Anesthetic management of a patient with methemoglobinemia. (Case Report).

Abstract: Methemoglobinemia results from the oxidation of the ferrous iron in hemoglobin to the ferric iron state. Methemoglobin is incapable of carrying[O.sub.2], and high levels may impact on [O.sub.2] delivery to the tissues. Methemoglobinemia may result from congenital deficiencies of enzymes that normally convert methemoglobin to hemoglobin, alterations in the hemoglobin molecule itself or, most commonly, from the ingestion of medications or toxins that oxidize the ferrous iron of hemoglobin. Several issues must be considered when anesthetizing patients with methemoglobinemia, including the potential for decreased [O.sub.2] delivery, which may be exacerbated by intraoperative blood loss and anemia, interference with normal intraoperative monitoring devices, and the potential for medications to cause or exacerbate methemoglobinemia. We describe a patient with acquired methemoglobinemia from dapsone therapy who required anesthetic care for shoulder arthroscopy. The patient's drug-induced methemoglobinemia was diagnosed intraoperatively during previous anesthesia on the basis of discrepancy between the [O.sub.2] saturation noted by pulse oximetry and that obtained from arterial blood gas analysis. Anesthetic care for patients with methemoglobinemia is discussed and a review of methemoglobinemia presented.

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Key Points

* Methemoglobinemia results from the oxidation of the ferrous iron in hemoglobin to the ferric iron state; because it is incapable of carrying [O.sub.2], high levels of methemoglobin may impact [O.sub.2] delivery to the tissues, resulting in tissue hypoxia.

* The causes of methemoglobinemia include congenital forms (deficiencies of enzymes that normally convert methemoglobin back to standard hemoglobin, or amino acid substitutions of the hemoglobin molecule [M hemoglobinopathies]) and various acquired forms (caused by drugs or toxins).

* Perioperative care includes arrival at the correct diagnosis when there is a discrepancy between pulse oximetry and clinical findings, determination of the cause with removal of the offending agent, treatment with intravenous methylene blue, and maintenance of [O.sub.2] delivery to tissue.

* Perioperative monitoring may be problematic because of the interference of methemoglobin with pulse oximetry function.

Hemoglobin normally contains an iron molecule in the ferrous state. The oxidation of ferrous iron to ferric iron results in methemoglobin (metHb). Interference with normal [O.sub.2] delivery occurs from two mechanisms: 1) metHb is incapable of carrying oxygen, and 2) metHb shifts the oxyhemoglobin dissociation curve to the left, thereby further decreasing [O.sub.2] delivery to the tissues. (1) During metabolic processes, oxidants are produced that result in the conversion of hemoglobin to metHb, accounting for the normal 1 to 2% concentration of circulating metHb. (2) Methemoglobin is then converted back to the ferrous state by the body's normal reducing system, the reduced form of nicotinamide adenine dinucleotide (NADH) cytochrome [b.sub.5]-reductase. Excessive oxidant stresses from toxins/drugs, congenital enzyme deficiencies of the NADH cytochrome [b.sub.5]-reductase system, or alterations in the hemoglobin structure can result in higher concentrations of metHb. Given its potential to interfere with [O.su b.2] delivery, interference with normal intraoperative monitoring devices such as the pulse oximeter, and the propensity for certain medications to cause or exacerbate preexisting methemoglobinemia, significant perioperative issues may be present in patients with this condition. We present the case of a 62-year-old woman with chronic methemoglobinemia related to dapsone therapy for dermatitis herpetiformis who required shoulder arthroscopy for persistent shoulder pain and decreased range of motion.

