The Role of the Nephrologist in Management of Poisoning and Intoxication: Core Curriculum 2022
|Change in Serum Osmolality
|Common Clinical Features
|Notable Laboratory Features
|3.09 mOsm/L per 10 mg/dL of alcohol
|Windshield washer fluid, carburetor cleaner, octane boosters, racing fuels, adulterated ethanol (“moonshine”)
|Abdominal pain; decreased vision, blindness; rarely, Parkinson-like features
|Osmolal gap; HAGMA
|1.60 mOsm/L per 10 mg/dL of alcohol
|Antifreeze, engine coolants, deicing fluids
|Osmolal gap, HAGMA, calcium oxalate crystalluria, elevated lactate with point-of-care analyzer
|1.66 mOsm/L per 10 mg/dL of alcohol
|Rubbing alcohol, hand sanitizers
|Inebriation, depressed sensorium, abdominal pain
|Osmolal gap, acetonemia, spurious increase in creatinine (Jaffe reaction)
|1.31 mOsm/L per 10 mg/dL of alcohol
|Diluent in parenteral medications, “nontoxic” automotive antifreeze
|Liver or kidney disease may increase likelihood of more severe toxicity
|Increased osmolal gap; lactic acidosis; rarely, AKI
|0.9 mOsm/L per 10 mg/dL of alcohol
|Automotive brake fluids, hydraulic fluids, adulterated liquid medications
|Abdominal pain, nausea/vomiting, acute pancreatitis, AKI
|Osmolal gap, HAGMA, AKI
Abbreviations: AKI, acute kidney injury; +EtOH, with coingested alcohol; −EtOH, without coingested alcohol; HAGMA, high anion gap metabolic acidosis; MW, molecular weight; NA, not applicable
- Ethylene glycol or methanol concentration > 50 mg/dL without ADH inhibitor (fomepizole or ethanol)
- Ethylene glycol concentration > 200-300 mg/dL with ADH inhibitor and normal kidney function
- Methanol concentration > 70 mg/dL with ADH inhibitor and normal kidney function
- Isopropanol concentration > 400-500 mg/dL
- Any toxic alcohol: severe acidemia (pH < 7.2) or AKI
- Concentration > 7.2 mmol/L (100 mg/dL)
- Concentration > 6.5 mmol/L (90 mg/dL) with AKI or CKD
- Concentration > 6.5 mmol/L (90 mg/dL) after IV fluids, sodium bicarbonate, and potassium
- Concentration > 5.8 mmol/L (80 mg/dL) after IV fluids, sodium bicarbonate, and potassium and with AKI or CKD
- Altered mental status
- Respiratory distress or new hypoxemia requiring supplemental oxygen
- pH ≤ 7.2
- Lactate > 10 mmol/L
- pH < 7.2
- Failure of standard supportive measures (IV fluids, sodium bicarbonate)
- Decreased level of consciousness
- Concentration > 5.0 mEq/L
- Concentration > 4.0 mEq/L with AKI or CKD
- Decreased level of consciousness, seizures, or life-threatening dysrhythmias at any lithium concentration
- Estimated time to reach lithium concentration < 1 mEq/L exceeds 36 hours
- Concentration > 1,000 mg/L (6,620 μmol/L)
- Concentration > 700 mg/L (4,630 μmol/L) with altered mental status, metabolic acidosis, or elevated lactate
Based on information from www.extrip-workgroup.org and additional readings. Abbreviations: ADH, alcohol dehydrogenase; AKI, acute kidney injury; CKD, chronic kidney disease; IV, intravenous.
Methanol and Ethylene Glycol
Gastric decontamination is usually not helpful because the absorption of methanol and ethylene glycol in the gastrointestinal tract is so rapid. Intravenous administration of base (sodium bicarbonate) corrects metabolic acidosis and increases methanol’s ionization to formic acid. This promotes its urinary excretion and reduces its penetration into the optic nerve. Treatment guidelines recommend an ADH inhibitor when the serum methanol or ethylene glycol concentration exceeds 20 mg/dL (6 mmol/L of methanol or 3 mmol/L of ethylene glycol). Other indications include a high suspicion of toxic alcohol ingestion with either an osmolal gap greater than 10 mOsm/kg H2O or metabolic acidosis of unknown cause. Intravenous ethanol was the major therapy before FDA approval of fomepizole (4-methylpyrazole) 2 decades ago. It remains an alternative, particularly when fomepizole is not available. It is an effective competitive inhibitor of ADH. The target ethanol concentration is 100 mg/dL (22 mmol/L). Ethanol is generally available and inexpensive but requires compounding by a pharmacist for intravenous (IV) use. The serum ethanol concentration requires careful monitoring, so patients usually require hospitalization in the intensive care unit. It may be difficult to discern the degree of inebriation attributable to the toxic alcohol and to the ethanol used as an antidote.
