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Common Problems Solving:
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A. Note symptoms of weakness, lethargy, and vomiting. Identify predisposing factors or conditions such as diuretic therapy, steroid therapy persistent vomiting (pyloric stenosis), and renal disorders. Treatment of diabetic ketoacidosis may lead to hypokalemia because of bicarbonate therapy and loss of potassium secondary to an osmotic diuresis. An inadequate potassium concentration in intravenous fluids also produces hypokalemia.
B. Note vital signs, especially blood pressure. The presence of a moon face, purplish striae, and central adiposity suggests Cushing’s syndrome or prolonged steroid therapy.
C. ECG findings of hypokalemia include flattened T-waves, depressed ST segments, and a prominent U wave.
D. Plasma renin activity is suppressed by primary mineralocorticoid excess (adrenal hyperactivity). Obtain a 24-hour urine collection and test for aldosterone (serum aldosterone level is fluctuate widely during the day). Clinical conditions characterized by suppressed rerun activity without an elevation in daily aldosterone production included inborn errors of metabolism within the adrenal gland such as 11- or 17-hydroxylase deficiency. These inborn errors and Cushing’s syndrome result in increased levels of mineralocorticoids which may produce hypokalemia.
E. Clinical signs of adrenal excess (Cushing’s syndrome) have different etiologies. Bilateral adrenal hyperplasia caused by inappropriate secretion of adrenocorticotropic hormone (ACTH) by the pituitary is Cushing’s disease. Possible treatments of Cushing’s disease include pituitary irradiation transsphenoidal hypophysectormy, and the use of an antipituitary drug (cyproheptadine) or, alternatively, drugs that are toxic to the adrenal gland. Other causes of Cushing’s syndrome include excessive ingestion of steroid hormones, ectopic production of ACTH by tumors such as pheochromocytoma, neuroblastoma, pancreatic tumors, and Wtlms’ tumors, and tumors of the adrenal gland (carcinomas or adenomas). Consider total adrenalectomy when Cushing’s syndrome is caused by an adrenal carcinoma or adenoma or only when all other more conservative treatments are ineffective.
F. Congenital adrenal hyperplasia due to either 11- or 17-h~droxylase deficiency differs from 21-hydroxylase deficiency in that mineralocorticoid synthesis is not impaired. Treat with glucocorticoid replacement. Boys and girls with the latter disorder require testosterone or estrogen replacement respectively, during the pubertal and postpubertal periods because the 17-hydroxylase enzyme is necessary for the synthesis of androgen and estrogens.
G. Plasma renin activity is high in certain renal disorders. A defect in chloride absorption (Bartter’s syndrome) produces a hypokalemic metabolic alkalosis that is associated with a normal serum sodium and blood pressure. Elevated plasma renin activity, hypokalemia, and hypertension are characteristic of renal vascular disease and tumors of the juxtaglomerular cells. The work-up for these disorders includes renal scan and digital subtraction angiography.
H. Treat Banter’s syndrome with a sodium-wasting/ potassium-retaining diuretic such as spironolactone and the supplemental use of potassium and magnesium as their respective chloride salts. This regimen blocks endogenous aldosterone action and restores electrolyte balances in potassium, magnesium, and chloride. Success has been reported recently with the use of a prostaglandin synthesis-inhibitor, indomethacin.
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METABOLIC ACIDOSIS
Metabolic acidosis results in a decrease in the buffering capacity (sodium bicarbonate) of the blood; this decrease is produced either by an abnormal increase in circulating acid (endogenous or exogenous) or by loss of bicarbonate from the gastrointestinal tract or kidneys. The kidneys are the major sites for the regulation of buffer capacity. The lungs also alter pH by determining the clearance of carbon dioxide, an end product of normal metabolism. Acidosis associated with the diminished clearance of carbon dioxide from the lungs is called respiratory acidosis.
