Given his continued deterioration, 14 h after arrival he received OHCob 5 g i.v. for presumed CN toxicity. Methemoglobin concentration 4 min prior to OHCob administration was 0.7%, and 2 h after administration was 4.2%. This subsequently increased to 14.3% (16 h after OHCob administration) and peaked at 16.3% (47 h after OHCob administration), at which time he was administered a dose of methylene blue 50 mg i.v., 60 h after ED arrival. His methemoglobin concentrations fluctuated until a consistent downward trend starting at 92 h from ED arrival. He continued to deteriorate and expired on hospital day 5 with a methemoglobin concentration of approximately 6.0%. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: CN toxicity requires immediate recognition and treatment. The antidote, OHCob, is believed to not induce methemoglobinemia. However, this potential side effect must be considered by emergency physicians when treating suspected CN toxicity, especially if the patient does not improve after antidotal therapy.

Effect of hydroxocobalamin on surface oximetry in nonexposed humans

INTRODUCTION

The newer cyanide antidote, hydroxocobalamin, due to its pigmentation, has been found to cause interferences in some laboratory assays. Co-oximetry may also be affected by hydroxocobalamin, leading to false elevations in hemoglobin concentration, methemoglobin, carboxyhemoglobin, and false decreases in oxyhemoglobin. The Masimo Radical-7 is a medical device that performs noninvasive oximetry and estimates hemoglobin (Hb) concentration and percent carboxyhemoglobin (COHb), methemoglobin (MetHb), and oxyhemoglobin saturation (O2Hb).

STUDY OBJECTIVES

The study sought to determine the effect of hydroxocobalamin on noninvasive measurement of hemoglobin indices using the Masimo Radical-7 monitor.

METHODS

Seven asymptomatic volunteers who were unexposed to cyanide had baseline heart rate (HR), blood pressure (BP), and oximeter measurements recorded followed by an infusion of five grams of hydroxocobalamin over 15 minutes. The above parameters were subsequently recorded at: 5, 10, 15, 30 and 60 minutes post infusion. Data were analyzed by calculating the area under the curve (AUC) for each variable and comparing the results to expected values by paired t tests. Expected AUC values were calculated by extrapolating baseline values across the entire time period.

RESULTS

The mean differences from baseline values with 95% confidence intervals and t tests of mean difference were: SBP: 11 mm Hg (95% CI, 0-22; P = .051); HR: -9 (95% CI, -15 to -3; P = .01); Hb: -0.1 (95% CI, -0.7 to 0.4; P = .57); O2Hb: 0 g/dL (95% CI, -1 to 1; P = .41); COHb: -1 (95% CI, -3 to 1; P = .25); MetHb: -0.2 (95% CI, -0.3 to 0; P = .03).

DISCUSSION

After infusion of hydroxocobalamin there was a significant elevation of systolic blood pressure and decrease in heart rate. There were no significant differences in Hb, O2Hb, and COHb. Although percent methemoglobin concentrations were statistically lower, the authors feel this difference is of trivial clinical significance.

CONCLUSION

The administration of hydroxocobalamin does not significantly impact noninvasive oximetry.