Fomepizole is a strong inhibitor of ADH with very high enzyme affinity (8,000 times that of ethanol). It is effective at micromolar concentrations, does not have serious side effects, and patients do not need to be monitored in an intensive care unit. The loading dose is 15 mg per kilogram of body weight and the subsequent maintenance dose is 10 mg/kg every 12 hours. Given that the drug may induce its own metabolism by cytochrome P450 enzymes, the maintenance dose is raised to 15 mg/kg every 12 hours after 48 hours. Because hemodialysis removes fomepizole, patients undergoing hemodialysis should receive it immediately after a hemodialysis session.
Both fomepizole and ethanol are effective inhibitors of ADH, but there is no controlled clinical trial directly comparing their outcomes. In the United States, fomepizole is the most common antidote to treat methanol and ethylene glycol poisonings. By contrast, outside the United States, fomepizole is less readily available, and ethanol (either IV or oral) is more frequent. Although fomepizole is more expensive than ethanol, fomepizole results in lower mortality and fewer adverse effects. Oral ethanol is an effective first aid treatment until a poisoned patient can reach a hospital with access to fomepizole or hemodialysis.
Methanol and ethylene glycol both have low molecular weights, high water solubility, low protein binding, and small volumes of distribution. These characteristics favor their rapid removal by extracorporeal toxin removal (ECTR).
The Extracorporeal Treatments in Poisoning Workgroup (EXTRIP, www.extrip-workgroup.org) has guidelines for the use of hemodialysis in the treatment of methanol poisoning. They recommend hemodialysis with severe metabolic acidosis, serum methanol concentrations higher than 50 mg/dL (16 mmol/L), deteriorating vital signs despite supportive care, and AKI or problems with vision. Intermittent hemodialysis with a large (>2.1) surface area dialyzer and high-flux membrane is highly effective in the rapid removal of the toxic alcohols. Continuous kidney replacement therapy (CKRT) removes toxic alcohols more slowly but may be safer for hemodynamically unstable patients. Antidotal treatment of methanol or ethylene glycol intoxication without hemodialysis has been reported without adverse consequences. However, fomepizole (without hemodialysis) extends the half-lives for elimination of methanol and ethylene glycol to as long as 71 hours and 16 hours, respectively (these values are only 2.5 and 2.7 hours, respectively, with hemodialysis). Hemodialysis may shorten the total hospital stay but requires more critical care resources. The comparative costs of the 2 treatment strategies (ADH inhibition with or without hemodialysis) varies according to several factors, including absorbed dose of the toxic alcohol, drug costs, cost of hemodialysis, and room costs. Although controlled studies are not available, treatment in children is similar to that in adults. EXTRIP has reviewed ECTR for ethylene glycol. We expect that a published guideline for ECTR in ethylene glycol poisoning is forthcoming.
Before the advent of fomepizole, any concentration of a toxic alcohols above 50 mg/dL warranted urgent hemodialysis in addition to antidotal treatment. Clinical experience in the 2 decades since the FDA approval of fomepizole has demonstrated that higher hemodialysis thresholds are possible when using fomepizole. This is more clearly true with ethylene glycol, which undergoes renal clearance, but less so for methanol, which has much lower renal clearance and little clearance in exhaled breath. Experts suggest that fomepizole allows a hemodialysis threshold as high as 300 mg/dL for ethylene glycol when acidosis is mild or not present. EXTRIP guidelines indicate a hemodialysis threshold of 70 mg/dL for methanol poisoning treated with fomepizole. Adjunctive treatments may promote conversion of toxic metabolites to less toxic metabolites. For ethylene glycol poisoning, thiamine and magnesium promote conversion of glyoxylate to α-ketoadipate. Pyridoxine (vitamin B6) promotes conversion of glycolate to glycine. For methanol poisoning, high-dose folic acid or folinic acid (1 mg/kg of either) promotes conversion of formic acid to carbon dioxide and water. No prospective trials show the magnitude of these effects.
