Methemoglobinemia

[Methemoglobinemia]

Methemoglobinemia (MetHb) refers to the state of oxidation of the iron ion "ferrous" (Fe2+) to iron "ferric" (Fe3+) within the heme molecule that makes up hemoglobin (Hb). This state is physiological if its level remains controlled. The ferrous state of the heme molecule occurs in the event of significant oxidative stress. The pathophysiology of MetHb involves NADH, NADPH and glucose cycle enzymes such as cytochrome-b5-reductase. MetHb can be acquired or more rarely, congenital. Acquired causes include drug-induced effects such as topical anesthetics, or toxic effects such as nitrites. Primary causes are linked to enzyme deficiencies or constitutional Hb abnormalities. Excessively high MetHb causes symptoms of varying intensity, depending on the level of MetHb and associated comorbidities. Clinical signs are dominated by cyanosis, indicative of tissue hypoxia, which can be complicated by severe metabolic disorders leading to death. Diagnosis can be complex, as the resulting biological abnormalities may go undetected. Treatment is mainly based on identifying the etiology and restoring the heme molecule to its physiological state. Methylene blue is the main antidote in cases of elevated MetHb, but precautions must be taken in its use, and its physico-chemical effects must be understood. We provide an update on methemoglobinemia, summarizing its pathophysiology and clinical presentations, complementary tests and therapeutic principles.

Hepatoprotective Effect Assessment of C-Phycocyanin on Hepatocellular Carcinoma Rat Model by Using Photoacoustic Spectroscopy

Hepatocellular carcinoma (HCC) is the most common primary neoplasia of the liver with elevated mortality. Experimental treatment with antioxidants has a beneficial effect on the experimental models of HCC. Arthrospira maxima (spirulina) and its phycocyanin have antitumoral action on different tumoral cells. However, it is unknown whether phycocyanin is the responsible molecule for the antitumoral effect on HCC. Photoacoustic spectroscopy (PAS) stands out among other spectroscopy techniques for its versatility of samples analyzed. This technique makes it possible to obtain the optical absorption spectrum of solid or liquid, dark or transparent samples. Previous studies report that assessing liver damage in rats produced by the modified resistant hepatocyte model (MRHM) is possible by analyzing their blood optical absorption spectrum. This study aimed to investigate, by PAS, the effect of phycocyanin obtained from spirulina on hepatic dysfunction. The optical absorption spectra analysis of the rat blood indicates the damage level induced by the MRHM group, being in concordance with the carried out biological conventional studies results, indicating an increase in the activity of hepatic enzymes, oxidative stress, Bax/Bcl2 ratio, cdk2, and AKT2 expression results, with a reduction in p53 expression. Also, PAS results suggest that phycocyanin decreases induced damage, due to the prevention of the Bax, AKT2, and p53 altered expression and the tumor progression in a HCC rat model.

Case report: Methemoglobinemia caused by nitrobenzene poisoning

Nitrobenzene poisoning is uncommon, with most cases occurring in the dye, paint, and other chemical industries. Nitrobenzene enters the body mainly through the skin, respiratory tract, and oral cavity. Nitrobenzene poisoning symptoms include hypermethemoglobinemia, hemolytic anemia, liver and kidney dysfunction, cardiogenic pulmonary edema, and toxic encephalopathy, which endanger people's lives. Therefore, we present a case of nitrobenzene poisoning caused by skin absorption, focusing on its clinical characteristics and treatment outcomes. A 58 years-old man presented to our department with confusion and cyanosis. He has a history of hypertension and cerebral infarction. The patient was diagnosed with moderate occupational acute benzene poisoning with nitro compounds. Symptomatic support, methylene blue, and other antioxidant treatments were commenced after diagnosis. After treatment, the patient's condition gradually improved, and he was discharged.