Discussion

Methemoglobin can arise from a variety of sources including genetic or congenital forms (enzyme deficiencies or alterations in the hemoglobin molecule), dietary causes (eg, ingestion of well water contaminated with nitrates) or, most commonly, the ingestion of toxins or medications, as was the cause in our patient. Under normal homeostatic conditions, the 3% metHb that is formed daily from normal metabolic processes is rapidly converted back to the ferrous state by intracellular enzyme systems. (5) These mechanisms include NADH cytochrome-[b.sub.5] metHb reductase and NADP phosphate (NADPH) metHb reductase. Cytochrome-[b.sub.5] reductase, the predominant pathway for the reduction of metHb, catalyzes the transfer of electrons from NADH to cytochrome [b.sub.5] and then to metHb. NADPH metHb reductase, under normal circumstances, is unimportant for the reduction of metHb, but this enzyme has a high affinity for methylene blue and is a rapid reducer of metHb when methylene blue is present (see below). (6) Infant s and neonates are more susceptible to the development of methemoglobinemia, because fetal hemoglobin is less resistant to oxidant stresses than adult hemoglobin and the enzymatic pathways for the conversion of metHb back to hemoglobin are somewhat deficient in the neonatal period.

Hereditary methemoglobinemia results from one of two basic alterations: 1) an alteration or deficiency in the enzymes responsible for the conversion of iron from the ferric back to the ferrous state, or 2) amino acid substitutions in the hemoglobin molecule, making the molecule itself more susceptible to oxidant stresses. Enzymatic causes of congenital methe-moglobinemia, which are inherited in an autosomal recessive pattern, can be further divided into four subtypes on the basis of the specific enzyme deficiency and the clinical presentation (Table 1). (7)

Hereditary methemoglobinemia can also occur because of hemoglobin M hemoglobin variants, in which the proximal or distal histidine residue is replaced with tyrosine. The alteration of the amino acid sequence predisposes the hemoglobin to oxidant stresses, thereby increasing the metHb concentration. Patients with hemoglobin M appear cyanotic, but are otherwise asymptomatic. Like the enzymatic forms of methemoglobinemia, the metHb concentration usually does not exceed 20%, and therefore patients remain asymptomatic unless additional factors compromise [O.sub.2] delivery to tissue. The hemoglobin M diseases are autosomal dominant, and they do not respond to treatment with methylene blue.

Methemoglobinemia after exposure to drugs or toxins is the most frequently encountered form of the condition. Exposure may involve ingestion, skin contact, or inhalation. The formation of metHb involves the direct or indirect oxidation of hemoglobin to such an extent that the reductive pathways are overwhelmed, thereby resulting in high concentrations of metHb. Direct oxidation involves the toxin, drug, or its metabolite reacting directly with the hemoglobin molecule, whereas indirect oxidizers are reducing agents that reduce [O.sub.2] to form a free radical, which then oxidizes hemoglobin to methemoglobin. Several different medications and toxins, including many used in anesthetic practice, have been associated with methemoglobinemia (Table 2). (8) Those used most commonly in anesthetic practice include the local anesthetic agents (prilocaine, lidocaine, benzocaine), metoclopramide, nitric oxide, nitroglycerin, and sodium nitroprusside.

The severity of acquired methemoglobinemia depends on the dose of the offending agent, the capacity of the reductive pathways, and the patient's metabolism (if it is a metabolite of the drug that is the oxidizing agent responsible for metHb formation). (8) Patients with congenital forms of methemoglobinemia, although asymptomatic at baseline, may have an increased tendency to develop symptoms when exposed to metHb-inducing drugs or toxins. (7) Therefore, in patients with congenital or acquired chronic methemoglobinemia, it is important to recognize which drugs may induce metHb and avoid them during anesthetic care. Likewise, these patients may be less able to tolerate additional stresses to [O.sub.2] delivery (eg, decreased cardiac output, anemia), because part of their hemoglobin store is ineffective in [O.sub.2] delivery.

Symptoms of methemoglobinemia are related primarily to the percentage of metHb but are also influenced by other patient factors, including the total hemoglobin concentration, cardiovascular status and ability to increase cardiac output, and respiratory function ([PaO.sub.2] and therefore [O.sub.2] saturation). Concentrations of metHb between 15 and 20% generally result only in the appearance of cyanosis. Symptoms including headache, lethargy, tachycardia, weakness, and dizziness occur with metHb levels of 20 to 45%. Levels greater than 45% may result in dyspnea, acidosis, cardiac dysrhythmias, heart failure, and seizures. A high mortality rate is associated with metHb levels >70%. (2)