Toxicity of propylene glycol is low. If hyperosmolality and lactic acidosis occur, discontinuation of the medication containing propylene glycol and administration of IV fluids are generally sufficient treatment. Fomepizole is generally unnecessary. Hemodialysis may be helpful in significant lactic acidosis with AKI.
Most experts recommend fomepizole. However, because AKI in patients with this poisoning is often severe enough to produce marked kidney failure, intermittent hemodialysis along with administration of fomepizole is often necessary.
Generally, supportive measures are sufficient. Intermittent hemodialysis has been recommended when the serum isopropanol concentration is ≥500 mg/dL (83 mmol/L), or if hypotension is present or severe lactic acidosis develops. In contrast to the other toxic alcohols, alcohol dehydrogenase inhibitors are unnecessary.
Review of Case 1
Returning to question 1, (e) is the best answer. Correction of acidemia and inhibition of ADH are both early critical actions. This is true for both ethylene glycol and methanol. It has been suggested that provision of sdoium bicarbonate will increase the urinary excretion of formate and glycolate. Timely ethylene glycol concentrations are seldom available, so critical treatment must precede diagnostic certainty. The garage in the example may have antifreeze (ethylene glycol), windshield washer fluid (methanol), or brake fluid (diethylene glycol, glycol ethers).
Case 2: A 48-year-old man arrives by ambulance after a large dose of unknown medication. In the emergency department, he has altered mental status; blood pressure, 110/70 mm Hg; respiratory rate of 30; heart rate of 130, and temperature of 39°C. He complains of hearing a loud “buzzing” sound. His blood chemistry values reveal the following: [Na+], 135 mEq/L; [K+], 3.1 mEq/L; [total CO2], 12 mEq/L; [Cl−], 102 mEq/L; pH 7.40; and Pco2, 18 mm Hg.
Question 2: Which of the items presented above is most suggestive of salicylate intoxication rather than some other intoxication?
a) Altered mental status
b) Buzzing sound in his ears
For the answer to the question, see the following text.
Salicylate intoxication can be either acute or chronic. Acute salicylate intoxication occurs after ingestion of ≥100 to 150 mg/kg salicylate or ingestion of small amounts of methyl salicylate (as low as 5 mL of oil of wintergreen). Rarely, repeated topical use of topical analgesic cream (up to 30% methyl salicylate) may cause serious poisoning. Headache powders (most commonly used in the southeastern United States) provide a rapidly absorbable form of acetylsalicylic acid with caffeine.
The most common source of salicylate poisoning is acetylsalicylic acid or aspirin (the latter is a trade name in some countries but a generic name in others, including the United States). Acetylsalicylic acid undergoes rapid hydrolysis to salicylate in the gastrointestinal tract, liver, and bloodstream. Acute salicylate intoxication most commonly occurs in adults taking salicylates in a suicide attempt or in children after unintentional exposure. US poison centers record approximately 25,000 salicylate exposures annually. Chronic poisoning is more common in elderly individuals. Preexisting kidney disease or compromise of kidney function produced by the salicylate itself can lead to an increase in blood salicylate concentrations and worsen toxicity.
Salicylates directly stimulate the respiratory center of the medulla, producing an increase in both the rate and depth of respiration resulting in respiratory alkalosis. They also uncouple oxidative phosphorylation and inhibit citric acid cycle dehydrogenases, causing a shift in metabolism due to increased glycolysis. This leads to generation of lactic acid and stimulation of hormone-sensitive lipase, leading to increased lipolysis and increased ketone production. Organic acid transporters in the proximal tubule of the kidney excrete these products, which compete for uric acid excretion. The kidney freely excretes only about 10% of unchanged salicylic acid.
Patients with acute salicylate intoxication can present with confusion, agitation, disorientation that can progress to coma, shortness of breath, and tinnitus (or other hearing disturbances). Physical findings can include hyperventilation, evidence of volume depletion, noncardiogenic pulmonary edema, hematemesis, and petechiae. The latter may result from platelet dysfunction. A chest radiograph can reveal pulmonary opacities. Symptoms and signs can be subtle and insidious and may initially be misattributed to other causes such as sepsis.
When toxicity occurs in individuals taking salicylates for treatment of chronic diseases, there may be no overt evidence of excess ingestion. Agitation, confusion, hallucinations, slurred speech, seizures, and coma appear to be more frequent in those with chronic salicylate poisoning than with acute intoxication. These patients often have a delay in diagnosis of salicylate intoxication. A delay in diagnosis and initiation of therapy may explain, in part, the higher morbidity reported for chronic intoxication than with acute intoxication.