Bacterial safety study of the production process of hemoglobin-based oxygen carriers

Hemoglobin microparticles (HbMP) produced with a three-step procedure, including coprecipitation of hemoglobin with manganese carbonate, protein cross-linking, and dissolution of the carbonate template were shown to be suitable for application as artificial oxygen carriers. First preclinical safety investigations delivered promising results. Bacterial safety plays a decisive role during the production of HbMP. Therefore, the bioburden and endotoxin content of the starting materials (especially hemoglobin) and the final particle suspension are intensively tested. However, some bacteria may not be detected by standard tests due to low concentration. The aim of this study was to investigate how these bacteria would behave in the fabrication process. Biocidal effects are known for glutaraldehyde and for ethylenediaminetetraacetic acid, chemicals that are used in the fabrication process of HbMP. It was shown that both chemicals prevent bacterial growth at the concentrations used during HbMP fabrication. In addition, the particle production was carried out with hemoglobin solutions spiked with Escherichia coli or Staphylococcus epidermidis. No living bacteria could be detected in the final particle suspensions. Therefore, we conclude that the HbMP fabrication procedure is safe in respect of bacterial contamination.

Determination of Methemoglobin in Hemoglobin Submicron Particles Using NMR Relaxometry

Methemoglobin (MetHb) is a hemoglobin (Hb) derivative with the heme iron in ferric state (Fe³⁺), unable to deliver oxygen. Quantification of methemoglobin is a very important diagnostic parameter in hypoxia. Recently, novel hemoglobin microparticles (Hb-MP) with a narrow size distribution around 700 nm, consisting of cross-linked Hb were proposed as artificial oxygen carriers. The cross-linking of Hb by glutaraldehyde (GA) generates a certain amount of MetHb. Due to the strong light scattering, quantitative determination of MetHb in Hb-MP suspensions by common spectrophotometry is not possible. Here, we demonstrate that ¹H₂O NMR relaxometry is a perfect tool for direct measurement of total Hb and MetHb concentrations in Hb-MP samples. The longitudinal relaxation rate 1/T₁ shows a linear increase with increasing MetHb concentration, whereas the transverse relaxation rate 1/T₂ linearly increases with the total Hb concentration. In both linear regressions the determination coefficient (R²) is higher than 0.99. The method does not require time-consuming pretreatment or digestion of the particles and is not impaired by light scattering. Therefore, it can be established as the method of choice for the quality control of Hb-MP and similar hemoglobin-based oxygen carriers in the future.

Novel assay for the toxicity evaluation of nanoscale zero-valent iron and derived nanomaterials based on lipid peroxidation in bacterial species

Nano-scale zero-valent iron (nZVI) began attracting research attention in remediation practice in recent decades as a prospective nanomaterial applicable to various contaminated matrices. Despite concerns about the negative effects of nanomaterials on ecosystems, the number of reliable toxicity tests is limited. We have developed a test based on the evaluation of oxidative stress (OS). The test employed the analysis of a typical OS marker (malondialdehyde, MDA), after exposure of six bacterial strains to the tested nanomaterial. We also attempted to use other OS and cell membrane damage assays, including the determination of glutathione and lactate dehydrogenase, respectively. However, we found that the components of these assays interfered with nZVI; therefore, these tests were not applicable. The MDA assay was tested using nZVI and three newly engineered oxide shell nZVI materials with different oxide thicknesses. Six different bacterial species were employed, and the results showed that the test was fully applicable for the concentrations of nanomaterials used in remediation practice (0.1-10 g/L). MDA was produced in a dose-response manner, and the bacteria showed a similar response toward pure pyrophoric nZVI, reaching EC₅₀ values of 0.3-1.1 g/L. We observed different responses in the absolute production of MDA; however, the MDA concentrations were correlated with the cell membrane surfaces of the individual strains (R > 0.75; P < 0.09). Additionally, the EC₅₀ values correlated with the thickness of the oxide shells (except for Escherichia coli: R > 0.95; P < 0.05), documenting the reliability of the assay, where reactivity was confirmed to be an important factor for reactive oxygen species production.

Characterization of the methemoglobin forming metabolites of benzocaine and lidocaine

