USA Surgery Today

Although not directly related to fluid and electrolyte management, acidв base disorders are important to consider in the care of surgical infants and children because shifts in hydrogen or bicarbonate will effect distribution and total body content of the major electrolytes. Acid base disorders are termed either metabolic or respiratory, based on the pathogenesis of the imbalance. Metabolic acidosis is either from excess acid production or administration, or renal losses of bicarbonate. Metabolic alkalosis is usually from volume contraction or loss of hydrogen ions. Respiratory acidosis results from carbon dioxide retention, whereas alkalosis is the consequence of hyperventilation. Disorders are termed simple if there is only one primary disorder, or mixed if two or more are involved. The body attempts to correct metabolic disorders in the short term by altering respiration; however, respiratory defects can only be compensated by metabolic processes over longer periods of time. Compensatory mechanisms never overcorrect the primary derangement.

Metabolic acidosis occurs when exogenous acid is administered, endogenous acids are produced, or bicarbonate is lost in either gastrointestinal fluid or in the urine. It is defined by a plasma pH less than 7.35. The body attempts to compensate by increasing minute ventilation in an effort to lower dissolved carbon dioxide content. There are two major types of metabolic acidosis, anion gap and nonanion gap acidosis. The anion gap is the difference between unmeasured cations and anions and is usually around 8 to 16 meq per L. It is estimated by the formula:

Anion gap (meq/L) = sodium concentration - (chloride + bicarbonate concentrations)

When the calculated gap is in the normal range, it is termed nonanion gap acidosis, which is usually related to bicarbonate loss. If the anion gap is above the normal range, it is due to excess acid production or administration.

The most common nonanion gap acidosis in children is associated with profuse diarrhea. The stool contains large amounts of bicarbonate and potassium. Excessive bicarbonate losses may also be encountered with high-output fistulae or stomas, especially pancreatic or biliary fistulae. Diversion of the urinary stream to bowel in either ileal conduits or bladder augmentations with bowel may result in nonanion gap acidosis as bicarbonate in the mucosa exchanges with chloride in the urine. The other major abnormalities associated with bicarbonate loss and nonanion gap acidosis result from renal tubule dysfunction and are termed renal tubular acidosis.

There are three types of clinically significant renal tubular acidosis (RTA) based on etiology and pathophysiology. Type I (distal) RTA is idiopathic and results from inability of the distal tubule to secrete hydrogen ions. Bicarbonate wasting is seen during rapid growth phases, but not universally. The urine pH is generally above 6.0 in mild or severe cases. Untreated children develop hypokalemia, hyperchloremia, hypercalciuria, and sometimes nephrocalicnosis. Type II or proximal RTA is characterized by bicarbonate wasting in the urine. The urine pH is greater than 6.0 in mild cases. In severe cases the urine pH drops below 5.5. Carbonic anhydrase inhibition or deficiency is the proposed derangement in this circumstance. Hypokalemia is almost always seen with type II RTA, and patients receive large amounts of potassium and bicarbonate to correct the metabolic disorders. The third clinically relevant RTA is type IV or hyperkalemic RTA. It is characterized by aldosterone resistance in the distal tubules, and thus potassium and hydrogen are resorbed instead of sodium. Patient who suffer from type IV RTA are usually hyponatremic and volume depleted despite increases in renin and aldosterone activity. Daily sodium chloride replacement is the mainstay of therapy and some patients may have spontaneous resolution in early childhood.

Increased anion-gap acidosis is usually associated with normal chloride levels and is secondary to overproduction of endogenous acids, ingestion of acids, or decreased clearance of acids as part of renal failure. The most commonly produced endogenous acids are ketoacids in either insulin-dependent diabetes or the inborn errors of metabolism and glycogen storage diseases. Treatment of the underlying condition in these disorders will correct the acidosis. Lactic acidosis is of particular importance in surgical practice and can result from poor tissue perfusion, especially the gut. Lactic acidosis per se is not correlated with high morbidity or mortality; however, the underlying cause of lactic acidosis may carry an extremely high morbidity and mortality. Again, therapy must be directed at correction of underlying reason for lactate production. This generally requires excision of dead tissue and/or improvement in tissue perfusion.