Prominent laboratory abnormalities include acid-base disturbances. In children, there is often an early, transient respiratory alkalosis followed by metabolic acidosis. In adults, approximately 20% will have respiratory alkalosis alone, and 56% will have combined respiratory alkalosis and high anion gap metabolic acidosis. In both groups, the respiratory alkalosis results from stimulation of the respiratory center by salicylate.
Less commonly, a normal anion gap metabolic acidosis can develop due to excretion of sodium and potassium salts in the urine with subsequent retention of chloride. The normal anion gap metabolic acidosis can also result in a false hyperchloremia due to salicylate interference with the measurement of chloride. The latter effect can rarely cause a negative anion gap. Hypokalemia results from increased losses of potassium in the urine due to increased excretion of the organic acid anions, augmented aldosterone concentrations, and increased distal sodium delivery. Measurement of salicylate concentration best confirms the diagnosis. Tests for salicylate are simple and generally available in all clinical laboratories.
Question 3: Which of the following is true?
a) Hemodialysis is usually necessary only if the salicylate concentration is greater than 100 mg/dL.
b) Tachypnea is a sign of respiratory distress, and the patient requires intubation before considering hemodialysis.
c) Hemodialysis should commence as quickly as possible.
d) Hemodialysis is only necessary if a trial of sodium bicarbonate fails to lower the salicylate concentration.
e) Hemodialysis is only necessary if the serum potassium concentration is high in a salicylate-poisoned patient.
For answer to the question, see the following text.
Aggressive volume resuscitation with normal saline or lactated Ringer solution is important as fluid losses are common. However, once euvolemia is achieved, large quantities of fluid to induce forced diuresis is not recommended. Oral activated charcoal reduces further salicylate absorption when given within 1 to 2 hours of ingestion and may be useful beyond 2 hours if persistently high salicylate concentrations suggest the possibility of a gastric bezoar of acetylsalicylic acid.
Salicylic acid is a weak acid (pKa 2.97), and therefore alkalinization of the blood and urine with IV sodium bicarbonate is important. Alkalinizing the serum decreases salicylate concentrations in the central nervous system. Alkalinizing the urine (target urine pH of 7.5 or greater) will increase excretion of salicylate. Increasing urine pH by 1 unit from 6.5 to 7.5 can triple urinary salicylate clearance. Oral bicarbonate should be avoided because it might enhance gastrointestinal absorption. Because bicarbonate administration can exacerbate any systemic alkalemia present, blood gases should be monitored carefully during therapy. Also, alkalemia can worsen any hypocalcemia, making monitoring of ionized calcium important. Salicylate-poisoned patients generally present with mild hypokalemia. Potassium replacement (both IV and orally) should accompany sodium bicarbonate administration. This prevents worsening hypokalemia and facilitates urinary alkalinization.
Hemodialysis is the fastest and most effective method of eliminating salicylate from the body. Salicylate has low molecular weight, low volume of distribution, high water solubility, and limited protein binding. Hemodialysis should occur early when indications are present. Delaying hemodialysis increases mortality. Box 1 shows indications for emergency hemodialysis. Absolute indications for hemodialysis include a salicylate blood concentration of 90 mg/dL (6.5 mmol/L), regardless of the presence of signs or symptoms. Other indications for hemodialysis regardless of concentration include altered mental status, decreased kidney function, or acute respiratory distress. Promptly removing the drug at this stage can limit accumulation in tissue and prevent severe toxic effects. Intermittent hemodialysis is the preferred method, but hemoperfusion and CKRT are acceptable should intermittent hemodialysis not be available or if the patient is hemodynamically unstable. Observation of the patient with close monitoring of serum salicylate concentrations and blood pH are important until the patient has clinically improved and serum salicylate concentration has fallen considerably.
Review of Case 2
Returning to question 2, the best answer is (b) the buzzing sound in his ears (tinnitus). The other findings are typical of salicylate poisoning but overlap with other conditions such as diabetic ketoacidosis, sepsis, methylxanthine poisoning, and alcoholic ketoacidosis. The best answer to question 3 is also (b): hemodialysis should commence as soon as possible because delays in hemodialysis can result in a fatal outcome. Tachypnea, altered mental status, and hyperthermia indicate severe toxicity in this patient.
