Hyperchloremic Acidosis

Etiology

The human body is very good at remaining balanced ionically under most scenarios. As a result, with loss of bicarbonate (the negatively charged ion), the negatively charged chloride (Cl) ion is displaced to the extracellular space. This leads to a narrow anion gap, an electrically neutral state without correcting the pathology that induced the acidosis. Likewise, increased Cl may displace bicarbonate intracellularly. To determine the exact etiology of a narrow anion gap, hyperchloremic acidosis requires another test, the urine anion gap. The urine anion gap is calculated as follows:

• Urine anion gap = (Na + K) - Cl

Where Na is urine sodium, K is urine potassium, and Cl is urine chloride. The urine anion gap provides an estimate of urinary ammonium (NH4) excretion. The normal renal response to metabolic acidosis is to increase acidic NH4 excretion renally. Therefore, a positive urine anion gap between 20 and 90 mEq/L is indicative of low or normal NH4 excretion, seen in renal causes, such as distal renal tubular acidosis. A negative urine anion gap between -20 and -50 mEq/L is indicative of increased NH4 excretion. This occurs in patients with metabolic acidosis generated by extrarenal causes, such as profuse watery diarrhea. A urine anion gap approaching 0 is indeterminate.[4]

Epidemiology

The prevalence of hyperchloremic acidosis is unknown. The exact frequency and distribution of disease are dependent upon the etiology. Likewise, the morbidity and mortality rates are dependent on the etiology of the disease.

Pathophysiology

Hyperchloremic metabolic acidosis is a pathological state that results from bicarbonate loss, rather than acid production or retention. Bicarbonate loss leading to hyperchloremic metabolic acidosis occurs in a variety of ways: gastrointestinal (GI) causes, renal causes, and exogenous causes. GI loss of bicarbonate occurs through severe diarrhea, pancreatic fistula, nasojejunal tube suctioning from the duodenum, and chronic laxative use. Renal sources of hyperchloremic acidosis include proximal renal tubular acidosis, distal renal tubular acidosis, and long-term use of carbonic anhydrase inhibitors. Exogenous causes include the ingestion of acids such as ammonium chloride and hydrochloric acid and volume resuscitation with 0.9% saline.[5][6][7][8][9]

Gastrointestinal Causes

Normally, there is a degree of bicarbonate secreted into the intestinal lumen to allow for neutralization of the acidic environment of food from gastric emptying. Over the distance of the small intestines, this bicarbonate is reabsorbed as bile. However, in pathologies with profuse watery diarrhea, bicarbonate within the intestines is lost through the stool due to increased motility of the gut. This leads to further secretion of bicarbonate from the pancreas and intestinal mucosa, leading to net acidification of the blood from bicarbonate loss. Likewise, pancreatic fistula leads to excessive bicarbonate secretion from the pancreas into the intestines. This excess bicarbonate is ultimately lost in stools. Nasojejunal suctioning removes bicarbonate from the duodenal or jejunal space via direct suctioning of the luminal contents. The overarching theme with these pathologies is the loss of bicarbonate from the gastrointestinal spaces, which leads to an acidotic state in the blood via unopposed hydrogen in the buffering system as above.

Renal Causes

Distal renal tubular acidosis (type 1) is a failure of the distal nephron to secrete hydrogen appropriately into the urine. This results in alkalotic urine and acidosis of the blood. Failure to secrete hydrogen directly correlates with the NH4 levels in urine and is able to be deduced via a positive urine anion gap as above. Proximal renal tubular acidosis (type 2) is a pathology where bicarbonate is failed to be reabsorbed appropriately. This leads to the loss of bicarbonate in the urine. The net result is acidosis of blood and alkalotic urine. Both types of renal tubular acidosis are associated with hypokalemia. Carbonic anhydrase inhibitors such as acetazolamide create a medically induced type 2 proximal renal tubular acidosis scenario by inhibiting bicarbonate reabsorption in the proximal nephron.

Exogenous Causes

Many of the exogenous causes of hyperchloremic acidosis are logical evaluations. When substances such as ammonium chloride and hydrochloric acid are supplemented into the body, they react with bicarbonate in an attempt to buffer the pH. However, this will deplete bicarbonate stores leading to an acidotic state. Large volume resuscitation with normal saline leads to an overload of chloride ions into the blood. As stated previously, chloride and bicarbonate work together to maintain an ionic balance of the cellular space. Hyperchlorhydria forces bicarbonate to move intracellularly to maintain ionic equilibrium, thus reducing the available bicarbonate for the pH buffering system leading to net acidosis.