1. Topical anesthesia with benzocaine or lidocaine occasionally causes methemoglobinemia, an uncommon but potentially fatal disorder where the blood has a reduced ability to transport oxygen. Previous in vitro studies using human whole blood have shown that benzocaine causes more methemoglobin (MetHb) formation than lidocaine, and that both compounds require metabolic transformation to form the MetHb producing species. In the current investigation, the active species of benzocaine forming the MetHb was investigated. 2. HPLC analysis of benzocaine samples incubated with human hepatic S9 showed the formation of a peak with the same UV spectrum and retention time as benzocaine hydroxylamine (BenzNOH). To confirm the activity of BenzNOH, MetHb production following exposure to the compound was determined in whole human blood using an Avoximeter 4000 CO-oximeter. 3. BenzNOH produced MetHb in a concentration dependent manner without the need for metabolic activation. Benzocaine in the presence of metabolic activation required a concentration of 500 μM to produce a similar degree of MetHb formation as 20 μM BenzNOH without activation. Previous work suggested that two metabolites of lidocaine may also form MetHb; N-hydroxyxylidine and 4-hydroxyxylidine. Of these two metabolites 4-hydroxyxylidine produced the most MetHb in whole blood in vitro in the absence of metabolic activation, however BenzNOH produced up to 14.2 times more MetHb than 4-hydroxyxylidine at a similar concentration. 4. These results suggest that the ability of benzocaine to form MetHb is likely to be mediated through its hydroxylamine metabolite and that this metabolite is inherently more active than the potentially MetHb-forming metabolites of lidocaine.

Acute cardiogenic pulmonary oedema with multiorgan dysfunction--still to learn more about nitrobenzene poisoning

Nitrobenzene is a nitrite compound often used in polishes or solvents. Its toxic effects are due to its ability to induce methaemoglobinaemia. The clinical presentation of this poisoning varies according to the concentration of methaemoglobin level in blood. The importance of early identification of the compound on the basis of clinical suspicion corroborative with methaemoglobin level with timely intervention is required to prevent fatal outcome. It is also important to take care of the secondary cycling of nitrobenzene from body stores to prevent secondary recurrence of symptoms in patients after heavy exposure. Here author reports a rare case of accidental poisoning with nitrobenzene presented with respiratory distress and cyanosis. On investigation, he was diagnosed to have cardiogenic pulmonary oedema and multiorgan dysfunction. The urgent institution of methylene blue as specific antidote along with haemodynamic and ventilatory support was proved crucial for life saving of the patient.

Intraoperative Diagnosis of Unsuspected Methemoglobinemia Due to Low Pulse Oximetry Values

Methemoglobinemia results from the oxidation of the iron in the hemoglobin molecule from the ferrous to the ferric state. Methemoglobinemia may result from congenital deficiencies of enzymes that normally convert methemoglobin (metHb) to hemoglobin, alterations in the hemoglobin molecule itself or, most commonly, from medications or toxins. As metHb cannot carry oxygen, clinical sequelae result when the concentration of metHb is high enough to compromise oxygen delivery to the tissues. With low levels, the patient may be asymptomatic or only symptomatic during periods of increased tissue oxygen demands such as exercise. With higher levels, symptoms may occur at rest. We describe an adolescent with acute leukemia who presented to the operating room for placement of a Broviac catheter for permanent central venous access. Given a persistently low oxygen saturation as measured by pulse oximetry (92% to 94%) with no response to changes in the inspired oxygen concentration and the lack of physical findings on auscultation to explain the low oxygen saturation, the diagnosis of metHb was entertained and confirmed by laboratory analysis.

Patent blue sentinel node mapping in cervical cancer patients may lead to decreased pulse oximeter readings and positive methaemoglobin results

BACKGROUND AND OBJECTIVE

Patent blue (4-[(4-diethylaminophenyl)-(4-diethylazaniumylidencyclohexa-2,5-dienyliden) methyl]-6-hydroxy-3-sulfo-benzolsulfonate, sodium salt) is a contrast dye used for the intraoperative detection of the primary lymphatic nodes draining the area of tumour infiltration. The dye is known to interact with pulse oximeter readings. However, the degree of alteration seems to be moderate and predictable when patent blue is injected into the perimammilar region during breast surgery.

METHODS

Here we report severe interference with the anaesthetic monitoring when patent blue was injected into the cervix prior to laparoscopy-assisted radical vaginal hysterectomy for cervical cancer.

RESULTS

Injection of patent blue into the cervix induced a rapid (within 14 +/- 9 min after the injection) and severe (from 98% to 89 +/- 2%) decrease in pulse oximeter readings, accompanied by positive methaemoglobin values of 7.3 +/- 2.5% (arterial co-oximetry, Bayer Rapidlab 865 blood gas analyser; Bayer, Fernwald, Germany). Control of these values by a different device (Radiometer ABL co-oximeter blood gas analyser; Radiometer, Willich, Germany) yielded negative methaemoglobin results (<1.7%, mean 0.9 +/- 0.6%). The arterial PO2 was normal in all patients throughout the procedure.

