7 FLUIDS, ELECTROLYTES, GATORADE, AND SWEAT
Alden H. Harken M.D.

1. What is hypertonic saline?

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Normal saline is 0.9% sodium chloride. Hypertonic saline is 7.5% sodium chloride (eight times as concentrated as normal saline).

KEY POINTS: ION CONCENTRATIONS IN CRYSTALLOID SOLUTIONS

1. ½ NS or 0.45% NaCl: 77 mEq of Na+, 77 mEq of Cl-
2. NS or 0.9% NaCl: 154 mEq of Na+, 154 mEq of Cl-
3. Hypertonic NS or 7.5% NaCl: 1283 mEq of Na+, 1283 mEq of Cl-
4. Lactated Ringer’s: 130 mEq of Na+, 110 mEq of Cl-, 38 mEq of lactate, 4 mEq of K+, and 3 mEq Ca+

2. What is hypertonic saline good for?

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Resuscitation. The initial hypothesis was that a little hypertonic saline would pull extravascular water into the intravascular compartment, rapidly restoring volume. It now appears that an osmotic jolt (even a transient jump from 140 to 180 mOsm) would pacify circulating neutrophils so that they do not stick to the endovasculature and provoke posttraumatic inflammation.

3. Is hypertonic saline good for anything else?

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Pacification of “primed” neutrophils should decrease the risk of posttraumatic multiple organ failure.

4. How do you convert 1 g of sodium into milliequivalents (mEq)?

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Divide by the atomic weight of sodium:

1g (1000 mg) of sodium ÷ 23 = 43.5 mEq

5. How many mEq of sodium are in 1 teaspoon of salt?

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104 mEq (or 2400 mg).

6. How many mEq of sodium are in an 8-oz bottle of Gatorade?

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5 mEq.

7. How much does a 40-lb block of salt cost?

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$3.40 at the feed store.

8. What is the electrolyte content of IV fluids?

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See Table 7-1.

Table 7-1. ELECTROLYTE CONTENT OF INTRAVENOUS FLUIDS

*Lactate is converted immediately to bicarbonate.

9. How do these concentrations relate to body fluid and electrolyte compartments?

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See Table 7-2.
Table 7-2. ELECTROLYTE CONCENTRATIONS IN BODY FLUIDS

10. What are the daily volumes (mL/24 h) and electrolyte contents (mEq/L) of body secretions for a 70-kg medical student?

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See Table 7-3.
Table 7-3. DAILY VOLUMES AND ELECTROLYTE CONTENTS OF BODY SECRETIONS

*See question 6.

11. Are sweat glands responsive to aldosterone? Can they be trained?

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Yes and yes. Archie Bunker’s sweat contains 100 mEq/L sodium, whereas an Olympic marathon runner retains sodium (sweat sodium may be as low as 25 mEq/L).

12. Is Gatorade really just flavored athlete’s sweat?

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Yes.

13. What are the daily maintenance fluid and electrolyte requirements for a 70-kg medical student?

14. Does the routine postoperative patient require IV sodium or potassium supplementation? Routine serum electrolyte testing?

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No and no.

15. Can a patient with a good heart and kidneys overcome all but the most woefully incompetent fluid and electrolyte management?

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Yes.

16. Can one throw a healthy medical student into congestive heart failure by IV infusion of 100 mL of 5% dextrose in saline solution per kg per hour?

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No. One will simply be ankle-deep in urine.

17. What is subtraction alkalosis?

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Vigorous nasogastric suction of a patient with a lot of gastric acid eliminates hydrochloric acid, leaving the patient alkaloti

c.
18. Which electrolyte is most useful in repairing a hypokalemic metabolic alkalosis?

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Chloride.

19. List the best indicators of a patient’s volume status.

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* Heart rate
* Blood pressure
* Urine output
* Big-toe temperature

20. Does a warm big toe indicate a hemodynamically stable patient?

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Most likely. The vascular autoregulatory ability of a young healthy patient is huge. The carotid and coronary circulations are maintained until the bitter end. Conversely, if the patient’s big toe is warm and perfused, the patient is stable.

21. What is the minimal adequate postoperative urine output?

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0.5 mL/kg/h.

22. What is a typical postoperative urine sodium?

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< 20 mEq/L.

23. Why?

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Surgical stress prompts mineralocorticoid (aldosterone) secretion so that the normal kidney retains sodium.

24. Explain paradoxical aciduria.

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Postoperative patients, by virtue of nasogastric suction (loss of gastric acid), blood transfusions (the citrate in blood is converted to bicarbonate), and hyperventilation (decreased Pco2), are typically alkalotic. Patients also are stressed, and their kidneys retain sodium and water. The renal tubules must exchange some other cations for the retained sodium. The kidney chooses to exchange potassium and hydrogen ions. Even in the face of systemic alkalosis, the postoperative kidney absorbs sodium and excretes hydrogen ions, producing a paradoxical aciduria.
KEY POINTS: MECHANISMS OF PARADOXICAL ACIDURIA

1. Nasogastric suction or refractory vomiting results in loss of gastric acid.
2. Physiologic stress promotes renal retention of sodium and water.
3. To hold on to sodium, the kidney must release other cations (potassium and hydrogen).
4. Counterintuitively, the kidney will release hydrogen ions to keep sodium, resulting in acidic urine.

25. What is third spacing?

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Hypotension and infection prime neutrophils (CD11 and CD18 receptor complexes), promoting adherence to vascular endothelial cells. Subsequent activation of adherent neutrophils spews out proteases and toxic superoxide radicals, blowing big holes in the vascular lining. Water and plasma albumin leak through the holes. The volume pulled out of the vascular space into the third space of the interstitial and hollow viscus (gut) creates relative hypovolemia and requires additional fluid replacement.

26. What is a Lasix sandwich?

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25% albumin followed by 20 mg of furosemide (Lasix) IV. If the patient is edematous, the IV albumin theoretically sucks water osmotically out of the interstitial third space. As the excessive water enters the vascular compartment, Lasix produces a healthy diuresis. In most intensive care unit patients, however, the infused albumin rapidly equilibrates across the damaged vascular endothelium. No additional water is pulled into the blood volume. Although surgeons often order Lasix sandwiches, they probably work only in healthy patients who do not need them.

References
BIBLIOGRAPHY
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