Case 3: A 29-year-old woman comes to the emergency department after texting a friend that she had ingested “handfuls” of acetaminophen from a container bought earlier in the day at a “big box” retailer. She arrives approximately 6 hours after ingestion. Her blood chemistry values reveal the following: [Na+], 140 mEq/L; [K+] 3.5 mEq/L; [Cl−], 100 mEq/L; [total CO2], 10 mEq/L; [SUN], 25 mg/dL; serum creatinine concentration ([Scr]), 1.5 mg/dL (134 μmol/L); pH 7.21; and Pco2, 26 mm Hg. Her serum acetaminophen concentration is 980 mg/L (6,500 μmol/L). Her serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are 122 and 115 IU/L, respectively.
Question 4: Which one of the following regarding hemodialysis in acetaminophen overdose is true?
a) Acetaminophen has a high protein binding in the plasma and is not easily dialyzable.
b) Hemodialysis is only indicated if antidotal treatment with acetylcysteine fails to reduce the acetaminophen concentration.
c) Indications for hemodialysis include serum acetaminophen concentration > 900 mg/L (6,000 μmol/L) or the presence of severe metabolic acidosis.
d) AST activity greater than 1,000 IU/L is an indication for hemodialysis.
For answer to the question, see the following text.
Acetaminophen (paracetamol) is the most frequent pharmaceutical agent involved in human poisonings. It accounts for approximately 5% of the over 100,000 cases reported annually to US poison centers and remains a leading cause of poisoning death (overshadowed only by street drugs such as heroin or illicit fentanyl).
The principal toxic effects occur in the liver. Overdoses of acetaminophen saturate the glucuronide and sulfate conjugation pathways, and a larger fraction of the drug undergoes oxidation (mainly by cytochrome P450 2E1 isozyme [CYP2E1]) to N-acetyl parabenzoquinonimine (NAPQI). NAPQI is a potent oxidant that binds sulfhydryl (-SH) groups on intracellular proteins and denatures them, causing hepatocellular injury. Glutathione, an antioxidant sulfhydryl donor, binds NAPQI to prevent cellular injury. This process consumes glutathione, and hepatotoxicity results when NAPQI depletes available glutathione.
Three aspects of acetaminophen poisoning may come to the attention of a nephrologist. The first is severe metabolic acidosis. Extremely large overdoses (eg, >30 g) cause early metabolic acidosis due to mitochondrial dysfunction and impaired oxidative phosphorylation developing well before the hepatic transaminase activities reach their peaks. These cases involve acetaminophen concentrations above 700 mg/L (4,600 μmol/L).
The second is AKI after acute acetaminophen overdose. This occurs in approximately 2% of all acetaminophen overdoses but may occur more frequently in severe overdoses. In most cases, Scr slowly rises to a peak within 1 week of the overdose. Scr gradually returns to normal; patients seldom require hemodialysis.
The third and rarest is high anion gap metabolic acidosis due to 5-oxoproline (pyroglutamic) in a small number of patients with chronic acetaminophen exposure (including therapeutic doses). The actual prevalence of this complication is unknown. Formation of 5-oxoproline reflects disordered glutathione production and metabolism. Most of these cases likely reflect pyroglutamic acidosis due to inherited enzyme deficiencies or malnutrition. In case of unexplained high anion gap metabolic acidosis (HAGMA) with concern for 5-oxoproline, a reference laboratory that screens newborns for inherited metabolic defects can detect and measure 5-oxoproline. Although some authors have suggested using N-acetylcysteine (NAC) for this problem, supportive care and discontinuing acetaminophen exposure are likely the mainstays of treatment.
On the first day after an acetaminophen ingestion, the patient may have nausea and abdominal pain or may be asymptomatic. Rising AST and ALT activities will become apparent on day 2, with peak values around day 3. The international normalized ratio (INR) may rise about 1 day after the rise in AST and ALT. Peak toxicity occurs around day 3 or 4. Severe cases may have hepatic encephalopathy and cerebral edema. AKI with acute tubular necrosis may appear. Following this, patients will either recover or die (unless they receive a liver transplant).
Kings College Criteria for liver transplantation include a blood pH < 7.30 at any time or a composite of Scr > 3.3 mg/dL, prothrombin time >100 seconds, and severe (grade III or IV) hepatic encephalopathy. Lactic acidosis and hypoglycemia are sensitive indicators of severe hepatotoxicity. Blood lactate concentrations above 3.0 mmol/L on initial assessment or above 2.5 mmol/L after fluid resuscitation are highly sensitive for acute liver failure.