CONCLUSION

Injection of patent blue into the cervix uteri interferes dramatically with pulse oximeter readings. This situation is further complicated by device-dependent arterial co-oximetry methaemoglobin results. For the time being it is recommendable to monitor adequate oxygenation of the patient in the presence of patent blue by regular control of the arterial PO2. Clearly, the unresolved issue of reliable methaemoglobin determination in the presence of patent blue remains a matter of clinical concern for anaesthetists.

Aniline-induced methaemoglobinaemia in a glucose-6-phosphate dehydrogenase enzyme deficient patient

A case of methaemoglobinaemia following ingestion of an aniline-containing material is described. The detrimental effect of methylene blue, the classical antidote of methaemoglobinaemia, in a patient with glucose-6-phosphate dehydrogenase deficiency is highlighted.

Risk factors for prilocaine-induced methaemoglobinaemia following peripheral regional anaesthesia

BACKGROUND

The local anaesthetic prilocaine has a low systemic toxicity mainly because of a high absorption in the lung and a large volume of distribution and thus is associated with a lower risk of neurological or cardiac side-effects. However, the major disadvantage is the formation of methaemoglobin by its metabolite o-toluidine. This prospective observational study was performed to identify factors that are associated with increased prilocaine-induced methaemoglobinaemia.

PATIENTS AND METHODS

One Hundred and sixty two patients undergoing major knee surgery under general anaesthesia combined with peripheral nerve blocks (femoral nerve block, combined femoral/sciatic nerve block or lumbar plexus block) received a single bolus injection of 300 or 400 mg prilocaine about 30 min before surgery via a catheter. The proper placement was verified using nerve stimulation via a stimulating catheter. Three hours after prilocaine injection, venous blood samples were drawn and methaemoglobin levels were measured by standard photometric technique. Data was subjected to a stepwise multiple regression analysis.

RESULTS

The mean methaemoglobin for all patients was 2.7% (range: 0.9-15.4%). A higher dose of prilocaine and younger age were the most important predictive factors for higher methaemoglobin formation. Female sex and to a lesser extent the use of high-concentration/low-volume prilocaine also increased methaemoglobin levels. These four factors of the model explain 36% of the total variance. Other investigated factors, including the patient's height, weight, body mass index, the site of catheter insertion, the anaesthetist's judgement concerning the difficulty of catheter placement, duration of catheter placement or an inadvertent puncture of a venous or arterial vessel, had no significant impact on the concentration of methaemoglobin.

CONCLUSION

The use of prilocaine for regional block is safe, since the older patients who might be more susceptible to suffer from clinical symptoms of methaemoglobinaemia usually form less methaemoglobin. However, since prediction of high methaemoglobin levels is difficult, anaesthesiologists performing regional blocks in patients who might be jeopardized by a decreased oxygen transport capacity should avoid high doses of prilocaine.

[Methemoglobinemia due to prilocaine after plexus anesthesia. Reduction by prophylactic administration of ascorbic acid?]

OBJECTIVE

This study investigated in vivo and in vitro kinetics of o-toluidine-induced methemoglobinemia and the influence of ascorbic acid on resulting methemoglobin concentrations. o-Toluidine is a metabolite of prilocaline and ascorbic acid is recommended for treatment of methemoglobinemia as an alternative to methylene blue.

METHODS

We measured the formation of methemoglobin in vitro in a whole blood culture system of 8 healthy individuals 30, 60, and 360 min after the addition of different concentrations of o-toluidine (0.5, 5, 50 micrograms/ml) with and without addition of ascorbic acid (0.5 and 5 mg/ml). In a prospective randomized clinical study, a total of 72 patients of ASA risk I-III were investigated. The 3 groups of 24 patients received either an axillary, an infraclavicular vertical brachial plexus, or a combined femoral and ischiadic blockade. In each plexus anesthesia group, 12 patients were given 2,000 mg ascorbic acid intravenously before applying the local anesthetics. For surgery of the upper limb the patients received 40 ml 1% prilocaine and 10 ml 0.5% bupivacaine, for surgery of the lower limb they received 60 ml 1% prilocaine and 0.25 mg adrenaline. Blood samples for measurement of methemoglobin concentrations were taken before and 30, 60, 120, 180 and 360 min after the injection of the regional anesthetic. A p < 0.05 was considered to be significant.