Ingestion of toxic materials and the resultant metabolic acidosis is relatively common in young children. Aspirin (salicylic acid) overdose initially causes stimulation of the respiratory centers and a respiratory alkalosis. Later in the clinical course, buffering with bicarbonate ensues and ketoacidosis and lactic acidosis follow. Nausea and vomiting either induced by the overdose or from the treatment may complicate the condition. Methanol and ethylene glycol (antifreeze) are metabolized by the liver to form organic acids and their accumulation may be lethal. Ethylene glycol is converted to glycolic acid, which may be metabolized to oxalate and result in crystal deposition in the kidney, brain, heart, and lungs, leading to organ failure and death. Gastric lavage followed by ethanol infusion is the recommended therapy. Ethanol is a competitive inhibitor for both methanol and ethylene glycol, and allows time for hemodialysis to remove the toxins. Hemodialysis is also used for chronic maintenance therapy in patients with renal failure. Hydrogen ion generated during normal metabolic processes cannot be excreted and an anion gap acidosis develops.

Metabolic alkalosis is defined by a loss of hydrogen ions, a net gain of bicarbonate or by chloride loss in excess of bicarbonate loss. Persistent emesis (as in pyloric stenosis) is the most common cause of metabolic alkalosis in children, although any condition with prolonged nasogastric suction or loss of gastric fluids may result in a hypochloremic metabolic alkalosis. Volume depletion as a result of gastric losses induces renal sodium absorption in exchange for first potassium and then hydrogen as serum potassium falls. A paradoxical aciduria ensues that may worsen or at least perpetuate the alkalosis. Diuretics promote sodium and chloride excretion. When given in excess, they can lead to volume contraction that stimulates the secretion of aldosterone and subsequent sodium retention in exchange for potassium and hydrogen. Similar to pyloric stenosis, the end result is hypokalemic metabolic alkalosis. Adrenal adenomas or hyperplasia may result in increased aldosterone secretion and induce a hypokalemic metabolic alkalosis in euvolemic or hypervolemic patients.

Determination of urinary chloride concentration may be very useful in determining the cause of metabolic alkalosis if no obvious explanation is evident. Chloride levels less than 10 meq per L in the urine indicate maximal tubular resorption, meaning that the likely source of the alkalosis is chloride loss from a gastrointestinal source in this circumstance. The optimal therapy in these conditions is volume expansion, usually with isotonic saline. Urinary chloride levels greater than 20 meq per L usually indicate a pathogenesis other than chloride loss and volume contraction, and thus saline administration will not correct the alkalosis. Other causes must be sought and corrected for the metabolic alkalosis to improve, especially in cases of extreme hypokalemia.

Under normal circumstances, respiratory compensation is attempted by the body to correct any metabolic acid base disturbance. However, respiratory imbalances can exacerbate metabolic disorders and produce life-threatening acidosis or alkalosis. Respiratory acidosis occurs when there is insufficient minute ventilation and carbon dioxide retention occurs. Any ailment that decreases alveolar ventilation (i.e., pneumonia, airway obstruction, bronchiolitis, cystic fibrosis, narcotic overdose) may result in respiratory acidosis. Overfeeding with carbohydrates may generate enough carbon dioxide to overwhelm the pulmonary ventilatory capacity and may promote respiratory acidosis. An increase in plasma carbon dioxide content is initially buffered by intracellular systems. Thereafter, renal reabsorption of bicarbonate begins and attempts to correct the acidosis. However, with chronic respiratory acidosis, serum bicarbonate levels may reach 40 meq per L. Treatment is directed to the underlying pulmonary condition, unless narcotic overdose or carbohydrate administration are involved.

Respiratory alkalosis is usually related to hyperventilation. Common causes include anxiety, fever, sepsis, primary CNS abnormalities, and mechanical ventilation. Acute changes in altitude initially may cause hyperventilation, but after chronic compensation by the kidney, people living at high altitude have normal acid–base physiology. As with respiratory acidosis, initial buffering by intracellular proteins gives way to renal compensatory mechanisms after 24 to 48 hours. Obviously, the mainstay of therapy is correction of the underlying disorder.