Intraoperative monitoring of these patients can be problematic because of the effect of metHb on the accuracy of [S.sub.p][O.sub.2]. Pulse oximetry works by measuring light absorbance at two wavelengths (660 and 940 nm) and using the ratio of the absorbance at those two wavelengths to determine [O.sub.2] saturation. These two wavelengths are chosen because the difference of the absorption of oxyhemoglobin and deoxyhemoglobin is greatest at these points. Because metHb absorbs light equally at both wavelengths, as the metHb concentration rises, the pulse oximeter approaches 85%. With low concentrations of metHb (10-20%), the pulse oximeter will read erroneously low, whereas even toxic or lethal concentrations of metHb will still result in an [O.sub.2] saturation by [S.sub.p][O.sub.2] of 85% (Table 3). (9)

Standard arterial blood gas analysis relies on a change in electric current to measure the [P.sub.a][O.sub.2], which is not impacted by alterations in the metHb concentration. Depending on the institution and the exact test ordered, the [O.sub.2] saturation that is reported on blood gas analysis may be calculated on the basis of the [P.sub.a][O.sub.2] and the temperature, so that the actual individual hemoglobin species (oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin) are not measured. (10) In such cases, an erroneous assumption can be made that there is no significant problem. The measurement of levels of metHb and other hemoglobin species requires the use of cooximetry. The co-oximeter has the ability to measure light absorbance at the four different wavelengths that characterize deoxyhemoglobin, oxyhemoglobin, carboxyhemoglobin, and methemoglobin." The peak absorbance of light at 630 nm is used to characterize metHb. Given the inherent inaccuracy of [S.sub.p][O.sub.2], we used transcu taneous [O.sub.2] and [CO.sub.2] monitoring in our patient. The technique of transcutaneous [O.sub.2] [CO.sub.2] monitoring and its potential for intraoperative use has been discussed in greater detail elsewhere. (3,4,12-14) Although there are problems regarding its use in adult patients, the transcutaneous [O.sub.2] value is generally less than that of the [P.sub.a][O.sub.2], so that an acceptable transcutaneous value reflects a higher [P.sub.2[O.sub.2] value and provides some assurance that tissue oxygenation is adequate. (12-14)

For more involved procedures, intra-arterial access with either intermittent sampling or the use of continuous, in vivo, blood gas monitoring systems may be considered. Because these devices use the same technology as standard arterial blood gas systems, no interference should be noted with elevated metHb levels; however, these devices do not directly measure [O.sub.2] saturation. Rather, [O.sub.2] saturation is calculated from the [P.sub.a][O.sub.2] and body temperature using standardized nomograms. As such, erroneous values occur with elevated metHb levels, because the [O.sub.2] saturation is calculated from the [P.sub.a][O.sub.2] without direct measurement of the various hemoglobin species.

The diagnosis of metHb should be suspected in patients that appear cyanotic and have a low pulse oximetry reading yet have no apparent respiratory or cardiovascular problems to explain the low [O.sub.2] saturation. Arterial blood gas analysis will reveal a normal [P.sub.a][O.sub.2] that is not consistent with the pulse-oximeter-measured saturation. The diagnosis is confirmed by measurement of metHb levels using co-oximetry. Patients with congenital forms of methemoglobinemia may have been previously diagnosed or there may be a family history of the condition. Recent history may reveal exposure to a metHb-including drug or toxin.

When an elevated metHb level is reported, it should be recognized that sulthemoglobin (sulfHb), which has an absorbance at 614 am, may overlap with metHb measured by co-oximetry. Therefore, elevated levels of metHb may reflect sulfhemoglobinemia. (15) Sulfhemoglobin production occurs when a drug or toxin with oxidative properties produces metHb, followed by the incorporation of a sulfur atom into the hemoglobin. (16) Sulfhemoglobin, like metHb, is incapable of transporting oxygen; however, unlike metHb, [O.sub.2] delivery is somewhat increased because of a rightward shift of the [O.sub.2] dissociation curve, thereby decreasing [O.sub.2] affinity for the unaffected hemoglobin and increasing [O.sub.2] delivery at the tissue level. Sulfhemoglobin cannot be reduced by methylene blue and lasts the lifetime of the red blood cell. (16) Drugs associated with sulfhemoglobinemia include phenacetin, dapsone, sulfonamides, and metoclopramide; however, any drug that can cause methemoglobinemia can also cause sulfhemoglobi nemia, if sulfur is available. Techniques used to differentiate sulfHb from metHb include new generation co-oximeters and spectrophotometry. A bedside test for differentiating metHb form sulfHb involves adding potassium cyanide to the blood sample. The metHb will react with the cyanide to form cyanomethemoglobin that is bright red, but the sulfHb will remain chocolate brown. (9)