RESULTS

There was a dose-dependent increase of methemoglobin due to addition of o-toluidine after 360 min in vitro. The application of 0.5 mg/ml ascorbic acid to the whole blood samples with 0.5 and 5 micrograms/ml o-toluidine resulted in a further increase of methemoglobin formation whereas there was no difference in the samples with 50 micrograms/ml. The higher concentration of 5 mg/ml ascorbic acid attenuated the methemoglobin formation only with 50 micrograms/ml o-toluidine. No effect was observed with lower concentrations of o-toluidine. In the in vivo study plexus anesthesia with prilocaine resulted in an increase of the methemoglobin concentration with a maximum after 120-180 min. The highest measured methemoglobin concentration found was 11.3%. The methemoglobin concentration already showed a decrease 360 min after the application of the regional anesthetic 2,000 mg ascorbic acid given intravenously before plexus anesthesia was not able to influence the resulting methemoglobin concentrations.

CONCLUSIONS

In vitro high concentrations of ascorbic acid are able to reduce the resulting methemoglobin concentration 360 min after addition of 50 micrograms/ml o-toluidine. The application of 2,000 mg ascorbic acid i.v. before plexus anesthesia with prilocaine does not reduce the concentration of methemoglobin.

Methemoglobin formation in children with congenital heart disease treated with inhaled nitric oxide after cardiac surgery

OBJECTIVE

Inhaled nitric oxide (NO) is used as a therapy of pulmonary hypertension in children after cardiac surgery. Hemoglobin binds to NO with great affinity and forms methemoglobin by oxidation in the erythrocyte. Once produced, methemoglobin is unable to transport and unload oxygen in the tissues. The amount of available hemoglobin in the body for oxygen transport is thereby reduced. Anemia, acidosis, respiratory compromise and cardiac disease may render patients more susceptible than expected for a given methemoglobin level. The goal of the present study was to review the cumulative effect of inhaled NO on methemoglobin formation in critically ill children. We therefore looked for methemoglobin levels in children with congenital heart disease after cardiac surgery who were treated with inhaled NO in a range of 5-40 ppm.

METHODS

We retrospectively reviewed the medical charts of 38 children with congenital heart disease after cardiac surgery. We extracted demographic data and physiological measurements at the following time points: (1) T0 = before starting inhaled NO therapy, (2) T1 = 24 h after the beginning of inhaled NO therapy, (3) T2 = half-time therapy, (4) T3 = end of therapy, (5) T4 = 24 h after finishing inhaled NO therapy.

RESULTS

The median duration of inhaled NO therapy was 5.5 days (interquartile range 6, range 2-29), NO concentrations at T1 and T2 were 16 ppm (10, 5-40) and 12.5 ppm (12.3, 2-40), respectively. The median cumulative dose of inhaled NO was 1699 ppm (2313, 193-7018). Methemoglobin levels increased moderately, but significantly, during therapy ( T0 vs T1 p<0.05 and T0 vs T2 p<0.001). The highest methemoglobin level measured was 3.9%. Methemoglobin levels correlated positively with the inhaled NO doses applied at T1 ( r(2)=0.8376; p<0.01) and at T2 ( r(2)=0.8945; p<0.01). At T1 the methemoglobin level correlated negatively with the T1 blood pH value. The overall mortality rate was 13.2% (5 of 38 study patients died). There was no significant difference in methemoglobin levels between survivors and non-survivors.

CONCLUSION

We conclude from our data that the use of inhaled NO therapy for children with congenital heart disease after cardiac surgery in the described range of 5-40 ppm, resulting in a maximum of 4% methemoglobin blood level, is feasible and safe. However, we recommend the use of the minimal effective dose of inhaled NO and continuous monitoring of methemoglobin levels, especially in cases of anemia or sepsis in critically ill children.