History and Physical

Patients with hyperchloremic acidosis have no effects due to the hyperchloremia necessarily. However, acidosis can have many poor health effects. A headache, lack of energy, nausea, and vomiting are common complaints, however as acidosis worsens, stupor, coma, myocardial instability, or arrest may occur. It is expected to see an increase in respiratory rate as the body attempts to decrease carbon dioxide in compensation; however, in long-standing disease, this may lead to muscle fatigue and respiratory failure.

A physical exam may show altered mental status, tachycardia, tachypnea, accessory muscle use with respiration, neurological deficits, muscular weakness, cardiac arrhythmias, cardiac murmurs, respiratory wheezing, rales, or rhonchi.

Evaluation

As with any illness, a detailed history and physical exam is the most important initial step in evaluation. Hyperchloremic acidosis due to gastrointestinal bicarbonate loss or medication usage is apparent easily. A complete blood count (CBC) to evaluate for an infectious cause with elevated white blood count and fluid body status with hemoglobin and hematocrit values is useful. A complete metabolic panel is important with attention to sodium, potassium, and chloride levels as these can be used to calculate the anion gap value. Arterial blood gas measurement is needed to determine pH status and to identify that the acidosis is metabolic in origin. The urinary anion gap is an essential measurement in hyperchloremic acidosis to establish the urine ammonium excretion status, as discussed above. Distal renal tubular acidosis will have urinary pH greater than 5.3 and a positive urinary anion gap. In proximal renal tubular acidosis, urinary pH is usually less than 5.3, and the urinary anion gap is variable.[10]

Treatment / Management

In every case of hyperchloremic acidosis, the primary treatment is aimed at identifying and treating the inciting event of pathology. If respiratory fatigue and failure occur, these patients will need to be intubated and placed on mechanical ventilation. Hyperventilation of the patient on ventilator control can help reduce the acid load. In gastrointestinal causes, it is essential to administer intravenous (IV) normal saline to maintain fluid load as patients will easily dehydrate from diarrhea or suctioning of the intestines. Additionally, electrolytes need to be monitored and replenished as applicable. Of specific importance is the potassium level. The acidosis is moderated by supplementing bicarbonate into the saline fluids until the underlying pathology is repaired. In proximal renal tubular acidosis, large quantities of bicarbonate, vitamin D, and potassium administration are required. The usual dose of bicarbonate is 5-15 mE/kg/day. In hypokalemic distal renal tubular acidosis long-term alkali administration to counterbalance bicarbonaturia and endogenous acid production. Potassium supplements are also needed. In hyperkalemic distal renal tubular acidosis, obstructive uropathy must be identified. If fluid overload is a concern, diuretics with supplemental potassium may be administered for some effect. If the acidosis is resistant to therapy, it may be necessary to utilize dialysis therapy.[11][12][13][14](B3)

As always, a variety of medications are known to induce hyperchloremic acidosis and should be avoided or used with caution. Gastrointestinal bicarbonate loss is known to occur with calcium chloride, magnesium sulfate, and cholestyramine use. Proximal renal tubular acidosis is associated with streptozotocin, lead, mercury, arginine, valproic acid, gentamicin, ifosfamide, and outdated tetracycline usage. Distal renal tubular acidosis is associated with amphotericin B, toluene, nonsteroidal anti-inflammatory drugs, and lithium use.[15]

Differential Diagnosis

Conditions that may present with features similar to hyperchloremic acidosis include the following:

• Non-anion gap metabolic acidosis
• Vitamin D deficiency
• Renal tubular acidosis
• Vitamin D resistance
• Chronic diarrhea or other extrarenal loss of bicarbonate
• Monoclonal gammopathy and myeloma
• Secondary hyperparathyroidism
• Chronic hypocalcemia
• Lowe syndrome
• Sickle cell disease
• Obstructive uropathy
• Fabry disease
• Metachromatic leukodystrophy
• Methylmalonic acidemia

Prognosis

The prognosis of hypochloremic metabolic acidosis depends on the underlying cause of the condition. When the underlying cause is well managed, the prognosis is usually very good. Hyperchloremic metabolic acidosis can be detrimental in old age and those who already have a cardiopulmonary compromise.

Complications

If hyperchloremic acidosis is not corrected, acidemia can cause arrhythmias which can prove fatal and increase mortality. pH less than 7.2 can impair myocardial contraction and increase the risk of ventricular fibrillation and heart failure. Tachypnea results in compensation of acidosis can cause respiratory muscle fatigue and if not corrected promptly can lead to respiratory arrest.