The treatment of choice for symptomatic methemoglobinemia is immediate therapy with IV methylene blue and identification and removal of the offending drug or toxin. Patients with Type I enzymatopenic hereditary methemoglobinemia generally require treatment only for cosmetic purposes. In some patients, daily oral therapy with methylene blue, ascorbic acid, or riboflavin may be effective. Patients who fail to respond to oral therapy may require long-term therapy with IV methylene blue. Therapy is titrated on the basis of the patient's symptoms, the level of metHb, and other concurrent disease processes. General recommendations include the treatment of asymptomatic patients with a metHb level of 30% and symptomatic patients at a level of 20%. Patients with other disease processes that already limit [O.sub.2] delivery, such as heart disease, lung disease, carbon monoxide poisoning, or anemia, may need to be treated at metHb levels as low as 10% (eg, treatment may need to be initiated at lower levels in patients a t risk for myocardial ischemia). The metHb decreases the effective oxygen-carrying capacity of the blood, and may not be tolerated by these patients. The decreased oxygen-carrying capacity of the blood results in increased cardiac output to meet peripheral [O.sub.2] consumption and metabolic needs, thereby increasing myocardial [O.sub.2] consumption. Therefore, the treatment of methemoglobinemia with methylene blue in patients with ischemic heart disease or cerebrovascular disease must be individualized.

Methylene blue acts as a cofactor for NADPH metHb reductase, the alternative hemoglobin reduction pathway, and greatly increases the enzymatic reduction of metHb through this pathway. (17, 18) Methylene blue is administered IV in a dose of 1 to 2 ml/kg of a 1% solution; this dose may be repeated in 30 to 60 minutes if the methemoglobinemia persists. Adverse effects of methylene blue therapy include nausea, vomiting, diarrhea, dyspnea, diaphoresis and, rarely, anaphylactoid reactions. Doses of methylene blue greater than 4 to 15 ml/kg/d may actually cause metHb by direct oxidation of hemoglobin to metHb (Table 2). Patients with glucose-6-phosphate dehydrogenase deficiency may develop hemolytic anemia from methylene blue therapy. Life-threatening methemoglobinemia that is nonresponsive to treatment with methylene blue may occasionally respond to ascorbic acid therapy, although ascorbic acid works slowly. Anecdotal reports suggest additional therapies to increase [O.sub.2] delivery, such as homologous transfusio ns, exchange transfusions, and hyperbaric [O.sub.2] therapy. (2)

Conclusions

We describe the anesthetic care of a patient with acquired methemoglobinemia resulting from dapsone therapy. As was the case in this patient, the diagnosis may first be made intraoperatively. The diagnosis of methemoglobinemia should be suspected in patients who appear cyanotic and have a low [SpO.sub.2] reading, yet have no apparent respiratory or cardiovascular problems to explain the low 02 saturation. Arterial blood gas analysis will reveal a normal [PaO.sub.2] that is not consistent with the pulse oximeter-measured saturation. The diagnosis is confirmed by measurement of metHb levels using co-oximetry. Toxicity from methemoglobinemia relates to its interference with oxygen-carrying capacity. Causes include both hereditary, congenital forms (enzyme deficiencies and amino acid alterations of the hemoglobin molecule) and acquired forms from drugs and toxins (Table 2). The drugs and toxins that are known to cause metHb must be recognized and appropriate treatment with methylene blue administered when necessa ry. Additional therapy includes optimization of [O.sub.2] delivery with maintenance of cardiac output and consideration of the uninvolved hemoglobin concentration (total hemoglobin concentration minus methemoglobin) with transfusion therapy as needed (Table 4).