Lessons to be learned: a case study approach: prolonged methaemoglobinaemia due to inadvertent dapsone poisoning; treatment with methylene blue and exchange transfusion

The authors present a case of methaemoglobinaemia of acute onset, with an unusually protracted course. The long persistence of this disorder led to a search for the cause which was eventually traced to medication with dapsone. The latter was found to be inappropriately being taken by the patient instead of an antispasmodic that had been prescribed for a spinal condition; this was because the tablets had been incorrectly labelled and dispensed in a pharmacy. The patient took increasing doses of the presumed 'antispasmodic' tablets as they seemed to lack clinical effect, thus further exacerbating the toxic consequences. Moreover, the patient brought his wrongly labelled tablets into hospital and was allowed to use them there, contrary to normal hospital policy. As treatment for the methaemoglobinaemia both bolus and continuous infusions of methylene blue were used, which probably contributed to the severe haemolysis which followed. Furthermore, the development of a rare side effect of dapsone toxicity, namely that of a sensorimotor neuropathy, is reported.

Mode of antimalarial effect of methylene blue and some of its analogues on Plasmodium falciparum in culture and their inhibition of P. vinckei petteri and P. yoelii nigeriensis in vivo

The antimalarial action of methylene blue (MB) was first noted by Paul Ehrlich in the late 19th century. Although it has only sporadically been adopted as a serviceable drug, the resolution of its antimalarial action seems warranted, as it is currently used for the treatment of various methemoglobinemias. In this work we have used MB, and its analogues Azures A (AZA), B (AZB), C (AZC), and thionin (TH), as well as the oxazine Celestine blue (CB) and azine Phenosaphranin (PS). All MB analogues inhibit the growth of various strains of Plasmodium falciparum in culture with IC50s in the 2 x 10(-9)-1 x 10(-7) M range, with the rank order MB approximately AZA > AZB > AZC > TH > PS > CB. The IC50s for a mammalian cell line were in the 3 x 10(-6)-4 x 10(-5) M range, and the rank order was TH approximately AZB > AZA approximately PS > AZC approximately CB > MB. As MB could affect cell growth through the oxidation of NADPH, we tested the action of the various compounds on the hexose-monophosphate shunt activity. Appreciable activation of the shunt was observed at 1 x 10(-5) M in both cell types, thus accounting for inhibition of growth of mammalian cells but not of parasites. All compounds were found to complex with heme in a rank order similar to their antimalarial effect. It is therefore suggested that MB and its congeners act by preventing the polymerization of heme, which is produced during the digestion of host cell cytosol in the parasite food vacuole, into hemozoin. In this respect, these compounds seem to act similarly to the 4-aminoquinoline antimalarials. All compounds effectively suppressed the growth of P. vinckei petteri in vivo with IC50 in the 1.2-5.2 mg/kg range, and MB and AZB suppressed P. yoelii nigeriensis in the 9-11 mg/kg range (i.e. at doses similar to those of chloroquine). The potential toxicity of these compounds may restrict their clinical use, but their impressive antimalarial activities suggest that the phenothiazine structure could serve as a lead compound for further drug development.

Definition, significance and measurement of quantities pertaining to the oxygen carrying properties of human blood

A consistent set of definitions is given of the principal quantities pertaining to the oxygen transport by the blood, and of their mutual relationships, in relation to the methods used in their measurement. At the core is the correct definition of oxygen saturation, the deviation of which has recently been the cause of considerable confusion, especially concerning pulse oximetry. The occurrence, properties, determination, and pathophysiological and clinical significance of dyshaemoglobins, such as carboxyhemoglobin, methemoglobin, and sulfhemoglobin, are briefly described, together with possible consequences for diagnosis and therapy. In addition, attention is payed to the use of some haemoglobin derivatives, such as methaemoglobin and cyanmethaemoglobin, for clinical chemical and therapeutic purposes.

Methaemoglobin production in normal adults inhaling low concentrations of nitric oxide

OBJECTIVE

The study was performed to determine the changes in blood methaemoglobin level during the inhalation of nitric oxide.

DESIGN

The study was an unblinded dose-response study.

PARTICIPANTS

5 healthy adult volunteers aged 30-36 (4 male and 1 female) were studied on 4 occasions separated by at least one week.

INTERVENTION

Nitric oxide was inhaled at inspired concentrations of 32, 64, 128, and 512 volumes per million (vpm) in air. Venous blood samples were taken every 10 min for methaemoglobin determination. Inhalation continued for 3 h (32, 64 and 128 vpm) or until the methaemoglobin exceeded 5% of the total haemoglobin (512 vpm). The methaemoglobin levels were also recorded for 3 h after 512 vpm nitric oxide had been stopped.