Because there are no early signs or symptoms that reliably indicate toxicity, the diagnosis depends upon the serum acetaminophen concentration. The Rumack-Matthew nomogram determines the risk of hepatotoxicity by plotting serum acetaminophen concentration versus time after ingestion for any single serum acetaminophen concentration drawn at least 4 hours after ingestion. If the acetaminophen concentration exceeds the treatment line starting at 150 mg/L (993 μmol/L) at 4 hours, the patient should receive antidotal NAC either IV as Acetadote or orally as Mucomyst.
The main therapeutic measures are supportive care and the administration of NAC, which as a sulfhydryl donor directly reduces NAPQI and repletes glutathione.
Severe cases with profound metabolic acidosis and very high acetaminophen concentrations (exceeding 700 mg/L or 4,630 μmol/L) warrant hemodialysis. Acetaminophen is water soluble and has a low molecular weight (151.2 Da), low protein binding, and a low volume of distribution (0.9 L/kg). Hemodialysis removes acetaminophen and corrects acidosis. Intermittent hemodialysis is the preferred modality, but CKRT is acceptable if the patient is hemodynamically unstable or if hemodialysis is unavailable.
Fomepizole, the ADH antagonist used in toxic alcohol poisoning, also blocks CYP2E1, the main enzyme that oxidizes acetaminophen to NAPQI. Case reports and case series suggest that fomepizole may be useful in patients with extremely high acetaminophen concentrations (>700 mg/L or >4,630 μmol/L) with metabolic acidosis.
Review of Case 3
The best answer for question 4 is (c), the indications for hemodialysis include serum acetaminophen concentration > 900 mg/L or the presence of severe metabolic acidosis. One of the major indications for initiation of hemodialysis is a markedly elevated serum concentration of acetaminophen or the presence of a severe metabolic acidosis.
Case 4: A 63-year-old man comes to the emergency department reporting a headache. The patient has a history of type 2 diabetes mellitus for which he takes metformin and small doses of insulin. He is awake and responsive. His blood pressure is 110/70 mm Hg without orthostatic changes. His blood chemistry values reveal the following: [Na+], 138 mEq/L; [K+], 3.0 mEq/L; [total CO2], 8 mEq/L; [Cl−], 100 mEq/L; [SUN], 25 mg/dL; [Scr], 2.5 mg/dL; pH, 7.15; and Pco2, 24 mm Hg. His initial lactate concentration is 15 mmol/L. A point-of-care ketone measurement (β -hydroxybutyrate) is 0.5 mmol/L.
Question 5: Which of the following statements is true?
a) This patient has diabetic ketoacidosis and requires insulin in addition to IV fluid resuscitation.
b) Lactic acidosis only occurs in patients with metformin overdose.
c) Lactic acidosis can develop in patients taking therapeutic doses of metformin.
d) Hemodialysis does not remove metformin.
For the answer to the question, see the following text.
Metformin is one of the most frequently prescribed diabetic medication in the world and is first-line therapy in the treatment of type 2 diabetes mellitus. Complications appearing in as many as 20% to 30% of patients are not life threatening and usually include nausea, vomiting, and decreased appetite. Metformin-associated lactic acidosis (MALA) occurs in 3 to 10 cases per 100,000 patient-years. Mortality can be as high as 61% in some cases. Predisposing conditions include hepatic and acute or chronic kidney disease. For this reason, metformin is contraindicated in patients with an estimated glomerular filtration rate (eGFR) of ≤30 mL/min/1.73 m2 (CKD 4 or 5). Also, clinicians should be cautious in administering metformin to individuals with an eGFR ≤45 mL/min/1.73 m2 (CKD 3b) or who have potentially unstable kidney function.
Metformin inhibits glycerol-3-phosphate dehydrogenase and the glycerophosphate shuttle. This decreases the mitochondrial redox state and increases the cytosolic redox state, reducing conversion of lactate to pyruvate in the cytosol. Also, inhibition of the mitochondrial respiratory chain complex in peripheral tissues augments lactic acid production.