Table 1

Types of enzymatic deficiencies resulting in methemoglobinemia (a)

Enzymatic
deficiency              Characteristics

Type I      Most common form of congenital
             methemoglobinemia, with only isolated
             case reports of other types.

            Deficiency of NADH cytochrome-[b.sub.5]
             reductase occurs only in the red blood
             cells.

            Patients appear cyanotic (due to the
             10-20% circulating methemoglobin
             concentrations).

            No other clinical manifestations at
             baseline; however, they may be more
             susceptible to toxic methemoglobinemia
             after exposure to methemoglobin-
             inducing drugs, chemicals, or toxins.

            Patients may be less tolerant of
             factors that decrease oxygen delivery,
             such as decrease in hemoglobin
             concentration, hypoxemia, or decrease
             in cardiac output.

Type II     More severe, generalized disorder.

            Defective NADH cytochrome-[b.sub.5]
             reductase in all tissues, including the
             central nervous system.

            Progressive neurologic dysfunction and
             mental retardation in addition to
             methemoglobinemia.

Type III    Deficiency of NADH cytochrome-[b.sub.5]
             reductase in all of the hematopoietic
             cell lines, including erythrocytes,
             leukocytes, and platelets, without
             central nervous system involvement.

Type IV     Deficiency of the cofactor cytochrome
             [b.sub.5] itself, without cytochrome-
             [b.sub.5] reductase deficiency.

(a)NADH, reduced form of nicotinamide adenine dinucleotide.

Table 2

Medications potentially implicated in causing methemoglobinemia


Acetanilid         Phenacetin            Aniline dyes
Metoclopramide     Phenazopyridine       Benzene derivatives
                                          (including chlorobenzene)
Methylene blue     Phenol                Naphthalene
Phenylhydrazine    Chlorates             Nitrates and nitrites
                                          (including nitroglycerin)
Phenytoin          Sodium nitroprusside  Nitric oxide
Chloroquine        Primaquine            Dapsone
Nitroalkanes       Dinitrophenol         Acetaminophen
Nitrofurantoin     Sulfamethoxazole
Local anesthetics
 Benzocaine
 Procaine
 Prilocaine
 Lidocaine

Table 3

Hemoglobin species and absorption spectra

Substance          Absorption

Oxyhemoglobin and  Absorption at both 660 nm and 940 nm;
  deoxyhemoglobin    differential absorption of these two
                     hemoglobin species is greatest at these
                     two wavelengths

Methemoglobin      Equal absorption at 660 nm and 940 nm

Carboxyhemoglobin  Absorption equal to oxyhemoglobin at
                     660 nm

Methylene blue     Absorption at 668 nm (results in
                     erroneous decrease in pulse oximetry
                     after intravenous administration)

Table 4

Perioperative management of methemoglobinemia (a)



1. Arrive at the diagnosis

   Suspect when [SpO.sub.2] is low despite no clinical reasons

   Confirm by low [SpO.sup.2] and high [PaO.sub.2] and normal
   saturation by co-oximetry

2. Determine the cause

   Congenital enzymatic deficiency

   Hemoglobin M due to amino acid alteration of hemoglobin molecule

   Drug-induced

3. Treatment Remove offending agent

   Methylene blue for levels >20-30%

   Ensure adequate tissue oxygen delivery (cardiac output
   plus effective hemoglobin concentration)

4. Intraoperative care

   Avoid medications that may exacerbate methemoglobinemia Consider
   additional monitoring (transcutaneous oxygen/in vivo devices)

(a)[SpO.sub.2], pulse oximetry.

Accepted April 25, 2002.

References

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(2.) Curry S. Methemoglobinemia. Ann Emerg Med l982;ll:2l4-221.

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(4.) McBride DS Jr, Johnson JO, Tobias JD. Noninvasive carbon dioxide monitoring during neurosurgical procedures in adults: End-tidal versus transcutaneous techniques. South Med J 2002;95:870- 874.

(5.) Eder HA, Finch C, McKee RW. Congenital methemoglobinemia: A clinical and biochemical study of a case. J Clin invest 1949;28:265-268.