MEASUREMENTS AND RESULTS

Both the increase in methaemoglobin fraction during nitric oxide inhalation and the decay after ceasing inhalation fitted well with a first order model describing methaemoglobin elimination. The calculated time constants were between 39-91 min. The predicted mean maximum methaemoglobin levels that would be achieved during inhalation of 32, 64, 128, and 512 vpm nitric oxide were 1.04% (0.92-1.16), 1.75% (1.80-1.90%), 3.75% (3.58-4.05), 6.93% (5.70-8.16) respectively (95% confidence interval of estimate in brackets).

CONCLUSIONS

In normal individuals inhalation of up to 128 vpm of nitric oxide, greater than any dose used clinically to date, does not result in clinically significant methaemoglobinaemia. Maximum methaemoglobin levels are likely to be reached in 3-5 h after inhalation begins. However, these figures may not apply to critically ill adults and infants. Nitric oxide may have other toxic effects not examined in this study.

Incidence of subclinical methemoglobinemia in infants with diarrhea

STUDY HYPOTHESIS

Infants with diarrhea are at a greater-than-recognized risk of developing methemoglobinemia.

DESIGN

Prospective clinical study.

SETTING

A university hospital pediatric emergency department.

PARTICIPANTS

Consecutive infants under 6 months of age with a history of diarrhea of more than 24 hours' duration not associated with vomiting.

INTERVENTIONS

Blood samples were obtained for methemoglobin (MHgb) assay (normal, 0.4% to 1.5%) and electrolytes. Treatment interventions were performed as clinically indicated. Patients with elevated MHgb levels subsequently underwent hemoglobin electrophoresis to exclude congenital methemoglobinemia.

RESULTS

Forty-three patients were studied; 27 (64%) had elevated MHgb levels and 13 were cyanotic. Five patients received infusions of methylene blue for methemoglobinemia. All patients recovered without sequelae. There was a strong correlation between weight at or below the tenth percentile for age and the development of methemoglobinemia. Contrary to previous studies, there was no correlation between incidence or severity of methemoglobinemia and acidosis, hyperchloremia, or positive microbiologic studies.

CONCLUSION

In ill infants with diarrhea, particularly those who are small for age, consideration should be given to screening for methemoglobinemia.

Methemoglobinemia secondary to automobile exhaust fumes

Methemoglobinemia is an uncommon cause of cyanosis. A 28-year-old male presented to the emergency department cyanotic and short of breath after exposure to noxious automobile fumes. He did not improve with the administration of 100% oxygen therapy. The initial arterial blood gas with cooximetry was: pH of 7.38, PaCO2 of 43 mm Hg, PaO2 of 118 mm Hg, measured oxygen saturation of 70%, and a methemoglobin level of 24.8%. Methylene blue was given (2 mg/kg intravenously) and the patient's symptoms resolved. On the following day he was discharged home without complication. A comprehensive review of the literature revealed no reported cases of methemoglobinemia secondary to accidental exposure to exhaust fumes.

Varying presentations of methemoglobinemia: two cases

The cases of two patients with methemoglobin levels approaching 30% are presented. No history of exposure to an oxidant was ever determined for the first case; the second patient had taken amyl nitrate orally along with alcohol as well as a self-injected narcotic and antihistamine. Due to their symptomatology, both patients were treated with methylene blue with good results; however, the patient described in case one had a brief episode of symptoms and pulse oximetry changes, not previously emphasized, consistent with the administration of methylene blue. The pathophysiology of methemoglobinemia, the utility of bedside diagnostic techniques, and the use of pulse oximetry are discussed.

Methemoglobinemia as a cause of coma

A 50-year-old man presented to the emergency department in a coma with fixed and dilated pupils. Skin and mucous membranes were noted to be an ashen grey color. Initial vital signs were blood pressure of 104/70 mm Hg; pulse, 110; and respirations, 12. Initial arterial blood gases were pH of 7.25; PaCO2, 26.6 mm Hg; PaO2, 22.1 mm Hg; oxygen saturation, 15.2%; and methemoglobin level, 81.5%. Venous and arterial blood samples were chocolate brown, and it was noted that the color did not change when 100% oxygen was bubbled through the blood. Methylene blue 140 mg was given intravenously, and the patient gradually became more responsive. He was discharged three days later with no neurologic deficits. A comprehensive literature search revealed no reported cases of complete patient recovery with this high a methemoglobin level.