A. Note the presence of vomiting, diarrhea, failure to thrive, polyuria, fever, or altered neurologic status (seizures). Identify predisposing factors such as renal disease, diabetes mellitus, diarrhea and vomiting, hypothermia, toxin ingestion, known hypoglycemic syndrome, or inborn errors of metabolism. Ask about family history of neonatal death or metabolic disorders.
B. Note the patient’s vital signs; assess the circulatory status. Note signs consistent with respiratory distress, sepsis, or a CNS disorder.
C. Calculate the anion gap by determining the difference between the serum sodium concentration and the sum of the serum chloride and bicarbonate concentration. Since the sums of all serum anions and cations are equal because of the electroneutrality of the body, the anion gap represents the difference between the unmeasured serum anions (anions except chloride and bicarbonate) and unmeasured serum cations (cations except sodium). The normal value is usually 12 mEq/L ±4. Normal anion gaps are generally seen in situations in which sodium and bicarbonate are lost from the body, e.g., in diarrhea or through the kidney in proximal renal tubular acidosis (type II). An increase in the calculated anion gap is frequently seen with increased acid production by the body, such as in inborn errors of metabolism or diabetic ketoacidosis, or with the ingestion of acid-producing toxins, such as methanol and ethylene glycol. Aspirin intoxication is associated with a variable anion gap, due to the accumulation of organic acids, secondary to aspirin’s effect on intermediary metabolism. A normal calculated anion gap may be present in the early stages of any of the conditions listed that traditionally have an increased anion gap; therefore, the clinician should be aware of the vagaries in using the anion gap alone in the differential diagnosis of metabolic acidosis. Periodic reassessment of the patient’s electrolyte status enables the clinician to recalculate the anion gap if the diagnosis proves elusive.
D. Hypoglycemia, positive urinary ketones, and an elevated anion gap suggest a disorder of glycogenolysis or gluconeogenesis. Excessive ketone production is due to the utilization of fat stores (to provide energy in the
absence of glucose) in excess of the liver’s ability to metabolize the breakdown products of fatty acid oxidation. Patients with type I glycogen storage disease also produce excess lactic and uric acid that contribute to the metabolic acidosis. Premature infants may develop metabolic acidosis when they receive an acid load that exceeds their renal capacity (excessive phosphate or protein from cow’s milk).
E. Abnormal renal function produces metabolic acidosis because of impaired acid excretion and/or inability to retain sodium bicarbonate. Renal tubular disease results in acidosis due to the failure to reabsorb sodium bicarbonate or excrete acid in normal amounts. Renal tubular disorders include Fanconi’s syndrome and proximal and distal renal tubular acidoses. In proximal renal tubular acidosis there is a lowered threshold for the reabsorption of bicarbonate; patients can acidify their urine only when plasma bicarbonate falls below their lowered renal threshold (approximately 15 mEqIL). While the exact mode of inheritance is unknown, it appears to have a genetic component. In distal renal tubular acidosis, the kidney is unable to excrete an acid urine at any plasma bicarbonate concentration. Impaired distal tubular hydrogen ion excretion results in excessive urinary loss of both potassium and phosphorus. This leads to hypokalemic acidosis. Distal renal tubular acidosis also may result in mild to moderate antidiuretic hormone unresponsiveness at the renal tubule; the subsequent polyuria accentuates both hypokalemia and hypophosphatemia.
F. Inborn errors of metabolism that can produce a metabolic acidosis include deficiencies in pyruvate dehydrogenase enzyme complex and enzyme defects between pyruvate and phosphoenol-pyruvate, leading to lactic acidosis. Lactic acidosis may also be a nonspecific finding related to long periods of hypoxia, producing anaerobic tissue metabolism. Other inborn errors of metabolism associated with metabolic acidosis are errors in amino acid oxidation (e.g., maple syrup urine disease) and disorders of fatty acid oxidation, which may be secondary to carnitine deficiency or the absence of specific enzymes crucial to beta oxidation of fatty acids. In these latter two groups of conditions, ketosis is generally absent and the determination of organic acids in the urine (qualitatively and quantitatively) may bean invaluable guide to the diagnosis.
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Hofuf Medical Forum |
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