HY lecture notes:
For majority of electrolyte disturbance Qs on the USMLE, answer will be simple normal saline (0.9% NaCl or Ringer lactate).
For low K (normal range 3.5-5.0 mEq/L), simply give K. You will see flattened T-waves or U-waves on ECG. Hypokalemia is classically seen in vomiting and diarrhea. It can also be caused by low magnesium. Low K is also the most common cause of death (arrhythmia) in eating disorders.
For high K, if there are ECG changes (peaked T waves classic, but can progress to PR and QRS abnormalities), answer = IV calcium to stabilize myocardium; do not choose mere saline first in this setting. Literature and previous resources fixate on specifically IV calcium gluconate, but there’s a new NBME Q for 2CK where the answer was IV calcium chloride (calcium gluconate wasn’t listed). So the bottom line is, just give IV calcium for hyperkalemia if there are ECG changes. In this above lecture I say calcium carbonate, but I meant to say calcium chloride.
If you get high K and no ECG changes, just give normal saline.
In short, regarding potassium, it causes cardiac changes most classically.
If you get any Q where the patient (usually an alcoholic) has low calcium or potassium not responding to supplementation, the answer = check serum magnesium levels. Alcoholics tend to get low Mg from dietary deficiency; the low Mg levels have nothing to do with malabsorption.
For low and high Na (normal range 135-145 mEq/L), give normal saline. Na needs to be corrected very very slowly (no more than 12-24 mEq / 24 hours) to prevent central pontine myelinolysis (correcting hyponatremia too quickly with hypertonic 3% saline) or cerebral edema (correcting hypernatremia too quickly with hypotonic 0.45% saline). Severe sodium disturbance leading to coma can be corrected with very small amounts of dilute or concentrated saline, but I’m yet to see it as an answer on the NBME. I’ve seen in the literature that up to 150 mL of hypertonic saline can be given super slowly in patients who have coma and hyponatremia.
Low Na is classic in diarrhea, SIADH, and psychogenic polydipsia. It can also be caused via a dilutional effect in hyperglycemia. High Na is classic in diabetes insipidus. In short, be aware that whilst K disturbance causes cardiac changes, Na disturbance causes CNS changes such as confusion and coma.
Low calcium (normal range 8.4-10.2 mEq/L) causes hypertonia and tetany (Chvostek sign: spasm of masseter muscle with palpation; or Trousseau sign: carpopedal spasm with inflated BP cuff). It is also seen in DiGeorge syndrome. Once again, low Ca that does not correct with supplementation, especially in an alcoholic, the answer = check serum Mg. Hypomagnesemia is a cause of low Ca and low K refractory to supplementation. Low Ca can also be due to rickets + osteomalacia (vitamin D deficiency in children vs adults, respectively) and in secondary hyperparathyroidism in renal failure. Black widow spider bites can also cause low Ca (HY for surg for some magical reason). To correct low Ca, simply supplement it.
High Ca is most of the time caused by primary hyperparathyroidism or metastatic malignancy leading to bone lysis. It can also be seen in multiple myeloma. High vitamin D due to granulomatous disease (sarcoidosis) is an important cause of high Ca (increased 1-alpha-hydroxylase activity by epithelioid macrophages). If Ca is 10.2-12 mEq/L, give saline; if 12-14, give saline and then add a bisphosphonate if symptomatic (confusion); if >14, give a bisphosphonate after the saline. Answers such as calcitonin and loop diuretics are almost always distractors. Calcitonin has a slight analgesic effect and can be given in someone who has hypercalcemia and bone pain.
Low phosphate (normal range 2.5-4.5 mEq/L) is seen in refeeding syndrome. Patient need not have a low BMI; it must simply be the case that patient has not eaten in awhile (anorexia; or if normal BMI, someone lost in the wilderness, etc.). The latter example is on the USMLE.
High phosphate is seen in tumor lysis syndrome. It’s also seen in renal failure with secondary hyperparathyroidism.