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RELATED ARTICLE: Case Report

A 62-year-old woman with acquired methemoglobinemia presented for right shoulder arthroscopy and lysis of adhesions. The patient was diagnosed with methemoglobinemia in 1995 during an elective laparoscopic cholecystectomy. Intraoperatively, the patient's [O.sub.2] saturation as measured by pulse oximetry (Sp[O.sub.2]) was 85 to 88% on room air, and increased to 91% on 100% [O.sub.2] by mask. Arterial blood gases obtained while the patient was intubated revealed a pH of 7.45, a Pa[O.sub.2] of 535 mm Hg, an oxyhemoglobin concentration of 78%, and a metHb concentration of 19.9%. Methylene blue was administered, and follow-up arterial blood gases revealed a pH of 7.35, a Pa[O.sub.2] of 586 mm Hg, an oxyhemoglobin concentration of 97.8%, and a metHb concentration of 4.0%.

The patient had several coexisting diseases, including non-insulin-dependent diabetes, congestive heart failure, celiac disease, hypothyroidism, sarcoidosis, dermatitis herpetiformis, hypertension, and osteoarthritis. She had been treated with dapsone since 1987 for dermatitis herpetiformis, and dapsone was also used for treatment of her sarcoidosis (diagnosed in 1992). Although this medication was identified as the cause of her methemoglobinemia after her previous anesthetic, the physicians caring for her felt that it was essential for the treatment of her dermatitis herpetiformis and sarcoidosis. For long-term treatment of the methemoglobinemia, she initially received oral methylene blue, which was ineffective despite dose escalation. Therapy was subsequently changed to intravenous (IV) methylene blue through a Broviac catheter at home. She was receiving 150 mg/d of methylene blue IV at the time of presentation, and her metHb concentration ranged from 2 to 7%. Physical examination revealed perioral cyanosi s. Her vital signs were stable, and the Sp[O.sub.2] was 88 to 91% and did not change with supplemental [O.sub.2] treatment. Airway examination revealed an edentulous patient with a Mallampati Class I airway. Preoperative laboratory values included a hemoglobin of 7.8 mg/dl, a hematocrit of 24.5%, and platelet count of 259,000/[mm.sup.3]. Electrolyte levels, renal function tests, and coagulation parameters were within normal limits. The patient was given nothing to eat or drink after midnight before surgery. Regional anesthesia was chosen for the procedure. The patient was seen in the holding area, where her central line was accessed for IV fluid infusion. The patient also had a transcutaneous [O.sub.2] monitor attached using our previously described technique. (3,4) The patient was prepared and draped in a sterile fashion for an interscalene block. The patient was sedated with a combination of midazolam and fentanyl. Despite the use of a nerve stimulator and elicitation of a twitch in a distal muscle group wi th low amplitude, effective surgical anesthesia was not obtained. With evidence of a failed block, general anesthesia was chosen as the route of anesthesia administration. The patient was transported to the operating room and routine monitors were applied; monitoring of the transcutaneous [O.sub.2] saturation was continued as well. The patient was placed on 2 L [O.sub.2] through a nasal cannula on arrival in the operating room. The SP[O.sub.2] was 91%. Transcutaneous [O.sub.2] tension was 90 to 100 mm Hg. After preoxygenation with 100% [O.sub.2] through the anesthesia mask for 3 minutes, a combination of propofol and fentanyl was administered to induce anesthesia. Tracheal intubation was facilitated with administration of cisatracurium besylate. Maintenance anesthesia consisted of giving desflurane in 50% air and [O.sub.2] and fentanyl (3-5 [micro]g/kg). Ongoing neuromuscular blockade was provided by administering intermittent doses of cisatracurium. Intraoperatively, the Sp[O.sub.2] ranged from 91 to 94%, wi th transcutaneous [O.sub.2] saturations of 75 to 100 mm Hg. At the completion of the procedure, residual neuromuscular blockade was reversed and the patient's trachea extubated. The remainder of her postoperative course was unremarkable.

From the Division of Pediatric Critical Care/Pediatric Anesthesiology, Departments of Child Health and Anesthesiology, University of Missouri, Columbia, MO.

Reprint requests to Joseph D. Tobias, MD, Department of Anesthesiology, University of Missouri, One Hospital Drive, 3W40H, Columbia, MO 65212. Email: [e-mail]

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