Methemoglobin reduction under near physiological conditions

Pure methemoglobin was prepared from fresh red cells and was used as substrate for methemoglobin reduction reaction. Two sources of methemoglobin reductase were used: (a) red cell hemolysate which was prepared by freezing and thawing of unwashed red cells; (b) purified methemoglobin reductase from bank blood. Methemoglobin reduction rate was measured in a mixture of pure methemoglobin (substrate) and hemolysate (enzyme). In other experiments the rate of methemoglobin reduction was measured in the above mixture with the addition of various other compounds such as NADH, cytochrome b5, and pure methemoglobin reductase. Only the addition of pure enzyme accelerated the rate of methemoglobin reduction. In other experiments, the rate of methemoglobin reduction was measured when the reduction reaction was carried out in the presence of various amounts of deoxyhemoglobin, globin, or albumin. It was shown that all proteins tested here decreased the reduction rate. It is concluded that (a) in the red cell, under normal conditions, only the activity of the methemoglobin reductase controls the speed of methemoglobin reduction, and (b) the inhibition of methemoglobin reduction by reduced hemoglobin is mostly nonspecific suggesting a noncompetitive reaction.

Determination of NADH2-ferricyanide oxidoreductase (cytochrome b5 reductase, diaphorase) activity of human erythrocytes by an analysis of the time-dependence of NADH2 oxidation

The time-dependence of the reaction of human erythrocyte diaphorase activity has been studied by the use of NADH2 and ferricyanide as substrates. Reaction was found to be first-order with respect to NADH2 concentration, and zero-order with respect to ferricyanide concentration. These findings indicate that human erythrocyte diaphorase has a Km value for NADH2 by far higher than, and for ferricyanide by far lower than, the concentration of the substrates used, i.e. 0.1 and 0.2 mmol/l, respectively. The diaphorase activity determination method, described in the present communication, has been used in 19 healthy adults and children. Diaphorase activity was found to be 7.29 +/- 3.69 1 SD mumol NADH2 oxidized/ml packed cells per min, at 25 degrees C, and pH 7.00.

The effect of oxygen on in vitro studies on methemoglobin production in man and dog blood using 4-dimethylaminophenol

Methemoglobin production induced by the addition of 4-dimethylaminophenol to human or beagle blood in vitro is inhibited at high oxyhemoglobin levels. The effect is similar in the two species and probably results from conformational change in hemoglobin consequent on oxygen binding.

Species differences in methaemoglobin production after addition of 4-dimethylaminophenol, a cyanide antidote, to blood in vitro: a comparative study

Methaemoglobin production after addition of DMAP to blood of various species, has been studied in vitro. The study was undertaken both with blood, as taken, and after equilibration with atmospheric oxygen. Considerable interspecies variation in methaemoglobin production was found. When the initial rate of methaemoglobin formation was considered only marmoset and human blood showed any marked degree of inhibition by equilibration with atmospheric oxygen.

Hemoglobin autoxidation at physiological concentrations

Methemoglobin formation was studied at near physiological hemoglobin concentration. The reaction proceeds at a faster rate when the concentration of hemoglobin is high (15-18 mM in heme) than when it is low (2 mM). Constant shaking of hemoglobin preparations during the incubation decreases the differences seen in the rates of autoxidation between concentrated and dilute samples. When red cell hemolysate is used instead of pure hemoglobin, similar results are obtained. A comparison of rates of methemoglobin formation in hemoglobin solutions under low air pressure (1/2 atm) with those under normal air pressure (1 atm) shows no differences between concentrated and dilute samples. There is also no significant difference between the rates of autoxidation of dilute and concentrated solutions when the reactions are carried out under one atmosphere of oxygen (100 percent O2). The study of one patient with hereditary spherocytosis demonstrated higher hemoglobin autoxidation rate in spherocytes, which have higher hemoglobin concentration, than in normal biconcave red cells. These results suggest that: a) the rate of hemoglobin autoxidation at red cell hemoglobin concentration is significantly faster than rates obtained by studying dilute solutions; b) although the accelerated oxidation might be related to multiple factors, one seems to be less accessibility of oxygen when the hemoglobin solution is highly concentrated.