Gastrointestinal symptoms are common in patients with metformin-related lactic acidosis, but nonspecific signs may dominate. In severe cases, patients may have serious hemodynamic instability and depressed consciousness. Abdominal tenderness may mimic an acute abdomen. Laboratory findings include elevated lactate concentrations (>5 mmol/L) and acidemia. Because hepatic and kidney disease are predisposing factors, it is common to find evidence of both kidney and liver disease.
Treatment includes sodium bicarbonate to treat the acidemia and supportive therapy to stabilize the blood pressure. Mortality approaches 40% with high blood concentrations (above 50 mg/L or 388 μmol/L), but most clinical laboratories cannot measure metformin concentrations. Early hemodialysis is the most effective therapy to remove metformin and to correct the acidosis. Box 1 shows indications for emergency hemodialysis. Intermittent hemodialysis is effective in hemodynamically stable patients with a drug clearance of 200 mL/min. CKRT is acceptable in hemodynamically challenged patients.
Review of Case 4
The best answer to question 5 is (c), lactic acidosis occurs most often among patients taking therapeutic doses of metformin. Lactic acidosis does not require acute overdose of metformin. Hemodialysis removes metformin and corrects acidosis.
Case 5: A 55-year-old woman arrives to the emergency department via the emergency medical service after neighbors found her with confusion. She had been well until she developed an acute diarrheal illness for the past 2 days. Her basic metabolic panel reveals AKI with a [Scr] of 2.4 mg/dL (compared with 1.2 mg/dL 1 month earlier). Her anion gap is 6 mmol/L (compared with 10 mmol/L 1 month earlier). Further history reveals that she takes lithium carbonate for bipolar disorder and that her psychiatrist increased her dose 1 month earlier.
Question 6: Which of the following statements are true?
a) Forced diuresis with normal saline and IV furosemide is the appropriate treatment.
b) Sodium polystyrene resin (Kayexalate) is effective in removing lithium.
c) The indication for hemodialysis depends solely on the serum concentration of lithium.
d) Central nervous system dysfunction is an indication for hemodialysis regardless of the lithium concentration.
For the answer to the question, see the following text.
Lithium salts were first shown to have benefit in mania and bipolar disorder in 1949 in Australia. Subsequent studies in the 1950s to the 1970s corroborated its use. The exact mechanism of action remains incompletely understood but may involve inositol metabolism or modulation of serotonin release. Lithium (usually as lithium carbonate) has a very narrow therapeutic range (serum lithium concentration [Li+] usually between 0.6 and 1.3 mmol/L) and is sensitive to modest changes in kidney function. Acute-on-chronic lithium toxicity most often results from AKI from other causes (such as dehydration from diarrhea) or from rapid escalation of the dose. Acute overdose can rapidly produce high lithium concentrations. Hemodialysis effectively clears lithium, which has a molecular weight of 7 Da. Its volume of distribution is near 1 L/kg. Due to high intracellular concentrations, [Li+] often rebounds after hemodialysis due to redistribution from the intracellular space.
Indications for hemodialysis depend upon the [Li+], kidney function, and neurological symptoms. The EXTRIP Workgroup recommends hemodialysis when [Li+] > 4.0 mEq/L or if the patient has a decreased level of consciousness, seizures, or life-threatening dysrhythmias, regardless of the [Li+]. CKRT is an acceptable alternative if hemodialysis is unavailable or inadvisable.
When measuring [Li+], using the wrong tube produces an unexpectedly high apparent concentration. Green-top tubes (both Kelly green and mint green) contain lithium heparin. The heparin prevents coagulation of blood in the analyzer. Heparin has many negative charges and requires cations to neutralize the charge without interfering with chemistry measurements. Because lithium is an unmeasured cation in the basic metabolic panel, it is the cation used (instead of sodium) to neutralize the charge on heparin in the tubes. If the laboratory uses a green-top tube to measure lithium, the lithium heparin will produce a factitious elevation in the apparent [Li+]. The magnitude of the apparent [Li+] varies inversely with the amount of blood in the tube, but the range of concentrations overlaps the toxic range. Blood specimens for lithium measurement should go in red-top tubes.
Returning to question 6, (d) is the best answer. Central nervous system dysfunction is an indication for hemodialysis regardless of [Li+]. This is one EXTRIP criterion for hemodialysis. Other EXTRIP criteria include [Li+] > 5.0 mmol/L, [Li+] > 4.0 with AKI, estimated time to [Li+] < 1.0 mmol/L exceeds 36 hours, confusion, seizure, or cardiac arrhythmia.